The cores of most Cycladic Islands are formed as consequences of early Paleogene high-pressure subduction processes of the African plate beneath Europe, and Miocene extensional exhumation of the subduction trench. The island of Thera (Santorini) resides in the Hellenic Volcanic Arc, and is dominated by Quaternary eruptive volcanic material atop a pre-volcanic basement. The position of the island has led to debate as to the nature of the pre-volcanic basement, with connections drawn to either the Cycladic Blueschist Unit (CBU) or the Phyllite-Quartzite Unit (PQU). Field observations, which document a top-to-SSE detachment, in conjunction with geochronological techniques have been applied to assess the tectonostratigraphy of the pre-volcanic basement. The results resolve the pre-volcanic basement as belonging to the CBU which has been juxtaposed against Sub-Pelogonian marble by the SSE directed detachment. This firmly establishes Thera within the Cycladic realm, documents Miocene deformation, and changes the geologic map of the Hellenides.

The 190-180 Ma Aishihik batholith is located in southcentral Yukon in the North American Cordillera. Low-temperature (U-Th)/He thermochronology is used to assess the thermal history of the batholith and the surrounding host rock to provide an understanding of the Jurassic-Cretaceous tectonism. The Yukon Tanana terrane (YTT) shows a steady cooling of ~5°C/m.y from the Early Jurassic to the Late Cretaceous, when it reaches the surface. Sections of YTT adjacent to the batholith exhibits moderate cooling rates of 7°C/m.y from the Early to Late Jurassic and then slows down to 1.5°C/m.y until reaching the surface in the Late Cretaceous. The batholith itself exhibits relatively fast cooling from the Early to Late Jurassic (15°C/m.y) and then slows down (1.5°C/m.y) until it reaches surface conditions in the Late Cretaceous. All rocks are at the surface at the latest by the Late Cretaceous with no evidence that there were any subsequent substantial thermal events, which indicates that the Yukon has a mature landscape compared to the rest of the North American Cordillera.

The Weepah Hills Area (Nevada) exposes exhumed metamorphic and plutonic rocks and upper-plate (supradetachment) volcano-sedimentary rocks that have experienced a complex, multi-stage deformational and depositional history. The Weepah Hills metamorphic core complex (WHMCC) is located in a region of the western Cordillera that was affected by both Miocene Basin-and-Range style E-W extension and Mio-Pliocene Walker Lane transcurrent shearing. Mio-Pliocene transcurrent deformation is transferred across a ~175 km releasing bend, known as the Mina Deflection, that kinematically links dextral strike-slip faults of the Death Valley-Fish Lake Valley with the central Walker Lane Belt. Progressive Mio-Pliocene transtension is characterized by core complex detachment faulting and younger high-angle normal faults. Timing of detachment faulting is constrained by both (U-Th)/He thermochronometry of footwall rocks and detailed chronostratigraphy of upper-plate strata to between 9-6 Ma. This age is supported by deformation recorded in the upper-plate strata that is attributed to progressive folding of the detachment associated with corrugation development. Earlier Miocene Basin-and-Range style extension is characterized by N-S trending high-angle normal faults and half-grabens that are strongly overprinted by Mio-Pliocene structures. (U-Th)/He zircon cooling ages from the detachment footwall range from ~12-20 Ma and are attributed to exhumation and unroofing related to E-W Basin-and-Range extension. New detailed sedimentological and geochronologic data show that, in contrast to previous research, the WHMCC upper-plate strata do not form a single supradetachment package, but rather three temporally distinct Miocene stratigraphic packages bounded by angular unconformities. The stratigraphic, structural, and exhumational record preserved in the WHMCC elucidates the timing of deformation and sedimentary basin evolution related to both Basin-and-Range E-W extension and Walker Lane related NW-directed transtension. / text

The architecture and morphology of a mountain range is fundamentally controlled by the combination of rock uplift and distribution of precipitation. This relationship attributes fluctuations in climate to the erosion of orogens, sedimentation rates, and geodynamics of the crust. Glaciers are the most effective climate driven erosive processes, where the frequency of glacial periods has a direct impact on the structure of mountain ranges through time. The late Cenozoic global cooling period was the beginning of a series of many glaciations which increased erosion in orogens experiencing fast rock uplift. We characterize the threshold for the onset of effective glacial erosion and record the increase in erosion rate during the late Cenozoic in the Chugach and Kenai Mountains of Alaska. We utilized low temperature thermochronometry and cosmogenic dating to constrain the spatial and temporal distribution of exhumation and glacial history in order to characterize the net effect of glaciers on an orogen that experiences slow rock uplift. We constrain the spatial distribution of exhumation and characterize the landscape along the Kenai Peninsula, underlain by the transition from flat slab to normal subduction. The region is characterized by old AHe ages which mimic the subduction angle of the down going plate and decrease away from an exhumational hotspot at a syntax in the Chugach Mountains. We attribute the long term exhumational characteristic of the Kenai Peninsula to subduction and underplating of sediment shed from the accreting Yakutat microplate to the east. A delineation of the glacial history using 10Be cosmogenic dating depict a series of glacial advances which date to the early and late Wisconsin. We find that the asynchronuity of glaciation across maritime and continental Alaska is controlled by steep orographic precipitation gradients which result from upper plate deformation. Finally, we observe an increase in erosion since the late Cenozoic using both AHe and cosmogenic dating and conclude that it is possible for the onset of effective glacial erosion in regions that experience slow to moderate rock uplift and that climate drives erosion rates in these regions. / Ph. D. / The formation of mountain ranges is controlled by how fast they grow and the precipitation they receive. Fluctuations in climate change influence the architecture and erosion of the crust, especially in regions where there are glaciers. The beginning of global glacial activity approximately five million years ago, increased erosion in mountain ranges that experience fast growing rates. We characterize the effect of glaciers on mountain growth and record the related erosion in the Chugach and Kenai Mountains of Alaska. We used dating techniques to measure the spatial and temporal distribution of erosion in mountain ranges that grow slowly. We measured erosion rate variations in the Kenai Mountains, and discovered that erosion rate mimics the change in tectonic morphology along the subduction zone. The glacial history of the Chugach Mountains was delineated and shows a series of glacial advances from 50-14 thousand years ago. We found that the characteristics of the glacial advances were dependent upon the distribution of precipitation along the mountain ranges, and that the latest glacial ice age was dryer than older events. Finally, we observed an increase in erosion in the mountain ranges and concluded that it is possible for glaciers to effectively erode slowly growing mountains.

The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

abstract: The collision of India and Eurasia constructed the Himalayan Mountains. Questions remain regarding how subsequent exhumation by climatic and tectonic processes shaped the landscape throughout the Late Cenozoic to create the complex architecture observed today. The Mount Everest region underwent tectonic denudation by extension and bestrides one of the world’s most significant rain shadows. Also, glacial and fluvial processes eroded the Everest massif over shorter timescales. In this work, I review new bedrock and detrital thermochronological and geochronological data and both one- and two-dimensional thermal-mechanical modeling that provides insights on the age range and rates of tectonic and erosional processes in this region. A strand of the South Tibetan detachment system (STDS), a series of prominent normal-sense structures that dip to the north and strike along the Himalayan spine, is exposed in the Rongbuk valley near Everest. Using thermochronometric techniques, thermal-kinematic modeling, and published (U-Th)/Pb geochronology, I show exhumation rates were high (~3-4 mm/a) from at least 20 to 13 Ma because of slip on the STDS. Subsequently, exhumation rates dropped drastically to ≤ 0.5 mm/a and remain low today. However, thermochronometric datasets and thermal-kinematic modeling results from Nepal south of Everest reveal a sharp transition in cooling ages and exhumation rates across a major knickpoint in the river profile, corresponding to the modern-day Himalayan rainfall transition. To the north of this transition, exhumation histories are similar to those in Tibet. Conversely, < 3 km south of the transition, exhumation rates were relatively low until the Pliocene, when they increased to ~4 mm/a before slowing at ~3 Ma. Such contrasting exhumation histories over a short distance suggest that bedrock exhumation rates correlate with modern precipitation patterns in deep time, however, there are competing interpretations regarding this correlation. My work also provides insights regarding how processes of glacial erosion act in a glacio-fluvial valley north of Everest. Integrated laser ablation U/Pb and (U-Th)/He dating of detrital zircon from fluvial and moraine sediments reveal sourcing from distinctive areas of the catchment. In general, the glacial advances eroded material from lower elevations, while the glacial outwash system carries material from higher elevations. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2017

The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

<p>  </p> <p>In this thesis, I investigate whether (U-Th)/He thermochronology on the bioapatite of fossilized gar scales can be used to reconstruct the thermal histories of sedimentary basins. I acquired 37 (U-Th)/He dates from fossil gar scale ganoine, two (U-Th)/He dates from fossil gar scale bone, and 18 (U-Th)/He dates from detrital apatite grains within sandstones from the upper Cretaceous and lower Paleogene rocks of two sedimentary basins with distinct thermal histories: the Williston basin and San Juan basin. I also obtained spatially resolved trace element concentrations by laser ablation-inductively coupled plasma mass spectrometry, as well as scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction data to assess the chemical and structural impacts of diagenesis on gar scale bioapatite. Timescales of diagenesis in bioapatite are relevant to (U-Th)/He thermochronology since the parent nuclides are taken up by bioapatite during fossilization but could also be taken up or lost much later, which could complicate (U-Th)/He data interpretation. Trace element concentrations and profiles vary spatially between ganoine and bone. Ganoine trace element profiles commonly show exponentially decreasing concentration profiles from the surface to 20 µm depth, and occasionally display more complex concentration depth profiles. Trace element profiles in bones are complex and spatially heterogeneous. (U-Th)/He data from ganoine and bone show an inverse relationship between (U-Th)/He age and parent nuclide concentrations, indicating open system behavior and late-stage uptake of parent nuclides. Helium loss via diffusion through pore spaces, fractures, and growth layers could also provide a reason for young fossil gar scale (U-Th)/He ages. Detrital apatite (U-Th)/He thermochronology from two Williston Basin samples suggest heating of upper Cretaceous and lower Paleogene rocks to ~100 ℃ at ~50 Ma. Detrital apatite (U-Th)/He thermochronology from the San Juan Basin sample yielded modeled maximum temperatures of ~150 ℃ at ~30 Ma. The discrepancies between fossil gar scale and detrital apatite (U-Th)/He ages suggest that protracted diagenesis of gar scale bioapatite prevents gar scales from being a suitable material for (U-Th)/He thermochronology.</p>