Globally, deformation along obliquely converging plate margins produce a wide variety of complex fault patterns, including crustal pop-ups, fault duplex structures, restraining bends, and flower structures. Depending on the plate velocity, plate obliquity, crustal rheology, length-scale, and climate, the evolution of faulting into translational and vertical strain can range in complexity and fault slip partitioning (i.e. vertical vs. horizontal strain). In this dissertation I studied two restraining bends to understand how these factors influence patterns of deformation along two major plate boundaries: The North American-Caribbean and the North AmericanPacific plate boundaries. First, I estimate the exhumation and cooling history along the Blue Mountains restraining bend in Jamaica using multiple thermochronometers. Three phases of cooling have occurred within Jamaica: 1) initial rock crystallization and rapid emplacement of plutons from 75-68 Ma, 2) slow cooling from 68-20 Ma, and 3) two-stage exhumation from 20 Ma – Present. During the most recent phase of Jamaica’s cooling history, two stages of exhumation have been identified at 0.2 mm/yr (20 – 5 Ma) and ~1 mm/yr (5 Ma – Present). Given the plate velocity to exhumation rate ratio during the most recent phase, we suggest that the climate of Jamaica increases the erosivity of the Blue Mountain suite, whereby the Blue Mountains may be in an erosional stead-state. Second, I studied the long-term evolution of a restraining bend at San Gorgonio Pass in southern California by relating fault kinematics within the uplifted San Bernardino Mountains to the nearby Eastern California shear zone. Using highresolution topography (i.e. UAV and lidar surveys), I studied the plausibility of faulting along two potentially nascent faults within the San Bernardino Mountains, namely the Lone Valley and Lake Peak faults. We found that while both faults display evidence for Quaternary faulting, deciphering true fault slip rates was challenging due to the erosive nature of the mountainous landscape. Coupled with evidence of Quaternary faulting along other faults within the San Bernardino Mountains, we suggest a western migration of the Eastern California shear zone. / PHD / The deformation of rocks along tectonic plate boundaries provides insight into how the upper crust behaves, and is dependent on the crustal strength, plate velocity, temporal and spatial scales, and climate. At most convergent plate boundaries, plate motion is oblique to the plate boundary, resulting in zones of transpression: compression and translation. Geologists refer to these features as restraining bends. What factors dictate how faults within restraining bends evolve is a major question in the field of tectonics. In this dissertation I studied two major restraining bends which differ in both scale (i.e. length to width ratio) and climate, namely the Blue Mountains restraining bend in Jamaica and the restraining bend at San Gorgonio Pass in southern California. Along the Blue Mountains restraining bend, it was not understood when or how fast this mountain range was being exhumed due to the tectonic forces being applied to the plate boundary. I use a technique called thermochronometry, whereby instead of measuring the age of rock crystallization, I measure when the rock cools below a certain temperature. Different minerals have different closure temperatures, and by using multiple minerals, I determined the cooling path of the rocks in the Blue Mountains since they crystalized in the late Cretaceous (~75 million years ago). We found that the rocks experienced three different phases of cooling, with a more recent phase being divided into two stages since 20 Ma: Blue Mountain rocks being exhumed at a rate of 0.2 mm/yr from 20 – 5 Ma (relatively slow) and ~1 mm/yr from 5 – 0 Ma (relatively fast). I concluded that the climate of Jamaica weathers and erodes rocks so efficiently that the Blue Mountains are in an erosional balance between plate tectonic forces and climatic forces. My second chapter identifies small, unstudied faults within the San Bernardino Mountains, and determined that these faults display enough evidence that they should be considered a earthquake hazard. The restraining bend itself is migrating towards the southeast and is being influenced by other faults in the area. What once was a predominantly transpressional system, is now being influenced mainly by strike-slip faulting.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/96018 |
| Date | 25 June 2018 |
| Creators | Cochran, William Joseph |
| Contributors | Geosciences, Spotila, James A., Romans, Brian W., Hole, John A., Gazel, Esteban, Law, Richard D. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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