The CAFE Experiment : a joint seismic and MT investigation of the Cascadia Subduction System

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013In this thesis we present results from inversion of data using dense arrays of collocated seismic and magnetotelluric stations located in the Cascadia subduction zone region of central Washington. In the migrated seismic section, we clearly image the top of the slab and oceanic Moho, as well as a velocity increase corresponding to the eclogitization of the hydrated upper crust. A deeper velocity increase is interpreted as the eclogitization of metastable gabbros, assisted by fluids released from the dehydration of upper mantle chlorite. A low velocity feature interpreted as a fluid/melt phase is present above this transition. The serpentinized wedge and continental Moho are also imaged. The magnetotelluric image further constrains the fluid/melt features, showing a rising conductive feature that forms a column up to a conductor indicative of a magma chamber feeding Mt. Rainier. This feature also explains the disruption of the continental Moho found in the migrated image. Exploration of the assumption of smoothness implicit in the standard MT inversion provides tools that enable us to generate a more accurate MT model. This final MT model clearly demonstrates the link between slab derived fluids/melting and the Mt. Rainier magma chamber.Funding for this work was made possible by the American Society for Engineering education through a National Defense Science and Engineering Fellowship, and by the National Science Foundation through two grants for the CAFE and CAFE MT projects

Seismic imaging of the mantle transition zone

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.Cataloged from PDF version of thesis.Includes bibliographical references.In this thesis, we developed a generalized Radon transform of SS precursors for large-scale, high-resolution seismo-stratigraphy of the upper mantle transition zone. The generalized Radon transform (GRT) is based on the single scattering approximation and maps singularities (reflections) in broad-band data into singularities (reflectors/scatters) in the medium. It is able to detect and characterize mantle discontinuities at a lateral resolution of several hundred kilometers. Synthetic tests with realistic source-receiver distributions demonstrate that the GRT is able to detect and image deep mantle interfaces at correct depths, even in the presence of noise, depth phases, phase conversions, and multiples generated by reverberation within the transition zone. We apply the GRT to ~1,600,000 broadband seismograms to delineate transition zone interfaces beneath distinct tectonic units, including a cross-section in the northwest Pacific Ocean that is far away from known down- and up-wellings, the volcanic islands of Hawaii, and the northwest Pacific subduction system. We account for smooth 3D mantle heterogeneity using first-order perturbation theory and independently derived global tomography models. Through integration with mineral physics data, the GRT seismic sections can put important constraints on the mantle temperature and mineralogy of the transition zone. Our GRT imaging results beneath the Central Pacific (including the Hawaii hotspot) reveal a more complicated mantle convection picture than a thin narrow vertical mantle "plume" passing through the transition zone. We found an 800- to 2000-kilometer-wide thermal anomaly (with a maximum temperature increase of -300 to 400 kelvin) deep in the transition zone west of Hawaii, by explaining the 410 and 660 km discontinuity topographies with olivine and garnet transitions in a pyrolitic mantle. According to our geodynamical modeling study of mantle upwellings, this might suggest that the hot materials feeding the Hawaii volcanoes do not rise from the lower mantle directly through a narrow vertical plume but may accumulate near the base of the transition zone before being entrained in flow toward Hawaii. In the GRT images of the subduction system, we found a deepened 660 km discontinuity in the slab that penetrates directly into the lower mantle according to tomography results. In another cross-section, where tomography results show that the slab is stagnant above the top of the lower mantle, we found broadening of the 660 km discontinuity signals at both edges of the slab. No corresponding uplift of the 410 km discontinuity is found. However, deepening of the 410 km discontinuity is observed beneath the continental side of the subduction system in both cross-sections, indicating hot anomalies at 410 km depth at the continental side if only the thermal effect is playing a role.by Qin Cao.Ph.D

Advances in crosshole seismic reflection processing

In recent years there have been significant advances in the acquisition and processing of crosshole seismic reflection data, and the method has been shown to be a high resolution imaging technique. However, the fidelity of the final images produced by this technique needs to be considered carefully to avoid incorrect interpretation. This thesis concerns the imaging capability of crosshole surveys, as well as advances made in processing techniques for application to crosshole seismic reflection surveys. In a migrated seismic section, a meaningful image is only obtained if a range of dips around the local structural dip is sampled at each image point. For crosshole seismic reflection surveys, the distribution of dips sampled at an image point is controlled principally by the survey geometry, including source and receiver array lengths and their element spacings. By considering the dips sampled, the imaging capability of crosshole reflection surveying is discussed, with suggestions as to how to ensure optimal imaging of the target zone. To overcome problems encountered in applying standard processing procedures, two new processing techniques are presented which enhance the imaging potential of crosshole reflection seismics. Generalised Berryhill migration has been developed as a full generalised Kirchhoff migration to include the near-field term, with the aim of improving image accuracy close to the source and receiver arrays. 3-D f-k-k filtering is an improved method of wavefield separation for crosshole seismic data. Finally, the results of processing three types of dataset are presented. One is from a site in the Groningen gas field, another was acquired through a model interrogated at ultrasonic frequencies in a water tank, and the third type was acquired using coal exploration boreholes in Yorkshire. The results demonstrate the imaging capability of the crosshole reflection method, and the success of the two new processing schemes

Joint seismic and MT investigation of the Cascadia subduction system

Thesis (Ph. D. in Geophysics)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013.Page 176 blank. Cataloged from PDF version of thesis.Includes bibliographical references.In this thesis we present results from inversion of data using dense arrays of collocated seismic and magnetotelluric stations located in the Cascadia subduction zone region of central Washington. In the migrated seismic section, we clearly image the top of the slab and oceanic Moho, as well as a velocity increase corresponding to the eclogitization of the hydrated upper crust. A deeper velocity increase is interpreted as the eclogitization of metastable gabbros, assisted by fluids released from the dehydration of upper mantle chlorite. A low velocity feature interpreted as a fluid/melt phase is present above this transition. The serpentinized wedge and continental Moho are also imaged. The magnetotelluric image further constrains the fluid/melt features, showing a rising conductive feature that forms a column up to a conductor indicative of a magma chamber feeding Mt. Rainier. This feature also explains the disruption of the continental Moho found in the migrated image. Exploration of the assumption of smoothness implicit in the standard MT inversion provides tools that enable us to generate a more accurate MT model. This final MT model clearly demonstrates the link between slab derived fluids/melting and the Mt. Rainier magma chamber.by R. Shane McGary.Ph.D.in Geophysic

Thermo-mechanical evolution of the subcontinental lithospheric mantle in an extensional environment Insights from the Beni Bousera peridotite massif (Rif belt, Morocco)

The mantle deformation processes that control the thinning and break-up of continental lithosphere remain poorly understood. Our knowledge is restricted to either lithospheric scale thermo-mechanical models --that use experimentally derived flow laws--, geophysical imaging and/or rare xenoliths from active continental rifts, such as the East African Rift System. The originality of this work relies on the study of the two largest outcrops of diamond facies subcontinental lithospheric mantle in the world: the Beni Bousera and Ronda peridotite massifs in N Morocco and S Spain, respectively. The structures and petrologic and metamorphic zoning preserved in these massifs --implying a polybaric and polythermal evolution-- provides a unique opportunity to investigate the thermo-mechanical evolution of thick subcontinental lithospheric mantle in extensional settings. In this thesis we studied the deformation mechanisms in both peridotites and pyroxenites to constrain the modes of exhumation of subcontinental lithospheric mantle from garnet-, to spinel-, and finally, to plagioclase lherzolite facies conditions. We combined field mapping of tectonometamorphic domains and structural mapping of ductile structures, microstructural analysis, crystal preferred orientations (CPO) measurements and conventional thermobarometric calculations and thermodynamic modeling (Perple_X) to unravel the pressure and temperature conditions of deformation. We showed that exhumation from garnet- to spinel lherzolite facies conditions was accommodated by fast shearing --in thermal disequilibrium--along a lithospheric scale transtensional shear zone. In this context, the petrological zoning and the large temperature gradient (ca. 100ºC/km) preserved in the Beni Bousera massif represent the mechanical juxtaposition of progressively deeper and hotter lithospheric levels at depths of ca. 60 km in the latest Oligocene (ca. 25 Ma). Final exhumation from spinel- to plagioclase facies lherzolite and emplacement into the crust is best recorded in the Ronda massif where it occurred by inversion and lithospheric scale folding of the highly attenuated continental lithosphere in a back-arc region, probably in relation with southward slab rollback and subsequent collision with the palaeo-Maghrebien passive margin in the early Miocene (21-23 Ma).Les processus de déformation contrôlant l'amincissement de la lithosphère continentale sont encore mal contraints. Nos connaissances sont principalement basées sur la modélisation thermomécanique d'extension à l'échelle de la lithosphère--utilisant des lois rhéologiques derivées expérimentalement, l'imagerie géophysique et l'analyse de xénolithes provenant de rift continentaux actifs à ce jour, tels que le Rift Est-Africain. L'originalité de ce travail reside dans l'étude des deux plus grands massifs de péridotites sous-continentales ayant enregistrées des conditions primaires du facies à diamant: les massifs de Beni Bousera au nord du Maroc et de Ronda au sud de l'Espagne, respectivement. Les structures et la zonation petrologique et métamorphique --impliquant une évolution polybarique et polythermique-- préservéees dans ces massifs offrent une opportunité unique pour étudier l'évolution thermo-mécanique du manteau sous-continental dans un contexte extensif. Dans ce travail, nous avons étudié les mécanismes de déformation des péridotites et des pyroxénites afin de contraindre les modes d'exhumation du manteau lithosphérique sous-continental, depuis des conditions du facies des lherzolites à grenat, jusqu'au facies à spinelle et enfin à plagioclase. Nous avons combiné la cartographie des faciès tectono-métamorphiques et des structures ductiles de déformation, l'analyse des microstructures, la mesure d'orientations préférentielles de réseau (OPR), et la géothermobarométrie conventionelle couplée à la modélisation thermodynamique (PerpleX) afin de contraindre les conditions de pression et température de la déformation. Nous avons montré que l'exhumation précoce du facies à grenat au facies à spinelle était accomodée par une faille transtensive affectant le manteau lithosphérique. Dans ce contexte, la zonation tectono-métamorphique et le gradient thermique important (ca. 100ºC/km) préservés à Beni Bousera résultent de la juxtaposition mécanique de domaines lithosphériques initialement équilibrés à différentes pressions et températures, fossilisée à une profondeur de ca. 60 km durant l'Oligocène supérieur (ca. 25 Ma). L'exhumation finale du facies de lherzolite à spinelle au facies à plagioclase et l'emplacement final dans la croûte, mieux enregistrés dans Ronda, se sont produits par inversion et plissement de la section lithosphérique fortement amincie dans un contexte arrière-arc, probablement lors du retrait vers le sud de la lithosphère subduite et la collision de l'arc avec les paléo-marges maghrébines au Miocène inférieur (21-23 Ma)

Constraining the Timing of Evolution of Shear Zones in Two Collisional Orogens: Fusing Structural Geology and Geochronology

In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust

Laser writable high-K dielectric for van der Waals nano-electronics

This is the author accepted manuscript. The final version is available from American Association for the Advancement of Science via the DOI in this record.Like silicon-based semiconductor devices, van der Waals heterostructures will require integration with high-K oxides. This is needed to achieve suitable voltage scaling, improved performance as well as allowing for added functionalities. Unfortunately, commonly used high-k oxide deposition methods are not directly compatible with 2D materials. Here we demonstrate a method to embed a multi-functional few nm thick high-k oxide within van der Waals devices without degrading the properties of the neighbouring 2D materials. This is achieved by in-situ laser oxidation of embedded few layer HfS2 crystals. The resultant oxide is found to be in the amorphous phase with a dielectric constant of k~15 and break-down electric fields in the range of 0.5-0.6 V/nm. This transformation allows for the creation of a variety of fundamental nano-electronic and opto-electronic devices including, flexible Schottky barrier field effect transistors, dual gated graphene transistors as well as vertical light emitting and detecting tunnelling transistors. Furthermore, upon dielectric break-down, electrically conductive filaments are formed. This filamentation process can be used to electrically contact encapsulated conductive materials. Careful control of the filamentation process also allows for reversible switching between two resistance states. This allows for the creation of resistive switching random access memories (ReRAMs). We believe that this method of embedding a high-k oxide within complex van der Waals heterostructures could play an important role in future flexible multi-functional van der Waals devices.F.W acknowledges support from the Royal Academy of Engineering. J.D.M. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). S.R. and M.F.C. acknowledge financial support from EPSRC (Grant no. EP/K010050/1, EP/M001024/1, EP/M002438/1), from Royal Society international Exchanges Scheme 2016/R1, from The Leverhulme trust (grant title “Quantum Revolution” and "Quantum Drums"). A.P Rooney and S.J Haigh acknowledge support from the EPSRC postdoctoral fellowship and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement ERC-2016-STG-EvoluTEM-715502) and the Defence Threat Reduction Agency (HDTRA1-12-1-0013). I.A. acknowledges financial support from The European Commission Marie Curie Individual Fellowships (Grant number 701704)

Mesozoic to Tertiary evolution of the southwestern proto-Pacific Gondwana margin

The Andean margin of South America formed part of the proto-Pacific margin of Gondwana, and has been the site of long-lived subduction. The tectonic evolution of the margin has been influenced by the breakup of that supercontinent and interactions with the subduction zone and the subducting proto-Pacific plates. Since Late Jurassic times the margin has experienced two distinct tectonic regimes. The first was principally controlled by extension associated with Gondwana dispersal, resulting in the development of extensional basins along the length of the margin. The second regime, principally controlled by convergence with minor episodes of extension, initiated at different times along the margin and continues today. During this second period the margin experienced major uplift, leading particularly to the development of the Altiplano-Puna Plateau system, as well as numerous fold and thrust belts. The role of subduction along the proto-Pacific margin in contributing to the development of these events is quantitatively examined in the framework of a recently developed kinematic plate model. The absolute velocity of the overriding South American plate, the convergence velocity between the downgoing oceanic slab and the South American plate, and the age of the subducting slab are resolved along the margin back to 170 Ma to determine correlations between these parameters and deformation along the Andean margin. Any single parameter examined in isolation did not produce a correlation with deformation in the overriding plate, indicating that more complex interactions among several factors are required to produce the observed pattern of deformation. The development of extensional basins floored by mafic/oceanic crust was associated with periods when the motion of the South American plate was directed away from the trench in conjunction with the subduction of oceanic crust older than c.50 Myr. Contraction events, such as the development of fold and thrust belts and of the Altiplano-Puna Plateau, were associated with convergence velocities greater than 4 cm/yr. The absolute motion of South America was directed trenchward during these events, although the magnitude of motion was very small. The development of extensional basins that did not progress to the formation of mafic crust was accompanied by diverse conditions along the subduction zone, as were large magmatic fluxes leading to batholith emplacement, indicating that these events are not primarily controlled by any of the parameters investigated. In addition, no correlations could be established related to the structural style of fold and thrust belts. Whereas plate scale processes control the tectonic regime along the margin, other mechanisms that act within the overall context of the imposed plate setting can affect deformation at more regional or local scales. The Rocas Verdes Basin in southern Patagonia is an unusual setting as it is one of only two extensional basins floored by ocean crust in a string of Mesozoic extensi onal ensialic basins developed along the Andean margin. The Rocas Verdes Basin is interpreted to have undergone different evolutional histories along the length of the basin. Negative (i.e. extensional) convergence rates were present in the southern portion of the basin during opening, while positive convergence rates affected the north, are implicated as contributing factors resulting in different magnitudes of extension. The Cordillera Darwin metamorphic complex in southernmost Patagonia exposes amphibolite facies kyanite-staurolite metapelitic schist, the highest grade metamorphic rocks in the Andean chain south of Ecuador. Closure of the Rocas Verdes Basin in this area resulted in continental underthrusting beneath the arc, resulting in the amphibolite facies metamorphism. Thrust-controlled exhumation terminated metamorphism. Garnet in metapelitic schist has patchy textures where regions of clear grossular-rich garnet with fine-grained S1 inclusion trails are cut by turbid regions of spessartine-pyrope-rich garnet containing! inclusi ons of biotite, muscovite, plagioclase, and quartz that align with S2 in the matrix. Aqueous micro-inclusions in the turbid regions suggest that garnet recrystallization to form the patchy textures was facilitated by fluid ingress; recrystallization appears contemporaneous with the growth of matrix kyanite and staurolite. Pseudosection modelling in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) describes a P-T path for rocks of the Cordillera Darwin of nearly isothermal decompression from 12 to 9 kbar at T≈620°C. Along this path, garnet mode is predicted to decrease from a maximum of c.5% to <1%, driving garnet dissolution. In situion-probe U-Th-Pb dating of S2 matrix monazite, texturally associated with the growth of S2 kyanite and staurolite, shows that exhumation of the Cordillera Darwin was underway by 72.6 ± 1.1 Ma. Later granitic intrusions produced contact aureoles including sillimanite bearing migmatites in the adjacent rocks at P≈6 kbar. The Seno Otway region lies midway between the comparatively well studied regions of Ultima Esperanza in the north, and Cordillera Darwin in the south. The area includes rocks that reflect the two distinct tectonic regimes of the South American margin: there is a complete record from the development of the Jurassic – Late Cretaceous Rocas Verdes marginal basin to the Cretaceous Magallanes foreland basin succession. Sedimentation into the Rocas Verdes Basin consisted of the hemipelagic mudstones and fine-grained sandstones of the Zapata and Canal Bertrand formations. The formations are mudstone dominated with an increasing abundance of sand in the upper portions recording the onset of contraction and basin closure. Thrusting that obducted portions of the mafic marginal basin floor onto the South American continent was contemporary with the development of the Magallanes foreland basin. Sedimentation into this basin included repeated fining upward sequences of in terbedded mudstone and sandstone turbidites called the Latorre forma tion, and the overlying Escarpada Formation, composed of interbedded sandstone, mudstone, and conglomerate. Emergent portions of the Rocas Verdes Basin and Late Jurassic volcanic rock, exposed through thrusting, were eroded to form parts of the Late Cretaceous sedimentary units. The Magallanes fold and thrust belt involved the metamorphic Paleozoic basement in this region. Folding styles are tight to isoclinal near the main obduction thrusts and broaden to become close to open toward the foreland. An accompanying axial planar cleavage is fine-scaled and pervasive in regions of tight folding and is only weakly developed toward the foreland. Metamorphic grade in the region does not exceed greenschist facies. Deformation in portions of the Latorre and Escarpada formations is low enough to allow for the preservation of original sedimentary features, including various paleocurrent indicators. These indicators record the presence of multiple submarine fans in the Magallanes forelan d basin in the Late Cretaceous. The Andean margin of South America is commonly considered the type example of an ocean – continent convergent margin. Geological variability along its length provides a unique opportunity to validate macroscopic models for lithospheric weakening leading to the formation and destruction of marginal basins. During the Early Mesozoic most of the Andean margin lay below sea level as a series of extensional basins formed above a subduction zone. Cretaceous compression related to intraplate shortening resulted in basin inversion, with highly variable thickening and uplift. Crustal thickening in the area now known as Cordillera Darwin resulted in high-grade metamorphism from tectonic processes commonly associated with regions of continent – continent convergence. Such diversity in the Rocas Verdes Basin thus provides valuable insights into the evolution of a margin perhaps too commonly regarded as uniform and coherent. Basin development and inversion reflect related but distinct extensional and contractional regimes along the length of the Andean margin, and events in the history of the basin are demonstrably linked with plate-scale processes