Influence of Pre-Andean Crustal Structure on Cenozoic Thrust Belt

00923 1st pages / page 1 of 17 Infl uence 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 1Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA 2Department of Geosciences, Idaho State University, 921 South 8th Avenue, Pocatello, Idaho 83209, USA 3Facultatea de Geologie Geofi zica, Universitatea Bucuresti, Strada N. Balcescu Nr 1, Bucuresti, Romania ABSTRACT shortening at this latitude is ~142 km, which in northernmost Argentina and Bolivia (Fig. 1; is moderate in magnitude compared to the 17–23°S), Cenozoic thin-skinned shortening The retroarc fold-and-thrust belt of the 250–350 km of shortening accommodated in within a thick Paleozoic basin exceeds 300 km Central Andes exhibits major along-strike the retroarc thrust belt of southern Bolivia (Fig. 2; e.g., McQuarrie, 2002). Southwest of variations in its pre-Cenozoic tectonic con- to the north. This work supports previous Salta, Argentina (Fig. 1; ~25°S), where this fi guration. These variations have been pro- hypotheses that the magnitude of shorten- thick Paleozoic basin was absent prior to Ceno- posed to explain the considerable southward ing decreases signifi cantly along strike away zoic time, steeply dipping reverse faults that are decrease in the observed magnitude of Ceno- from a maximum in southern Bolivia, largely locally inverted normal faults are suggested to zoic shortening. Regional mapping, a cross as a result of the distribution of pre-Ceno- have accommodated <100 km of shortening section, and U-Pb and (U-Th)/He age dating zoic basins that are able to accommodate a (Fig. 2; e.g., Allmendinger et al., 1983; Grier of apatite and zircon presented here build large magnitude of thin-skinned shortening. et al., 1991). Despite this large along-strike upon the preexisting geological framework A major implication is that variations in the variation in shortening magnitude and struc- for the region. At the latitude of the regional pre-orogenic upper-crustal architecture can tural style, a corresponding major southward transect (24–25°S), results demonstrate that infl uence the behavior of the continental lith- decrease in elevation and crustal thickness does the thrust belt propagated in an overall east- osphere during later orogenesis, a result that not accompany this transition in the Central ward direction in three distinct pulses during challenges geodynamic models that neglect Andes (e.g., Isacks, 1988), prompting specula- Cenozoic time. Each eastward jump in the upper-plate heterogeneities. tion that magmatic addition, tectonic underplat- deformation front was apparently followed ing, and/or crustal fl ow may have contributed by local westward deformation migration, INTRODUCTION signifi cantly to the crustal volume south of the likely refl ecting a subcritically tapered oro- thin-skinned Bolivian fold-and-thrust belt (Kley genic wedge. The fi rst eastward jump was at Cordilleran-style orogens form during con- and Monaldi, 1998; Husson and Sempere, 2003; ca. 40 Ma, when deformation and exhuma- vergence of oceanic and continental plates and Gerbault et al., 2005). tion were restricted to the western margin are characterized by long belts of continental Although inversion of rift faults and the dis- of the Eastern Cordillera and eastern mar- magmatism and shortening. An active example tribution of pre-orogenic basins have long been gin of the Puna Plateau. At 12–10 Ma, the of such an orogenic system is in South America, suggested to infl uence the style of deformation thrust front jumped ~75 km toward the east where shortening of the overriding plate results in the Central Andes (e.g., Allmendinger et al., to bypass the central portion of a horst block in continued growth of the Andes. In spite of the 1983), only recently have workers integrated of the Cretaceous Salta rift system, followed documentation of major along-strike variations geo-thermochronological results with structural by initiation of new faults in a subsystem in the style and magnitude of Cenozoic shorten- analysis in southern Bolivia to show that the spa- that propagated toward the west into this ing in the Andes (e.g., Allmendinger et al., 1983; tial extent of the Altiplano Plateau was largely preexisting structural high. During Pliocene Isacks, 1988; Kley and Monaldi, 1998; Kley controlled by the distribution of Mesozoic rift time, deformation again migrated >100 km et al., 1999), there is not a considerable along- faults and was established by ca. 25 Ma (Sem- eastward to a Cretaceous synrift depocenter strike difference in the relative convergence pere et al., 2002; Elger et al., 2005; Ege et al., in the Santa Bárbara Ranges. The sporadic velocity of the oceanic and continental plates 2007; Barnes et al., 2008). However, the infl u- foreland-ward propagation documented here nor in the age of the subducting oceanic Nazca ence of these pre-Cenozoic heterogeneities in may be common in basement-involved thrust plate (Oncken et al., 2006). In contrast, some infl uencing the kinematics of the thrust belt has systems where inherited weaknesses due to of the observed spatial variations in the style not been suffi ciently investigated in northwest- previous crustal deformation are preferen- and magnitude of Cenozoic retroarc shortening ern Argentina, despite the observation that early tially reactivated during later shortening. match with changes in pre-Andean stratigraphy Andean deformation spatially correlates with The minimum estimate for the magnitude of and structure of the South American plate (e.g., Cretaceous rift basins (Kley and Monaldi, 2002; Mpodozis and Ramos, 1989; Allmendinger and Carrera et al., 2006; Hongn et al., 2007; Insel *E-mail: [email protected]. Gubbels, 1996; Kley et al., 1999). For example, et al., 2012). One study at ~25.75°S, utilizing Geosphere; December 2013; v. 9; no. 6; p. 1–17; doi:10.1130/GES00923.1; 11 fi gures; 1 table; 2 supplemental fi les. Received 1 March 2013 ♦ Revision received 30 August 2013 ♦ Accepted 31 October 2013 ♦ Published online XX Month 2013 For permission to copy, contact [email protected] 1 © 2013 Geological Society of America uencing structural style - 00923 1stpages / page2 of 17 studies, evaluating the importance of pre-orogenic crustal architecture (e.g., Allmendinger et al., 1983; Allmendinger and Gubbels, 1996; Kley et al., 1999) is critical for understanding the main factors infl relative to other models that largely neglect preexisting upper-plate heterogeneities and instead implicate climate (e.g., Lamb and Davis, 2003; uenced the evolution Strecker et al., 2007), mantle dynamics (e.g., Russo and Silver, 1994; Sobolev and Babeyko, 2005; Schellart et al., 2007; Husson et al., 2012), or buoyant anomalies within the down- going plate (e.g., Jordan et al., 1983; Isacks, 1988; Ramos, 2009). Also, in spite of the hypothesized importance of shallow subduction beneath the Central Andes during Miocene time (e.g., Ramos, 2009), few workers have evalu- ated the spatio-temporal correlation between the kinematic history of the thrust beltThis and paper an east- focuses on an E-W transect across uenced the evolution ward migration of retroarc magmatism thought to indicate shallow subduction. the Eastern Cordillera tectonomorphic province of the Andean retroarc thrust belt of northwestern Argentina (Fig. 1). Results presented here provide new constraints on the style, timing, kinematics, and magnitude of shortening of the fold-and-thrust belt at ~24.75°S latitude. These results: (1) indicate that the northwestern Argen- tine thrust belt was deformed above a W-dipping décollement that transferred slip to a system of E-dipping back thrusts; (2) constrain the timing of eastward deformation propagation within the Pearson et al. Eastern Cordillera and suggest that the Creta- ceous rift architecture infl of the thrust belt at this latitude; and (3) increase the estimate of the magnitude of shortening at this latitude, but they still suggest that signifi cantly less shortening was accommodated south of the thin-skinned Bolivian fold-and-thrust belt. This work complements existing work and underscores the importance of the preexisting tectonic framework in controlling the spatial distribution of shortening, particularly during the nascent stages of thrust belt development. This, in turn, strongly infl of the orogenic system. Tectonomorphic provinces of the central GEOLOGICAL BACKGROUND 63°W 50 Andean retroarc include, from west to east (Fig. 1A; Jordan et al., 1983): (1) the Puna Plateau, a Lomas del Olmedo relatively low-relief, topographically high (aver- age elevation ~4 km) region of internal drain- age, where Paleogene thrust belt structures are mostly buried by Cenozoic sedimentary and volcanic rocks (this province is the southern continuation of the broad, lower-relief Altiplano Plateau of Bolivia); (2) the Eastern Cordillera, 0Major thrust faults 100 km Cenozoic sediment Cenozoic igneous rocks Mesozoic sed. rocks Mesozoic igneous rocks Paleozoic sed. rocks Paleozoic granitoids 64°W Mostly Cambrian rocks Subandes Fig. 2 64°W ect regional depo- SB ranges SB Eastern Cordillera Eastern Fig. 1B location Salta Bolivia Puna-Altiplano 65°W uence of older structures on Ceno- Argentina 68°W QdT Chile lack of infl zoic thrust belt propagation (Siks and Horton, 2011). These and similar studies are focused upon Cenozoic strata that refl 20°SA systems and evolving sediment source areas. 21°S In contrast, the approach taken here involves (U-Th)/He apatite and zircon analysis of reverse fault hanging walls that were uplifted and exhumed during fault displacement. In addition B Bolivia to resolving the potential spatial heterogeneity 66°W of thrust belt kinematics implied by these prior 21°S 25°S L Argentina ur ac atao Range 22°S Chile Traannssppamamppeeaan Archch uence of pre- 23°S 24°S 67°W 25°S Figure 1.

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