No description available.

The widespread recognition of (ultra)high-pressure ((U)HP) metamorphic rocks in orogens worldwide suggests that subduction and exhumation of crustal rocks from mantle depths are normal processes at convergent plate margins. However, the dynamics of these processes, in particular the comparative roles of erosion and crustal extension, and the driving forces of extension during (U)HP rock exhumation, remain controversial. This thesis presents numerical modeling and field/analytical studies that address the geodynamics of crustal subduction and exhumation in two intensely studied orogens, the Alps and the Caledonides. The 2D numerical models show how different scales and durations of orogeny and plate motions can lead to marked contrasts in the style of orogenic growth, crustal subduction, and (U)HP exhumation. In the Western Alps, rapid exhumation (1-3 cm/a) can be explained by local, syn-orogenic extension driven by the buoyant ascent of deforming (U)HP crust from the subduction channel. Later trans-crustal exhumation probably resulted from the combined effects of syn-convergent thrusting, local extension, and erosion. The low temperatures (500-700°C) of Alpine (U)HP metamorphism are attributable to the small size of the orogen and short duration of subduction/exhumation. Contrary to recent suggestions, neither erosion nor absolute extension is required to explain (U)HP exhumation in the Alps. The Western Gneiss Region (WGR) (Norwegian Caledonides), in contrast, can be explained by subduction to (U)HP conditions followed by plate divergence. Gravitational spreading of a thick, hot orogenic wedge leads to a short period of coeval thrusting and extension. Exhumation of (U)HP crust from the subduction channel is achieved by normal-sense shearing along the top of the (U)HP terrane, with minor associated shortening. Trans-crustal exhumation by vertical thinning of the orogenic wedge results from continued absolute extension and erosion. The comparatively high temperatures (700-800°C) achieved by Caledonian (U)HP rocks reflect the orogen's greater size, slower exhumation rates, and possible stalling of the (U)HP terrane at depth. These contrasting models underscore the variety of possible mechanisms responsible for (U)HP exhumation, and represent new benchmarks in the understanding of Alpine and Caledonian tectonics and (U)HP rock exhumation in general.

Thesis (Ph. D.)--University of Adelaide, Dept. of Geology and Geophysics, 1995. / Copies of author's previously published works inserted. Includes bibliographical references.

Undergraduate honors paper--Mount Holyoke College, 2006. Dept. of Astronomy. / Includes bibliographical references (42-44 leaves ).

Thesis (Ph.D.) - James Cook University, 2004. / Typescript (photocopy) Bibliography: leaves 89-97 (volume 1)

Thesis (M.S. in Earth and Environmental Sciences)--Vanderbilt University, Aug. 2008. / Title from title screen. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1966. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1967. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The Gander Lake Subzone of northeast Newfoundland preserves a complex tectonothermal evolution resulting from continental collision of Gondwana and Laurentia following closure of the Iapetus ocean. Field, petrographic, geothermobarometric studies and isotopic age data define five northeastsouthwest trending domains, each with a characteristic P-T-t-d evolutionary path, which reveal elements of the overall tectonothermal evolution in this sector of the Appalachians. Domain I preserves deformed low grade metasediments and east vergent flat-lying 02 folds formed at c. 470 Ma. Domain 2 preserves focusing of later progressive deformation (D3wEST) into a steep, predominantly sinistral high strain zone characterised by andalusite ~ kyanite ~ sillimanite indicative of a clockwise metamorphic path (peak conditions c. 650°C, 5.5 kbar). In domain 3, deformed metasediments (D2-D3EAsT) display an eastward increase in structural complexity and metamorphic grade to a peak of c. 600°C. Domain 4 displays progressive amphibolite facies deformation (D3EAsT) characterised by prograde andalusite ~ sillimanite-bearing (c. 425 Ma) migmatites with peak conditions of c. 700°C, 4.5 kbar. Retrograde 04 deformation and metamorphism is concentrated in steep narrow high strain zones. S4WES~amphibolite to greenschist facies shear fabrics (predominantly dextral) overprint prograde fabrics (S3WEST)within domain 2 and are cross-cut by the c. 427 Ma Middle Brook Granite, Locally in domains 3 and 4 prograde (D3EAsT) fabrics are overprinted by amphibolite to upper greenschist facies S4EAST fabrics which also form the dominant fabric in c. 417 Ma syntectonic granites. D5-06 retrogressive deformation is pervasive in a c. 2 km wide mylonitic zone adjacent to the Dover Fault. D5 dextral greenschist-facies ductile structures are cut by the c. 385 Ma Newport Granite which in tum is cut by 06 sub-greenschist facies brittle dextral faults. In combination, the domains preserve A) low grade deformation (Ordovician?) associated with easterly thrusting of the Dunnage Zone over the Gander Zone, B) Silurian rIletamorphism and deformation progressively partitioned into high strain zones and, C) Devonian retrograde ductile-brittle shearing and brittle faulting local to the Dover Fault. The spatial and temporal coincidence of transpressive deformation, moderate to high grade metamorphism and voluminous granite magmatism in the east portion of the Gander Zone is taken to relate to sinistrally oblique collision between two major crustal blocks during the Silurian. Devonian reactivation juxtaposed part of the high grade Gander Zone against the low grade Avalon block across the brittle-ductile Dover Fault.

Subsequent to Lower Eocene (ca. 50Ma) collision of the Indian and Asian plates, continental subduction occurred along the N-dipping Main Central Thrust (MCT) of the Himalaya. In western Garhwal, NW India, upper amphibolite facies Vaikrita Group gneisses of the High Himalayan Slab (HHS) were thrust southwards over unmetamorphosed to greenschist facies Garhwal Group quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a ca. 10km thick shear zone composed of mylonitic Munsiari Group augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the N by the Vaikrita roof thrust (VT) and by the Munsiari floor thrust (MT) to the S. The VT is a diffuse high-temperature shear zone recognised through a difference in lithology, metamorphic history, and tectonic style between the Vaikrita and Munsiari Groups. The MT is a relatively discrete fault formed at conditions approaching the brittle-ductile transition. N of the MCT zone, the Jhala Normal Fault (JNF) is a ductile to brittle N-dipping extensional shear zone that was responsible for the downthrow of HHS gneisses and Tethyan sediments in response to gravitational instability of the uplifting orogen. Garnet compositional zoning was produced during growth in both the MCT zone and the lower HHS. In the central and upper HHS it resulted from high-temperature homogenization followed by retrogressive re-equilibration. Diffusion studies suggest rapid cooling of the upper HHS garnets may have been caused by crustal thinning across the JNF. The inverted metamorphic sequence is the cumulative result of polyphase metamoiphism. M1 was a post-collisional Barrovian event of garnet to sillimanite grade restricted to the HHS. M2 was contemporaneous with D2 MCT kinematics and was prograde only in the MCT zone and lower HHS possibly as a result of conductive footwall heating. M3 resulted from nearly isothermal decompression of the upper HHS as a consequence of JNF activation. Thermobarometic transects reveal a significant increase in both P and T across the VT with subsequent decreases accompanying structural height in the HHS. Reliable K-Ar (muscovite) cooling ages from a transect through the MCT zone and HHS are progressively younger towards the S. Ages of ca. 22Ma to ca. 8Ma reflect the piggy-back style deformation sequence; disruptions to the younging sequence are interpreted as localised resetting of ages due to out-of-sequence shearing events. Biotite ages commonly suffered from excess argon and were unreliable. An40Ar/39Ar (hornblende) cooling age suggests rocks of the lower MCT zone were not heated above ca. 500°C since the Precambrian. A ca. 20Ma age dates the last high-temperature motion in the upper MCT zone. The decrease in cooling rate obtained from cooling ages for specific mineral blocking temperatures for the upper MCT zone may be hnked to a return to erosion-controlled denudation after JNF extension.