Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude: Northwestern Argentina

Abstract

The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic confi guration. 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 refl ecting a subcritically tapered orogenic wedge. The fi rst 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 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 signifi cantly 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 thinskinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can infl uence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities.

Rapid Geodetic Shortening Across the Eastern Cordillera of NW Argentina Observed by the Puna‐Andes GPS Array

“…It is composed of predominantly lower Paleozoic sedimentary, metasedimentary, and igneous rocks, and is the locus of late Eocene‐Miocene crustal shortening associated with slip on both west and east vergent reverse faults at the latitudes of this study (Carrapa et al, ; Carrapa & DeCelles, ; Grier et al, ; Pearson et al, ). (3) The Santa Barbara ranges are a system of bivergent, inverted Cretaceous to Paleogene rift‐related normal faults that were reactivated at ~10 Ma when the thrust front jumped ~75 km eastward to its modern position (Carrapa et al, ; Kley & Monaldi, ; Pearson et al, ; Reynolds et al, ). Total crustal shortening estimates for the combined Eastern Cordillera and Santa Barbara system range from about 60 km to 140 km based on regional balanced cross sections (Kley & Monaldi, ; Pearson et al, ).…”

“…Blue curve shows the contribution to the thrust‐perpendicular velocity field from the best‐fit décollement model. Faults from Pearson et al () (thin black lines) overlay the scaled 2‐D PPD showing the normalized probability for the location of décollement model parameters d and ξ (figure adapted from Brooks et al (, Figure 2)).…”

“…We assume that the residual velocity field is dominated by interseismic strain accumulation due to a frictional transition from steady sliding to fully locked on a basal décollement buried beneath the fold‐thrust belt. Building on the work of Kley and Monaldi (), Pearson et al () used a restored balanced regional cross section along the profile shown in Figure to infer the existence of such a detachment at ~10 km depth with a regional dip of 2° to the west. We use the results of Pearson et al () in this analysis to prescribe the dip ( δ ) of our edge‐dislocation model to 2° to the west (Figure b), but we estimate the depth of the décollement using the GPS velocity data.…”

“…Building on the work of Kley and Monaldi (), Pearson et al () used a restored balanced regional cross section along the profile shown in Figure to infer the existence of such a detachment at ~10 km depth with a regional dip of 2° to the west. We use the results of Pearson et al () in this analysis to prescribe the dip ( δ ) of our edge‐dislocation model to 2° to the west (Figure b), but we estimate the depth of the décollement using the GPS velocity data. We performed a constrained random search (CRS) (Brachetti et al, ) to evaluate >50,000 models with four model parameters.…”

“…The Puna is characterized by relatively high topographic relief, has thinner than expected crust, and is bounded to the east by the bivergent fold and thrust belt of the Santa Barbara system (Allmendinger et al, ; Kley & Monaldi, ). While it has long been understood that roughly east‐west horizontal crustal shortening has been integral to the development of the broad, high‐elevation central Andean Plateau, providing mass for crustal thickening (Allmendinger et al, ; Isacks, ; Sheffels, ), estimates of cumulative crustal shortening of the South American plate are as much as 2 times greater for the Altiplano than they are for the Puna (McQuarrie, ; Pearson et al, ). Furthermore, it has been found that along strike, there is not a one‐to‐one correlation between shortening estimates and estimates of crustal thickness (Kley & Monaldi, ), complicating the notion that crustal shortening is the sole control on the topographic expression of orogenic processes.…”