RESUMO
According to their respective temperature sensitivities, Apatite (U-Th)/He (AHe) and apatite fission-track (AFT) thermochronology records the thermal evolution of the upper crust (<5 km) and is a key for distinguishing between different exhumation mechanisms through time-evolving rock uplift, and landscape evolution. We applied these methods to extract the thermal evolution of the upper crust in the Abancay Deflection at the northern edge of the Altiplano (southern Peru). We present 120 single-crystal AHe ages (from 31 samples) and 27 AFT central ages obtained from magmatic bodies across the study area. AHe ages range from 0.6 ± 0.1 to 35.8 ± 2.9 Ma with a satisfactory reproducibility of single-crystal AHe ages with less than 10% averaged dispersion. AFT ages range from 2.6 ± 1.9 to 38.2 ± 4.4 Ma with P( χ 2) values >5%. This dataset allows exploring the crust evolution from the late-Eocene to the Quaternary. Data processed and interpreted in the related article published in Tectonics[6] are stored in PANGAEA repository (108 AHe single-grain ages and 27 AFT ages). We furthermore present in this article 12 extra single-grain AHe ages obtained after the related article publication. We also present the details of fission-track length measurements published in the related article. Thermochronological ages could be reused for testing He diffusion or fission track annealing processes or investigating the broader tectonic/geodynamic evolution of the Andes.
RESUMO
Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude M(w) = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50-70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with M(w) = 8.0 in the Pisco area is estimated to be 250 years.
