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

Abstract

We present crustal velocities for 29 continuously recording GPS stations from the southern central Andes across the Puna, Eastern Cordillera, and Santa Barbara system for the period between the 27 February 2010 Maule and 1 April 2014 Iquique earthquakes in a South American frame. The velocity field exhibits a systematic decrease in magnitude from ~35 mm/yr near the trench to <1 mm/yr within the craton. We forward model loading on the Nazca‐South America (NZ‐SA) subduction interface using back slip on elastic dislocations to approximate a fully locked interface from 10 to 50 km depth. We generate an ensemble of models by iterating over the percentage of NZ‐SA convergence accommodated at the subduction interface. Velocity residuals calculated for each model demonstrate that locking on the NZ‐SA interface is insufficient to reproduce the observed velocities. We model deformation associated with a back‐arc décollement using an edge dislocation, estimating model parameters from the velocity residuals for each forward model of the subduction interface ensemble using a Bayesian approach. We realize our best fit to the thrust‐perpendicular velocity field with 70 ± 5% of NZ‐SA convergence accommodated at the subduction interface and a slip rate of 9.1 ± 0.9 mm/yr on the fold‐thrust belt décollement. We also estimate a locking depth of 14 ± 9 km, which places the downdip extent of the locked zone 135 ± 20 km from the thrust front. The thrust‐parallel component of velocity is fit by a constant shear strain rate of −19 × 10−9 yr−1, equivalent to clockwise rigid block rotation of the back arc at a rate of 1.1°/Myr.

Late Pleistocene to Recent Deformation in the Thick‐Skinned Fold‐and‐Thrust Belt of Northwestern Argentina (Central Calchaquí Valley, 26°S)

“…GPS‐derived horizontal surface velocities projected onto a structure‐perpendicular profile in NWA gradually decrease from west to east (Figure 1). A similar velocity profile presented in McFarland et al (2017) located slightly to the north of our new profile exhibits an abrupt velocity decrease between the CGPS site near the Golgóta fault (GOLG) (Strecker & Marrett, 1999) and the Universidad de Salta site (UNSA) (Figure 1) superposed on the overall pattern of a gradual eastward velocity reduction. As we discuss below, the abrupt change is used by McFarland et al (2017) to argue for the presence of a locked décollement underlying the ranges of the EC, similar to what has been observed in the fold‐and‐thrust belt of southern Bolivia (Brooks et al, 2011; Weiss et al, 2016).…”

“…Gray dashed lines are handdrawn fits to the data intended to emphasize features in the velocity field that we refer to in the text. Two velocities for UNSA are shown in (c) with the smaller value (UNSA*) corresponding to the time interval used to estimate the interseismic velocity by McFarland et al (2017) and the revised, larger value of the time interval used in this study. Note the rapid decrease in velocity between GOLG and UNSA does not exist if our revised estimate is considered.…”

“…Here we take this last approach and generate a new NWA interseismic surface-velocity field and structure-perpendicular velocity profile (Figs. 1 and 9) that also includes velocities for four additional CGPS stations (JBAL, TUC1, TERO, CATA) that are close to our study area and were not analyzed by McFarland et al (2017) (Fig. 1).…”

“…We also thank Rick Bennett and Philip McFarland for installing and maintaining the PAGA CGPS network in NWA and UNAVCO for serving the associated data. 1 and 10 can be found in Weiss et al (2016) and McFarland et al (2017). OSL ages for the El Mollar Area come from Reger (2018).…”

“…Blue circles indicate GPS sites in Bolivia from Weiss et al (2016) with CGPS horizontal velocity vectors and 2‐sigma error ellipses. Red circles are NWA CGPS site locations from McFarland et al (2017) with revised horizontal velocities from this study. Yellow CGPS velocities are newly added.…”