Assessing Quaternary shortening through trishear kinematic models at the Andean Orogenic Front, Southern Precordillera, Argentina

The active orogenic front of Southern Central Andes, at the latitude of ~32°-33°S, is located in the foothills of the Southern Precordillera. This region lies within a flat-slab subduction setting, which defines an area of very high seismic hazard characterized by Quaternary deformation and intense shallow-crustal seismicity. The active deformation in this area is focused on the easternmost thrusts of the Southern Precordillera, particularly along the Las Peñas-Las Higueras range (32°10’-32°45’S). The Las Peñas Thrust System (LPTS) bounds the range to the east and propagates towards the piedmont through both surface-reaching and blind thrusts. Numerous fault and fold scarps, characterized by a N-S strike and eastward vergence, have been active since Pliocene-Pleistocene times, with the most recent expressions located at the easternmost piedmont.

Toward the southern end of the Las Higueras-Las Peñas range, the thrust front corresponds to a transposed east-verging anticline, which becomes blind in the study area of Baños Colorados Creek. Before its geomorphic signature is fully lost towards the south, the morphotectonic expression of the LPTS in this creek shows discontinuous remnants of deformed quaternary alluvial deposits lying unconformably over neogene units. These deposits define fold-limb scarps ~300 m long and with scarp heights ranging from 20 to 45 m. Such exposures provide a unique opportunity to estimate shortening in neotectonic blind thrusts that exhume the bedrock. They also allow quantification of deformation in the hanging wall, where geological markers are commonly removed by erosion. This setting provides an exceptional opportunity to estimate deformation by considering the contribution of adjacent blocks (off-fault analysis), offering key insights into how quaternary deformation is distributed along the SCLP. Moreover, until now, the activity of this thrust system had been evaluated exclusively through indicators obtained directly at the fault zone and its immediate surroundings (on-fault), so this analysis represents a complementary and significant contribution.

We calculated quaternary shortening applying fault-propagation fold models based on the trishear concept using both the reconstructed topography of alluvial surfaces and stratigraphic layers as deformation markers, surveyed with high-resolution techniques (UAV and DGNSS). Shortening rates of 0.17-0.50 mm/yr were obtained for 13-16 ka surfaces, while minimum shortening of 15.6-36.76 m was estimated for an older surface (>13-16 ka and likely <200 ka).

Although estimating shortening rates on blind thrusts involves significant uncertainties, our results refine the characterization of the seismogenic sources affecting the surroundings of Mendoza city, one of the most populated in Argentina, where hazard assessments remain outdated and do not adequately incorporate blind-fault activity.