Tectonic implications of early Miocene extensional unroofing of the Sierra Mazata´n metamorphic core complex, Sonora, Mexico Martin S. Wong* Phillip B. Gans
Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA
ABSTRACT Sierra Mazata´n in northwest Mexico is the southernmost metamorphic core complex in the North American Cordillera. New geologic, structural, and 40Ar/39Ar thermochronologic data demonstrate that the core complex detachment fault accommodated 15–35 km of slip at a rate of 3.3–7.7 mm/yr between 20.5 and 16 Ma. These data also suggest that the lower plate was tilted eastward 208–508 since 20 Ma, indicating that the detachment fault initially dipped 308–608. Rapid slip on the fault occurred concurrently with subduction in Sonora and thus was not related to the ca. 12 Ma plate boundary change at this latitude or the late Miocene opening of the Gulf of California. Rapid extension at Sierra Mazata´n was synchronous with core complexes along the length of the Cordillera, suggesting a distinct 20–15 Ma core complex event, the fundamental causes of which remain a mystery. Keywords: metamorphic thermochronology.
core
complexes,
INTRODUCTION Vast areas of continental crust in northern Mexico were extended during the Cenozoic (Fig. 1) (Henry and Aranda-Gomez, 1990), but it remains unclear what forces drove extension in this region. Stock and Hodges (1989) proposed that areas adjacent to the *E-mail:
[email protected].
extension,
detachment
faults,
Ar/Ar,
Gulf of California, termed the Gulf extensional province, were extended during a distinct phase of middle to late Miocene protogulf (12–6 Ma) extension. In their model, subduction offshore of northwest Mexico ceased ca. 12 Ma. Oblique Pacific–North American plate motion was then partitioned between several hundred kilometers of strike-slip faulting on the Tosco-Abreojos fault outboard of Baja
Figure 1. Map of Gulf of California region showing areas of inferred Cenozoic extension and limits of Gulf extensional province (from Stock and Hodges, 1989). SM—Sierra Mazata´n. Modified from Henry and Aranda-Gomez (2000).
California and 160 6 80 km of orthogonal extension inboard within the Gulf extensional province. In contrast, Gans (1997) argued that most extension in the province occurred prior to 12 Ma during subduction and that distributed transtension in the province accommodated most of the 12–6 Ma Pacific–North American plate motion. These two models have been difficult to evaluate owing to the lack of data on the timing and magnitude of deformation in the province, especially in Sonora and Sinaloa. Sierra Mazata´n, 70 km east of Hermosillo in Sonora, Mexico, is the southernmost metamorphic core complex in the Cordillera (Coney, 1980) and is a key locality for understanding Neogene deformation in Sonora. The basic elements of the core complex were previously described (e.g., Anderson et al., 1980; Nourse et al., 1994; Vega-Granillo and Calmus, 2003), but little was known about the timing or magnitude of extension and whether the formation of Sonoran core complexes was related to earlier Basin and Range–type extension or to late Miocene rifting in the gulf. We present new geologic, structural, and 40Ar/ 39Ar thermochronologic data that constrain the timing, magnitude, rate, and style of extension at Sierra Mazata´n. These results shed new light on the Neogene extensional history of northwestern Mexico and reveal a striking synchroneity of core complex development in Sonora with the rest of the Cordillera. ´N GEOLOGY OF SIERRA MAZATA Geologic units in the core complex are divided into upper and lower plates separated by a low-angle detachment fault (Fig. 2). Most of the upper plate has been removed by erosion, and the lower plate forms a broad dome whose morphology mimics the bounding detachment fault. The lower plate is composed mainly of variably deformed Laramide granitic rocks, the main phase of which is a 58 6 3 Ma granite (Anderson et al., 1980). Subordinate lowerplate lithologies include banded gneisses, pegmatitic dikes and sills, and steeply dipping postmylonitic mafic dikes. Lower-plate rocks are weakly deformed in the eastern and central parts of the range but are increasingly mylonitized toward the west. The mylonites have a west-southwest– trending, shallowly plunging lineation defined by ribbon quartz and comminuted trails of mica and feldspar, and a subhorizontal folia-
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ern flank of the range. The fault is broadly arched over the range, dipping ;108WSW on the western side and 108–208NW on the northern flank (Fig. 2). The fault forms a broad synform north of the range and continues at least 50 km north to the town of Puerta del Sol (Vega-Granillo and Calmus, 2003). Erosional remnants of a subhorizontal, 5– 10-m-thick, densely welded ignimbrite unconformably overlie granitic footwall rocks north of the range. This unit contains phenocrysts of quartz, feldspar, and minor clinopyroxene and has yielded an 40Ar/39Ar age of 12.4 6 0.1 Ma on sanidine (Fig. DR1 and Table DR11). Petrographic and age similarities strongly suggest that this unit is correlative with the ca. 12.5 Ma Tuff of San Felipe reported by Oskin et al. (2001) in coastal Sonora. Regardless, this field relationship demonstrates that at least the eastern footwall had been completely exhumed by 12.4 Ma. 40Ar/39Ar
Figure 2. Simplified geologic map of Sierra Mazata´n. Lower-plate granites and mylonites that form range are separated from upper-plate limestone and clastic and volcanic rocks by low-angle detachment fault on western side of range. Stereonet projection (lower left) of mylonitic fabrics shows subhorizontal foliation (open circles) and west-southwest–trending lineation (dots). Thermochronologic samples M1–M4 were taken along transect subparallel to slip direction. Location of cross section in Figure 3 is A-A9.
tion defined by micas and flattened quartz (Fig. 2). Ubiquitous kinematic indicators, including S-C fabrics and sigma porphyroclasts, consistently indicate a top-to-the-southwest sense of shear. Petrographic observations, including ribbon and dynamically recrystallized quartz, brittle deformation of feldspars, and synkinematic growth of white mica and chlorite, suggest that ductile deformation occurred mainly at greenschist-grade conditions (300– 500 8C). Preserved remnants of the upper plate include Paleozoic limestone, altered Late Cretaceous volcanic rocks, and a sequence of 954
northeast-dipping Oligocene–Miocene(?) conglomerate, sandstone, shale, and minor andesite flows. The conglomerate contains mainly volcanic clasts in the lower part of the sequence and nonmylonitic granitic clasts in the upper part, suggesting syntectonic deposition and progressive unroofing of the footwall. A similar (correlative?) sedimentary sequence unconformably overlies footwall rocks ;20 km east of Sierra Mazata´n (east of area shown in Fig. 2) and may bracket the total amount of slip on the detachment fault. The bounding detachment fault is best exposed beneath limestone klippen on the west-
THERMOCHRONOLOGY We analyzed four K-feldspar samples from the western and central lower plate to assess the footwall’s cooling history. Samples were collected along an ;11-km-long transect parallel to the inferred west-southwest slip direction (Fig. 2). Several important features are evident in the 40Ar/39Ar K-feldspar age spectra (Fig. 3). (1) Apparent K-feldspar ages young toward the western (deepest) part of the footwall, ranging from 20–26 Ma for eastern sample M4 but only 15.5–18 Ma for western sample M1. (2) Age spectra from eastern samples M3 and M4 have low-temperature (T) flat segments ca. 20 Ma followed by steeply climbing segments at high T, whereas western samples M1 and M2 have flatter spectra. (3) The M1 age spectrum has a pronounced flat at 16 Ma but drops to 15 Ma at the lowest T steps. We applied multidomain diffusion analysis to the 40Ar/39Ar K-feldspar data, following Lovera (1992), to determine a thermal history for the samples from ;350 to 150 8C (e.g., Richter et al., 1991) (Appendix, Fig. DR2– DR5 and Tables DR2–DR5 [see footnote 1]). Our modeling indicates that eastern sample M4 cooled slowly to ;300 8C from 26 to 20.5 Ma and then was rapidly cooled (Fig. 3), although the duration and rate of this cooling event are not well constrained by this sample. Sample M3 resided at slightly higher temperatures than M4 and records a similar onset of rapid cooling at 20 Ma. Western samples M1 and M2 were well above K-feldspar closure temperature (;350 8C) prior to 20 Ma because 1GSA Data Repository item 2003144, 40Ar/39Ar tabulated data and spectra, is available online at www.geosociety.org/pubs/ft2003.htm, or on request from
[email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 803019140, USA.
GEOLOGY, November 2003
ples that are currently separated by 11 km horizontally but only 1 km vertically suggests that the footwall has been tilted 208–508 eastward since 20 Ma, depending on the assumed paleogeothermal gradient. Postmylonitic basaltic dikes in the footwall dip 708–758W, indicating .158–208 of postmylonitic eastward tilt. Middle Tertiary sedimentary rocks east of Sierra Mazata´n (east of area shown in Fig. 2) that unconformably overlie footwall rocks dip 308–508 eastward. Taken together, these data suggest that the footwall has been tilted 208– 508 eastward since 20 Ma, such that the currently low-angle (;108) fault would restore to a steeper initial dip of 308–608.
Figure 3. A: Cross section of Sierra Mazata´n, showing sample locations and paleogeothermal gradients (dashed lines) at 20 Ma interpreted from K-feldspar data. Gray lines are postmylonitic mafic dikes. B: 40Ar/39Ar K-feldspar spectra for samples M1–M4. C: Cooling histories of M1–M4 based on multidomain diffusion models of K-feldspar data (solid black lines) and estimated error (gray). Rapid footwall cooling began ca. 20.5 Ma and continued until at least 16 Ma, bracketing timing of major tectonic unroofing. Western footwall (M1 and M2) was above K-feldspar closure temperature at 20 Ma based on younger K-feldspar age spectra. Projected cooling rates (dashed lines) imply ~150–200 8C temperature difference between eastern and western footwall at 20 Ma.
they record only post–20 Ma rapid cooling. Sample M2 cooled rapidly from 350 to 200 8C between 19.5 and 17.5 Ma, whereas M1 cooled through the same temperature interval between 18 and 16 Ma, with the suggestion that cooling rates slowed abruptly after 16 Ma. These data suggest a simple footwall thermal history that includes (1) slow cooling and/or prolonged isothermal residence prior to 20.5 Ma followed by the inception of rapid cooling; (2) rapid cooling (.50 8C/m.y.) from 20.5 to 16 Ma; and (3) a western footwall that was hotter (deeper) than the eastern part at the inception of rapid cooling and cooled through any given temperature at a younger time. DISCUSSION An inevitable consequence of rapid, large displacement on a normal fault is that the footwall will cool as it is tectonically exhumed and juxtaposed against a progressively cooler hanging wall. Footwall thermochronology has been widely applied in extensional terranes to assess a variety of extensional parameters (e.g., Foster and John, 1999), and we use the same approach to assess the timing, magnitude, rate, and style of tectonic unroofing at Sierra Mazata´n. The abrupt change to rapid cooling of footwall rocks at 20.5 Ma suggests that slip on the detachment fault began at this time. Continued cooling of the western footwall indicates that slip on the fault occurred until at least 16 Ma. The steep, low-T age gradient from 15 to 16 Ma for sample M1 implies a change back to GEOLOGY, November 2003
slower cooling after 16 Ma and may bracket the end of rapid slip. In any case, slip on the fault had likely ceased by 12.4 Ma, when the ignimbrite was deposited directly on the eastern footwall. These data also broadly constrain the magnitude of slip on the fault. The western footwall cooled at least 200 8C and probably 300– 350 8C during rapid tectonic exhumation. The amount of footwall cooling is the measure of the total exhumation, the precise relationship being dependent on the fault geometry and geothermal gradient. To rapidly cool the footwall 350 8C requires 15–35 km of fault slip for a range of initial fault dips (308–608) and paleogeothermal gradients (20–30 8C/km). These slip estimates are supported by an apparent offset of a distinctive Tertiary sedimentary sequence that unconformably overlies footwall rocks ;20 km east (east of area shown in Fig. 2) of where correlative hanging wall rocks crop out north of Sierra Mazata´n. If 15–35 km of slip occurred between 20.5 and 16 Ma, then average slip rates were 3.3– 7.7 mm/yr, similar to those determined for other Cordilleran core complexes (e.g., Foster and John, 1999). Several lines of evidence indicate that the detachment fault was rotated to a lower angle after it formed. The cooling data indicate that the western footwall resided at higher temperatures than the eastern footwall at the inception and throughout the duration of slip. The implied 150–200 8C difference between sam-
REGIONAL IMPLICATIONS Large-magnitude extension at Sierra Mazata´n took place while subduction was still occurring beneath northwest Mexico (Stock and Hodges, 1989) and the core complex was situated in an intra-arc or backarc setting. This study adds to mounting evidence that extension in eastern and central Sonora occurred mainly during the late Oligocene to early Miocene (Gans, 1997). Significant extension is largely bracketed in eastern Sonora between 26 and 18 Ma (Gans, 1997; Blair and Gans, 2003) and in central Sonora between 25 and 12 Ma near Suaqui Grande (McDowell et al., 1997) and 20.5 and 16 Ma at Sierra Mazata´n. Late Miocene extension appears to be mainly limited to coastal Sonora, where deformation largely occurred between 12 and 9 Ma (MoraAlvarez and McDowell, 2000; MacMillan et al., 2003). These studies also support an apparent westward migration of the locus of extension during the Miocene that lagged behind but mimicked the inferred migration of the arc (Gans et al., 2003). Although the Stock and Hodges (1989) tectonic model for the gulf region invokes 160 6 80 km of protogulf (12–6 Ma) orthogonal extension in the Gulf extensional province, most of the extension from Sierra Mazata´n to eastern Sonora is too old to contribute to this proposed phase of deformation. Late Miocene deformation in Sonora may be largely restricted to an ;100-km-wide coastal belt and appears to be strongly transtensional (Gans et al., 2003). We do not rule out the suggestion by Henry and Aranda-Gomez (2000) that protogulf extension was distributed over a much broader area making it more difficult to recognize. However, the limited distribution and transtensional character of known deformation of protogulf age in Sonora makes us favor Gans’ (1997) model, wherein late Miocene Pacific–North American plate motion was not partitioned onto discrete outboard strike-slip and inboard extensional components, but rather was largely accommodated inboard within Baja California, Sonora, and Sinaloa. 955
Figure 4. Timing of rapid extensional slip in Cordilleran core complexes for which precise thermochronologic data are available. These data indicate nearly synchronous pulse of rapid extension in core complexes along length of much of Cordillera during early to middle Miocene despite profound differences in both plate-tectonic and local geologic settings.
Although Cordilleran core complex development was originally postulated to represent a distinct middle Tertiary event (e.g., Coney, 1980), improved timing constraints reveal a remarkable synchroneity of rapid slip on bounding detachment faults along the length of the Cordillera during the early to middle Miocene (mainly 20–15 Ma). While some core complexes clearly had an earlier history (e.g., Friedman and Armstrong, 1988), a pulse of rapid slip began in the Raft River core complex in Utah between 25 and 20 Ma (Wells et al., 2000), in the Snake Range in Nevada ca. 17 Ma (Miller et al., 1999), in the Chemehuevi Mountains in California (Foster and John, 1999) and the South Mountains in Arizona (Fitzgerald et al., 1994) ca. 22 Ma, and at Sierra Mazata´n in Sonora, Mexico, at 20.5 Ma (Fig. 4). These observations suggest the occurrence of a ca. 20–15 Ma Cordilleran-wide core complex event. This event appears to have been independent of the plate-tectonic setting at any particular latitude (cf. Dokka and Ross, 1995), in that the northern and southern core complexes were inboard of a convergent margin, whereas those in the central Cordillera were inboard of a developing transform. Although the significance of such an event remains unclear, it suggests that plate-boundary effects may not have played an instrumental role in the development of Cordilleran metamorphic core complexes. ACKNOWLEDGMENTS This work was funded by National Science Foundation grant EAR-0230439 and University of California Institute for Mexico and the United States Collaborative Grant (Gans and J. Roldan) and Dissertation Grant (Wong). We thank J. Roldan for logistical support, R. Vega and T. Calmus for valuable
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discussions and a preprint of their paper, and D. Foster and G.H. Davis for helpful reviews. REFERENCES CITED Anderson, T.H., Silver, L.T., and Salas, G.A., 1980, Distribution and U-Pb isotope ages of some lineated plutons, northwestern Mexico, in Crittenden, M.D., et al., eds., Cordilleran metamorphic core complexes: Geological Society of America Memoir 153, p. 269–283. Blair, K., and Gans, P.B., 2003, Stratigraphy of the Sahuaripa basin and preliminary comparison to the Rio Yaqui basin, east-central Sonora, Mexico: Geological Society of America Abstracts with Programs, v. 35, no. 4, p. 27. Coney, P.J., 1980, Cordilleran metamorphic core complexes: An overview, in Crittenden, M.D., et al., eds., Cordilleran metamorphic core complexes: Geological Society of America Memoir 153, p. 7–31. Dokka, R.K., and Ross, T.M., 1995, Collapse of southwestern North America and the evolution of early Miocene detachment faults, metamorphic core complexes, the Sierra Nevada orocline, and the San Andreas fault system: Geology, v. 23, p. 1075–1078. Fitzgerald, P.G., Reynolds, S.J., Stump, E., Foster, D.A., and Gleadow, A.J.W., 1994, Thermochronologic evidence for the timing of denudation and rate of crustal extension of the South Mountains metamorphic core complex and Sierra Estrella: Nuclear Tracks and Radiation Measurements, v. 21, p. 555–563. Friedman, R.M., and Armstrong, R.L., 1988, Tatla Lake metamorphic complex: An Eocene metamorphic core complex on the southwestern edge of the intermontane belt of British Columbia: Tectonics, v. 7, p. 1141–1166. Foster, D.A., and John, B.E., 1999, Quantifying tectonic exhumation in an extensional orogen with thermochronology: Examples from the southern Basin and Range province, in Ring, U., et al., eds., Exhumation processes: Normal faulting, ductile flow and erosion: Geological Society [London] Special Publication 154, p. 343–364. Gans, P.B., 1997, Large-magnitude Oligo–Miocene
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