Late Cenozoic geomorphic and tectonic evolution ... - GeoScienceWorld

Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 428S and 468S: An appraisal based on fission-track results from the transpressional intra-arc Liquin˜e-Ofqui fault zone

Stuart N. Thomson* Institut fu¨r Geologie, Mineralogie, und Geophysik, Ruhr-Universita¨t Bochum, D-44780 Bochum, Germany

ABSTRACT Fission-track (FT) thermochronology has been applied to investigate the low-temperature cooling and denudation history of the Patagonian Andes along the southern part of the intra-arc transpressional Liquin˜eOfqui fault zone between 428 and 468S. The Liquin˜e-Ofqui fault is shown to have been the focus of enhanced cooling and denudation initiated between ca. 16 and 10 Ma. Several fault blocks with different cooling histories are identified; these are separated by major oblique- or reverse-slip faults proposed to form the eastern part of a major (crustal-scale) dextral transpression zone. Local very fast rates of cooling and denudation between ca. 7 and 2 Ma were coeval with collision of the Chile Rise (an active mid-oceanic ridge) with the PeruChile Trench between ;478 and 488S. This location is close to the southern termination of the Liquin˜e-Ofqui fault, implying that the collision of the ridge was a major force driving late Cenozoic transpression. The lack of significant cooling and denudation before ca. 16 Ma is indicative of pure strike-slip or transtensional movement along the Liquin˜e-Ofqui fault before the collision of the ridge. Digital landscape analysis supports glacial and periglacial erosion as the main contributor to denudation since ca. 7 Ma, leading to restriction of topographic development. The combination of transpression-induced rock uplift and glacial erosion is shown to be very effective at causing localized denudation. Anomalously young FT ages along the Liquin˜e-Ofqui fault are attributed to the ex*E-mail: [email protected].

istence of a late Cenozoic localized heatflow anomaly along the fault. Keywords: Cenozoic, denudation, fissiontrack dating, landscape evolution, Patagonian Andes, transpression. INTRODUCTION Large (crustal-scale) intra-arc strike-slip faults are a common feature in the overriding plate at convergent-plate boundaries where subduction convergence is oblique to the plate margin (Fitch, 1972; Jarrard, 1986). Their existence is most commonly explained by interplate coupling causing partitioning of the oblique plate-convergence vector into two orthogonal components: trench-orthogonal compression—usually confined to the forearc and actual plate boundary—and trench-parallel strike-slip motion accommodated by discrete transcurrent faults and/or distributed wrench and shortening strain in the overriding plate (Beck, 1983; McCaffrey, 1992; Tikoff and Teyssier, 1994; Saint Blanquat et al., 1998; Chemenda et al., 2000). Because of the interaction of strike-slip offset and compression in the overriding plate, intra-arc faults are typically transpressional in nature (Fossen and Tikoff, 1998) and accompanied by significant rock uplift in the form of positive flower structures and crustal-scale pop-ups (e.g., Schreuers and Colletta, 1998). If high erosion rates are prevalent, then substantial, but localized denudation can occur across such transpressional fault zones. This study investigates the Liquin˜e-Ofqui fault zone of southern Chile. This fault zone is a classic intra-arc transpressional dextral strike-slip fault system situated within the southern Andes in the overriding plate above the obliquely subduct-

ing Nazca oceanic plate (Fig. 1). The Liquin˜eOfqui fault extends for ;1000 km from close to the Nazca–Antarctic–South American plate Chile triple junction at ;488S to near Liquin˜e in the Andean Cordillera at ;388S. This fault has been subject to numerous geologic studies (e.g., Thiele et al., 1986; Cembrano et al., 1996, 2000; Lavenu and Cembrano, 1999). However, the long-term nature and timing of movement and denudation along this important fault system remain poorly understood owing to a lack of detailed geochronologic and structural data. To address this, fissiontrack (FT) analysis has been applied on a regional scale around the southern part of the Liquin˜e-Ofqui fault. This technique is especially useful in defining fault history where other information such as kinematic and metamorphic data related to fault motion is poor or absent (e.g., Thomson, 1998). Several proposed, but contradictory tectonic models that attribute Cenozoic activity along the Liquin˜eOfqui fault to either strike-slip partitioning related to oblique subduction or late Miocene to Pliocene collision of the subducting Chile Rise spreading center with the Peru-Chile Trench are reassessed in light of the new data. ˜ E-OFQUI FAULT ZONE THE LIQUIN Tectonic Setting The present-day tectonic setting of the southern Andes and southeast Pacific at the latitude of the southern Liquin˜e-Ofqui fault is illustrated in Figure 1. Since ca. 25 Ma, the convergence vector between the Nazca and South American plates in the southern part of South America has retained a largely constant east-northeastward trend, leading to an angle of oblique convergence with respect to the

GSA Bulletin; September 2002; v. 114; no. 9; p. 1159–1173; 10 figures; Data Repository item 2002113.

For permission to copy, contact [email protected] q 2002 Geological Society of America

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Figure 1. Tectonic map of the southeast Pacific and southern Chile close to the Chile triple junction.

Peru-Chile Trench normal between 108 and 308 (Cande and Leslie, 1986; Pardo-Casas and Molnar, 1987; Somoza, 1998). The presence of the dextral Liquin˜e-Ofqui fault is commonly explained as being the result of strike-slip partitioning of weak crustal lithosphere in the region of the Andean magmatic arc (Beck, 1988; Rojas et al., 1994; Diraison et al., 1998; Cembrano et al., 1996, 1999; 2000; Lavenu and Cembrano, 1999). Regional seismicity confirms present-day trench-orthogonal shortening and a small component of trenchparallel dextral strike-slip motion north of the Chile triple junction (Cifuentes, 1989; Dewey and Lamb, 1992; Murdie et al., 1993). However, several authors have proposed that late Cenozoic activity of the southern segment of the Liquin˜e-Ofqui fault (Forsythe and Nelson, 1985; Nelson et al., 1994; Murdie et al., 1993) was caused by collision of the Chile Rise spreading center with the South American continental margin in the past 10 m.y. In this model, the Chile Rise acted as an indenter to the continental margin and caused a forearc sliver of the overriding plate to move northward, bounded on its eastern margin by the dextral strike-slip Liquin˜e-Ofqui fault. However, Cembrano et al. (2000) considered this

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model to describe a second-order mechanism that explains only the more recent dextral motion along the southern part of the fault zone. Regional Geology The geology around the southern part of the Liquin˜e-Ofqui fault is illustrated in Figure 2. It is dominated by calc-alkalic plutonic rocks of the Patagonian batholith. The northern part of the batholith (448–478S) exhibits a complex episodic pattern of Early Cretaceous to early Miocene emplacement ages (Pankhurst et al., 1999). Close to the fault, small bodies of late Miocene (ca. 10–5 Ma) peraluminous leucogranite crop out; they bear synmagmatic foliation related to dextral movement along the main fault trace (Herve´ et al., 1993). Alin-hornblende geobarometry and magmatic spessartine-rich garnet (Herve´ et al., 1996) indicate crystallization of one late Miocene pluton at a depth of 11 6 2 km, implying a late Cenozoic average denudation rate of ;1 mm/ yr close to the Liquin˜e-Ofqui fault. Despite reservations that Al-in-hornblende geobarometry only provides a maximum depth of crystallization (Anderson and Smith, 1995), Herve´ et al. (1996) applied this technique to the Cre-

taceous parts of the Patagonian batholith and derived crystallization depths of ;13 km at its eastern margin and as deep as ;20 km on its western margin, both implying low average denudation rates since intrusion of ;0.2 mm/ yr. Further evidence of low overall postCretaceous denudation is indicated by the presence of low-pressure metamorphic aureoles at the batholith margins (Pankhurst et al., 1999; Willner et al., 2000). Late Eocene to early Miocene marine volcano-sedimentary rocks of the Triagueń Formation (Herve´ et al., 1995) and Aycara Formation (Rojas et al., 1994) exist close to the Liquin˜e-Ofqui fault north of ;468S. Herve´ et al. (1995) proposed that these were deposited in pull-apart or asymmetric extensional basins in a transtensional setting along a precursor to the present Liquin˜e-Ofqui fault during a period of extremely oblique subduction. These rocks were deposited unconformably upon older parts of the Patagonian batholith and then were themselves intruded by younger Miocene plutons. Pankhurst et al. (1999) pointed out that the older parts of the batholith must have been denuded and exposed during sedimentation in the Eocene to early Miocene, reburied before intrusion by plutons at ;10

Geological Society of America Bulletin, September 2002

LATE CENOZOIC GEOMORPHIC AND TECTONIC EVOLUTION OF THE PATAGONIAN ANDES

Figure 2. Geologic map of southern Chile between 428S and 498S. The numbers are estimated emplacement ages of various granitoid plutons of the Patagonian batholith taken from Pankhurst et al. (1999) and Thomson et al. (2001), determined by several techniques for radiometric age dating.


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km depth during the late Miocene, and finally reexposed following a second phase of denudation that continues to the present day. The maximum depth of burial of the Traigueń Formation rocks is defined by pre–late Miocene amphibolite-facies contact metamorphism as being ,5 kilobars or 17 km (Herve´ et al., 1995). Fault History The Liquin˜e-Ofqui fault zone north of ;418S is characterized by a 1–2-km-wide belt of mylonite and cataclastic rocks. Here Cembrano et al. (1996, 2000) have described sinistral reverse shear-sense indicators that predate a ca. 100 Ma undeformed crosscutting dike. Farther south (;418309S), a complex pattern of brittle dextral strike-slip and reverse faulting is dominant. Here the fault rocks are much younger and cut late Miocene plutons. Cembrano et al. (2000) speculated that the lack of ductile deformation in this segment reflects a shallow level of exposure of fault rocks. At 428S, a 4–5-km-wide zone of ductile to brittle dextral strike-slip faulting disrupts the rocks of the Patagonian batholith. Here 40 Ar-39Ar ages from synkinematic muscovite and biotite have been interpreted by Cembrano et al. (1999, 2000) to date ductile movement to between 10 Ma and 4 Ma. More recent brittle deformation cuts rocks as young as 3.3 Ma. South of ;438S, the Liquin˜e-Ofqui fault is expressed as a single linear fault with both a brittle and ductile dextral oblique-slip component. Synkinematic biotites at ;448S gave 40Ar-39Ar ages of 4.2–3.8 Ma (Cembrano et al., 1999). Similar ages of 4.4 6 0.3 Ma were obtained from mylonites farther south at ;458–468S. East of the main Liquin˜e-Ofqui fault between ;448 and 468S, two major northnortheast–trending faults—the Azul-Tigre fault (after Romero, 1983) and the Rıó Man˜ihuales fault (J. Cembrano, 2001, written commun.)—are conspicuous as major features in satellite imagery and digital elevation models (Fig. 3). Across the Azul-Tigre fault Romero (1983) noted that K-Ar and Rb-Sr ages obtained by Halpern and Fuenzalida (1978) increase suddenly from younger than ca. 20 Ma to older than ca. 80 Ma, implying late Cenozoic relative vertical offset. One segment of the Azul-Tigre fault has been mapped (J. Cembrano, 2001, written commun.) as a several-hundred-meter-wide shear zone (labeled the Queulat shear zone) with dominant dextral reverse (top-to-the-northeast) shear-sense indicators. Some of these faults may be responsible for the complex pattern of counterclock-

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Figure 3. Obliquely viewed shaded relief map of the southern part of the Liquin˜e-Ofqui fault zone. The elevation data are taken from the public-domain U.S. Geological Survey GTOPO30 worldwide 30-arc-second-resolution digital elevation model (U.S. Geological Survey EROS Data Center, 1996). The elevation is shown with five times vertical exaggeration, and the map is lit at a 308 angle from the west. The region of digital elevation data analyzed to produce Figure 10 is outlined. The horizontal scale varies in this perspective image. wise rotation west of the main Liquin˜e-Ofqui fault and clockwise rotation within and to its east, revealed by paleomagnetic studies (Garcıá et al., 1988; Beck et al., 1993; Rojas et al., 1994; Beck et al., 2000). At the extreme southern end of the Liquin˜eOfqui fault near the Golfo de Penas at ;468309S, Quaternary displacement has been documented by Muir Wood (1989) and revealed by several seismic lines in the Golfo de Penas (Forsythe and Prior, 1992). Forsythe and Nelson (1985) and Murdie et al. (1993) interpreted the Golfo de Penas as a pull-apart basin at the southern end of the Liquin˜e-Ofqui fault, the formation of which can be related to the collision and subduction of ridge segments of the Chile Rise since ca. 10 Ma. Recorded historic seismic activity along the fault zone is low, although several small, shallow earthquakes demonstrate a dextral strike-slip faultplane solution (Cifuentes, 1989; Nelson et al., 1994; Lavenu and Cembrano, 1999). Several major Quaternary arc volcanoes, postglacial cinder cones, and numerous hot springs are also situated along the Liquin˜e-Ofqui fault. FISSION-TRACK THERMOCHRONOLOGY Apatite and zircon crystals were separated, mounted, polished, and etched according to

the techniques outlined by Hurford et al. (1991). The samples were analyzed by using the external-detector method and irradiated at the R3 graphite reflector facility at the Risø National Laboratory, Roskilde, Denmark. The neutron fluence was monitored by using Corning uranium-dosed glasses CN-5 for apatite and CN-2 for zircon. Spontaneous and induced FT densities were counted by using a Zeiss Axioplan microscope at 12503 magnification. Apatite FT lengths were measured by using an attached drawing tube and digitizing tablet calibrated against a stage micrometer. Central ages (Galbraith and Laslett, 1993) were calculated with the zeta-calibration approach of Hurford and Green (1983), recommended by the International Union of Geological Sciences. CN-5 apatite and CN-2 zircon zeta calibration factors of 354.1 6 5.6 and 130.7 6 2.8, respectively, were obtained by using recommended age standards (Hurford, 1990). GSA Data Repository Table DR11 presents 32 zircon and 28 apatite FT ages as well as 14 apatite confined FT-length analyses. All 1 GSA Data Repository item 2002113 is available on the web at http://www.geosociety.org/ pubs/ft2002.htm. Requests may also be sent to [email protected].



samples, with one exception, were collected from granitoid rocks of the Patagonian batholith. Detailed locations of the FT ages and track-length measurements are illustrated in Figure 4 together with a number of other FT ages previously presented in Thomson et al. (2001). All the apatite and zircon FT ages pass the chi-squared test and show low individual grain-age dispersions (,4%). INTERPRETATION OF THE FISSIONTRACK RESULTS Preamble Attempting to quantify amounts and rates of denudation from cooling histories derived from FT thermochronology requires knowledge of the upper-crustal geotherm over geologic time. In the southern Andes the regional surface heat flow is currently ;80–100 mW/ m2 (Hamza and Munõz, 1996); a local value of 97 6 15 mW/m2 was measured at Lago General Carrera at ;478S (Fig. 2) (Munõz, 1999). As the rocks investigated are largely granitoids with an upper-crustal thermal conductivity typically between 2.5 and 3.5 W/ (m·K) (e.g., Seipold, 1998), then the present near-surface average geothermal gradient must be between 23 and 45 8C/km (34 6 11 8C/ km). The crustal heat flow in the past is more difficult to assess. However, as the overall tectonic situation of the southern Andes has changed little during the late Cenozoic, significant regional digression from present-day values is unlikely, except close to shallow late Cenozoic plutons. The wide range of values also allows for effects of advection of heat in the upper crust during rapid denudation. Such advection can lead to an increased geothermal gradient and hence to overestimation of true denudation rates derived from cooling rates (Stu¨we et al., 1994; Mancktelow and Grasemann, 1997). Estimates of the timing of changes in denudation rates based on cooling histories are, in contrast, little affected by changes in the geothermal gradient. In this study, all denudation attributed to lowtemperature cooling is presumed to be the result of erosion. Denudation attributable to unroofing by extensional faulting occurs only on a very localized scale in the Patagonian Andes (e.g., Forsythe and Nelson, 1985; Muir Wood, 1989). Thermochronologic histories derived from FT data are based on the knowledge that fission tracks shorten or anneal with increased temperature and duration of heating. Models of annealing of fission tracks in apatite are well defined by experimental and borehole

data (e.g., Green et al., 1989; Ketcham et al., 1999). When the models are extrapolated to geologic time, annealing is predicted to occur at a sufficient rate to be measurable above ;60 8C. Above ;100–120 8C—depending on the chlorine content—the annealing rate is so high that the apatite FT age and mean track length are effectively reduced to zero. This range of temperatures is labeled the apatite partial-annealing zone (APAZ). In zircon, estimating the temperature limits of the partialannealing zone (ZPAZ) is complicated by the effects of radiation damage on the annealing process (see Brandon et al., 1998). In rocks where zircons are rapidly cooled from high temperatures, such as those investigated in this study, low accumulated radiation damage can be expected (Rahn et al., 2000). This circumstance favors application of the fanning and parallel kinetic-annealing models derived by Tagami et al. (1998) from experiments on pristine zircon. Estimates of the temperature limits of the ZPAZ for different heating durations based on these two annealing models are illustrated in Figure 5. The ZPAZ is defined by the 20% and 60% track-length reduction isopleths implied by the reduction in FT age across a contact-metamorphic aureole (Tagami and Shimada, 1996). Despite these annealing models being derived from experiments on zircons with low radiation damage, the models closely predict observed tracklength data in several ultradeep boreholes (Coyle and Wagner, 1996; Green et al., 1996; Tagami et al., 1996) where temperature and heating duration are well known. Independent estimates of the ZPAZ from natural zircons in high-pressure–low-temperature metamorphic rocks from Crete, Greece (Brix et al., 2002), are also consistent with the Tagami et al. (1998) models. Prior to Cenozoic denudation, the majority of the batholithic rocks in this study would have maintained relatively stable temperatures for as long as ;50 m.y. following their intrusion in the Late Cretaceous. For both Tagami et al. (1998) zircon FT-annealing models, such a heating duration corresponds to a ZPAZ with temperature limits of 250 6 10 8C to 310 6 20 8C. Regional Cooling and Denudation Pattern The regional pattern of FT ages (Fig. 4) is dominated by an east to west decrease in both apatite and zircon FT ages toward the main trace of the Liquin˜e-Ofqui fault. Two eastwest transects (Fig. 6) demonstrate that this decrease is not gradual, but characterized by abrupt Cretaceous to late Miocene apatite or zircon FT-age reductions coincident with seg-

ments of the Rıó Man˜ihuales fault, the AzulTigre fault, and the Liquin˜e-Ofqui fault. Other features of the FT-age distribution include an increase in zircon FT ages between the AzulTigre fault and the Liquin˜e-Ofqui fault north of ;448209S and some anomalously young apatite and zircon ages located close to some of the main faults. The drop to much younger apatite or zircon FT ages implies that the rocks on the western side of the major faults have cooled from higher pre–late Miocene paleotemperatures, and hence denuded from greater crustal depths relative to those rocks east of the fault. Where the age change occurs, the fault must represent a structure with considerable relative vertical offset. Indeed, late Miocene and Pliocene reverse-slip and reverse oblique-slip shear sense indicators have been identified along the Liquin˜e-Ofqui fault and several west-dipping shear zones to its east (Cembrano et al., 1999; J. Cembrano, 2001, written commun.). Differential cooling and denudation as recorded by the FT data on either side of a fault with a reverse-slip component is best achieved by the preferential erosion of the uplifted hangingwall block. However, where isolated Miocene magmatism has occurred, younger FT ages may indicate resetting or postmagmatic cooling and do not necessarily reflect an area of increased denudation and higher denudation rates. A schematic east-west profile across the Rıó Man˜ihuales fault and Azul-Tigre fault at 448409S, and across the Liquin˜e-Ofqui fault at 448S (Fig. 7) depicts interpretation of the FT data in terms of differentially uplifted and eroded fault blocks vertically offset by the major faults. Apart from the easternmost crustal block, each fault block is named after the fault on its eastern side to which the block forms the hanging wall. The Easternmost Crustal Block The oldest apatite FT ages in the study area, varying between 86 Ma and 33 Ma (Thomson et al., 2001), occur east of the Rıó Man˜ihuales fault (Fig. 4). These ages require residence at temperatures lower than the APAZ (;,60 8C) since the Late Cretaceous. Such temperatures are equivalent to depths of not more than ;1.3–2.6 km, if the present-day geothermal gradient of 34 6 11 8C/km is assumed. The average long-term cooling rate of these rocks is ,1 8C/m.y. (an equivalent time-averaged denudation rate of ,0.02–0.04 mm/yr). Al-inhornblende geobarometry of nearby Cretaceous batholithic rocks also implies low longterm average denudation rates of ,0.08 mm/


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Figure 4. Map showing the location of fission-track age and track-length data and the place names referred to in the text. The ages in gray were presented in Thomson et al. (2001). The location of the west-northwest–east-southeast age profiles of Figure 6, A and B, are outlined.

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n˜ihuales fault block, and between 6 and 3 Ma in the northern part at average cooling rates of ;20 8C/m.y., equivalent to denudation rates of between ;0.4 and 0.9 mm/yr. If such a rate had remained constant since the onset of accelerated cooling from initial temperatures of ,250 6 10 8C to ca. 9 Ma when the sample entered the APAZ at 110 6 10 8C, then accelerated cooling must have begun in the Rıó Man˜ihuales fault block more recently than ca. 16 Ma, but before ca. 10 Ma (middle to late Miocene) The young 10 6 1 Ma zircon FT age from the Rıó Man˜ihuales fault block can be explained either by the presence of a localized paleo–heat-flow anomaly sufficient to cause annealing of zircon fission tracks (.310 6 20 8C), a yet-undated local Miocene intrusion, or significant, but localized late Miocene uplift and denudation since ca. 10 6 1 Ma. The Azul-Tigre Fault Block Figure 5. Plot of the temperature limits against the heating duration of the zircon fissiontrack partial-annealing zone (ZPAZ) defined by 20% and 60% track-length reduction (Tagami and Shimada, 1996) calculated for both the fanning (gray) and parallel (black) kinetic-annealing models of Tagami et al. (1998). The open triangles are the measured temperature and estimated heating-duration values from several ultradeep boreholes (see text for references). The gray and black triangles are the temperatures predicted by the fanning and parallel kinetic-annealing models, respectively, based on the measured track length and estimated heating duration of each borehole. The temperature estimates of the ZPAZ based on fission-track results from high-pressure–low-temperature metamorphic rocks from Crete (Brix et al., 2002) are shown for comparison. KTB, Kola, and Vienna are the ultradeep boreholes described in the text. yr (Herve´ et al., 1996). However, post–Late Cretaceous cooling and denudation were not monotonic. Tertiary sedimentary rocks that overlie parts of the eastern Patagonian batholith (Marshall and Salinas, 1990) necessitate at least one cycle of erosion, burial, and further erosion. Zircon FT ages to the east of the Rıó Man˜ihuales fault vary between 98 Ma and 67 Ma (Thomson et al., 2001) and probably reflect the time of conductive cooling to below the ZPAZ following the intrusion of the batholith in the Cretaceous. The Rıó Man˜ihuales Fault Block All samples from the Rıó Man˜ihuales fault block have young (late Miocene) apatite FT ages of ,9 6 1 Ma and long mean FT lengths of .13.7 mm; with one exception, all the samples have Late Cretaceous zircon FT ages. The apatite data indicate accelerated cooling to the surface since the late Miocene from paleotemperatures of at least 110 6 10 8C (the high temperature limit of the APAZ), equivalent to a minimum denudation of at least ;2.5–5 km.

As the rocks east of the Rıó Man˜ihuales fault were at temperatures below ;60 8C in the Miocene, then the post–late Miocene relative vertical offset of that fault must be at least the width of the APAZ (50 6 10 8C), i.e., a minimum vertical displacement component of at least ;1 km. The zircon FT ages, with one exception, are unaffected by post-Cretaceous cooling. Accelerated Miocene cooling must have thus been initiated while the rocks resided at temperatures of less than ;250 6 10 8C (below the low-temperature limit of the ZPAZ) or at depths of less than ;6–11 km. The FT results thus limit the maximum permissible vertical offset of the Rıó Man˜ihuales fault to being ,190 6 10 8C or ;8.5 km (i.e., the lower limit of the ZPAZ west of the fault minus the lower limit of the APAZ east of the fault). Quantitative thermal modeling of the apatite FT data (Fig. 8A) using the inverse approach of Gallagher (1995) and the apatiteannealing model of Laslett et al. (1987) predicts cooling through the APAZ between 9 and 4 Ma in the southern part of the Rıó Ma-

Zircon FT ages decrease abruptly from Cretaceous to late Miocene westward across the southern segment of the Azul-Tigre fault south of ;448209S. A similar drop in Rb-Sr and K-Ar biotite ages has also been recognized (Halpern and Fuenzalida, 1978; Romero, 1983; Herve´, 1984). In contrast, north of ;448409S, Cretaceous zircon FT ages were obtained from both sides of the Azul-Tigre fault. Similar late Miocene and Pliocene apatite FT ages were acquired on both sides of the fault. South of ;458S, Miocene zircon FT ages indicate cooling from at least 310 6 20 8C to surface temperatures in the past 12 6 1 m.y. at an average minimum cooling rate of at least 25 8C/m.y. This cooling is equivalent to a minimum total denudation of ;7–14 km at a rates of at least 0.5–1.3 mm/yr. Cooling from even greater temperatures since ca. 16 Ma is implied by Miocene Rb-Sr and K-Ar biotite ages from Cretaceous granitoids in the southern part of the Azul-Tigre fault block (Herve´, 1984). Apatite FT-age and -length data demonstrate continued and near monotonic rapid cooling to below ;60 8C at ca. 5 6 1 Ma supported by apatite FT quantitative time-temperature modeling (Fig. 8B). The difference in zircon FT ages either side of this southern segment of the Azul-Tigre fault establishes that its post–late Miocene relative vertical displacement was at least the width of the ZPAZ (;60 8C or 1.3–2.6 km). A northward decrease in the relative vertical displacement is implied by the disappearance of the difference in zircon and apatite FT ages either side of the fault.


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rate of 1 mm/yr. These values are similar to those calculated from the Azul-Tigre fault block farther south away from the influence of Miocene magmatism. North of Puyuhuapi (448409S), older Cretaceous zircon FT ages indicate a northward decrease in the total post-Miocene cooling of the Azul-Tigre fault block, although some accelerated Miocene cooling is implied by the young apatite FT ages. Quantitative timetemperature modeling (Fig. 8B) supports rapid cooling through the APAZ between ca. 6 and 3 Ma at a rate of ;20 8C/m.y. At these latitudes, two Late Cretaceous apatite FT ages (70 and 68 Ma) from a Cretaceous granitoid in Argentina ;50 km east of the postulated trace of the Azul-Tigre fault indicate that the total amount of late Miocene cooling and erosion increase substantially westward toward the main trace of the Liquin˜e-Ofqui fault. Whether this east-to-west change is gradual or punctuated by faulting and preferential erosion of the Azul-Tigre fault block cannot be ascertained at present. The Liquin˜e-Ofqui Fault Block

Figure 6. Approximate west-northwest–east-southeast age-distance profiles across several major faults of the Liquin˜e-Ofqui fault zone, demonstrating well some of the marked age changes that are referred to in the text. The positions of the profiles are given in Figure 4. Between 448S and 448409S (Fig. 4), the Miocene cooling history west of the Azul-Tigre fault is complicated by extensive late Miocene magmatism, including the 10 6 1 Ma Rıó Cisnes leucogranite (Herve´ et al., 1993). The associated increased geothermal gradient and heat flow mean that cooling rates derived from the FT data are very high. Rates of .60 8C/m.y. are implied by the very young 4 6 1 Ma zircon FT ages from the Rıó Cisnes pluton

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and its surroundings. Similarly high rates of 50–135 8C/m.y. were determined by Herve´ et al. (1993) on the basis of 40Ar-39Ar hornblende and biotite ages. Late Cenozoic denudation rates, however, were not similarly increased in this part of the Azul-Tigre fault block. Geobarometric estimates of the depth of crystallization Rıó Cisnes leucogranite (Herve´ et al., 1993, 1996) reveal a maximum denudation of ;11 6 2 km in the past 10 m.y. at an average

Samples from west of the main trace of the Liquin˜e-Ofqui fault (i.e., in the Liquin˜e-Ofqui fault block) were collected only north of ;448S (Fig. 4). The Miocene and Pliocene apatite FT ages are similar or slightly younger on the west side of the fault, whereas the zircon FT ages are Cretaceous east of the fault and Miocene or younger to the west. Along the trace of the fault, several very young apatite ages (younger than 2 Ma) were obtained. The zircon FT ages require that this part of the Liquin˜e-Ofqui fault block has cooled from at least ;310 8C (or ;7–14 km) since ca. 9 Ma. The difference in zircon FT ages across the fault between ;448 and 428309 is evidence for a previously unidentified significant component of post-Miocene vertical displacement at least the width of the ZPAZ (i.e., at least ;60 8C or 1.3–2.6 km). The maximum amount of late Cenozoic cooling of the Liquin˜e-Ofqui fault block cannot be determined from the FT data. However, the maximum depth of burial, and hence denudation, of the Cenozoic Triagueń Formation sedimentary rocks immediately west of the Liquin˜e-Ofqui fault is defined by Miocene contact metamorphism to be ;5 kilobars or ;17 km (Herve´ et al., 1995). West of the Liquin˜e-Ofqui fault at ;438S, near-concordant zircon and apatite FT ages signify very fast cooling rates of at least 63 8C/m.y. (equivalent to denudation rates of .1.4–2.7 mm/yr), particularly between ca. 7 and 4 Ma. Deformation in fault



Figure 7. East-west profiles across the Azul-Tigre and Rıó Man˜ihuales faults at ;448409 and across the Liquin˜e-Ofqui fault at ;448S. Shown is the interpretation of the fission-track results in terms of pre–ca. 16 Ma fossil apatite and zircon partial-annealing zones (FAPAZ and FZPAZ, respectively). The relative vertical offsets and total amounts of denudation are visualized by assuming that no denudation occurred. The estimated cooling and denudation rates for each of the fault blocks are summarized in the lower part of the figure. rocks at ;428S has been independently dated to have occurred at 4.3 6 0.3 Ma (Cembrano et al., 1999). This result implies that rapid cooling and denudation of the Liquin˜e-Ofqui fault block were contemporaneous with fault activity. The very young apatite FT ages along the Liquin˜e-Ofqui fault require that these rocks were above 110 6 10 8C at ca. 1–2 Ma, while samples a few kilometers either side of the fault were at temperatures below ;60 8C. Such localized younger apatite FT ages can be produced by either (1) very localized rapid cooling and denudation at 1–2 Ma, (2) a shortlived heating event that reset the apatite ages (.110 6 10 8C) but not the zircon ages (;,260 8C) between 1 and 2 Ma, or (3) a long-lived heat-flow anomaly along the fault. Very localized rapid denudation can be effectively ruled out, as the other FT data obtained in this study show fast denudation to be a regional process. The other two possibilities are

interrelated, as they require the presence of either a short-lived or long-lived thermal anomaly along the Liquin˜e-Ofqui fault. Rapid monotonic cooling of rocks from one locality close to the fault near Puyuhuapi is indicated by a ca. 4 Ma biotite 40Ar-39Ar age (Cembrano et al., 1999), a 2.6 6 0.2 Ma zircon FT age, and a 1.0 6 0.3 Ma apatite FT age. This result favors existence of a relatively stable longterm thermal anomaly, supported by the presence of late Miocene to Pliocene magmatism (Pankhurst et al., 1992; Herve´ et al., 1993) and numerous hot springs and Holocene volcanism along the Liquin˜e-Ofqui fault. Older apatite and zircon FT ages between 9 and 6 Ma, ;5–10 km distant from the fault, are older than the ca. 4 Ma 40Ar-39Ar biotite ages (Cembrano et al., 1999) from the actual fault rocks. Thus, at ca. 4 Ma, the localized thermal anomaly along the fault must already have been present, because at this time the fault rocks were undergoing ductile deformation at

a temperature sufficient to totally anneal zircon fission tracks (.310 6 20 8C), while just a little away from the Liquin˜e-Ofqui fault, temperatures were below the APAZ (;,60 8C). DISCUSSION Transpression and Erosion: An Effective Denudation Mechanism The results of FT thermochronology have revealed large late Cenozoic relative vertical displacement of regional-scale fault blocks at the southern end of the Liquin˜e-Ofqui fault (south of ;438S). This block movement is related to reverse-slip and oblique-slip rather than purely strike-slip fault movement and clearly demonstrates the transpressional nature of the Liquin˜e-Ofqui fault system. The different rates and amounts of cooling and denudation revealed in the individual fault blocks


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Figure 8. Time-temperature (T-t) paths from four selected samples that closely predict the observed apatite fission-track data by using the MonteTrax inverse-modeling procedure (Gallagher, 1995) from (A) the northern and southern parts of the Rıó Man˜ihuales fault block and (B) the northern and southern parts of the Azul-Tigre fault block. Solid lines are the best-fit T-t paths, whereas those in gray match the observed FT data with 95% confidence. The observed FT data and those predicted by the best-fit paths are compared in the accompanying track-length histograms.

can be best interpreted as having been driven by the combined process of rock uplift induced by transpression and contemporaneous high erosion rates. FT thermochronology has additionally identified the Azul-Tigre and Rıó Man˜ihuales faults as being important tectonic features active in the late Miocene and Pliocene where enhanced cooling and denudation were initiated between ca. 16 and 10 Ma and where locally very high rates existed between ca. 7 and 2 Ma. Rock uplift of the discrete

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fault blocks was driven by transpressional tectonism (e.g., Fossen and Tikoff, 1998) in the overriding plate to the obliquely subducting Nazca plate. More localized mechanisms for rock uplift along strike-slip faults, such as compressional fault stepovers or restraining bends (e.g., Spotila et al., 1998), may have caused locally enhanced rock uplift and denudation, but could not have been responsible for the observed regional-scale cooling and denudation patterns. Oblique-slip and contrac-

tional shear zones in the eastern and western parts of the main Patagonian batholith have also been identified by Cembrano et al. (1999). These authors similarly proposed that transpression, in the form of a crustal-scale pop-up structure, caused post-Miocene uplift and denudation of the deeper ductile levels of the Liquin˜e-Ofqui fault. A revised schematic model is presented here to account for the results of FT thermochronology (Fig. 9). The model proposes the Azul-Tigre and Rıó Ma-



Figure 9. Three-dimensional block diagram interpretation of the Liquin˜e-Ofqui fault zone at ;448–458S, illustrating the relative differential block movements, if it is assumed that no denudation occurred since ca. 10 Ma. The Liquin˜e-Ofqui fault zone and the major oblique-slip or reverse faults to the east are interpreted as a positive transpressional flower structure. n˜ihuales faults forming the eastern part of a major upper-crustal-scale positive flower structure that roots to a broad, ductile, dextral transpressional shear zone at depth centered along the main trace of the Liquin˜e-Ofqui fault. Implications for Late Cenozoic Landscape Development in the Patagonian Andes One important aspect of the Patagonian Andes is the relative uniformity of the topography across the individual fault blocks of the Liquin˜e-Ofqui fault system, despite their different cooling and denudation histories. Such uniform topography implies a landscape close to steady state, whereby the recorded denudation has managed to approximately balance differential rock uplift since the late Miocene. What surface process has allowed denudation to counteract the different amounts and rates of rock uplift in each of the individual fault blocks? In the absence of significant extensional tectonism, the most obvious late Cenozoic erosional surface process in the Patagonian Andes is glacial erosion. The present landscape is totally dominated by periglacial and glacial landforms. Late Cenozoic glacialtill deposits between 7 and 4.6 Ma in age (Mercer and Sutter, 1982) record the first recognizable late Cenozoic glaciation in southernmost South America and the Patagonian

Andes. This deposition was followed by as many as 40 further glaciations until the Last Glacial Maximum. Other important erosional surface processes such as fluvial incision and mass movement by landsliding (e.g., Burbank et al., 1996) must have been relatively minor contributors to late Cenozoic denudation, as even in present interglacial times they generally act only to remove material generated by glacial erosion. In fact, the effects of widespread glacial overdeepening mean that the products of erosion become stored in lakes and fjords, and significant amounts of material are not transported away from the mountain belt and the areas of most rapid rock uplift. Hallet et al. (1996) estimated that basin-wide long-term average glacial erosion rates can approach 10–100 mm/yr for fast-moving, high ice-flux, temperate-glacier landscapes in Alaska, very similar to those of the Patagonian Andes. These rates are more than enough to counteract the high rock-uplift rates of the crustal fault blocks of the Liquin˜e-Ofqui fault system. The influence of glaciation and climate on landscape development in the northwestern Himalaya was investigated by Brozovic et al. (1997). Their observations support the late nineteenth century ideas of A. Penck and M. Dawson (cited in Brozovic et al. [1997]) that in mountain belts that intersect the snowline, glacial processes operating above the mean glacial-equilibrium-line alti-

tude (ELA) place an upper bound to the development of topography through which only a relatively small amount of material is allowed to pass irrespective of the rate of rock uplift. The Patagonian Andes provide convincing support for this proposal, especially as very rapid local rates of cooling and denudation between ca. 7 and 2 Ma occurred during a period of widespread glaciation. This proposal is also supported by the excellent correlation between several topographic characteristics and the late Cenozoic ELA (e.g., Brozovic et al., 1997). The present-day ELA of the North Patagonian ice field at ;478S varies from 1200 m on its western side to 1350 m on its eastern side (Lliboutry, 1999). During the Last Glacial Maximum—the second most extensive glacial maximum during the late Cenozoic in Patagonia according to Lliboutry (1999)—Hulton et al. (1994) calculated that the ELA was ;560 m lower at 408S and 160 m lower than at present at 508S. The long-term average ELA during the late Cenozoic must have been somewhere between these two values (i.e. ;100–300 m lower than at present). The topography of the Patagonian Andes has been analyzed (Fig. 10A) from the public-domain GTOPO30 global 30-arc-second DEM (U.S. Geological Survey EROS Data Center, 1996) from a region near Ayseń (see Fig. 3) where good-resolution data exist that incorporate both the Azul-Tigre fault and Rıó Man˜ihuales fault. The modal elevation for this region is ;1000 m, whereas the mean elevation of 818 m approximates the average late Cenozoic ELA value. From the hypsometric curve (Fig. 10B), the increase in glacial area resulting from the lowering of the ELA can be calculated (Brozovic et al., 1997). In the Patagonian Andes, lowering the ELA by between 160 and 560 m would increase the proportion of glaciated landscape by ;14%– 54%. Increased rock uplift can also lead to a relative lowering of the ELA (Brozovic et al., 1997). Any increase in the overall glaciated area should lead to enhanced ice flux and glacial-erosion rates sufficient to balance even very high rates of rock uplift and hence maintain the landscape close to a topographic steady state. Were the onset of glaciation and glacial erosional processes in the Patagonian Andes caused by climate change or an increase in elevation as the result of transpressioninduced rock and surface uplift? The evidence for significant transpressional tectonism along the Liquin˜e-Ofqui fault related to the intrusion of the Rıó Cisnes granite at ca. 10 Ma (Herve´ et al., 1993) signifies that transpression-induced rock uplift was under way well before


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surfaces (Fig. 3) capped by late Miocene flood basalts (Ramos, 1989). Here, Late Cretaceous apatite FT ages (Fig. 4) imply very little erosion since the late Miocene (Thomson et al., 2001). The maintenance today of the east-towest change from steady-state to pre–steadystate landscape, despite the large amounts and fast rates of post-Miocene rock uplift and denudation localized close to the Liquin˜e-Ofqui fault, implies that transpressional tectonism has had little long-term influence on the post– late Oligocene topographic development of the Patagonian Andes (see Thomson et al., 2001). Localized Elevated Heat Flow Along the Liquin˜e-Ofqui Fault: Fluid Flux and/or Shear Heating?

Figure 10. Topographic analysis of USGS GTOPO30 30 arc-second digital-elevation-model data (U.S. Geological Survey EROS Data Center, 1996) from the Patagonian Andes between lat 458S and 468S and long 728W and 738W in the region of Puerto Ayseń and Coyhaique, southern Chile. These analyses assume no postglacial isostatic rebound and/ or sea-level change. (A) Elevation hypsometry and elevation vs. slope distributions showing the correlation between the present-day modal elevation and the present and Last Glacial Maximum glacial equilibrium line altitudes. The anomalous peak at ;500 m corresponds to a large lake (Lago Yulton) in the west of the sampled area. (B) Hypsometric curve to demonstrate the large increase in the proportion of ice-covered area during the Last Glacial Maximum.

the initiation of widespread glaciation. This finding favors climate change rather than an increase in elevation as the most likely cause for the onset of late Cenozoic glaciation in the Patagonian Andes.

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In contrast to the topography close to the main faults, most of the landscape of the easternmost Patagonian Andes has clearly not yet reached steady state, as is evident from the preservation of abundant planar relict land

The presence of the long-lived localized thermal anomaly along the main trace of the Liquin˜e-Ofqui fault may be explained by several possible mechanisms: Heat flux via hydrothermal fluids and magmatism concentrated along the fault zone (e.g., Strong and Hanmer, 1981; Hutton and Reavy, 1992), localized crustal thickening (e.g., D’Lemos et al., 1992, and Hutton and Reavy, 1992), or shear heating generated by sustained fault- or shear-zone slip (e.g., Brun and Cobbold, 1980; Lachenbruch and Sass, 1980, 1992; Thatcher and England, 1998; Leloup et al., 1999). The presence of hot springs, volcanism, and late Miocene to Pliocene magmatic rocks along parts of the Liquin˜e-Ofqui fault supports localized heat advection as having contributed significantly to late Cenozoic localized elevated heat flow. However, several of the granitoid samples that yield the young (1–2 Ma) apatite FT ages show little or none of the alteration that would be expected had these rocks been subject to significant long-term hydrothermal fluid throughput. Also, to produce such a localized hydrothermal fluid flux requires the presence of a preexisting thermal anomaly in the deeper parts of the fault zone generated by an endogenic process such as crustal thickening or shear heating. Crustal thickening along transpressional shear zones can lead to granite generation and the production of a thermal anomaly (D’Lemos et al., 1992; Hutton and Reavy, 1992). However, Leloup et al. (1999) argued that without mantle delamination, this process can only occur late in the development of a strike-slip fault zone because of the time lag required for sufficient granite-melt generation after thickening to occur. However, high heat flow and late Miocene leucogranite generation clearly developed early in the history of late Cenozoic



transpressional activity along the Liquin˜eOfqui fault. Numerical thermomechanical modeling of frictional and ductile shear heating caused by sustained shear-zone slip (e.g., Lachenbruch and Sass, 1980, 1992; Thatcher and England, 1998; Leloup et al., 1999) can generate localized anomalies in surface heat flow and temperature increases at depth. In the numerical models, the heat generated by this process is assumed to be transported by conduction. This assumption, if accurate, could explain why the samples close to the Liquin˜eOfqui fault have remained unaltered by hydrothermal fluid circulation, but were still heated sufficiently to give rise to localized younger apatite FT ages. Leloup et al. (1999) proposed that heat advection by rising fluids and melts in large-scale strike-slip fault zones can cause local accentuation of the thermal anomaly, although initial heat input provided by shear heating is still required. The temporal coincidence of granitic melts with transpressional tectonism in many major strike-slip shear zones led Leloup et al. (1999) to further argue that shear heating, perhaps combined with local decompression, can provide a sufficient increase in the geotherm in the lower crust and upper mantle to promote partial melting and generation of syntectonic granitic melts without the need for crustal thickening. This model provides a viable alternative for the genesis of the several peraluminous leucogranites found along the Liquin˜e-Ofqui fault, including the late Miocene Rıó Cisnes leucogranite. Previously, Herve´ et al. (1993) proposed that this granite was generated by partial melting in the lower crust or upper mantle induced by heat input from subduction-related mantle-derived granitic magmas followed by decompression melting related to rapid uplift and high erosion rates along the Liquin˜e-Ofqui fault. However, the FT results show that the most rapid denudation rates in the rocks surrounding the Rıó Cisnes granite occurred well after its emplacement. This model also does not adequately explain why all the known peraluminous leucogranites within the Patagonian batholith have been intruded only since the late Miocene, coeval with the initiation of the main phase of late Miocene activity of the Liquin˜e-Ofqui fault. Leloup et al. (1999) demonstrated that shear heating can cause an increase in the lowercrustal geotherm to temperatures above that required for melt generation by muscovite and even biotite dehydration, hence providing an environment for the generation of leucogranitic melts early in the history of transpressional tectonism. The anomalously young (ca. 1–2 Ma) apa-

tite FT ages from along the Liquin˜e-Ofqui fault require that a fault-localized thermal anomaly still existed at this time, signifying ongoing activity along this fault related to either shear heating or pervasive heat advection by rising hydrothermal fluids and melts. The continued ascent of melts along the fault is marked by Holocene volcanism in the Puyuhuapi area (448209S). Present-day dextral strike-slip seismic activity along the fault has also been recorded in a few minor earthquakes (Cifuentes, 1989). Tectonic Implications Locally, the very rapid cooling and denudation between ca. 7 and 2 Ma related to transpression and erosion along the southern part of the Liquin˜e-Ofqui fault correspond temporally with the collision of several short segments of the Chile Rise spreading center with the Pacific margin of South America at ;488– 498S, close to the present southernmost termination of the Liquin˜e-Ofqui fault (Cande and Leslie, 1986; Murdie et al., 1993; Nelson et al., 1994). This temporal correspondence implies that spreading-center collision acted as a major driving force sustaining the late Cenozoic dextral transpressional tectonism along the southern part of the Liquin˜e-Ofqui fault. No significant cooling or denudation that could be linked by using FT thermochronology to transpressional tectonism along the Liquin˜e-Ofqui fault has been identified prior to the collision of the ridge. This result contradicts the suggestion of Cembrano et al. (1999) that a dextral transpressional regime driven by plate coupling related to oblique subduction of the Nazca plate has existed in the Patagonian Andes for much of the Cenozoic. Instead it is likely that the Liquin˜e-Ofqui fault was purely transcurrent or even transtensional during much of this period and that compression in the overriding plate was accommodated by an alternative mechanism such as accretion at the plate margin, more regional compression and crustal thickening, or backarc folding and thrusting. A transtensional environment along the Liquin˜e-Ofqui fault up to the middle Miocene is further indicated by deposition of the Eocene to middle Miocene volcaniclastic Ayacara and Traigueń Formations in strike-slip–related pull-apart basins (Rojas et al., 1994; Herve´ et al., 1995). CONCLUSIONS The application of FT thermochronology along the intra-arc Liquin˜e-Ofqui fault zone in the Patagonian Andes of southern Chile be-

tween lat 428S and 468S has led to the following main conclusions: 1. Since between ca. 16 Ma and 10 Ma the Liquin˜e-Ofqui fault was the focus of enhanced late Cenozoic cooling and denudation caused by transpression-induced rock uplift balanced by high rates of glacial erosion. Very fast cooling occurred locally close to the Liquin˜eOfqui fault between 7 and 2 Ma. 2. FT thermochronology has identified a considerable vertical- or oblique-slip component of movement along the Liquin˜e-Ofqui fault and two other steeply dipping faults— the Azul-Tigre fault and the Rıó Man˜ihuales faults—between lat ;438S and 468S. The amount and rate of late Cenozoic cooling and denudation increase from east to west toward the fault zone, with each fault block showing varied rates and total amounts of cooling and denudation. 3. A structural model is proposed whereby the main faults link at depth somewhere below the main trace of the Liquin˜e-Ofqui fault to form the eastern part of a crustal-scale dextral transpressional flower structure or crustal popup. 4. The onset of the most rapid phase of denudation agrees with tectonic models that propose collision of segments of the Chile Rise spreading center as the main driving force behind late Cenozoic transpression along the Liquin˜e-Ofqui fault. The lack of apparent denudation recorded by the FT data before ca. 16 Ma implies that the fault zone was earlier dominated by horizontal or transtensional strike-slip tectonism. This interpretation is supported by the presence of Eocene to middle Miocene sedimentary rocks deposited in pull-apart basins west of the Liquin˜e-Ofqui fault. 5. Digital-elevation-model analysis indicates that late Cenozoic topographic development of the Patagonian Andes was limited by glacial-erosion processes. The similarity of the topography between the various fault blocks, despite their different cooling and denudation histories, implies that glacial erosion has acted at such a rate so as to completely balance any rock uplift caused by transpression and has maintained the landscape close to steady state. 6. Anomalously young apatite FT ages along the Liquin˜e-Ofqui fault indicate the existence of a long-lived late Cenozoic heat-flow anomaly localized within a few kilometers of the fault. This anomaly can be attributed to a combination of late Cenozoic shear heating and advection of fluids and granitic melts up the main fault trace.


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S.N. THOMSON ACKNOWLEDGMENTS This study would not have been possible without the kind support, advice, and collaboration of Francisco Herve´ and Jose´ Cembrano at the Universidad de Chile. This work was supported by a German Science Foundation (DFG) Stipendium Th 573/2-1. Field work was funded by DFG grants Sto 196/111 and Sto 196/11-2. The zircon fission-track age of sample Br286 is published with the permission of Manfred Brix. Sample preparation was carried out at the Ruhr-Universita¨t Bochum under the supervision of Frank Hansen. This manuscript has also benefited greatly from discussions with Bernhard Sto¨ckhert, Manfred Brix, and Alberto Adriasola. The helpful comments and suggestions of reviewers Matthew Burns, Diane Seward, and David Foster are also much appreciated. REFERENCES CITED Anderson, J.L., and Smith, D.R., 1995, The effects of temperature and ƒO2 on the Al-in-hornblende barometer: American Mineralogist, v. 80, p. 549–559. Beck, M.E., 1983, On the mechanism of tectonic transport in zones of oblique subduction: Tectonophysics, v. 93, p. 1–11. Beck, M.E., 1988, Analysis of Late Jurassic–Recent paleomagnetic data from active plate margins of South America: Journal of South American Earth Sciences, v. 1, p. 39–52. Beck, M.E., Rojas, C., and Cembrano, J., 1993, On the nature of buttressing in margin-parallel strike-slip fault systems: Geology, v. 21, p. 755–758. Beck, M.E., Burmester, R., Cembrano, J., Drake, R., Garcia, A., Herve´, F., and Munizaga, F., 2000, Paleomagnetism of the North Patagonian batholith, southern Chile. An exercise in shape analysis: Tectonophysics, v. 326, p. 185–202. Brandon, M.T., Roden-Tice, M.K., and Garver, J.I., 1998, Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State: Geological Society of America Bulletin, v. 110, p. 985–1009. Brix, M.R., Sto¨ckhert, B., Seidel, E., Theye, T., Thomson, S.N., and Ku¨ster, M., 2002, Thermobarometric data from a fossil zircon partial annealing zone in highpressure–low temperature rocks of eastern and central Crete, Greece: Tectonophysics, v. 349, p. 309–326. Brozovic, N., Burbank, D.W., and Meigs, A.J., 1997, Climatic limits on landscape development in the northwestern Himalaya: Science, v. 276, p. 571–574. Brun, J.P., and Cobbold, P.R., 1980, Strain heating and thermal softening in continental shear zones: A review: Journal of Structural Geology, v. 2, p. 149–158. Burbank, D.W., Leland, J., Fielding, E., Anderson, R.S., Brozovic, N., Reid, M.R., and Duncan, C., 1996, Bedrock incision, rock uplift, and threshold slopes in the northwestern Himalaya: Nature, v. 379, p. 505–510. Cande, S.C., and Leslie, R.B., 1986, Late Cenozoic tectonics of the Southern Chile Trench: Journal of Geophysical Research., v. 91, p. 471–496. Cembrano, J., Herve´, F., and Lavenu, A., 1996, The Liquin˜e-Ofqui fault zone: A long lived intraarc fault system in southern Chile: Tectonophysics, v. 259, p. 55–66. Cembrano, J., Lavenu, A., Arancibia, G., Lo´pez, G., and Sanhueza, A., 1999, Crustal-scale pop-up structure at the southern Andes plate boundary zone: A kinematic response to Pliocene transpression: Go¨ttingen, Germany, Fourth International Symposium on Andean Geodynamics (ISAG), abstract volume, p. 151–154. Cembrano, J., Schermer, E., Lavenu, A., and Sanhueza, A., 2000, Contrasting nature of deformation along an intraarc shear zone, the Liquin˜e-Ofqui fault zone, southern Chilean Andes: Tectonophysics, v. 319, p. 129–149. Chemenda, A., Lallemand, S., and Bokun, A., 2000, Strain partitioning and interplate friction in oblique subduction zones: Constraints provided by experimental

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modeling: Journal of Geophysical Research, v. 105, p. 5567–5581. Cifuentes, L.I., 1989, The 1960 Chilean earthquake: Journal of Geophysical Research, v. 94, p. 665–680. Coyle, D.A., and Wagner, G.A., 1996, Fission-track dating of zircon and titanite from the 9101 m deep KTB: Observed fundamentals of track stability and thermal history reconstruction: Geological Institute, University of Gent, Belgium, International Workshop on Fission Track Dating, abstract volume, p. 22. D’Lemos, R.S., Brown, M., and Strachan, R.A., 1992, Granite magma generation, ascent and emplacement within a transpressional orogen: Journal of the Geological Society, London, v. 149, p. 487–490. Dewey, J.F., and Lamb, S.H., 1992, Active tectonics of the Andes: Tectonophysics, v. 205, p. 79–95. Diraison, M., Cobbold, P.R., Rossello, E.A., and Amos, A.J., 1998, Neogene dextral transpression due to oblique convergence across the Andes of northwestern Patagonia, Argentina: Journal of South American Earth Sciences, v. 11, p. 519–532. Fitch, T.J., 1972, Plate convergence, transcurrent faults, and internal deformation adjacent to southeast Asia and the western Pacific: Journal of Geophysical Research, v. 77, p. 4432–4460. Forsythe, R., and Nelson, E., 1985, Geological manifestations of ridge collision: Evidence from the Golfo de Penas–Taitao Basin, southern Chile: Tectonics, v. 4, p. 477–495. Forsythe, R., and Prior, D., 1992, Cenozoic continental geology of South America and its relations to the evolution of the Chile triple junction, in Behrmann, J.H., Lewis, S.D., Musgrave, R.J., et al., eds., Proceedings of the Ocean Drilling Program, Initial Reports, Leg 141: College Station, Texas, Ocean Drilling Program, v. 141, p. 23–31. Fossen, H., and Tikoff, B., 1998, Extended models of transpression and transtension, and application to tectonic settings, in Holdsworth, R.E., Strachan, R.A., and Dewey, J.F., eds., Continental transpressional and transtensional tectonics: Geological Society of London Special Publication 135, p. 15–33. Galbraith, R.F., and Laslett, G.M., 1993, Statistical models for mixed fission track ages: Nuclear Tracks, v. 21, p. 459–470. Gallagher, K., 1995, Evolving temperature histories from apatite fission-track data: Earth and Planetary Science Letters, v. 126, p. 421–435. Garcıá, A., Beck, M.E., Burmester, R.F., Herve´, F., and Munizaga, F., 1988, Paleomagnetic reconnaissance of the Regioń de Los Lagos, southern Chile, and its tectonic implications: Revista Geolo´gica de Chile, v. 15, p. 13–30. Green, P.F., Duddy, I.R., Laslett, G.M., Hegarty, K.A., Gleadow, A.J.W., and Lovering, J.F., 1989, Thermal annealing of fission tracks in apatite. 4. Quantitative modelling techniques and extension to geological timescales: Chemical Geology (Isotope Geoscience Section), v. 79, p. 155–182. Green, P.F., Hegarty, K.A., Duddy, I.R., Foland, S.S., and Gorbachev, V., 1996, Geological constraints on fission track annealing in zircon: Geological Institute, University of Gent, Belgium, International Workshop on Fission Track Dating, abstract volume, p. 44. Hallet, B., Hunter, L., and Bogen, J., 1996, Rates of erosion and sediment evacuation by glaciers: A review of field data and their implications: Global and Planetary Change, v. 12, p. 213–235. Halpern, M., and Fuenzalida, R., 1978, Rubidium-strontium geochronology of a transect of the Chilean Andes between latitudes 458S and 468S: Earth and Planetary Science Letters, v. 41, p. 60–66. Hamza, V.M., and Munõz, M., 1996, Heat flow map of South America: Geothermics, v. 25, p. 599–646. Herve´, F., 1984, Rejuvenecimiento de edades radiometricas en la zona de falla Liquin˜e-Ofqui, en Ayseń: Departamento de Geologıá, Universidad de Chile, Comunicaciones, no. 34, p. 107–115. Herve´, F., Pankhurst, R.J., Drake, R., Beck, M.E., and Mpodozis, C., 1993, Granite generation and rapid unroofing related to strike-slip faulting, Ayseń, Chile: Earth and Planetary Science Letters, v. 120, p. 375–386.

Herve´, F., Pankhurst, R.J., Drake, R., and Beck, M.E., 1995, Pillow metabasalts in a mid-Tertiary extensional basin adjacent to the Liquin˜e-Ofqui fault zone: The Isla Magdalena area, Ayseń, Chile: Journal of South American Earth Science, v. 8, p. 33–46. Herve´, F., Pankhurst, R.J., Demant, A., and Ramirez, E., 1996, Age and Al-in-hornblende geobarometry in the North Patagonian batholith, Aysen, Chile: St. Malo, France, Third International Symposium on Andean Geodynamics (ISAG), abstract volume, p. 17–19. Hulton, N., Sugden, D., Payne, A., and Clapperton, C., 1994, Glacier modeling and the climate of Patagonia during the Last Glacial Maximum: Quaternary Research, v. 42, p. 1–19. Hurford, A.J., 1990, Standardization of fission track dating calibration: Recommended by the Fission Track Working Group of the I.U.G.S., Subcommission on Geochronology: Chemical Geology (Isotope Geoscience Section), v. 80, p. 171–178. Hurford, A.J., and Green, P.F., 1983, The zeta age calibration of fission-track dating: Isotope Geoscience, v. 1, p. 285–317. Hurford, A.J., Hunziker, J.C., and Sto¨ckhert, B., 1991, Constraints on the late thermotectonic evolution of the western Alps: Evidence for episodic rapid uplift: Tectonics, v. 10, p. 758–769. Hutton, D.H.W., and Reavy, R.J., 1992, Strike-slip tectonics and granite petrogenesis: Tectonics, v. 11, p. 960–967. Jarrard, R.D., 1986, Relations among subduction parameters: Reviews of Geophysics, v. 24, p. 217–284. Ketcham, R.A., Donelick, R.A., and Carlson, W.D., 1999, Variability of apatite fission-track annealing kinetics: III. Extrapolation to geological time scales: American Mineralogist, v. 84, p. 1235–1255. Lachenbruch, A.H., and Sass, J.H., 1980, Heat flow and energetics of the San Andreas fault zone: Journal of Geophysical Research, v. 85, p. 6185–6222. Lachenbruch, A.H., and Sass, J.H., 1992, Heat flow from Cajon Pass, fault strength, and tectonic implications: Journal of Geophysical Research, v. 97, p. 4995–5015. Laslett, G.M., Green, P.F., Duddy, I.R., and Gleadow, A.J.W., 1987, Thermal annealing of fission tracks in apatite: 2. A quantitative analysis: Chemical Geology (Isotope Geoscience Section), v. 65, p. 1–13. Lavenu, A., and Cembrano, J., 1999, Compressional- and transpressional-stress pattern for Pliocene and Quaternary brittle deformation in fore arc and intraarc zones (Andes of central and southern Chile): Journal of Structural Geology, v. 21, p. 1669–1691. Leloup, P.H., Ricard, Y., Battaglia, J., and Lacassin, R., 1999, Shear heating in continental stike-slip shear zones: Model and field examples: Geophysical Journal International, v. 136, p. 19–40. Lliboutry, L., 1999, Glaciers of the Wet Andes, in Williams, R.S., and Ferrigno, J.G., eds., Satellite image atlas of glaciers of the world: South America: U.S. Geological Survey Professional Paper 1386-I, http:// pubs.usgs.gov/prof/p1386i/index.html (last accessed on 16 May 2002). Mancktelow, N.S., and Grasemann, B., 1997, Time-dependent effects of heat advection and topography on cooling histories during erosion: Tectonophysics, v. 270, p. 167–195. Marshall, L., and Salinas, P., 1990, Stratigraphy of the Rio Frias Formation (Miocene) along the alto Rıó Cisnes, Aisen, Chile: Revista Geolo´gica de Chile, v. 17, p. 57–87. McCaffrey, R., 1992, Oblique plate convergence, slip vectors, and forearc deformation: Journal of Geophysical Research, v. 97, p. 8905–8915. Mercer, J.H., and Sutter, J.F., 1982, Late Miocene–earliest Pliocene glaciation in southern Argentina: Implications for global ice-sheet history: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 38, p. 185–206. Muir Wood, R., 1989, Recent normal faulting at Laguna de San Rafael, Aisen Province, southern Chile: Departamento de Geologıá, Universidad de Chile, Comunicaciones, no. 40, p. 57–68. Munõz, M., 1999, Tectonophysics of the Andes region: Relationships with heat flow and the thermal structure: Go¨ttingen, Germany, Fourth International Sym-


LATE CENOZOIC GEOMORPHIC AND TECTONIC EVOLUTION OF THE PATAGONIAN ANDES posium on Andean Geodynamics (ISAG), abstract volume, p. 532–534. Murdie, R.E., Prior, D.J., Styles, P., Flint, S.S., Pearce, R.G., and Agar, S.M., 1993, Seismic responses to ridge-transform subduction: Chile triple junction: Geology, v. 21, p. 1095–1098. Nelson, E., Forsythe, R., and Arit, I., 1994, Ridge collision tectonics in terrane development: Journal of South American Earth Sciences, v. 7, p. 271–278. Pankhurst, R.J., Herve´, F., Rojas, L., and Cembrano, J., 1992, Magmatism and tectonics in continental Chiloe´, Chile (428–428309S): Tectonophysics, v. 205, p. 283–294. Pankhurst, R.J., Weaver, S.D., Herve´, F., and Larrondo, P., 1999, Mesozoic–Cenozoic evolution of the North Patagonian batholith in Ayseń, southern Chile: Journal of the Geological Society, London, v. 156, p. 673–694. Pardo-Casas, F., and Molnar, P., 1987, Relative motion of the Nazca (Farallon) and South American plates since Late Cretaceous times: Tectonics, v. 6, p. 233–248. Rahn, M.K., Brandon, M.T., Batt, G.E., and Garver, J.I., 2000, Fission-track annealing in ‘‘zero damage’’ zircons: Field constraints and an empirical model: Ninth International Workshop on Fission Track Dating and Thermochronology, Lorne, Australia: Geological Society of Australia Abstracts Series, no. 58, p. 271. Ramos, V., 1989, Andean foothills structures in Northern Magallanes basin, Argentina: American Association of Petroleum Geologists Bulletin, v. 73, p. 887–903. Rojas, C., Beck, M.E., Burmester, R.F., Cembrano, J., and Herve´, F., 1994, Paleomagnetism of the mid-Tertiary Ayacara Formation, southern Chile: Counterclockwise rotation in a dextral shear zone: Journal of South American Earth Sciences, v. 7, p. 45–56. Romero, G.A., 1983, Geologıá del sector Alto Palena– Puerto Ramirez. Chiloe continental [Diploma thesis]: Santiago, Departamento de Geologıá, Universidad de Chile. Saint Blanquat, M., Tikoff, B., Teyssier, C., and Vigneresse,

J.L., 1998, Transpressional kinematics and magmatic arcs, in Holdsworth, R.E., Strachan, R.A., and Dewey, J.F., eds., Continental transpressional and transtensional tectonics: Geological Society of London Special Publication 135, p. 327–340. Schreuers, G., and Colletta, B., 1998, Analogue modelling of faulting in zones of continental transpression and transtension, in Holdsworth, R.E., Strachan, R.A., and Dewey, J.F., eds., Continental transpressional and transtensional tectonics: Geological Society of London Special Publication 135, p. 59–79. Seipold, U., 1998, Temperature dependence of thermal transport properties of crystalline rocks—A general law: Tectonophysics, v. 291, p. 161–171. Somoza, R., 1998, Updated Nazca (Farallon)–South America relative motions during the last 40 My: Implications for mountain building in the central Andean region: Journal of South American Earth Sciences, v. 11, p. 211–215. Spotila, J.A., Farley, K.A., and Sieh, K., 1998, Uplift and erosion of the San Bernardino Mountains associated with transpression along the San Andreas fault, California, as constrained by radiogenic helium thermochronometry: Tectonics, v. 17, p. 360–378. Strong, D. F., and Hanmer, S.K., 1981, The leucogranites of southern Brittany: Origin by faulting, frictional heating, fluid flux and fractional melting: Canadian Mineralogist, v. 19, p. 163–176. Stu¨we, K., White, L., and Brown, R., 1994, The influence of eroding topography on steady-state isotherms. Application to fission-track analysis: Earth and Planetary Earth Science Letters, v. 124, p. 63–74. Tagami, T., and Shimada, C., 1996, Natural long-term annealing of the zircon fission track system around a granite pluton: Journal of Geophysical Research, v. 101, p. 8245–8255. Tagami, T., Carter, A., and Hurford, A.J., 1996, Natural long-term annealing of the zircon fission-track system in Vienna basin deep borehole samples: Constraints upon the partial annealing zone and closure tempera-

ture: Chemical Geology (Isotope Geoscience Section), v. 130, p. 147–157. Tagami, T., Galbraith, R.F., Yamada, R., and Laslett, G.M., 1998, Revised annealing kinetics of fission tracks in zircon and geological implications, in Van den haute, P., and De Corte, F., eds., Advances in fission-track geochronology: Dordrecht, Netherlands, Kluwer Academic Publishers, p. 99–112. Thatcher, W., and England, P.C., 1998, Ductile shear zones beneath strike-slip faults: Implications fro the thermomechanics of the San Andreas fault: Journal of Geophysical Research, v. 103, p. 891–905. Thiele, R., Herve´, F., Parada, M.A., and Godoy, E., 1986, The Liquin˜e-Ofqui megafault at the Reloncavı´ Fiord (418309S), Chile: Departamento de Geologıá, Universidad de Chile, Comunicaciones, no. 46, p. 3–15. Thomson, S.N., 1998, Assessing the nature of tectonic contacts using fission-track thermochronology: An example from the Calabrian Arc, southern Italy: Terra Nova, v. 10, p. 32–36. Thomson, S.N., Herve´, F., and Sto¨ckhert, B., 2001, The Mesozoic–Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes: Tectonics, v. 20, p. 693–711. Tikoff, B., and Teyssier, C., 1994, Strain modeling of displacement field partitioning in transpressional orogens: Journal of Structural Geology, v. 16, p. 1575–1588. U.S. Geological Survey EROS Data Center, 1996, GTOPO30 (edition 1): Sioux Falls, South Dakota, U.S. Geological Survey. Willner, A.P., Herve´, F., and Massonne, H.-J., 2000, Mineral chemistry and pressure-temperature evolution of two contrasting high-pressure–low-temperature belts in the Chonos Archipelago, southern Chile: Journal of Petrology, v. 41, p. 309–330. MANUSCRIPT RECEIVED BY THE SOCIETY MAY 23, 2001 REVISED MANUSCRIPT RECEIVED JANUARY 31, 2002 MANUSCRIPT ACCEPTED MARCH 12, 2002 Printed in the USA


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