Thesis. 1977. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography : leaves 34-39. / by Norman C. Burr. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Microfiche copy available in Archives and Science. / Two maps and one illustration on 3 folded leaves in pocket. / Includes bibliographies. / by Brian Philip Wernicke. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1979. / Microfiche copy available in Archives and Science. / Bibliography: leaves 73-75. / by Steven J. Hellinger. / Ph.D.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography : leaves 202-217. / Ph.D.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 257-274. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981. / Microfiche copy available in Archives and Science. / Bibliography: leaves 102-106. / by Joann Miriam Stock. / M.S.

The timing, duration, and pressure-temperature (P-T) conditions of metamorphism provide a direct record of the physical and chemical evolution of the crust and inform our knowledge and understanding of plate tectonics. The characteristic timescales and length-scales of metamorphism vary by orders of magnitude, depending on the driving tectonic process. Two fundamental problems with the retrieval of this information from the metamorphic rock record are insufficient temporal resolution and processes that overprint or obscure the full record of metamorphism. Understanding what processes are recorded, and why they are recorded, is critical for accurate models of tectonics. This dissertation examines these processes in the metamorphic rock record in two settings: the central Appalachian orogen and the Western Alps fossil subduction zone. Chapters 2 and 3 focus on poly-metamorphic migmatites from the Smith River Allochthon (SRA) in the central Appalachians. A combination of petrography, thermodynamic modeling, and geochemistry is used to document and quantify the metamorphic evolution of the SRA and determine the petrologic processes that control metamorphic re-equilibration in high-temperature metamorphic systems. Chapter 2 presents new constraints for Silurian high-temperature (~750℃, 0.5 GPa) contact metamorphism in response to mafic magmatism and a cryptic Alleghanian metamorphism (~600℃, 0.8 GPa). A combination of extensive and highly variable melt loss followed by H2O-flux melting during contact metamorphism is shown to produce a range of modified bulk rock compositions and domains with variable fertilities for metamorphic re-equilibration during the Alleghanian. In chapter 3, monazite, allanite, and zircon laser ablation split-stream petrochronology are used to constrain the timing of poly-metamorphism and develop a tectonic model for the SRA. The SRA preserves evidence for at least three orogenic events, each with a relatively short duration (< 10 Myr.), likely due to repeated magmatic heating. The full record of this punctuated heating is obscured by dissolution-reprecipitation reactions that variably recrystallize monazite and decouple trace element chemistry from isotopic age and significantly restrict equilibrium length-scales. Chapters 4 and 5 examine the dynamic interplay between transient fluid flow, episodic metamorphism, and deformation in subduction zones. In chapter 4, diffusional speedometry is applied to eclogite breccias from the Monviso ophiolite to quantify the periodicity of transient deformation and metamorphism at eclogite facies P-T conditions. The maximum timescale for repeated fracturing is constrained to ~1 Myr., likely caused by cyclic variations in fluid pressure and strain rate (not necessarily seismicity). While difficult to preserve and detect in the rock record, this periodic metamorphism may play an important role in detachment and exhumation processes in subduction zones worldwide. Finally, in chapter 4 a combination of thermodynamic modeling and Sm-Nd garnet geochronology are used to construct a model for subduction and exhumation of the Voltri ophiolite. Garnet growth occurs rapidly and close to peak P-T conditions (~520℃, 2.4 GPa) across the ophiolite, with large (>10 km2) areas preserving near-identical ages, suggesting that the Voltri ophiolite was exhumed as several large coherent units, aided by the presence of buoyant serpentinites. / Doctor of Philosophy / Metamorphism provides a direct record of the physical and chemical evolution of Earth's crust and informs our knowledge and understanding of how plate tectonics works on Earth. Differences in the physical conditions (e.g. pressure, temperature) and timescales of metamorphism can provide clues for the operation of unique tectonic processes, such as the intrusion and cooling of magma deep underground or the collision of two tectonic plates and formation of a mountain range. The key is to correctly "read" the metamorphic rock record. One inherent difficulty in reading and interpreting metamorphic rocks is that few current methods are able to resolve very short timescale events (much less that 1 million years (Myr.) in duration), such as earthquakes, in the rock record. Moreover, metamorphic rocks experience numerous distinct 'events', which partly overprint one another and produce a complicated and near impossible puzzle for geologists to unravel. Solving this puzzle is critical to fully understand how plate tectonics works on Earth. This dissertation addresses these problems and examines metamorphism in two locations: the core of the ancient supercontinent Pangea (central Appalachians) and a fossil subduction zone (the Western Alps). Chapters 2 and 3 focus on the central Appalachians. Chemical and textural analysis of metamorphic rocks are used to understand the major heat sources that operated in the crust during the formation of the Appalachians and determine the processes that control metamorphic re-crystallization at extremely high temperatures. Chapter 2 presents new constraints for high-temperature (~750℃) metamorphism in response to magmatic heating and provides evidence for a younger metamorphic event that is cryptically recorded. A combination of compositional changes caused by earlier high-temperature metamorphism and the later addition of water along reactive grain boundaries are shown to be important factors in the cryptic record of the younger metamorphic event. In chapter 3, U-Pb geochronology is used to the determine the timing of metamorphism and construct a tectonic model for the central Appalachians, which preserves evidence for at least three tectonic events over ~200 Myr, but with each occurring over a relatively short duration (< 10 Myr.). These events are interpreted to represent repeated magmatic heating 'pulses' during the formation of Pangea. However, the full record of this punctuated heating is partly obscured by subsequent fluid alteration. Chapters 4 and 5 examine the dynamic interplay between transient fluid flow, earthquakes, and metamorphism deep in subduction zones. In chapter 4, fracture sets within metamorphic garnet crystals from the French Alps (Monviso) are used to determine the timescale of repeated fracturing and recrystallization during subduction. The fracture timescales are estimated to be much less than 1 Myr. and are interpreted to record repeated fluid "pulses" and possibly deep earthquakes. While difficult to preserve and detect in the rock record, this process may play an important role in bringing metamorphic rocks back from deep in subduction zones to Earth's surface. In chapter 4, a combination of mineral chemistry and geochronology are used to construct a tectonic model for the subduction and exhumation of a portion of the Italian Alps (Voltri). Metamorphic reactions occur synchronously and immediately before exhumation across a wide area (> 10 km2). This suggest that large (> 10 km2) pieces of oceanic crust can metamorphose, detach, and exhume deep in subduction zones.

Subduction zones, where one tectonic plate descends beneath another, are the most seismically active regions on Earth and have produced the largest earthquakes and some of the most destructive tsunamis ever recorded. Significant questions remain regarding the roles both the downgoing and overriding plates play in contributing to varying styles of rupture along the main seismogenic contact between the two plates, or megathrust, where such great (Mw > 8) earthquakes are generated. In the last few decades, the scientific community has recognized how different structural and compositional properties of both plates, and in particular the hydration state of the incoming plate can contribute to variations in megathrust slip behaviors. In this thesis, I show how marine multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data can be used to investigate structural controls on megathrust slip behavior including the different styles of great earthquakes and/or the generation of slow slip events. Offshore Alaska and Sumatra, we used long-streamer multichannel seismic data to create a high-resolution P-wave velocity (Vp) model of the upper oceanic crust prior to subduction. Using a differential effective medium theory, we place the first constraints on the amounts pore (free) water contained therein. Our results indicate that the uppermost oceanic crust of the incoming plates in both regions is significantly hydrated. Offshore Alaska, we show that pervasive faulting in the bending area allows seawater to penetrate into the uppermost crust. We propose that high water content in uppermost crust might contribute to observations of low coupling along the shallow plate interface in this area through the expulsion of pore fluids. Geochemical analyses of arc lavas in this segment of the Alaska subduction zone suggests significant fluid release from the downgoing crust compared to adjacent segments. Thus, we propose that during subduction, additional bending and high-temperature circulation of remaining pore fluids could further alter the upper oceanic crust that dehydrates around sub-arc depths. Offshore Sumatra, few bending-related faults are observed; however, evidence for significant and homogeneous hydration within the the uppermost crustal layer 2A (extrusives) suggests that plate bending plays a role in the shallow reopening cracks, facilitating the shallow penetration of seawater. In layer 2B (sheeted dikes) just below, our results suggest heterogeneous, yet significant, hydration that we attribute to the slow and diffuse deformation taking place in the Wharton Basin. We speculate that the large amounts of upper-crustal water carried into the Sumatra subduction zone can influence shallow slip behavior, as evidenced by recent records of a long-lasting slow slip event in the area. To further explore potential structural and compositional controls on spatial varia- tions in megathrust slip behavior in Alaska, we use OBS data to create a 3D Vp model of the Alaska Peninsula Subduction zone within a 500-by-400 km wide area with good resolution down to 20-25 km depths in both the incoming and overriding plates. Our model samples two subduction zone segments that exhibit differences in history and style of megathrust rupture. We interpret reductions in seismic velocities within the incoming plate as evidence for modest hydration of the Pacific oceanic plate resulting from a series of fracture zones and the formation of large seamounts and an associated basement swell, or platform. The bathymetry of the seamounts and platform in part modulates the distribution and lithology of subducting sediments across the margin that we propose might influence shallow slip behavior. Within the overriding North American plate, we see evidence for contrasting styles of deformation and variations in composition (i.e., rigidity) that agrees well with observed changes in plate coupling and great earthquake history. These results emphasize the importance of considering not only one, but several factors related to both the incoming and overriding plates which collectively contribute to along-strike and downdip variations in megathrust slip behavior between segments. Our final study looks at the incoming Cocos plate just before it subducts offshore Mexico beneath the North American plate. Here we jointly inverted 2D OBS and long-offset MCS data acquired parallel to the trench to derive a 270 km-long, high-resolution Vp model of the entire oceanic crust and uppermost mantle. We provide the first constraints on the quantities of both free and structural (i.e., mineral-bound) water contained within the Cocos plate outboard of the Guerrero Gap and adjacent segments of the Mexican subduction zone. The Guerrero gap hosts large slow slip events that are commonly explained through the release of water through the dehydration of altered sediments and upper oceanic crust downdip. Strikingly, our results show that while the Cocos plate is hydrated offshore Mexico, nearly all of the water is contained within the upper oceanic crust. Moreover, we see that most of the water by weight is present as free fluids in the pores and that the upper oceanic crust is only moderately altered (0.3-1.3 wt.%) compared to global averages (> 1.5-3 wt.%). While the upper crust appears hydrated everywhere across our profile, we find that ∼30% more water is subducting outboard the Guerrero seismic gap where large seamounts contribute to a thicker extrusive layer and more alteration. This, along with evidence for the subduction of seamounts in Guerrero might help explain observations of weak shallow plate coupling and a greater propensity for slow slip at greater seismogenic depths compared to adjacent segments. These results provide important new constraints on how much pore and structural water is carried in the Cocos plate offshore Mexico. We propose that global estimates of incoming structural water content are not applicable everywhere, as is commonly assumed by petrologic and thermal models. Much less structural water may be needed within the upper oceanic crust just before subduction to explain the occurrence of slow-slip events downdip in some subduction zones.

Graduation date: 1996

Thesis (Ph. D.)--University of Oregon, 2005. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 173-185). Also available for download via the World Wide Web; free to University of Oregon users.