Browsing by Subject "Subduction"

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Early Miocene high-pressure metamorphism in the Nevado-Filabride Complex of the Betic Cordillera, Spain: implications for subduction in the Western Mediterranean
(2014-08) Kirchner, Kory Lee; Behr, Whitney M.
The Betic Cordillera of southern Spain is an orogen formed in response to convergence between Africa and Iberia, from the late Mesozoic to the present. The orogen consists of three main tectonic complexes, two of which have been subducted to depth, then exhumed back to the surface over short timescales. Subduction in the structurally higher of these complexes is relatively well constrained to the Eocene, but the timing of high-pressure metamorphism in the structurally lower complex, known as the Nevado-Filabride Complex, has been a topic of debate for several years due to conflicting geochronological data. Several proposed tectonic models for the Nevado-Filabride Complex are based on ages of single mineral phases. For example, models based primarily on 40Ar/ 39Ar dating on white mica in high-pressure schists require that the Nevado-Filabride and the overlying tectonic unit, the Alpujarride Complex, were coevally subjected to high-pressure metamorphism in the Eocene, and subsequently exhumed at different rates. More recent models, based on Lu-Hf dating on prograde garnets in eclogites, separate the timing of high-pressure metamorphism of the Nevado-Filabride Complex from the Alpujarride Complex by at least 10 m.y. We examine the viability of these models using multimineral Rb-Sr dating of blueschist and eclogite facies rocks in the Nevado-Filabride Complex. The multimineral isochron method uses the whole high-pressure mineral assemblage rather than a single phase, which allows testing for isotopic disequilibrium. Statistically valid Rb-Sr ages of two schists and one eclogite from the Nevado-Filabride Complex yield ages of 15.78+/-0.47, 15.8+/-1.1, and 17.6+/-1.1 Ma, respectively. The early Miocene Rb-Sr ages are in agreement with garnet Lu-Hf ages and zircon U-Pb ages for high-pressure conditions in the Nevado-Filabride Complex. The new ages imply that two episodes of subduction, punctuated by a period of extension and exhumation, occurred in the Western Mediterranean.
Fluid-mobile element cycling (halogens, boron, lithium) and sourcing through the Costa Rican convergent margin
(2022-05-04) Helper, Jacob Pierce; Barnes, Jaime Danielle; De Moor, J. Maarten (Joost Marten); Cardenas, M. Bayani
The source of fluids in convergent margins can be traced using fluid-mobile elements such as Cl, Br, I, B, and Li. However, previous studies have often overlooked the forearc. Describing fluid loss from subducting oceanic crust to the forearc is difficult for several reasons including differences in pathways due to faulting, variability in the depth in which fluids are released, and compositional changes of the subducting slab. Seepage through the accretionary prism and incorporation into arc magmas are the primary outputs for fluids to leave the convergent margin; however, poor characterization of fluid-mobile element loss through the forearc has made it difficult to develop mass balance models and flux calculations. This study sampled subaerial forearc and volcanic arc springs along the Middle American Convergent Margin in Costa Rica to trace source fluids. Water samples were analyzed for stable isotopic compositions of Li, B, and Cl, and cation and anion concentrations. Concentrations of Cl (0.864-2420 mg/l), B (0.0081-40.172 mg/l), Li (0.000426-5.173 mg/l), Br (3.55-9421.99 µg/l), and I (0.651-2821.88 µg/l) varied greatly as a function of spring location with respect to the margin trench. Reliability in using Li, B, and Cl isotopes as fluid source tracers has been established due to limited isotopic fractionation resulting from incompatibilities with secondary mineral formation and host lithology interactions. ẟ⁷Li, ẟ¹¹B, and ẟ³⁷Cl values range from -2.4 ‰ to 27.3 ‰ (n = 32), -12.0 ‰ to 30.9 ‰ (n = 29), and -1.7 ‰ to 0.7 ‰ (n = 29), respectively. Isotopic compositions and concentration data support hypotheses that springs nearest to the trench reflect shallow sources (sediments and sedimentary pore-fluids), while those at the volcanic arc show trace representations of deeper fluid sources (altered oceanic crust, serpentinites, and mantle). Isotopic variabilities in springs on the Nicoya Peninsula, closest to the trench, result from either interaction between fluids and the host lithology, differences in fluid pathways due to faulting, or a combination of both. Limited data also supports along-arc geochemical differences between springs normal to oceanic crust created at the East Pacific Rise, Cocos-Nazca Spreading Center, and Cocos Ridge.
Kinematic restoration of the Costa Rican convergent margin : exploring the effects of a rough subducting seafloor
(2020-12) Gose, Brooklyn Nicole; Bangs, Nathan Lawrence Bailey; Barnes, Jaime Danielle
The Costa Rican convergent margin has been a primary site for studies of subduction erosion because it is a documented example of an erosional subduction zone with tectonic erosion by seamount, plateau, and ridge collision. Previous work has explored both modern-day and historical evidence for ongoing subduction of a rough seafloor at Costa Rica that has left erosional scars on seafloor bathymetry, but the impacts on slope sedimentation, the tectonic removal of pieces of the upper plate, and the long-term sub seafloor geologic record left behind from these events is poorly understood. To further explore the effects of subducting plate bathymetric features on the long-term structural and stratigraphic record, this study used three-dimensional kinematic restoration to examine trends in uplift and subsidence over time and potentially identify periods of seamount subduction. This study utilized a 3D seismic reflection volume along with interpreted well log data to create a finite element model of major structural and stratigraphic features within the study region. The 3D model was then kinematically restored using Paradigm’s SKUA-GoCAD Kine3D, providing a 2.2 Myr history of three-dimensional motion for the modeled area. Results start at 2.2 Ma when subaerial uplift elevated the margin 1.2 km corresponding to a regional unconformity established from drilling. This event was followed by a period of subsidence before a short pulse of uplift occurred in a ~9 km wide circular area on the shelf. Recent (1.3Ma – 0Ma) enhanced subsidence in the same region has led to the formation of a circular bathymetric depression on the seafloor that resemble similar features farther north interpreted as seamount impacts. This leads me to conclude that a subducted seamount (~9 km at the base, 1.5 km height) passed through the study region ~1.8 Ma leaving behind a weakened region of underplated sediments. Over time, basal erosion of this area has removed 180 km³ of material. The models reveal that seamount subduction causes erosion both by transfer of material from the upper plate in front of the seamount, and by weakening the upper plate to promote erosion in the wake of the seamount, resulting in distinctive seafloor indentations.
Mapping the Rivera and Cocos subduction zone
(2013-12) Suhardja, Sandy Kurniawan; Grand, Stephen P.
The crust and upper mantle seismic structure beneath southwestern Mexico was investigated using several techniques including teleseismic tomography using 3D raytracing, a joint tomographic inversion of teleseismic and regional data that included relocation of regional seismicity, and a P to S converted wave study. The data used in these studies came from a broadband seismic deployment called MARS. The seismic deployment lasted 1.5 years from January 2006 to June 2007 and the stations covered much of Jalisco and Colima states as well as the western part of Michoacan states. At depth less than 50 km, P-wave receiver function images show a clear dipping slow velocity anomaly above a fast velocity layer. The slow anomaly convertor seen in receiver functions is directly above a fast dipping seismic anomaly seen in regional tomography results. The slow velocity with high Vp/Vs ratio is interpreted as a high pore fluid pressure zone within the upper layer of subducting oceanic crust. Regional seismicity was located using the double difference technique and then relocated in a tomography inversion. The seismicity is located very close to the slow dipping boundary to depths of 30-35 km and thus along the plate interface between the subducted and overlying plate. Deeper events are below the slow layer and thus are intraplate. Receiver function results also show a weaker continental Moho signal above the dipping slab that I interpret as a region of mantle serpentinization in the mantle wedge. Inland of the subduction zone, a clear Moho is observed with a maximum thickness of near 42 km although it thins to near 36 km depth towards the north approaching the Tepic-Zacoalco Rift. Using H-K analysis to examine Vp/Vs ratios in the crust, I find a band of very high Vp/Vs along the Jalisco Volcanic lineament as well as beneath the Michoacan-Guanajuato volcanic field. These observations suggest the continental crust is warm and possibly partially molten over broad areas associated with these two magmatic regions and not just locally beneath the volcanoes. I also found seismicity associated with the Jalisco Volcanic Lineament but it was trenchward of the volcanoes. This may indicate extension in this region is part of the explanation for this magmatic activity. At depths below 100 km, the tomography results show clear fast anomalies, about 0.3 km/s faster than the reference model, dipping to the northeast that I interpret as the subducting Rivera and Cocos plates. Tomography models show that the Rivera slab is dipping much steeper than the Cocos plate at depth. Below 150 km depth, the Rivera plate shows an almost vertical dip supporting the interpretation that the slab has steepened through time beneath Jalisco leading to a coastward migration of young volcanism with mixed geochemical signatures. The location of the young volcanism of the Jalisco Volcanic Lineament is just at the edge of the steeply dipping slab seen in the tomography. The magmatism is thus likely a nascent arc. The models also display evidence of a gap between the Rivera and Cocos plates that increases in width with depth marking the boundary between the two plates. The gap lies just to the west of Colima graben and allows asthenosphere to rise above the plates feeding Colima volcano. Another interesting finding from this study is a possibility of a slab tear along the western edge of the Cocos plate at a depth of about 50 km extending 60 km horizontally. The tear is coincident with a lack of seismicity in this region although there are events below and above the tear.
Navajo Volcanic Field xenoliths of the Colorado Plateau : a window into subduction processes from the Proterozoic to the present
(2018-05-07) Marshall, Edward Wayne, IV; Lassiter, John C.; Barnes, Jaime Danielle; Smith, Doug; Behr, Whitney; Lee, Cin-Ty
Mantle xenoliths from the central Colorado Plateau record geochemical evidence of both ancient and modern subduction. These xenoliths are sampled from unusual serpentinized ultramafic microbreccia diatremes, and are both modally hydrated and metasomatized. The hydration and metasomatism of the xenoliths is related to Farallon flat-slab subduction beneath the Colorado Plateau. In Chapter 1, Sm-Nd and Re-Os isotopes systematics are used to see through Farallon hydration and metasomatism and learn more about pre-Farallon magmatic events. From Re-Os systematics, the lithospheric mantle beneath the Colorado Plateau has been retained since it formed between 2.0 to 1.6 Ga. Unmetasomatized xenoliths lie on a Sm-Nd isochron that is 1.45 Ga in age, suggesting a major isotopic resetting event at this time. Combining these two observations with those of previous studies suggests that subduction triggered the ~1.4 Ga granite magmatism event in Laurentia. In Chapter 2, oxygen isotope compositions of olivine and hydrogen isotope compositions of hydrous minerals were collected to investigate the sources and effects of Farallon flat-slab fluids on the Colorado Plateau lithospheric mantle. Hydrogen isotope compositions of hydrous minerals are consistent with equilibration with slab-derived fluids. Oxygen isotope compositions of olivines correlate with indices of metasomatism from the same xenoliths. From this correlation, metasomatism in the lithospheric mantle is related to fluids derived from the serpentinized lithosphere of the Farallon slab. In Chapter 3, hydrogen concentrations in nominally anhydrous minerals (NAMs; e.g. pyroxene) are measured and evaluated for connections between metasomatism, melt extraction, and hydrous mineral growth. There is no convincing clear connection between metasomatism, melt extraction, or hydrous mineral growth and NAM water content either in the Colorado Plateau xenoliths or in studies of NAM water contents from other localities. A key observation is that NAM water content has significantly less variability than similarly incompatible species, such as Ce. This can be explained if water diffuses rapidly through the mantle, smoothing out variability in concentration and decoupling it from indices of melt extraction, metasomatism, or hydrous mineral growth.
Seismic investigations of subduction and intra-arc rifting at the Hikurangi margin, New Zealand
(2021-12-08) Gase, Andrew C.; Van Avendonk, Harm J. A.; Bangs, Nathan Lawrence Bailey; Bassett, Dan; Barnes, Jaime; Spikes, Kyle; Tisato, Nicola
Subduction zones are dynamic systems that control the global distribution of large earthquakes and volcanism. Many interrelated factors can control tectonic, seismic, and magmatic processes within subduction zones, including mechanisms that vary stress, thermal regime, volatile supply, as well as inherited features within the lithosphere, but the relative importance of these factors are debated. North Island New Zealand, where the Pacific Plate subducts beneath the Australian Plate, is renowned for its unique patterns of seismicity and plate coupling in the forearc, the Hikurangi margin, and its magmatically productive intra-arc rift, the Taupo Volcanic Zone. In this dissertation I present three studies that use newly acquired controlled-source seismic data to evaluate (1) crustal and sedimentary controls on seismic behavior in the Hikurangi margin forearc, and (2) interplays between magmatism and crustal deformation in the offshore Taupo Volcanic Zone. In the first study, I explore the crustal structure of the northern Hikurangi margin, which is world renowned for its low seismic coupling, frequent shallow slow slip events, and strong ground-motion amplification during large earthquakes. I show that sharp along-strike variations in frontal accretion indicate variable sediment supply and past subduction of seamounts. Low velocities in the overthrusting plate indicate the presence of compliant materials that likely contribute to tsunamigenesis and enhanced ground motion during earthquakes. In a second study, I compare the structure of the megathrust fault across the interseismic coupling transition between the central and southern Hikurangi margin and reveal a clear correlation between sediment subduction and slip behavior. In the northern and central unlocked, slow slipping segments, the megathrust forms within pelagic carbonates and volcanic sediments. In contrast, the southern locked megathrust is localized to pelagic carbonates and is insulated from the effects of volcanics by ~0.5-1 km of subducting clastic sediment. I propose that slip behavior and coupling is controlled by the lithology and spatial distribution of frictional asperities along the megathrust. Finally, I determine the crustal structure of the offshore Taupo Volcanic Zone and demonstrate that crustal extension and recent magmatic activity are collocated. Deep-penetrating crustal normal faults overlie a ~40-kilometer-wide zone of sill-complexes and heterogeneous seismic velocities in the upper and middle crust. I propose that magmatic intrusions are localized by more permeable fractured crust and contribute to thermal weakening which facilitates rifting.
Structural and rheological evolution of subduction interface shear zones : insights from exhumed rocks
(2019-12-04) Kotowski, Alissa Jeanne; Behr, Whitney M.; Stockli, Daniel F.; Cloos, Mark; Barnes, Jaime; Soukis, Konstaninos; Wallace, Laura
The subduction interface is an inherently heterogeneous distributed shear zone occupying the boundary between a down-going (i.e., subducting) plate and overriding crust and mantle. The sinking of cold, dense oceanic lithosphere creates a slab pull force, which is a driving force for Plate Tectonics. Interface rheology (i.e., deformation or flow) exerts a first-order control on plate boundary strength, seismic style, and the propensity for rocks to exhume, or return to the surface of the Earth from mantle depths. When subducted rocks are exhumed, they provide rare snapshots of the rheological behavior of these complex plate boundary shear zones, and how rocks are subsequently modified by brittle and semi-brittle processes in the upper crust. In this dissertation, I combine a variety of observational and analytical techniques to investigate exhumed subduction-type rocks from Syros Island (Cyclades, Greece) and the sub-ophiolite metamorphic sole in Oman. This work provides constraints on the rheological behavior of subduction plate boundary shear zones, to better inform geodynamic models and to contextualize geophysical observations of active subduction zones. Rheological heterogeneities (i.e., outcrop- to regional-scale features producing strain gradients and/or significant differences in deformation mode or mechanism) are thought to be directly related to the seismic style that occurs along the subduction interface. At relevant depth and temperature conditions for Syros (~50-60 km, 500°C), the important seismic style is an enigmatic, coupled seismic-aseismic phenomena deemed Episodic Tremor and Slow Slip (ETS). ETS involves accelerated – but aseismic – slip over ~10’s-100’s km² of the interface, in conjunction with swarms of micro-seismicity, or tremor, and seems to occur nearly ubiquitously in subduction zones regardless of thermal structure, predicted depths of metamorphic dehydration reactions, or subducting rock type. In Chapter 2, I use exhumed blueschist- and greenschist-facies rocks on Syros to characterize the length scales, types, sources, and deformation mechanisms of rheological heterogeneities that occupy the deep subduction interface, and how they may contribute to enigmatic seismicity like ETS. Partial eclogitization of subducting rocks sets up stark rheological contrast across shear zones, which results in coupled brittle-viscous behavior assisted by near-lithostatic pore fluid pressures. Geologic observations are consistent with a mechanical model of ETS in which the deep interface comprises transiently brittle, potentially tremorgenic sub-patches, within a larger viscously creeping interface patch. These observations scale appropriately with geophysical constraints of tremor source areas and seismic moments. During a subduction-exhumation cycle, the length scales of mixing along the interface, maximum pressures (i.e., depths) that rocks reach, and mechanisms of rock exhumation depend in part on interface rheology. In Chapters 3 and 4, I combine structural and microstructural analysis, novel techniques in thermobarometry, and new interpretation of published metamorphic geochronology on Syros to understand bulk interface deformational style, progressive metamorphism, and rheology. The results are all consistent with a model of coherent subduction and underplating (i.e., transfer of subducting material to the overriding forearc), as opposed to large-scale, chaotic mixing in a mega-mélange. Exhumation of rocks on Syros occurred nearly entirely by buoyancy- and viscosity-driven subduction channel return flow, which accommodated vertical translations of ~40 km from peak depths (60 km) to the middle-lower crust. These inferences are consistent with calculated estimates of shear zone viscosity, and the balance between buoyancy forces and shear tractions at peak subduction depths. While exhumation mechanisms and mechanical behavior of thermally mature subduction zones are particularly important to an understanding of subduction tectonics, subduction initiation is even more enigmatic and poorly understood. The only geologic record of rocks deformed and metamorphosed in infant subduction zones are present as tectonic slivers beneath the world’s ophiolite sequences, deemed metamorphic soles. In Chapter 5, I investigate the structural and petrologic signatures of subduction, return flow, and ophiolite emplacement in a 100 m section of ”low-temperature” metamorphic sole acquired during Phase I of the Oman Drilling Project (Site BT-1B). The ”low-temperature” sole rocks acted as a mechanically coherent slice preserving various evidence for subduction and return flow prior to obduction. Intraoceanic subduction initiation is characterized by rapid cooling of the plate interface, which exerts primary control on metamorphic grade and changes in deformation mechanisms. Exhumation in the subduction channel accommodated at least 15 km of vertical translation, and occurred concurrently with early stages of ophiolite emplacement.
The behavior of halogens (F, Cl, Br, I) in altered oceanic crust during prograde subduction zone metamorphism and devolatilization
(2022-05-05) Beaudoin, Grace Margaret; Barnes, Jaime Danielle; Lassiter, John C; John, Timm; Penniston-Dorland, Sarah; Stockli, Daniel F
Halogens (F, Cl, Br, I) are volatile elements enriched in Earth’s surface reservoirs. Following seafloor alteration and tectonic convergence, halogens are carried into subduction zones. Prograde metamorphism of altered ocean crust (AOC) results in hydrous mineral breakdown and the release of volatiles. Halogen devolatilization is not well constrained, particularly for Br and I. To explore the halogens fluxes and behaviors during subduction of AOC, this study investigates bulk rock concentrations from seafloor AOC drill cores (n=21) and paleo-subduction settings that expose exhumed metamorphic rocks (n=44) that record the chemical evolution of AOC during prograde metamorphism. Chapter 2 juxtaposes eclogitic and AOC samples to gauge broadscale changes to the halogen budget between the trench and the depths of ~80 km. Chapter 3 focuses on a suite of petrogenetically-related meta-ophiolites from the Western Alps that record progressive metamorphism and shed light on halogen loss across metamorphic grade, especially at low P-T conditions. To study how halogens partition during phase breakdown and fluid transport, Chapter 4 examines a ~1 m transect of eclogitic samples that preserve evidence of high pressure vein formation and fluid-rock interactions. Findings from this work reveal that halogens are not evenly distributed in AOC. Basalts are F-rich and gabbros are Cl- and Br-rich. During subduction, F is decoupled from the heavy halogens (Cl, Br, I) displaying compatibility and immobility. Chlorine and Br are closely coupled; they preferentially partition into fluid phases and are efficiently removed from the mafic slab. Iodine behaviors are more cryptic, with many high-P samples preserving protolith-level abundances. Halogen mobility in devolatilizing AOC is as follows: Cl ≈ Br > I ≫ F. New flux estimates find that ≤40% of initial F and I and ~70% of initial Cl and Br is removed from subducting AOC prior to depths of arc magma genesis, with much of this loss occurring early in the forearc, before the transition to blueschist assemblages. Fluorine is broadly distributed among phases. The heavy halogen inventory is not controlled by a dominant phase (i.e., amphibole). Rather, these elements are distributed at low concentrations among many phases or are predominantly hosted in non-lattice sites.
The nature and causes of backstop deformation in the Northern Lesser Antilles subduction zone
(2000) Stachowiak, John Stephen; Bangs, Nathan Lawrence Bailey; Stoffa, Paul L., 1948-
Investigation of deformation mechanics in the northern Lesser Antilles subduction zone has, until recently, focused primarily on the pronounced variations in mechanical properties developing across the deformation front and the toe of the accretionary wedge. As a result, the major contrast in mechanical properties that exists between the island-arc crust, which forms the backstop to the accretionary wedge, and the accretionary wedge itself, has largely gone unstudied. In particular, the role of the backstop in forearc mechanics and the fate of sediments subducting beyond the toe of the accretionary prism have not been well discerned. A seismic experiment completed in 1998 was designed to address these two issues by imaging the Lesser Antilles subduction zone landward of the outerarc high. Analysis of the seismic reflection data reveals a seaward-dipping backstop with a morphology consisting of isolated domes and troughs (~15 km in diameter) underlying a forearc basin that spans approximately 60 km and contains up to 7 km of sediment. The localized uplift of the backstop has also caused deformation of the overlying forearc basin sediments. Structural and stratigraphic analysis of this deformation provided indirect insight into the deformation history of the backstop, showing that forearc basin sedimentation commenced prior to deformation and thus records the distribution and relative timing of the backstop deformation, uplift of the backstop has been a gradual process, uplift is currently ongoing and, deformation has recently shifted from near the center of the forearc basin to the basin's eastern margin. The three most likely backstop uplift mechanisms are the subduction of a basement ridge, interplate friction and sediment underplating. Based on the timing of backstop uplift (gradual, continuous) and the distribution of deformation (localized doming), the former mechanisms were precluded from being the primary cause of backstop uplift. Sediment underplating, however, can account for the observed deformation. Support for underplating as a primary backstop deformation mechanism is derived from: 1. A subducting sediment analysis, which showed that, on average, ~500 m of sediment reaches the backstop, providing sufficient material for underplating to occur and 2. A mass balance calculation suggesting that only about 5-10% of the subducting sediment would need to be underplated in order to produce the observed uplift. A model of deformation mechanics driven by sediment underplating beneath the backstop is proposed, incorporating all the foregoing observations and conclusions. The importance of this underplating model lies in its relevance to a margin type that is globally very common: convergent margins with a high sediment influx. The contribution of sediment underplating to backstop deformation probably surpasses that of subducting basement ridges, which are relatively rare and episodic. In addition, it may be as important or more important than interplate coupling, because of its pervasiveness. Sediment underplating plays a crucial role in the mechanics of a subduction zone that deserves further investigation at this and other convergent margins around the world
The tectonostratigraphy of the Cycladic Blueschist Unit and new garnet geo/thermochronology techniques
(2016-12) Seman, Spencer Mark; Stockli, Daniel F.; Barnes, Jaime; Cloos, Mark; Baxter, Ethan; Soukis, Konstantinos
Due to deformation and recrystallization at high-pressure low-temperature metamorphic conditions, blueschist and eclogite-facies rocks typically lack primary sedimentary features or fossils to constrain depositional age. Without this information, it is difficult to describe how these rocks evolved structurally through time and, therefore, to infer the processes at work during the exhumation. This work defines the tectonostratigraphy of the Cycladic Blueschist Unit (CBU) of Greece using detrital zircon geochronology, to correlate metasediments based on their provenance and estimate maximum depositional age. Two distinct types of metasediments are present within the CBU: those derived from the northern rifted margin of Gondwana and deposited during the Triassic-Early Jurassic, and flysch sourced from the Internal Hellenides deposited during the Late Jurassic-Late Cretaceous. In the Western Cyclades, younger flysch sediments are still found overlying older rift margin sediments. This is a primary depositional relationship at the scale of ~500 meters preserved through the subduction and exhumation process. On Syros Island, composed of a structural section of CBU ~12 km in thickness, I employ detrital zircon geochronology to define repetitions of tectonostratigraphy. Based on repeated Triassic volcanics, Syros is composed of thrust sheets ~3 km in thickness. Next, I use combination of detrital zircon geochronology and (U-Th)/He thermochronology to refine the tectonostratigrapy of southern Attica, specifically Hymittos Mountain and the Lavrion Peninsula. This work shows that two distinct low-angle normal fault systems are present in Attica, one active in the Middle Miocene and the other in the Late Miocene. Garnet is a common rock-forming mineral in both high-pressure low-temperature metamorphic rocks and magmatic-hydrothermal skarn. I have developed a garnet-based (U-Th)/He thermchronometric technique to constrain the timing of cooling of high-pressure low-temperature metamorphic rocks. This work shows that He diffusion in garnet is sensitive to temperatures between 200-300˚C at geologic timescales and characterizes a series of potential standards of grossular-andradite composition. Finally, in order to better understand the temporal evolution of magmatic-hydrothermal systems, I created a new U-Pb based technique to date the growth of grossular-andradite garnet. I characterize four different potential reference standards and apply to technique to hydrothermal skarn on Serifos Island, Greece.
Zircon U-Pb chronostratigraphy and provenance of the Cycladic Basement and CBU on Sikinos and Ios Islands, Greece
(2018-08) Poulaki, Eirini Maria; Stockli, Daniel F.
Sikinos and Ios Islands, located in the Southern Cyclades of the Aegean, are part of a Cenozoic metamorphic core complex system that exposes subduction-related rocks in the highly-extended back-arc region of the Hellenic subduction zone. The HP-LT metamorphic units exposed on the Islands are the Mesozoic metasedimentary rocks of the Cycladic Blueschist Unit (CBU) and Paleozoic ortho- and paragneisses of the Crystalline Basement Unit. While these units appear to share a common subduction metamorphic history, the magmatic and stratigraphic evolution of these two units and their relationship remain poorly understood. In particular, the nature of the contact between the CBU and Basement has been variably described as either an extensional shear zone or a subduction-related thrust that was potentially reactivated as an extensional top-to-the-north detachment. This study employed zircon U-Pb geochronometry on 59 samples to constrain (1) the crystallization ages of the Cycladic Basement on Sikinos Island, (2) the Maximum Deposition Ages (MDA) and the detrital provenance of the metasedimentary units. These new data allow to establish of a (chrono-) stratigraphic framework for the CBU and Cycladic Basement, elucidate the nature of the contact, and shed light on the subduction processes and pre-subduction paleogeography. The results reveal that (1) the Basement is composed of early Paleozoic metasedimentary rocks intruded by Carboniferous granites, (2) the Cycladic Basement is unconformably overlain by Permian and Mesozoic metasedimentary rocks of the CBU, (3) the CBU stratigraphy is clarified and its deposition spans from Permo-Triassic to Late Cretaceous times. This chronostratigraphic framework is also supported by the detrital zircon provenance U-Pb record that provides an additional stratigraphic fingerprint. These relationships in southern Sikinos demonstrate a nearly continuous stratigraphic record from the Cycladic Basement into the CBU and argue for a depositional contact and against both a subduction or extension-related structural juxtaposition of the CBU and the Cycladic Basement. On the other hand, the same stratigraphic constraints exhibit clear old-over-young relationships in NE Sikinos and Ios Islands, supporting structural repetition by thrust imbrication of the CBU slivers as a result of underplating during subduction in the Paleogene.