Investigation of the Effects of Nazca-South America Plate Collision along the Peruvian-Chilean Active Continental Margin through Teleseismic Receiver Function Analysis

Abstract

In this dissertation I present the results of three studies examining the Peruvian-Chilean Trench and Andes Mountains along the western margin of South America. Two studies focus on the southern end of the Peruvian flat slab (where the subducting Nazca oceanic plate descends at an angle of ~30° before flattening near 80 km to 100 km depth) and its effects on the overriding plate. The third study focuses on normal subduction (where the subducting Nazca oceanic plate descends continuously at an angle ~30°) in the Maule region of central Chile where I look at the Chile forearc associated with the Mw=8.8 2010 Maule earthquake. In all three studies I draw attention to similarities and contrasts between my observations of these regions and previous studies from similar tectonic settings. The first study uses teleseismic receiver functions to image the Mohos of the subducting Nazca oceanic plate and of the overriding South America continental plate. From these observations, I calculate and map surfaces representing the Moho of each plate to better understand their interactions. I find that the overthickened crust of the subducting Nazca Ridge forms the shallowest part of the southern segment of the Peruvian flat slab and lies at <80 km depth beneath the high elevations of the Peruvian Andes. This shallow segment extends several 100 km beyond the location where the subducted ridge crust appears to dehydrate and transform, at least partially, to eclogite. I show that the continental crust of the Andes thins from ~60 km to ~50 km above the shallowly subducted Nazca Ridge and that the ridge is in contact with the Andean crust along its southeastern edge. I interpret this to indicate that the ridge has removed the continental mantle lithosphere in this location and that the ridge has likely displaced the Andean lower crust. The second study builds on these observations, as well as prior seismic tomography observations of the deeper parts of the Nazca slab, to better constrain the origin of the Fitzcarrald Arch. This arch lies within the foreland of the Peruvian Andes and has previously been shown to have uplifted (without significant upper crustal faulting) at the same time the subducting Nazca Ridge arrived in this part of Peru. I show that the ~1 km of uplift of the arch is best explained in terms of a basal shear model. This model suggests that shear between the subducted Nazca Ridge and the base of the continental plate has thickened the mid-to-lower continental crust beneath the arch by ~5 km. This result suggests that the way in which basal shear has been assumed to operate in other flat slab settings may need to be reexamined. Rather than occurring over the full extent of a flat slab, basal shear is localized around the slab’s shallowest portions. The third study uses RFs to examine the relationship between the forearc Moho in the Maule region in Chile and the 2010 Maule Mw 8.8 earthquake. I find that the forearc can be divided into three distinct zones, which may reflect crustal structures inherited from earlier tectonic processes. I then examine the pre-Maule earthquake megathrust locking and the earthquake’s rupture in relation to these three zones. I then consider the implications of upper plate segmentation for prior models proposed to explain uplift along the South American active margin and to explain general characteristics of the seismogenic megathrust.