High seismic activity occurs along the Caribbean and the North American Plate boundary beneath the eastern part of Hispaniola. A large number of intermediate to deep earthquakes are clustered between the Puerto Rico Trench to the north and the Muertos Trench to the south suggesting the possibility of concurrent subduction from both north and south. The body wave inversion technique was used to analyze nine earthquakes, the largest teleseismically recorded events since the establishment of WWSSN (World Wide Standardized Seismograph Network) in 1963 in the geographic region between 72°W and 66°W latitude and 16°N and 21°N longitude. Their body-wave magnitude ranges from 5.6 to 6.1. Each event was inverted for strike, dip and slip of the two possible fault planes, as well as for the centroid depth, the total seismic moment and the source time function. In order to optimize the crustal structure parameters used in the body wave inversion method, a two-dimensional geophysical cross-section across Hispaniola was constructed by forward modeling of gravity and magnetics data. The inversion results are consistent and can be divided into groups according to the depth and the epicentral location of the events. The shallow events, with depths of 6 to 12 km, represent crustal deformation and show thrust mechanisms with large strike-slip component. The intermediate depth events range from 42 to 107 km in depth and occur to the south of eastern Hispaniola. They show clear thrust mechanisms with a consistent dip of the compressional P-axis at about 30° to the north and approximately north-south P-axis strike. The deep earthquakes occur between 110 and 177 km depth, have a steep-dipping tensional T-axis, and define another slab, possibly originating at the Puerto Rico Trench to the north. One m[subscript b]=6.l event, which occurred on 6/24/84, shows opposite orientations of the P- and T-axes from the surrounding intermediate events. It is interpreted as an interface event in the upper mantle. The southern subduction zone is well defined and indicates that the Muertos Trench is active, with the subducting plate dipping to the north beneath eastern Hispaniola. At a depth of about 110 km, the northward dipping slab collides with the almost vertical segment of the other slab. This deep vertical slab segment, extending to at least 200 km in depth, may be a remnant of an earlier subduction zone associated with the Puerto Rico Trench. Alternatively, it may be connected with a more gently dipping part of the slab towards the north or, even in some way, with subduction from the south. / Graduation date: 1990

The Chaochou Fault (CCF) is both an important lithologic boundary and a significant topographic feature in the Taiwan orogenic belt. It is the geologic boundary between the Slate Belt to the east, and the Western Foothills to the west. Although the fault is known to be a high angle oblique sinistral thrust fault in places, both its kinematic history and its current role in the development of the orogen are poorly understood. Field fabric data suggest that structural orientations vary along strike, particularly in the middle segment, the suspected location of the intersection of the on-land Eurasian continent-ocean boundary and the Luzon Island Arc. Foliation/solution cleavage is oriented NE-SW and in the northern and southern sections, but ESE-WNW in the middle segment. Slip lineations also reveal a change in fault motion from dip-parallel in the north to a more scattered pattern in the south. This correlates somewhat with recent GPS results, which indicate that the direction of current horizontal surface motion changes along strike from nearly perpendicular to the fault in the northern field area, to oblique and nearly parallel to the fault in the southern field area. The magnitude of vertical surface motion vectors, relative to Lanyu Island, decreases to the south. Surface morphology parameters, including mountain front sinuosity and valley floor width/valley height ratio indicate higher activity and uplift in the north. These observations correlate well with published apatite/zircon fission track data that indicate un-reset ages in the south, and reset ages in the northern segment. Geodetic and geomorphic data indicate that the northern segment of the CCF and Slate Belt are currently undergoing rapid uplift related to oblique arc-continent collision between the Eurasian continent and the Luzon arc. The southern segment is significantly less active perhaps because the orogen is not yet involved in direct arc-continent collision.

The interpretation of 3D seismic data from Ship Shoal South Addition Blocks 349-358, Gulf of Mexico shows a complex interaction between salt, faults, and sedimentary strata. Reconstruction of the geometry of early Pliestocene (about 3.65 Ma) through recent salt and associated sediments reveals the evolution of a supralobal basin in the study area. The basin depocenter shifted from the northeastern part to the center of the study area through time. A small, bulb-shaped, salt-stock structure occurs in the northwest, and a salt sheet structure is present in the southeastern part of the study area. Those structures are part of a pennant-shaped structure bounded by counter regional faults trending northeastward. Salt movements created instability and triggered extensive faulting of the overlying strata. Three-dimensional reconstruction suggests that salt blocked the sediment during the early Pleistocene. The sediment was diverted around the salt high on both east and west sides of the salt body to the southwest and southeast. Stratigraphic interpretation of the interval between 1.35 Ma and 1.95 Ma led to the identification of a highstand systems tract (HST), a transgressive systems tract(TST), and two lowstand systems tracts (LST). The strata are developed normally in the depocenter area, whereas the strata at the basin margin were deformed by salt movement and faulting. Each systems tract is uniquely associated with a certain seismic facies. Three seismic facies were identified associated with LST, TST, and HST. Additionally, seismic sections reveal channel geometries in the LST. Seismic attribute analysis elucidates facies distribution in the systems tracts. Because of its ability to move, to divert sediment, to create instability, and to block sediment transport pathways, salt exercises the main control on the sedimentary processes in the study area.

The Chaochou Fault (CCF) is both an important lithologic boundary and a significant topographic feature in the Taiwan orogenic belt. It is the geologic boundary between the Slate Belt to the east, and the Western Foothills to the west. Although the fault is known to be a high angle oblique sinistral thrust fault in places, both its kinematic history and its current role in the development of the orogen are poorly understood. Field fabric data suggest that structural orientations vary along strike, particularly in the middle segment, the suspected location of the intersection of the on-land Eurasian continent-ocean boundary and the Luzon Island Arc. Foliation/solution cleavage is oriented NE-SW and in the northern and southern sections, but ESE-WNW in the middle segment. Slip lineations also reveal a change in fault motion from dip-parallel in the north to a more scattered pattern in the south. This correlates somewhat with recent GPS results, which indicate that the direction of current horizontal surface motion changes along strike from nearly perpendicular to the fault in the northern field area, to oblique and nearly parallel to the fault in the southern field area. The magnitude of vertical surface motion vectors, relative to Lanyu Island, decreases to the south. Surface morphology parameters, including mountain front sinuosity and valley floor width/valley height ratio indicate higher activity and uplift in the north. These observations correlate well with published apatite/zircon fission track data that indicate un-reset ages in the south, and reset ages in the northern segment. Geodetic and geomorphic data indicate that the northern segment of the CCF and Slate Belt are currently undergoing rapid uplift related to oblique arc-continent collision between the Eurasian continent and the Luzon arc. The southern segment is significantly less active perhaps because the orogen is not yet involved in direct arc-continent collision.

Thesis (Ph. D.)--Open University. BLDSC no. DXN039344.

Differential loading induced deformation of a mobile substrate (e.g., salt tectonics) is an important process for the development of accommodation space and stratigraphic architectures in intra-slope minibasins. Numerous studies of minibasin systems have focused on either the tectonic processes involved in salt body deformation or the stratigraphic interpretation of the overburden sediment deposits. This study, however, focuses on coevolution of depositional and tectonic processes and provides a new insight of the linked evolution into the stratigraphic patterns. Using a silicone polymer to simulate a viscous mobile substrate, a series of 2D experiments were conducted to explore the effects of variation in 1) sedimentation rate, 2) depositional style (intermittent sediment supply), and 3) the thickness of the deformable salt substrate on subsidence patterns and minibasin evolution. Experiments results have shown that larger initial thickness of salt substrate as well as lower sedimentation rate caused greater amounts of subsidence for a given amount of deposit. Furthermore, increase in subsidence rate was observed as sedimentation continued, while decrease in subsidence rate occurred once sedimentation ceased. Due to the linked depositional and tectonic processes, higher sediment supply resulted in relatively slower subsidence and more actively widening minibasins. Lower sediment supply was observed to have the reverse effect, resulting in higher relative subsidence and a narrow basin width. A numerical model that captures viscous flow under the deposit is also presented here. The model for minibasin formation showed the effects of interaction of the two processes (deposition and tectonics) on the development of minibasin strata in the experiments. Experimental and modeled findings have resulted in a new model of minibasin development that incorporates the effects of sedimentation rates on subsidence patterns into basin evolution. / text

Explosive, caldera forming "super-eruptions" (an eruption of VEI 8 or larger, resulting in 1000+ km³ of volcanic ejecta in ignimbrite sheets) are the single most destructive natural disaster native to Earth. Super-eruptions require three elements to occur: 1-crustal magmatic fluxes above background solidification rates, 2-growth of a batholith scale magma chamber, and 3-an eruption trigger. This study addresses these requirements with new petrographic and geochemical analyses and numerical simulations of crustal magma bodies. Crustal magmatic fluxes up to 10x steady-state arc rates are required to form volcanic provinces that host super-eruptions. Super-eruptions can occur in continental hot-spots or rift environments. Why arcs "flare-up" is the subject of active debate. Arcs may follow a regular cycle of lithospheric thickening, delamination, and asthenospheric upwelling (the Andean cycle); alternatively fertilized lithospheric mantle may undergo rapid melting. Targeted sampling (n = 165) of mapped but unsampled mafic and lamprophyric magmas in the San Juan magmatic locus of Colorado, an archetypical ignimbrite province, over three years identified both the lithospheric mantle reservoir and the most primitive San Juan magmas using optical petrography, whole rock geochemistry (n = 50) and Pb, Sr, and Nd isotope geochemistry (n = 32). These mafic magmas more closely resemble the continental lithosphere geochemically. Mixing models based on Energy Constrained Assimilation/Fractional-Crystallization (EC-AFC) indicate that the San Juan magmatism is the product of lithospheric melts and 30-40% crustal assimilation rather than asthenospheric upwelling. The Farallon flat-slab "pre-fluxed" and refrigerated the Colorado lithospheric mantle; removal of that slab at around 40 Ma triggered the SJVF "flare-up." Numerical simulations of crustal magma chamber growth indicate giant magma chambers form when high magma fluxes raise upper crustal temperatures to 300-400 °C at 5-10 km depth. These simulations focus on chamber growth, convection, and cooling at the expense of geometry or chamber mechanical failure with realistic sill-like geometry at the expense of thermal modeling. New 3D finite difference simulations emphasize the importance of geometry on chamber lifespan and crustal heating. A spherical chamber (i.e. model construct) requires 10x the cooling time of a 2km caldera footprint sill of same volume. Increasing sill thickness by 1km can double chamber longevity. Focused intrusions (i.e. 1D modeling) locally produce higher thermal gradients and preserve larger primary basalt volumes. Random intrusions in 3D yield basalt to crust ratios of 3-4:1 (required in the EC-AFC models). Random intrusion in 3D into the upper crust at "flare-up" fluxes ([greater than or equal to]10 km³ per k.y.) elevate average crustal geotherms by 10 °C / km, allowing for growth of batholithic scale magma chambers a wider footprint. Once situated in the upper crust, sub-caldera magma chambers cool inward forming moving crystallization and fluid saturation fronts. If the saturation front propagates faster than the crystallization front, nucleating fluid bubbles have the opportunity to grow, ascend, and collect at the chamber roof. New 2D finite difference models couple magma chamber cooling to fluid production to explore the conditions of fluid escape and collection. Less silicic magma composition, equant geometry, high ambient thermal gradient, and a stock all aid in fluid pocket growth by slowing the advance of the crystallization front (a fluid trap) and triggering saturation at lower fluid concentrations. Fluid pockets that grow to certain sizes ( > 500 m hemispherical bubble) have the potential to trigger an eruption by propagation of a fluid fracture to the surface. This mechanism possibly triggered the eruption of the 5000+ km³ Fish Canyon Tuff as well as smaller, recent eruptions (Pinatubo, El Chichón). Caldera forming super-eruptions occur in regions that meet these three requirements: 1-high magmatic flux, 2-rapid growth to batholithic size, and 3-a delayed eruption trigger. For the SJVF of Colorado melting of the "pre-fluxed" lithosphere provided the magmatic pulse which melted and heated the crust, forming a broad batholith. As magmatism peaked and began to wane, upper crustal magma chambers started to crystallize, exsolving fluids. These fluids ascended, collected, and fractured their way to the surface, triggering the Fish Canyon Tuff and other eruptions. / text

published_or_final_version / Earth Sciences / Master / Master of Philosophy

Not available / text

The Pioneer Mountains of Idaho expose a lower plate core of Precambrian and Ordovician metasedimentary rocks, which are intruded by Cretaceous and Eocene plutonic bodies. The core is separated by a detachment fault from a surrounding upper plate of Paleozoic and Tertiary sedimentary and volcanic units. The detachment system developed during a Tertiary extensional event which overprinted Paleozoic and Mesozoic east-directed compressional features, and exhibits both brittle and ductile (mylonitic) deformation. Stretching lineations in the mylonite and striations along the detachment surface both cluster around N65W. Composite planar fabrics (s- and c-surfaces) in the mylonite and limited development of a mylonitic front along the NW side of the core both suggest a top-to-the-west sense of shear. Minimum translation is estimated at about 17 km. The Pioneer structural complex is one of a number of metamorphic core complexes present along the North American Cordillera. All exhibit Tertiary extensional deformation, expressed as detachment faults structurally adjacent to ductile mylonitic shear zones. Extension directions, as indicated by stretching lineations within mylonite and striations along detachment faults, fall into regional groups in which the directions are similar in trend throughout each group. Asymmetric fabrics on both small and large scales give senses of shear and indicate that tectonic vergence within each group is directed outward from a central axis. The regional consistency of extension directions implies a regional control of extension in metamorphic core complexes. Much of central Idaho, and possibly a large part of eastern Idaho as well, may be riding on the upper part of an extensive detachment terrane, of which the Pioneer complex exposes the deeper levels. The Pioneer complex, and other core complexes, owes its present elevation to isostatic uplift over an overthickened crustal welt of local scale. Larger-scale uplift may be due to a similar isostatic adjustment over a broad zone of crustal thickening from Mesozoic compressional tectonics and intrusion.