Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude: Northwestern Argentina David M. Pearson1,2,*, Paul Kapp1, Peter G. DeCelles1, Peter W. Reiners1, George E. Gehrels1, Mihai N. Ducea1,3, and Alex Pullen1 1
Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA Department of Geosciences, Idaho State University, 921 South 8th Avenue, Pocatello, Idaho 83209, USA 3 Facultatea de Geologie Geofizica, Universitatea Bucuresti, Strada N. Balcescu Nr 1, Bucuresti, Romania 2
ABSTRACT The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24–25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely reflecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12–10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of *E-mail:
[email protected].
shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250–350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can influence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities. INTRODUCTION Cordilleran-style orogens form during convergence of oceanic and continental plates and are characterized by long belts of continental magmatism and shortening. An active example of such an orogenic system is in South America, where shortening of the overriding plate results in continued growth of the Andes. In spite of the documentation of major along-strike variations in the style and magnitude of Cenozoic shortening in the Andes (e.g., Allmendinger et al., 1983; Isacks, 1988; Kley and Monaldi, 1998; Kley et al., 1999), there is not a considerable alongstrike difference in the relative convergence velocity of the oceanic and continental plates nor in the age of the subducting oceanic Nazca plate (Oncken et al., 2006). In contrast, some of the observed spatial variations in the style and magnitude of Cenozoic retroarc shortening match with changes in pre-Andean stratigraphy and structure of the South American plate (e.g., Mpodozis and Ramos, 1989; Allmendinger and Gubbels, 1996; Kley et al., 1999). For example,
in northernmost Argentina and Bolivia (Fig. 1; 17–23°S), Cenozoic thin-skinned shortening within a thick Paleozoic basin exceeds 300 km (Fig. 2; e.g., McQuarrie, 2002). Southwest of Salta, Argentina (Fig. 1; ~25°S), where this thick Paleozoic basin was absent prior to Cenozoic time, steeply dipping reverse faults that are locally inverted normal faults are suggested to have accommodated 6000 m), externally drained Cenozoic thrust belt with predominantly west-vergent structures in Argentina that transition northward into a bivergent system in Bolivia; and (3) the Santa Bárbara Ranges, a region near the modern deformation front that consists of mainly eastdipping reverse faults, transitioning along strike northward to the Subandes, a W-dipping thinskinned thrust belt in northernmost Argentina and southern Bolivia. Paleozoic Architecture The Paleozoic geology of western Bolivia consisted of >10 km of sedimentary rocks deposited in a back-arc setting during Cambrian to Carboniferous time (Fig. 2; e.g., Sempere, 1995). In northwestern Argentina, this Paleozoic basin was shallower and more limited in extent (Fig. 2; e.g., Starck, 1995; Egenhoff, 2007) and formed on the northeastern flank of a NNW-trending
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Paleozoic basement high (Transpampean arch; Figs. 1B and 2A; Tankard et al., 1995), thought to reflect a remnant Ordovician (“Ocloyic”) mountain belt (Mon and Salfity, 1995; Starck, 1995). Poorly constrained Late Devonian to Mississippian orogenesis from central Argentina to Peru (“Eohercynian/Chañic” orogenesis) eroded the original margins of the Ordovician to Carboniferous basin (Fig. 2A; Starck, 1995). Although Ordovician rocks are prevalent on the Puna Plateau west of this regional transect, Ordovician to Devonian rocks are not exposed in the Eastern Cordillera southwest of Quebrada del Toro (Fig. 1B), indicating that this locality may approximate the northeastern boundary of major Paleozoic deformation. Mesozoic Extension Widespread but low-magnitude Mesozoic extension affected much of western South America and has been variously attributed to
Geosphere, December 2013
Major crustal shortening in the Central Andes began after the South American plate overrode the subduction zone during opening of the South Atlantic Ocean (e.g., Coney and Evenchick, 1994). Deformation propagation has been sporadic through time, but most authors agree that shortening in the central Andean thrust belt began in Late Cretaceous to early Eocene time in northern Chile (Sempere et al., 1997; Arriagada et al., 2006; Jordan et al., 2007) and propagated in an overall eastward direction. In northwestern Argentina, growth strata and apatite fission-track data reflect 40–30 Ma deformation in the eastern Puna Plateau and western Eastern Cordillera, followed by an enhanced period of exhumation from 20 to 15 Ma (Andriessen and Reutter, 1994; Coutand et al., 2001; Deeken et al., 2006; Hongn et al., 2007; Carrapa and DeCelles, 2008; Bosio et al., 2009; Carrapa et al., 2011). A pre–15 Ma (Reynolds et al., 2000), possibly Eocene angular unconformity across the Santa Bárbara Ranges (Salfity et al., 1993) may reflect an early phase of shortening or the eastward migration of a flexural forebulge (DeCelles et al., 2011). In contrast to the 40–15 Ma evolution of the thrust belt, interpretations of the 15–0 Ma deformation history in northwestern Argentina vary significantly (Carrapa et al., 2011; Hain et al., 2011). Some authors use regional correlations of sedimentary rocks interpreted in the context of a flexural foreland basin to infer a progressive eastward migration of the thrust belt (e.g., DeCelles et al., 2011). Others suggest that an initially intact flexural depositional system was “broken” in mid- to late Miocene time as basement-involved reverse faults were initiated or reactivated away from what was once a continuous, along-strike thrust front and foreland basin (e.g., Hain et al., 2011).
Neoproterozoic to Middle Cambrian Puncoviscana and La Paya Formations: variably metamorphosed turbidites
Og
Єg
pЄp
Geosphere, December 2013
10DP07
40 Ma
A
116 km shortening (95 km in Eastern Cordillera + 21 km in SB Ranges)
belt in Eastern Cordillera. Section is thus semi-balanced
21 Lower detachment of Kley and Monaldi (2002) cropped for simplicity and consistency with thrust
seismic data (Kley and Monaldi, 2002)
20 Regional elevation of Cretaceous Balbuena Subgroup and deeper basin structure constrained by
19 Décollement depth constrained by thickness of Mojotoro fault hanging wall
antithetic fault that loses slip toward the south beneath the Lerma Valley.
18 This structure, with an open, S-plunging anticline in its footwall, is likely a lower-displacement
17 Here, a remnant of Cretaceous strata is truncated by the Lesser fault (new name)
along-strike, but accommodated less displacement.
16 The Quijano fault (new name) to the east apparently branches with the Pascha fault (new name)
10
5
0
5
3
A
4
Cachi fault
09DP071 40 Ma1
1
pЄp
Og
2
13.8 Max
13.8 Ma
09DP301
77
5 6
7
Calchaquí Valley
Cz
08DP081
57
Toro Muerto fault
66.25° W
62
KTb
Cz
32
La Poma
50
pЄp
60
Calchaquí fault
38 Ma*x
38 Ma* 09DP42
85
90
pЄp
61
5.2 Ma
66° W
7.3 ± 0.3 Ma
Cz
53
t
Cz
79
Q d a de
09DP46
pЄp
8
Capillas fault
34
ul
Cz
Ca pil l as
KTb
50
42
60
9
Solá fault
Gólgota fault
10 11 12
20
13
14
47
12.1 Ma
65
7.1 Ma
83
65.5° W
ЄOr
15
16
17 Lesser fault
9.1 ± 0.2 Ma 9.0 ± 0.1 Ma 12.8 ± 0.2 Ma
Cz
65
ЄOr
63
5.2 Ma
09DP15
29
10DP08 8.5 Ma 53
9.7 ± 0.2 Ma
37
QdT
65.75° W
70
QT42
6.3 ± 0.1 Ma
*81
ЄOr
Zamanca 4.4 ± 0.1 Ma fault
4.2 Ma 09DP47
79
28
16
4.7 Ma 10DP07
53
18
19
Lerma Valley
Lerma Valley
Salta
Cz
ЄOr
9.3 Ma
20 Ma
35
Cz
pЄp
45
50
?
10
65.25° W
Mojotoro fault
21 Max 52 Max 58 Max 74 Max 10.4 ± 0.2 Ma
Єg
83
66
Cz
Cz
20
65° W
Cz
Lavayén Valley
Lavayén Valley
General Güemes
Cz
Cz
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Kley
KTb
Cz
dM an
21
KTb
ЄOr
i, 2 ald on
p
ЄOr
ma 00 2
64.75° W
ЄOr
KTb
KTb
Cz
ЄOr
KTb
Modified from Kley and Monaldi, 2002
Cz
Figure 3. Regional map and balanced cross section, with sample locations and (U-Th)/He apatite and zircon age results. Superscript references: 1—Pearson et al. (2012); 2—Adams et al. (2008). Map and cross section in the Santa Bárbara Ranges are modified from Kley and Monaldi (2002); notably, we excluded their interpreted deeper décollement. See text for additional information. Abbreviations: QdT— Quebrada del Toro; SB—Santa Bárbara Ranges.
strata in Quebrada del Toro (DZ samples 08DP01 and 08DP03). * Growth U-Pb tuff sample 08DP04 = 9.4 Ma
*All (U-Th)/He apatite grains from this sample have very low eU
Youngest (U-Th)/He age from sample
Weighted mean (U-Th)/He apatite ages at 2σ uncertainty; xSingle grain
U-Pb detrital zircon sample locality
21 Ma
(U-Th)/He apatite age Cenozoic exhumation > 13.8 Ma thermo-chronometer closure depth
Neogene exhumation < thermo-chronometer closure depth
15
Cz
47
14
13
12
trending folds indicate minor pre-13 Ma shortening on the Gólgota fault and ~10 Ma above Solá fault Cambro-Ordovican strata and the W-verging anticline seen to the north are eroded farther south, indicating a southward increase in along-strike displacement Possible 5-10° angular unconformity between Cambro-Ordovician rocks and a sliver of structurally higher Cretaceous strata Displacement along the Pascha fault increases southward, with erosion of a W-verging hanging-wall anticline to the south Strata dip more steeply here than in hanging wall of Solá fault, suggesting that these structures were rotated during later displacement on the Solá fault
11 Subtle growth strata in the footwall of the W-dipping Solá pop-up fault within a series of ~N-S
42
89
(U-Th)/He zircon age
Overturned anticline axial trace
Overturned syncline axial trace
Cross section
Monocline axial trace
Syncline axial trace
Anticline axial trace
Og
A
pЄp
Og
36
A’
55
45
25 Overturned bedding
Bedding orientation
24.75° S
Cach i fault
09DP061
Czb
51
Geo- and Thermochronology
30
Dip of fault (triangle) and trend and plunge of striae on fault surface (diamond)
Bedding parallel cleavage, facing unknown
36
likely a truncated fault-propagation fold.
Reverse faults juxtaposed planar, ~30° E-dipping back limbs against overturned footwall synclines. Folding and faulting here likely represent deformation in the footwall of the E-dipping Capillas fault (new name) whose displacement increases toward the north. 9 Cenozoic sediment of possible Plio-Pleistocene age buried the E-dipping Zamanca fault (new name) that bounds the western margin of the Zamanca Range. 10 E-dipping Zamanca fault is dominant due to consistent along-strike hanging wall geometry; therefore W-dipping structure is pop-up
8
7 W-verging, locally overturned syncline in footwall of the Calchaquí fault (Hongn et al., 2007) is
an overturned footwall syncline along its trend. Folding of the Puncoviscana Formation is required to overturn this contact and folds are interpreted as fault-propagation folds. 6 Structural relief here constrains restored depth/dip of décollement
5 The Toro Muerto fault consists of a ~100 m wide zone of intense strain that structurally overlies
Cz
Lampasillos Range 85
Strike-slip fault with relative motion
Reverse fault; teeth on hanging wall
Lithologic contact
Solid where well located, dashed where poorly located, dotted where buried
Geological symbols
Mainly Upper Cambrian to Ordovician sedimentary rocks, with local Silurian to Devonian in the eastern part of the map area
Lower Ordovician granitoids
Cambrian granitoids
ЄOr
3 Pirgua Group conglomerates in the hanging wall of the Cachi fault south of the transect are up to
3 km thick adjacent to their western fault boundary; their absence to the west indicates that this is an inverted normal fault at the western boundary of the Salta Rift (Carrera et al., 2006). 4 Décollement dip constrained by structural relief in syncline in Calchaquí Valley (point 4)
2 E-dipping structure is dominant, therefore W-dipping structure is inferred antithetic fault
1 Eroded hanging wall geometry constrained by (U-Th)/He data (Pearson et al., 2012)
Mainly Cretaceous rocks of Pirgua and Balbuena Subgroups
Elevation (km)
KTb
Cachi Range
fault
rto ue M To ro
ea ar
Cross section notes
57
Cenozoic
64
Quaternary basalt flows
72
R a nge Pasch a
Ra ng e Lesse r
toro R ange
M ojo
Cz
a f a u lt
64
ult a fa sa d
an c
Me
Z am
í fault chaqu Cal
ge o t a f a ul t
Calchaquí Valley
R an G ól g
ult Pasch a f a u lt
ang e
Czb
Quijano fault
51
ul t
C
s illa ap
im R
C a p i ll as f a
n ca Lesser fault
a o Tor
Mojotoro fa
del
Unch
Zama
ang e
la s da
nton io R
a de brad Que bra
San A
64.5° W
ЄOr
a Ra nge ne l
Cen ti
e ng
Ra
e te
KTb
Cz
64.25° W
A’
Deformed
Cz
0
A’
Restored A’
25 km
No Vertical Exaggeration
Cz
Piq u
áf e Qu
m ap
53
02
S ol
67
ldi , 20
43
Mo na
Zapla
and
Rock Units
Kley
are a
The oldest unit exposed in the retroarc of the Central Andes is the Puncoviscana Formation, which consists of variably metamorphosed siltstone, argillite, and turbiditic sandstone, and it constitutes the majority of outcrop in the mapped area (Fig. 3). In the Cachi Range, the westernmost mountain range in this transect, the Puncoviscana Formation exhibits a gradational contact with the higher-metamorphic-grade La Paya Formation (Galliski, 1983). These rocks have Neoproterozoic to Cambrian protoliths and are correlative (Pearson et al., 2012); for this reason and for simplicity, these rocks are collectively referred to as the Puncoviscana Formation. In general, the Puncoviscana Formation exhibits finer grain size toward the west, including outcrops of chert south of La Poma (Fig. 3). Locally, however, metamorphic recrystallization has increased grain size. In contrast, in the eastern Lampasillos Range and Quebrada de las Capillas to the east (Fig. 3), the Puncoviscana Formation consists of 10–30-cm-thick fine-grained quartzites that are interbedded with 5–100-cm-thick slate beds. Closer to Quebrada del Toro to the east, these rocks alternate on the kilometer scale with low-grade metapelitic rocks characterized by 3–20-m-thick siltite and finegrained quartzite interspersed with slate. In the eastern Lampasillos Range and Quebrada de las Capillas, primary depositional features are common, including flute casts and ripple marks (Fig. 4A), and some rocks are volcaniclastic. In the Lesser and Mojotoro Ranges (Fig. 3), the Puncoviscana Formation mainly consists of low-grade, fine-grained quartzite and metapelite. Northwest of the Quebrada del Toro (Figs. 1 and 3), 526–517 Ma plutons (Hongn et al., 2010) intruded the Puncoviscana Formation. In the Cachi Range, 485–483 Ma granitoids (Fig. 2; Pearson et al., 2012) also intruded the Puncoviscana Formation and are among the northernmost outcrops of the Famatinian magmatic arc. East of Quebrada del Toro (Fig. 3), the Middle to Upper Cambrian angular unconformity between highly deformed rocks of the Puncoviscana Formation and overlying quartzite and shale of the Upper Cambrian Mesón Group (Adams et al., 2011) is exposed. Lower Ordovician shale and quartzite of the Santa Victoria Group overlie the Cambrian Mesón Group. Together, these rocks constitute the majority of the Pascha, Lesser, and Mojotoro Ranges (Fig. 3). Up to 2 km of synrift Cretaceous nonmarine conglomerate and sandstone of the Pirgua Subgroup disconformably overlie Paleozoic rocks in the Santa Bárbara Ranges (Salfity and Marquillas, 1994; Kley and Monaldi, 2002). In several localities, synrift depocenters corre-
Legend
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
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Pearson et al.
~N
~S
or
F
cordierite porphyroblasts
isca
na
Pu nc
Ba 0
u
n ti o
ov
lbu en
aS
ub
gro
p
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A
0
0.25 m
C
D
~5 m
W Toro Muerto fault
E n
Puncoviscana Formatio
overturned angular unconformity
tion rma o F ite ora Yac
0
E
E
W
brian Cam for eg rou n
Gólgota fault Pre 12.8 Ma angular unconformity
dc ol l uvi um
Qls co
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~15 m
~NW
fo
n
0
F ~SE
~
a en u b Ba l
p rou g b Su
~10 m
0
30 cm
Cambrian
0
~400 m
fo r egro und collu vium
rm
ab le
12
. 8 Ma lava flo wa Ba v rre ss an ds ton e
Figure 4. Outcrop photos. (A) Flute casts in Puncoviscana Formation turbidites in Quebrada de las Capillas; (B) major angular unconformity (>450 m.y.) between Puncoviscana Formation and Balbuena Subgroup rocks in the Quebrada de las Capillas; (C) irregular folding of likely early Paleozoic age within Puncoviscana Formation in Lampasillos Range; (D) overturned syncline and angular unconformity in the footwall of the Toro Muerto fault in the eastern Cachi Range; (E) likely fault-propagation fold in Balbuena Subgroup rocks south of La Poma; and (F) overturned syncline in footwall of Gólgota fault and >13 Ma (K-Ar age; Mazzuoli et al., 2008) angular unconformity below Barres sandstone demonstrating subtle early growth. Qls—Quaternary landslide.
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Geosphere, December 2013
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude spond to Cenozoic reverse fault hanging walls (Kley and Monaldi, 2002). Much of the mapped region west of the Santa Bárbara Ranges represents the Salta-Jujuy High, considered to be a horst block in the central part of the Salta rift (Salfity and Marquillas, 1994). However, a minor remnant of Pirgua Subgroup is exposed near the Pascha Range (Salfity and Monaldi, 1998), and up to 3 km of strata are exposed in the southern portion of the Cachi Range (Fig. 3; Carrera et al., 2006). Overlying the Pirgua Subgroup and adjacent structural highs, there is a thinner but more regionally contiguous package of postrift Upper Cretaceous to Lower Eocene sandstone, limestone, and shale of the Balbuena and Santa Bárbara Subgroups (Salfity and Marquillas, 1994). In the Quebrada de las Capillas, the depositional contact between previously deformed Puncoviscana Formation and overlying Balbuena Subgroup is exposed (Figs. 3 and 4B). Here, the angular discordance is 45°–90°, and overlying sandstone contains angular clasts of Puncoviscana Formation quartzite; minor fault slip has also occurred along the primarily depositional contact. Most workers attribute the accommodation space within which Balbuena rocks were deposited to postrift thermal relaxation and associated subsidence, an interpretation that is corroborated by the spatial coincidence of Paleogene depocenters with Cretaceous grabens (e.g., Starck, 2011). Elsewhere, workers have interpreted the Paleocene to Miocene succession as part of an eastward-advancing flexural foreland basin system (e.g., DeCelles et al., 2011). It is likely that the foreland basin related to growth of the Andes interacted in a complex way with waning thermal subsidence following Cretaceous extension. The Paleocene–Lower Eocene fluvial and lacustrine deposits are overlain regionally by Middle-Upper Eocene paleosols that transition across strike to a disconformity in the eastern part of the Eastern Cordillera (Salfity et al., 1993; DeCelles et al., 2011). In turn, the paleosols are overlain by 2–6 km of Upper Eocene to Lower Miocene upward-coarsening fluvial and eolian deposits, preserved within the current transect at the eastern side of Quebrada de las Capillas (Fig. 3), and capped locally by middle Miocene to Pliocene fluvial, lacustrine, and alluvial-fan deposits (Hernandez et al., 1996; Starck, 1996; Reynolds et al., 2000, 2001; Echavarria et al., 2003; DeCelles et al., 2011). In mid- to late Miocene time, retroarc magmatism migrated well east of the magmatic arc, with associated volcanic centers defining the NW-trending Calama–Olacapato–El Toro lineament that crosses the current transect near the Quebrada del Toro (Figs. 1 and 3; e.g., All-
mendinger et al., 1983). Although no volcanic centers are exposed along the current transect, tuff, volcaniclastic, and flow deposits of intermediate composition (Mazzuoli et al., 2008) occur as interbeds in a dominantly clastic sedimentary succession near the northern Quebrada del Toro; the Las Burras (Hongn et al., 2010) and Acay (Petrinovic et al., 1999) plutons likely represent the intrusive equivalents of these rocks and are exposed just north of the current transect. Miocene magmatism was followed by eruption of shoshonites of likely Pleistocene age near the northern part and eastern margin of the Cachi Range (Fig. 3; Kay et al., 1994; Ducea et al., 2013). In the higher-elevation regions along the transect, Pleistocene glacial deposits and landslides are abundant. Dark alluvial and landslide deposits, also of likely Pleistocene age (Trauth et al., 2000), unconformably blanket Cenozoic rocks and range-bounding reverse faults in many places at range margins. Terrace deposits of likely late Pleistocene age, in turn, overlie these sediments and are locally covered by Holocene alluvium. For simplicity, all Cenozoic sedimentary rocks are considered as one map unit (Fig. 3). STRUCTURAL GEOLOGY Work presented here was conducted along an ~130-km-long E-W transect across the Eastern Cordillera at 24.5–25°S latitude (Figs. 1 and 3). Field work involved geological mapping and structural analysis, coupled with sample collection for U-Pb and (U-Th)/He analysis of detrital and igneous zircon and apatite. Much of the field work was accomplished by multiday foot traverses across high-elevation mountain ranges for which minimal published data exist. This regional transect was then linked with a balanced cross section across the Santa Bárbara Ranges to the east (Kley and Monaldi, 2002) as well as thermochronological results and a balanced cross section across the Puna Plateau to the west (Coutand et al., 2001). This work also builds upon regional-scale mapping across the Salta province (1:500,000 scale; Salfity and Monaldi, 1998) and west of Quebrada de las Capillas (1:250,000 scale; Blasco et al., 1996), and more detailed mapping in the northern Calchaquí Valley (Hongn et al., 2007), Quebrada del Toro (Marrett and Allmendinger, 1990), and northern Cachi Range (Pearson et al., 2012). Paleozoic Structure Within the study area, cleavage intensity and the grade of metamorphism increase to a maximum toward the west in the Cachi Range. Here,
Geosphere, December 2013
bedding is transposed, and rootless isoclinal folds prohibit assessment of stratigraphic facing direction beyond the outcrop scale. Although open to tight chevron folds are common across the transect (Fig. 4C), rocks generally are less tightly folded toward the east. Bedding, bedding-parallel cleavage, and primary cleavage within slate, phyllite, and quartzite generally dip moderately to steeply to the northwest or southeast across the transect (Figs. 3 and 5), with the exception of within the Quebrada de las Capillas and southern Lesser Range, where foliations dip toward the northeast or southwest. Within the Cachi Range, fine-grained metamorphic minerals include ~1-mm-diameter anhedral cordierite that increases in size toward the core of the range; farther south, correlative rocks were subjected to granulite-facies metamorphism and anatexis (Pearson et al., 2012). Much of this deformation and metamorphism is thought to be Cambrian and Ordovician in age (Mon and Hongn, 1991; Mon and Salfity, 1995). N-S– striking ductile shear zones, also likely Ordovician in age, have also been documented in the Cachi Range (Pearson et al., 2012). East of the Cachi Range, rocks underwent lower-grade peak metamorphic conditions, and cordierite porphyroblasts are rare to absent. Although the orientations of bedding, bedding parallel cleavages, and primary cleavages are variable in orientation, on the scale of the transect, the Puncoviscana Formation usually dips more steeply than younger strata and is generally NE-SW striking and moderately to steeply NW-SE dipping (Fig. 5), an observation consistent with regional measurements of Puncoviscana Formation rocks (Piñán-Llamas and Simpson, 2006). Cenozoic Shortening Prominent Cenozoic faults consist of two types: (1) N-S–striking, mainly E-dipping reverse faults that are expressed in the modern topography, juxtaposed rocks of markedly different age, and are locally demonstrably inverted Cretaceous normal faults; and (2) mainly NWstriking sinistral faults with minimal displacement that are discontinuous and often en echelon. These near-vertical faults have likely been active during Holocene time. Major N-S–striking reverse faults generally dip 45°–60° toward the east and are characterized by up to 200-m-wide zones of intense fracturing, with rare through-going fault surfaces or fault gouge. In the footwalls of reverse faults where Upper Cretaceous and Paleogene rocks are preserved, overturned synclines are common, with steeply dipping axial surfaces that are subparallel to superjacent reverse faults (Fig. 3 and 4D). Corresponding hanging-wall anti-
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Pearson et al. Upper Cretaceous and Cenozoic Bedding (n=148) Folds (n=7)
Cambrian and Ordovician Bedding (n=104) Folds (n=10)
Poles to planes Trends/plunges of fold axes Cylindrical best fit fold axis Kamb contours Contour interval = 2σ Significance level = 3σ
Puncoviscana Formation Folds (n=62)
Bedding, bedding ll cleavage, primary cleavage (n=238)
brada del Toro in the footwall of the Solá fault. Analytical details are available in the Supplemental File.1 Detrital zircon ages are shown on relative ageprobability diagrams (Fig. 6). In accord with Dickinson and Gehrels (2009), we constrain maximum depositional ages using the youngest age cluster in a sample defined by three or more overlapping analyses. For the igneous sample, we attempt to minimize errors resulting from inclusion of inappropriate analytical data in age calculations by reporting a weighted mean age (Ludwig, 2001) of concordant and overlapping 206Pb/238U ages, with final uncertainties that include all random and systematic errors (Fig. 7). (U-Th)/He Apatite
Figure 5. Stereograms showing attitudes of sedimentary and metasedimentary rocks and structures.
clines were also observed in Cambrian–Ordovician, Cretaceous, and Cenozoic rocks and are consistent with fault-propagation folding being a dominant structural style in the Eastern Cordillera at this latitude (Fig. 3E). The (U-Th)/He zircon data obtained from the Cachi Range suggest that major antiforms in the hanging walls of reverse faults, also interpreted to be faultpropagation folds but at a more regional scale, accommodated the formation of up to 15 km of structural relief (Pearson et al., 2012). See notes in Figure 3 for descriptions of individual Cenozoic structures.
meter-scale offset of beds, brittle fault fabrics and kinematic indicators, tool marks, and NEstriking, antithetic dextral faults. In the Quebrada del Toro, portions of the Solá and Gólgota reverse faults (Fig. 3) that are coincident with the El Toro lineament (Salfity et al., 1976) are locally NW striking with horizontal slickensides, demonstrating that late strike-slip faulting exploited preexisting contractional structures (Marrett et al., 1994). Despite the prevalence of these faults, none are through-going at the regional scale, and the more significant of these faults are only characterized by tens of meters of displacement (Acocella et al., 2011).
Cenozoic Strike-Slip Faults METHODS NW-SE-striking lineaments are visible in aerial imagery across the southern Central Andes (Allmendinger et al., 1983) and are associated with some of the main retroarc magmatic complexes (e.g., Riller et al., 2001). In some cases, these lineaments are pre-Cenozoic in age (Monaldi et al., 2008) and appear to segment Cenozoic contractional structures (Coutand et al., 2001). At the latitude of this study, these NW-SE–trending lineaments were locally confirmed as sinistral strike-slip faults based upon
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U-Pb Zircon U-Pb zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), following methods described by Gehrels et al. (2008), was applied to eight detrital samples to better constrain provenance, ages of deposition, and deformation. A tuff was also collected for U-Pb zircon analysis to constrain the age of growth strata in the Que-
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Apatite (U-Th)/He thermochronology is used here to constrain the timing and magnitude of rock exhumation, which we infer resulted from rock deformation. Our results supplement earlier work in the region (e.g., Coutand et al., 2001; Deeken et al., 2006; Carrapa et al., 2011) by placing ages of low-temperature thermochronometers in a structural context. (U-Th)/He thermochronometry of apatite generally reflects the time since cooling of the apatite below ~70 °C (assuming an effective grain radius of 60 µm and a cooling rate of 10 °C/m.y.; Farley, 2000). Using apatite fission-track ages from vertical transects in the Cumbres de Luracatao (Fig. 1), Deeken et al. (2006) obtained a Miocene geothermal gradient of ~18 °C/km. Using stratigraphic exhumation depths and lack of complete closure of the apatite fission-track system, Coutand et al. (2006) calculated a similar value of 5 m.y. after major exhumation in the Cachi Range (Pearson et al., 2012) that was concurrent with the
establishment of a topographic high and internal drainage at the modern eastern margin of the Puna Plateau (Vandervoort et al., 1995; Coutand et al., 2006). Upper Cambrian and Ordovician zircons dominate detrital zircon populations of samples collected from the Agujas Conglomerate near this tuff (Fig. 6). Although recycling cannot be ruled out, this suggests that their sediment source during deposition was from the east, given that the main source of Ordovician grains on the Puna Plateau to the west was already hydrographically isolated. (U-Th)/He Apatite Forty-seven individual apatite grains analyzed by (U-Th)/He thermochronometry yielded mostly Miocene and early Pliocene ages, with a lesser number of Paleocene to Eocene dates (Table 1; Fig. 7). Two other grains yielded Proterozoic and Paleozoic ages, with low effective U (eU = U + 0.235Th) and Th concentrations, making it unlikely that these anomalously old ages result from radiation damage that enhanced He retentivity; instead, these old ages may have been compromised by He implantation (Spiegel et al., 2009). Multiple grains that generally form well-defined Upper Miocene to Lower Pliocene age clusters are considered here to represent recent exhumation and cooling of rock samples (Table 1; Fig. 8). Four Eocene (U-Th)/He apatite ages from the Lampasillos Range may represent an early signal of Cenozoic exhumation and cooling
U-Pb Zircon Longitude (DD) -66.5
-66.3
-66.1
-65.7
-65.5
-65.3 0
Westward younging 5 exhumation
10 15 20 25 30 35 40 45 50 55 (U-Th)/He apatite grain age 60 (U-Th)/He zircon grain age
2
Supplemental Table. Excel file of U-Pb (zircon) geochronologic analyses. If you are viewing the PDF of this paper or reading it offline, please visit http:// dx.doi.org/10.1130/GES00923.S2 or the full-text article on www.gsapubs.org to view the Supplemental Table.
-65.9
Age (Ma)
U-Pb analyses of detrital zircons and a U-Pb zircon age on a tuff help to constrain the provenance, timing of sediment source emergence, and the age of deposition of sedimentary and low-grade metasedimentary rocks in the region. U-Pb zircon results are available in the Supplemental Table.2 For Puncoviscana Formation rocks, the youngest zircon populations dominate and vary slightly in age (Fig. 6), indicating a continuous supply of young zircons during deposition, and suggesting that maximum depositional ages obtained from these rocks likely approximate depositional ages (Fig. 6). Cambrian zircons also dominate detrital zircon populations from Ordovician rocks of the Santa Victoria Group; these ages are comparable to those obtained from the Puncoviscana Forma-
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Figure 8. (U-Th)/He apatite (this study) and zircon (Pearson et al., 2012) ages (±2σ) versus longitude across the transect. Grayed ages lie outside of clusters and are considered partially reset.
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Pearson et al. and are consistent with subtle Eocene growth strata documented in the Luracatao and Calchaquí Valleys (Bosio et al., 2009; Hongn et al., 2007). Unfortunately, the quality of these ages is questionable, given their very low eU concentrations; these and eight other analyses with eU concentrations of 100 km) eastward jump in the location of deformation. The structural similarity between the Santa Bárbara and Zapla Ranges and the Mojotoro, Lesser, Pascha, and Zamanca Ranges to the west is intriguing: Both areas are bounded on the east by a major, W-dipping structure, and the latter region records a westward migration of exhumation toward the lesserdeformed Quebrada del Toro, where recent deformation has been documented (Hilley and Strecker, 2005). Progressive rotation of faults depicted by Kley and Monaldi (2002) also hints at a westward migration of deformation from the Piquete and Centinela synrift depocenters in the Santa Bárbara Ranges westward toward the minimally deformed Lavayén Valley, where active seismicity is focused within the growing Zapla Range (Cahill et al., 1992). This pattern of local migration of fault subsystems away from the foreland may be common during inversion of rift systems, as preexisting bivergent faults are reactivated, followed by new faults formed in footwalls of inverted structures as the subcritical orogenic wedge gains taper (Fig. 11). A sporadic eastward migration of deformation interspersed with local, in-sequence westward-migrating faulting is a scenario that differs markedly from that encountered ~100 km to the south (Carrapa et al., 2011). There, Carrapa et al. (2011) documented a progressive eastward migration of deformation. The discrepancy in results may reflect the influence of
Cretaceous rift faults?
Figure 11. Cartoon showing hypothesized reactivation of E-dipping Cretaceous normal faults, followed by local westward migration of shortening.
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude preexisting Salta rift architecture. To the south, mainly E-dipping, preexisting Cretaceous faults (Grier et al., 1991; Cristallini et al., 1997) were progressively inverted in front of the orogenic wedge in a roughly continuous E-W rift basin. In contrast, at 24–25°S, a lack of inversionprone Cretaceous rift faults within the central portion of the horst block may have promoted an eastward jump in the location of deformation (Figs. 10 and 11). The original configuration of the rift also likely influenced the topographic expression of mountains across the Eastern Cordillera. South of the present transect, where the rift basin is better developed, W-dipping, antithetic pop-up structures are less common; instead, rocks were deformed within a major eastward-propagating back-thrust belt, with subdued topography east of Cachi reflecting strain accommodation by primarily E-dipping structures. In contrast, at the latitude of the Salta-Jujuy High, several W-dipping pop-up structures form sharp topographic boundaries on the eastern margins of ranges (Fig. 3). Geodynamic Model Some workers have suggested that the segmented, “broken” nature of the Laramide and Sierras Pampeanas forelands reflects basement deformation that occurs during shallow subduction (e.g., Dickinson and Snyder, 1978; Jordan and Allmendinger, 1986). An eastward sweep of magmatism across the Altiplano-Puna Plateau from 25 to 15 Ma (e.g., Allmendinger et al., 1997) has led some researchers to suggest that southward-migrating shallow subduction occurred beneath much of the Central Andes during this time, presumably associated with oblique subduction of the Juan Fernández Ridge (Yañez et al., 2001). In the literature, shallow subduction is generally thought to cause magmatic lulls; the conventional model suggests that retroarc magmatism signals steepening of the subducting slab that follows shallow subduction (e.g., Dickinson and Snyder, 1978). However, retroarc magmatism occurred northwest of the Quebrada del Toro at ca. 15 Ma (Hongn et al., 2010), which predates by ~5 m.y. the interpreted location of the Juan Fernández Ridge beneath 24–25°S (Yañez et al., 2001) and the ~75 km eastward jump in the deformation front by the Eastern Cordillera. Similarly, in the southern Altiplano, enhanced retroarc deformation at 19–7 Ma followed the onset of retroarc magmatism by up to 8 m.y. (e.g., Allmendinger et al., 1997; Elger et al., 2005). Timing constraints suggest that the inferred interval of slab shallowing corresponds closely with enhanced deformation and thrust belt propagation in Bolivia and northwestern Argentina.
One possibility is that lithospheric delamination could result in enhanced magmatism due to an influx of asthenosphere, which may also create space beneath the upper plate for a shallowing slab, in turn promoting further forelandward propagation of the thrust belt. This model may explain some aspects of the Miocene kinematic history of northwestern Argentina whereby magmatism is followed by slab shallowing and an enhanced eastward propagation of shortening. Several workers have documented that the modern spatial extent of the Altiplano Plateau was already in place before 25 Ma (e.g., Horton et al., 2001), following rapid Eocene and Oligocene advancement of the thrust front to regions of preexisting rift depocenters (Sempere et al., 2002; Elger et al., 2005; Oncken et al., 2006; Ege et al., 2007). Additional constraints presented here refine the timing and kinematics of the retroarc thrust belt in northwestern Argentina. During shallow subduction, the upper plate accommodates increasing strain. As retroarc thrust belts commonly involve a craton-ward thinning wedge of sedimentary rocks, enhanced foreland-ward propagation of the deformation front during shallow subduction would thus be likely to encounter older basement rocks with less overlying strata and greater preexisting heterogeneities. Plateau formation may be enhanced in regions of pre-orogenic foreland heterogeneities because distal uplifts increase orography and the formation of internally drained basins, in turn providing a positive feedback for formation of an orogenic plateau (Sobel et al., 2003). With continued deformation in the Sierras Pampeanas, the Puna Plateau may grow southward as intramontane basins accommodate additional strain that follows initial reactivation of preexisting heterogeneities, much like within the Salta rift of northwestern Argentina. Implications of Along-Strike Variations in Shortening A primary observation by tectonicists working in the Andes is that the maximum magnitude of crustal shortening coincides with southern Bolivia, with shortening decreasing significantly along strike (Fig. 2B; Isacks, 1988; Kley and Monaldi, 1998). Hypotheses that seek to explain the along-strike change include the orientation of the relative convergence (Gephart, 1994), mantle flow beneath the long subducting slab (Schellart et al., 2007), and variations in the pre-orogenic stratigraphic architecture of the overriding plate (Fig. 2A; Allmendinger and Gubbels, 1996; Kley et al., 1999). Results from the present study suggest that the pre-Cenozoic structural and stratigraphic
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architecture strongly influenced the spatiotemporal evolution of the thrust belt (Figs. 2 and 10). There is a striking correlation between the magnitude of shortening and distribution of Paleozoic strata in the retroarc of the Central Andes, suggesting that it is not mantle flow or convergence parameters that control the ability of the upper plate to deform, but rather the preorogenic architecture of the upper plate (Fig. 2; Allmendinger and Gubbels, 1996; Kley et al., 1999). Due to the apparent decreased ability of the upper plate to accommodate a portion of the convergence between the South American and Nazca plates, the relative convergence along strike at the subduction interface would be predicted to increase away from the Central Andes, which may explain the formation of the Bolivian orocline (e.g., Isacks, 1988; Allmendinger and Gubbels, 1996; Kley et al., 1999; Arriagada et al., 2008). Coupled with existing estimates for the Puna Plateau (Coutand et al., 2001) and Santa Bárbara Ranges (Kley and Monaldi, 2002) near the latitude of the current transect, results presented here constrain a minimum estimate of 142 km for the total magnitude of shortening at 24–25°S (Figs. 2A and 3). These results also suggest that at least in the Eastern Cordillera at this latitude, this shortening estimate is not greatly underestimated. For comparison, the ~95 km of shortening within this domain is 100 km), thin-skinned shortening absorbed by the Bolivian Subandes because such a thick, pre-orogenic Paleozoic basin did not exist at this latitude (Fig. 2). The shortening estimate calculated here is greater than existing approximations in this region (e.g., Grier et al., 1991; Coutand et al., 2001), but it is still ~150 km less than predicted (Fig. 2; Isacks, 1988; Kley and Monaldi, 1998). Recent studies have suggested that local existence of anomalously thin crust beneath the Puna Plateau (~42 km; e.g., Yuan et al., 2002) may indicate Cenozoic crustal loss. However, at ~25°S,
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Pearson et al. most geophysical studies suggest an ~55-kmthick crust across much of the Andes (Yuan et al., 2002; Tassara et al., 2006; Wölbern et al., 2009). The initial crustal thickness in the Central Andes is poorly constrained. Assuming that the Andes are underlain by a 55-km-thick crust and had an initial crustal thickness of 35 km, ~190 km of shortening would be required at this latitude. This is 48 km more than the ~142 km of shortening documented here; given that shortening in the Puna and western Cordillera may be underestimated, we suggest that crustal addition (e.g., by crustal flow, magmatic underplating, etc.) may not be necessary to explain the observed crustal thickness at this latitude. CONCLUSIONS Regional geological mapping, structural analysis, and geo- and thermochronological results indicate that the northwestern Argentine thrust belt at 24–25°S was deformed above a W-dipping décollement that transferred slip to primarily E-dipping reverse faults in a major back-thrust belt that propagated in an overall eastward direction during Cenozoic time. Following rapid eastward propagation of the thrust belt at ca. 40 Ma, (U-Th)/He and U-Pb age dating of apatite and zircon constrains an ~75 km eastward propagation event at 12–10 Ma, when the thrust front bypassed the central portion of a horst block in the Cretaceous rift system, followed by subsequent initiation of new faults in a subsystem that propagated toward the west into this region. Subsequently, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges, likely followed by westward-migrating deformation to its current location in the Lavayén Valley. Approximately 100 km to the south, deformation migrated progressively toward the east through time with no local westward migration documented. This suggests that the architecture of the thrust belt was strongly influenced by the configuration of the Cretaceous rift, which likely influenced the discontinuous nature of deformation propagation in an overall foreland direction since Eocene time; these results are in accord with recent work in southern Bolivia. A regional balanced cross section across the Eastern Cordillera, coupled with existing shortening magnitude estimates for the Santa Bárbara Ranges and Puna Plateau, increases the estimate of the magnitude of shortening at this latitude to ~142 km, but it confirms that significantly less shortening was accommodated south of the thinskinned Bolivian fold-and-thrust belt. We suggest that greater shortening than has been previously documented was probably accommodated within the Puna Plateau and ancient retroarc
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of northern Chile, and that crustal shortening alone may explain the observed thick crust at 24–25°S. The overall along-strike decrease in shortening magnitude is well explained by the distribution of pre-Cenozoic basins that are able to accommodate large-magnitude thin-skinned shortening. Coupled with a likely correlation of Cenozoic thrust belt kinematics with the spatial distribution of the Cretaceous rift, this suggests that the pre-orogenic architecture strongly influenced the style, kinematics, and magnitude of shortening, which, in turn, influenced the geodynamic evolution of Andean orogenesis. ACKNOWLEDGMENTS
This research was conducted as part of the Convergent Orogenic Systems Analysis (COSA) project, in collaboration with and funded by ExxonMobil. National Science Foundation grant EAR-0732436 supported data acquisition at the Arizona LaserChron Center. This work benefited from discussions with many people, including M. McGroder, F. Fuentes, R. Waldrip, J. Kendall, G. Gray, R. Bennett, S. Lingrey, T. Hersum, T. Becker, and R.N. Alonso. B. Ratliff provided assistance with LithoTect® software. F. Shazanee, C. Hollenbeck, A. Abbey, I. Nurmaya, and M. Hearn helped with mineral separations. Constructive reviews by J. Barnes and G. Hilley improved the manuscript. REFERENCES CITED Acocella, V., Gioncada, A., Omarini, R., Riller, U., Mazzuoli, R., and Vezzoli, L., 2011, Tectonomagmatic characteristics of the back-arc portion of the Calama– Olacapato–El Toro fault zone, Central Andes: Tectonics, v. 30, no. 3, TC3005, doi:10.1029/2010TC002854. Adams, C.J., Miller, H., Toselli, A.J., and Griffin, W.L., 2008, The Puncoviscana Formation of northwest Argentina: U-Pb geochronology of detrital zircons and Rb-Sr metamorphic ages and their bearing on its stratigraphic age, sediment provenance and tectonic setting: Neues Jahrbuch für Geologie und Palaontologie: Abhandlungen, v. 247, no. 3, p. 341–352, doi:10.1127 /0077-7749/2008/0247-0341. Adams, C.J., Miller, H., Aceñolaza, F.G., Toselli, A., and Griffin, W.L., 2011, The Pacific Gondwana margin in the late Neoproterozoic–early Paleozoic: Detrital zircon U-Pb ages from metasediments in northwest Argentina reveal their maximum age, provenance and tectonic setting: Gondwana Research, v. 19, no. 1, p. 71–83, doi:10.1016/j.gr.2010.05.002. Allen, P.A., and Allen, J.R., 1990, Basin Analysis, Principles and Applications: Oxford, UK, Blackwell, 451 p. Allmendinger, R.W., and Gubbels, T., 1996, Pure and simple shear plateau uplift, Altiplano-Puna, Argentina and Bolivia: Tectonophysics, v. 259, no. 1–3, p. 1–13, doi:10.1016/0040-1951(96)00024-8. Allmendinger, R.W., Ramos, V.A., Jordan, T.E., Palma, M., and Isacks, B.L., 1983, Paleogeography and Andean structural geometry, northwest-Argentina: Tectonics, v. 2, no. 1, p. 1–16, doi:10.1029/TC002i001p00001. Allmendinger, R.W., Jordan, T.E., Kay, S.M., and Isacks, B.L., 1997, The evolution of the Altiplano-Puna Plateau of the Central Andes: Annual Review of Earth and Planetary Sciences, v. 25, p. 139–174, doi:10.1146 /annurev.earth.25.1.139. Andriessen, P.A.M., and Reutter, K.J., 1994, K-Ar and fission track mineral age determination of igneous rocks related to multiple magmatic arc systems along the 23°S latitude of Chile and NW Argentina, in Reutter, K.-J., Scheuber, E., and Wigger, P.J., eds., Tectonics of the Southern Central Andes: Structure and Evolution of an Active Continental Margin: Berlin, Springer-Verlag, v. 1, p. 141–153.
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