Alignment of post-Atlantic-rifting Volcanic Features on the Guinea Plateau, West Africa, and Present-Day Deformation in the Southwest United States from GPS Geodesy

Abstract

Analysis of the alignment of geologic features and the use of GPS strain measurements are very different approaches to understanding crustal deformation histories and crustal and upper mantle properties. In this dissertation, two study areas with markedly different environments are evaluated using these approaches. The first study area includes the Guinea Plateau offshore West Africa that is part of a complex passive-margin system formed during two phases of rifting during the Jurassic and Cretaceous. Circular features revealed in two 3D seismic reflection surveys are interpreted to be extrusive volcanic features or vents emplaced after the cessation of Cretaceous rifting and opening of the Equatorial Atlantic Ocean. Statistically significant alignments of the vents implies that their distribution was influenced by faults or fractures not obvious in the seismic data alone. The existence of inferred alignments provides additional information about possible structures in the area of the volcanic vents that can be compared to more regional structures, giving better insight to magma migration and extrusion and structure of the Guinea Plateau. The alignment in one of the 3D survey areas is sub-parallel to oceanic fracture zones and continental lineaments that may extend into the survey and could have influenced the distribution of the volcanic features. The alignment in a separate 3D survey area is sub-parallel to the shelf-break and thought be related to inferred oceanward crustal thinning. Employing a different approach to the analysis of deformation, the second study area focuses on the Southern Basin and Range and Colorado Plateau, a weakly deforming area that is still capable of producing large earthquakes such as the 1887 Mw 7.5 Sonoran earthquake. To better constrain crustal motions and investigate the distribution of strain rates several hundred kilometers from the Pacific-North American plate boundary, an expanded GPS network of 34 sites was installed to complement existing continuous and campaign networks. Coseismic and postseismic deformation associated with earthquakes outside the study area, including the 4 April 2010 Mw 7.2 El Mayor–Cucapah, affected the GPS time series resulting in time-varying crustal surface velocities that obscured the background tectonic deformation. Through a deformation model, viscosities of the lower crust and upper mantle are estimated and the effects of earthquakes dating back to 1887 are removed from the time series to yield a time-independent or background secular velocity. A total velocity uncertainty is calculated that includes uncertainty of the time-independent velocities related to uncertainty in the viscosity estimates. Displacement histories are used to illustrate how earthquakes along the Pacific-North American plate boundary can temporarily impede extension in the Southern Basin and Range, particularly in southwestern Arizona. The time-independent velocities are used to calculate strain rates using latitudinal and longitudinal velocity profiles on one-degree increments. On a statistically significant basis, the velocity profiles are modeled with two linear segments rather than a single linear segment. Using the break points dividing the segments, the study area can be separated into a relatively lower-strain-rate eastern domain and a relatively higher-strain-rate western domain. The break points are interpreted to signify a boundary zone approximately 1000 km in length that overlaps tectonic and deformational boundaries described in previous studies. Comparing the time-invariant velocities with cumulative extensional slip rates of Quaternary faults across the area reveals a discrepancy in a portion of the area that is difficult to explain, but may be related to the time-varying velocities resulting from earthquakes on the Pacific-North American plate boundary.