At collisional mountain ranges the tectonic history of crustal shortening and subsequent post-collisional erosion is preserved in the form of the presently observed gravity anomalies. In this study, models of erosion and isostatic rebound at various stages of collision illustrate the evolution of crustal structure, topography, and resulting gravity anomalies. The Ouachita Mountains of Arkansas, which show a low/high Bouguer gravity couple characteristic of the initial stages of collision, have undergone just 8 km of erosion during the process of completely rebounding the syn-orogenic crustal root. This minor rebound means that the Ouachitas retain a crustal geometry similar to the continental margin prior to collision, including thin transitional and oceanic crust. At more advances stages of collision Bouguer gravity anomalies show a broad low reflecting a thickened crustal root. The width of this low, which relates directly to the amount of crustal shortening, is retained during subsequent erosion and elastic rebound, but the amplitude decays gradually. Thus, the width and amplitude of the low can be used to estimate the degree of convergence and amount of erosion, respectively, for a specific mountain range. For the Scandinavian Caledonides results are consistent with 20 km of erosion following 200 km of crustal shortening. Following a larger magnitude of convergence, about 300 km, the southern Appalachians are estimated to have undergone 28 km of post-collisional erosion. Bouguer gravity profiles across the recently-active Alps compare with a model of 200 km of crustal shortening and 8 to 12 km of erosion. While the Alps have undergone a similar amount of shortening as that estimated for the Caledonides, erosion and post-collisional rebound is at an initial stage, such that a thick section of exotic crust still overlies the underthrusted European Platform. The results of these model comparisons suggest that the crustal geometry ofa collisional mountain range should be viewed as a consequence of the degree of crustal shortening as well as the amount of erosion and isostatic rebound. In models at moderate to advanced stages of shortening ( 200 km), and mature stages of erosion (e.g., Caledonides, Appalachians), the geometry of the crustal "suture" between overthrusting and underthrusting crusts is present as a shallow, subhorizontal de collement beneath the foreland. In the hinterland the suture abruptly steepens, a result of differential uplift during isostatic rebound. This crustal geometry, characteristic of seismic-reflection profiles across many ancient mountain belts, suggests: (1) that the "low angle detachment" observed beneath collisional mountain ranges was originally much deeper and steeper than it is at present; and (2) that steep-dipping seismic reflectors towards the hinterland represent the thrusted contact between converging crustal blocks, but have been steepened as a result of isostatic uplift following erosion. / Graduation date: 1992

The Far-Offset Active-Source Imaging of Mantle (FAIM) experiment was conducted along an 800-km-long transect in the Western Atlantic to study the evolution of 108-157 m.y. lithosphere. The main transect (Line 1) crosses a transition from slow (13-14 mm/yr in half rate) to ultra-slow (~8 mm/yr) paleo spreading rates, and thus represents an ideal setting to study spreading-rate dependent processes as expressed in preserved lithospheric structure. This thesis presents results of four analysis efforts along this transect. We present a crustal model based on seismic refraction and wide-angle traveltime modeling, we extend the crustal model to an upper lithosphere density model using gravity constraints, we constrain Poissons ratio in oceanic Layer 3 using converted shear-wave phases, and we consider regional lithospheric structure by analysis of geoid/topography ratios. The crustal model indicates that a transition in crustal thickness accompanies the spreading-rate change, with the crust produced at slow rates being 1.0-1.5 km thinner. The gravity modeling shows that a density model can be constructed that simultaneously satisfies observed gravity, seismic constraints on crustal thickness, and our expectation of isostacy if ~1.3 km of low-density material is distributed into the upper 30-60 km of the mantle. This amount of material (~1.3 km) roughly equals the difference in thickness between slow and ultra-slow spreading crust, suggesting that that the thinner crust formed during very slow spreading arises due to melt retention in the mantle rather than decreased mantle melting. Modeling of mode-converted S-wave phases reveals a uniform of Poissons ratio (~0.27) in the lower crust. Along with the observation of sharp crust/mantle boundary, this result suggests that crust along the FAIM transect is primarily melt-derived igneous crust. Geoid versus topography relationships along Line 1 and nearby parallel tracks show abrupt changes that may originate from lateral changes in mantle density, possibly in response to the transition from slow to ultra-slow spreading. This type of observation may enable us to extend our inferences to a more regional scale.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 127-141).

Thesis (Sc. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1983. / Includes bibliographical references (p. 229-244).

Extensive studies have been undertaking in exploring the tectonic evolution of the Himalayan Orogen. Various tectonic models were developed to explain and constraint spatially and temporally critical events including the collision of Indian Plate with the Eurasia Plate, crustal thickening in association with the indentation, crustal spreading of the Tibetan Plateau. Recent study by King et al., 2011 identified two distinct leucogranite suites which were formed by contrasting tectonic actions at Sakya. They are Equigranular Anastomosing Leucogranite (AEG) formed under prograde fluidpresent condition while the Discrete Porphyritic Pluton Leucogranite (DPP) formed with retro-grade fluid-absent environment. Based on the characteristics of AEG and DPP, this study started with the acquisition of geochemistry data of rock samples collected for researches at various locations of the Himalaya Orogen. The two leucogranite suites were characterized through the study of their geochemistry comprised major elements, trace elements and rare earth elements models. Results of the studies concluded the existence of AEGs and DPPs distributed over the eastern area of the Himalaya Orogen beyond longitude 85 degree East. DPPs are also found at the far West location of the orogen. AEGs are typically formed from around 38Ma to 23Ma, while DPPs are of young age from 23Ma to 15Ma. Based on the observation of missing, or paucity in data for AEG and DPPs available to the west of longitude 85 degree East, it is hypothesized that recent collision of the Arabia plate to the Iran Domain inhibited the northward indentation movement of the Indian plate that not only caused the anticlockwise rotation of the Indian plate but also decreased the rate of tectonic movement of the Indian plate in the West relative to Eurasia plate. The slow rate of tectonic movement may result in insufficient thickening/energy developed within the crustal layer to cause any melting. Further studies to examine and development of the hypothesis is recommended. / published_or_final_version / Applied Geosciences / Master / Master of Science

By studying volcanoes on Venus, Mark Jellinek is advancing the science of predicting volcanic activity on earth.

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Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1983. / Bibliography: leaves 441-472. / Microfiche. / xvii, 472 leaves, bound ill., maps 29 cm