The Bolivian Subandes and Beyond: Linking Wedge Deformation Processes Across Multiple Timescales

Abstract

Contractional wedges including subaerial fold-and-thrust belts and submarine accretionary prisms are fundamental features associated with mountain building and subduction and the largest earthquakes on Earth occur on the gently dipping fault planes (décollements) that underlie them. Despite a sound understanding of the basic mechanics that govern whole-wedge structure over geologic timescales and a growing body of studies that have characterized the deformation associated with historic to recent earthquakes, first order questions remain about wedge deformation processes active over intermediate seismotectonic timescales. In this dissertation I take a multidisciplinary approach to answering some of these questions while focusing on the active Subandean fold`and`thrust belt of southern Bolivia. In Chapter 1 I use elastic dislocation modeling of fault`related folds imaged in seismic reflection data to obtain probabilistic estimates of Bolivian wedge-front fault ages and slip rates. My results show that at least half and as much as all of the whole` wedge shortening rate is accommodated by wedge front fault system. The structures comprising this system pose a major source of seismic hazard. In Chapter 2 I use GPS data to examine the affect of subduction zone earthquakes on the backarc surface velocity field and to assess locking and strain accumulation rates on the Subandean décollement. The new velocity field reveals north to south changes in the deformation character of the backarc orogenic wedge including close to a factor of two decrease in the wedge loading rate and décollement locked width from north to south. A comparison of shortening rates over multiple timescales points towards a situation where the northern and southern wedge may be at different stages of the widening-thickening cycle. In Chapter 3 I use a continuum mechanics-based, finite difference method to investigate the spatiotemporal distribution of deformation during orogenic wedge growth. Model wedges match first` order critical taper predictions, exhibit widening-thickening cycles similar to active wedges, and show that as much as 75% of surface faulting occurs in the wedge interior. I explain thrust fault formation near the surface and subsequent down-dip propagation using a simple elastic solution and relate model results to the protothrust zones of active submarine accretionary prisms.