Lara S. Wagner

Upper mantle structure in the south central Chilean subduction zone (30° to 36°S)

[1]xa0Regional P and S wave travel time data were used to obtain three-dimensional seismic tomography models for Vp, Vs, and Vp/Vs above the subducting slab in central Chile and Argentina. In this region, there is an abrupt change from a normal subduction geometry south of 33°S to a flat subduction geometry to the north. We find low Vp, low Vs, and high Vp/Vs ratios in the southern half of our study area directly beneath the modern active volcanic arc, which we interpret as localized pockets of melt. In the northern half of our study area, above where the subducting Nazca plate flattens at 100 km depth, we find low Vp, high Vs, and low Vp/Vs ratios. These unusual results point to a lack of melt or hydrated mineralogies such as serpentine, both of which are characterized by high Vp/Vs values. The only mantle rocks that have low Vp/Vs and high Vs are Mg-rich compositions, such as dehydrated serpentinite or orthopyroxenite. We suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite, formed by a transient fluxing of silica-rich fluids. Such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at ∼10 Ma. This would imply that the hydration of mantle material above a flat slab can be a transient phenomenon, which leaves little residual-free water behind but significantly alters the mantle chemistry.

The June 23, 2001 Peru earthquake and the southern Peru subduction zone

[1]xa0The plate boundary between the South American and Nazca plate along the south-central Peru coast has been the site of large destructive earthquakes for many centuries, including the June 23, 2001 (MW=8.4) event. This underthrusting event has a fault area of 320 km by 100 km based on relocated aftershocks during the first three weeks following the mainshock. Modeling of the teleseismic broadband P waves of the 2001 Peru earthquake indicates two pulses of moment release with the larger second pulse located 130 km southeast of the mainshock initiation, indicating a unilateral rupture to the southeast. Based on intensity and tsunami reports, previous earthquakes in 1868 and 1604 were larger than the 2001 earthquake, while an event in 1784 was smaller. This provides further evidence that the size of earthquakes along the Peru coast has changed between successive earthquake cycles.

THE TECTONIC EVOLUTION OF THE CENTRAL ANDEAN PLATEAU AND GEODYNAMIC IMPLICATIONS FOR THE GROWTH OF PLATEAUS

Current end-member models for the geodynamic evolution of orogenic plateaus predict (1) slow-and-steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the lower lithosphere, or (2) rapid surface uplift following shortening, associated with punctuated removal of dense lower lithosphere and/or lower crustal flow. We will review results from a recent multidisciplinary study of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes that have formed the Plateau. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We will discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle-late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene-early Miocene and late Miocene-Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene-Pliocene crustal flow in the central and northern Altiplano.