ALBERT

Description: The Salt Range in Pakistan exposes Precambrian to Pleistocene strata outcropping along the Salt Range Thrust (SRT). To better understand the in‐situ Cambrian and Pliocene tectonic evolution of the Pakistan Subhimalaya, we have conducted low‐temperature thermochronological analysis using apatite (U‐Th‐Sm)/He and fission track dating. We combine cooling ages from different samples located along the thrust front of the SRT into a thermal model that shows two major cooling events associated with rifting and regional erosion in the Late Palaeozoic and SRT activity since the Pliocene. Our results suggest that the SRT maintained a long‐term average shortening rate of ~5–6 mm/yr and a high exhumation rate above the SRT ramp since ~4 Ma.

Description: The Central Andes are the product of contractional deformation related to subduction of the Nazca plate beneath the South American continent. In this geodynamic setting, thrust belt propagation in the Central Andean retroarc in NW Argentina and southern Bolivia has been linked with the critical wedge model implying progressive eastward migration of the deformation front during the Cenozoic. However, Eocene unconformities and growth strata within the orogenic hinterland of the Eastern Cordillera and Puna Plateau suggest selective reactivation of pre-Andean and Andean faults revealing a pattern of diachronous and spatially disparate range uplifts. The Calchaquí Valley in NW Argentina is characterized by basin-and-range morphology, which is intimately linked with the heterogeneous Neoproterozoic basement, Cretaceous rift structures, and the Cenozoic foreland. In this study, we analyze this complex deformation by reconstructing Cenozoic fault reactivation and sedimentation patterns in the Eastern Cordillera. Detailed structural mapping along the eastern Calchaquí border (Tonco area) reveals five Cenozoic unconformities in the former foreland deposits related to multiple episodes of fault reactivation during the Eocene-to-Pleistocene. A refined chronostratigraphy (U-Pb zircon), new AHe thermochronological data (apatite U-Th-Sm/He), and existing evidence for deformation indicate episodic out-of-sequence deformation and increased Eo-Oligocene (Puna Plateau) to Miocene (Eastern Cordillera) fault displacement apparently controlled by pre-Cenozoic heterogeneities. The coeval eastward propagation of deformation and continuing deformation in the west generally challenge the applicability of the critical wedge model.

Description: The Kohat fold and thrust belt in Pakistan shows a significantly different structural style due to the structural evolution on the double décollement compared to the rest of the Subhimalaya. In order to better understand the spatio-temporal structural evolution of the Kohat fold and thrust belt, we combine balanced cross sections with apatite (U–Th-Sm)/He (AHe) and apatite fission track (AFT) dating. The AHe and AFT ages appear to be totally reset, allowing us to date exhumation above structural ramps. The results suggest that deformation began on the frontal Surghar thrust at ~15 Ma, predating or coeval with the development of the Main Boundary thrust at ~12 Ma. Deformation propagated southward from the Main Boundary thrust on double décollements between 10 Ma and 2 Ma, resulting in a disharmonic structural style inside the Kohat fold and thrust belt. Thermal modeling of the thermochronologic data suggest that samples inside Kohat fold and thrust belt experienced cooling due to formation of the duplexes; this deformation facilitated tectonic thickening of the wedge and erosion of the Miocene to Pliocene foreland strata. The spatial distribution of AHe and AFT ages in combination with the structural forward model suggest that, in the Kohat fold and thrust belt, the wedge deformed in-sequence as a supercritical wedge (~15-12 Ma), then readjusted by out-sequence deformation (~12-0 Ma) within the Kohat fold and thrust belt into a sub-critical wedge.