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  • 1
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    Continental uplift through crustal hydration (2015)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2015-03-24
    Description: Isostatic surface uplift of large continental regions lacking deformation remains largely unexplained. Evidence from the eastern parts of the Cordilleran orogen in the western United States suggests that increased buoyancy in the lower crust supports the elevations of the High Plains and Wyoming craton. We suggest that hydration of the lower crust associated with the Laramide orogeny produced surface uplift by replacing dense mineral phases such as garnet with less dense phases such as amphibole and mica. Seismic and petrologic evidence from Wyoming and Montana is consistent with such changes. Comparable hydration in the Colorado Plateau is dated to the early Tertiary. Beyond establishing a newly recognized mechanism for broad continental uplift, such hydration suggests that interactions of subduction-derived fluids and the lithosphere can be more profound than previously envisioned.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 2
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    An exhumation history of continents over billion-year time scales (2012)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2012-01-10
    Description: The continental lithosphere contains the oldest and most stable structures on Earth, where fragments of ancient material have eluded destruction by tectonic and surface processes operating over billions of years. Although present-day erosion of these remnants is slow, a record of how they have uplifted, eroded, and cooled over Earth's history can provide insight into the physical properties of the continents and the forces operating to exhume them over geologic time. We constructed a continuous record of ancient lithosphere cooling with the use of uranium-lead (U-Pb) thermochronology on volcanically exhumed lower crustal fragments. Combining these measurements with thermal and Pb-diffusion models constrains the range of possible erosion histories. Measured U-Pb data are consistent with extremely low erosion rates persisting over time scales approaching the age of the continents themselves.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Blackburn, Terrence J -- Bowring, Samuel A -- Perron, J Taylor -- Mahan, Kevin H -- Dudas, Francis O -- Barnhart, Katherine R -- New York, N.Y. -- Science. 2012 Jan 6;335(6064):73-6. doi: 10.1126/science.1213496.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. terrence@mit.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22223803" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 3
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    Seismic structure and lithospheric rheology from deep crustal xenoliths, central Montana, USA (2012)
    Wiley
    Details
    Publication Date: 2012-10-18
    Description: Improved resolution of lower crustal structure, composition, and physical properties enhances our understanding and ability to model tectonic processes. The cratonic core of Montana and Wyoming, USA, contains some of the most enigmatic lower crust known in North America, with a high seismic velocity layer contributing to as much as half of the crustal column. Petrological and physical property data for xenoliths in Eocene volcanic rocks from central Montana provide new insight into the nature of the lower crust in this region. Inherent heterogeneity in xenoliths derived from depths below ∼30 km support a composite origin for the deep layer. Possible intralayer velocity steps may complicate the seismic definition of the crust/mantle boundary and interpretations of crustal thickness, particularly when metasomatized upper mantle is considered. Mafic mineral-dominant crustal xenoliths and published descriptions of mica-bearing peridotite and pyroxenite xenoliths suggest a strong lower crust overlying a potentially weaker upper mantle.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Transpressive uplift and exhumation of continental lower crust revealed by synkinematic monazite reactions (2013)
    Geological Society of America (GSA)
    Details
    Publication Date: 2013-09-18
    Description: Exposures of continental lower crust provide fundamental constraints on the thermal-mechanical behavior of continental lithosphere during orogeny. The applicability of field-based results, however, requires knowledge of whether these data pertain to deformation during lower-crustal residence or during uplift and exhumation of deep crust. Dating synkinematic monazite-producing reactions provides one way to evaluate deformation styles in the deep crust. We report on the implications of monazite reaction dating for the timing of fabric formation and movement along three crustal-scale shear zones in northern Saskatchewan, western Canadian Shield. The structures accommodated dextral transpressive strain during oblique- and thrust-sense displacement that was coeval with uplift and exhumation of 〉20,000 km 2 of continental lower crust (〉1.0 GPa) to middle-crustal levels (〈0.5 GPa). In situ Th-U–total Pb monazite data reveal that monazite rims in all three shear zones grew synkinematically at 1849 ± 6 Ma (2, mean square of weighted deviates = 0.8). The style of deformation involved localized strain concurrent with segmentation and translation of rheologically strong blocks of deep crust along mutually interacting shear zones during transpression.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 5
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    The Cora Lake shear zone, Athabasca granulite terrane, an intraplate response to far-field orogenic processes during the amalgamation of Laurentia (2014)
    National Research Council Canada (NRC)
    Details
    Publication Date: 2014-10-08
    Description: The Cora Lake shear zone (CLsz) is a 4–6 km wide localized high-strain zone that bisects the polydeformed Athabasca granulite terrane, northern Saskatchewan. It also coincides with the geophysical trace of the Snowbird tectonic zone. The CLsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of 〉20 000 km 2 of high-pressure granulites. Most rocks have a strong mineral lineation plunging moderately to the southwest. The Northwestern subdomain (hangingwall) is characterized by ca. 2.6 Ga plutonic rocks that contain an early, subhorizontal gneissic layering (ca. 2.57 Ga) that was overprinted by large amplitude folds and a partitioned, but pervasive, axial planar, dextral, shear fabric at ca. 1.9 Ga. Thermobarometry suggests metamorphic conditions of ~0.9 GPa and ~750 °C during both of the phases of tectonism. The footwall is predominantly underlain by the ca. 3.3–3.0 Ga Chipman tonalite, layers of intercalated mafic and felsic granulite, and the widespread 1.9 Ga Chipman mafic dyke swarm. Early subhorizontal layering in the footwall was also folded at ca. 1.9 Ga and transposed into a steeply dipping, northeast-striking axial planar shear fabric that corresponds with the metamorphic peak (1.1–1.2 GPa and 800–900 °C). These distinct domains were juxtaposed across the CLsz, which contains a gneissic foliation striking 231° and dipping moderately to steeply to the northwest. Abundant sinistral–normal kinematic indicators are consistent with the distinctly lower pressures to the northwest. The shear zone is characterized by very fine grain sizes, despite its high-temperature assemblages including clinopyroxene and garnet. Thermobarometry from the CLsz displays progressive decompression of reworked footwall rocks with increasing mylonitization. In situ monazite geochronology indicates shearing at 1.89–1.87 Ga shortly after the granulite facies metamorphic peak. The anomalous sinistral kinematics of the CLsz, bracketed in time between periods of dextral shearing, can be explained by changing regional stresses during alternating convergent tectonics to the west and to the southeast of the Athabasca granulite terrane.
    Print ISSN: 0008-4077
    Electronic ISSN: 1480-3313
    Topics: Geosciences
    Published by National Research Council Canada (NRC)
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  • 6
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    Strain localization in the Spanish Creek mylonite, Northern Madison Range, southwest Montana, U.S.A. (2014)
    University of Wyoming
    Details
    Publication Date: 2014-11-14
    Description: The Spanish Creek mylonite zone formed within multiply deformed granitic orthogneisses in the Northern Madison Range in southwestern Montana. An integrated approach incorporating fieldwork, microstructural analysis, electron backscatter diffraction (EBSD), and U-Pb zircon geochronology was utilized to characterize this mylonite zone and place its evolution into a broader regional context. The mylonitic rocks are characterized by a well-developed L-S fabric and a variably porphyroclastic character. U-Pb zircon geochronology yielded a magmatic crystallization age of 2824 ± 47 Ma (2) for the granitic protolith of the porphyroclastic Spanish Creek mylonite, indicating that the mylonitic deformation could have occurred during either a late Archean tectonic event or the late Paleoproterozoic Big Sky Orogeny, potentially activated and then reactivated with each successive event. Three distinct structural realms defined within the study area on the basis of structural observations and EBSD data reveal differences in the degree of mylonitic deformation. Two models are herein proposed to explain these significant differences, which are manifest in deformation microstructures and CPO fabrics between structural realms. In the first model, the intrusion and mylonitization of the granitic protolith for the rocks of the third structural realm were broadly coeval during a late Archean tectonic event, allowing for high-temperature mylonitic deformation. In an alternate model, the mylonitic deformation significantly postdated the ~2.8 Ga intrusion of the granitic protolith, with factors aside from temperature influencing the nature of the deformation. Strain localization provides a viable explanation for the observed differences in the intensity of deformation between structural realms.
    Print ISSN: 1555-7332
    Electronic ISSN: 1555-7340
    Topics: Geosciences
    Published by University of Wyoming
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  • 7
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    Electron Microprobe Petrochronology (2017)
    Mineralogical Society of America
    Details
    Publication Date: 2017-05-19
    Print ISSN: 1529-6466
    Electronic ISSN: 1943-2666
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 8
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    Deep crustal xenoliths from central Montana, USA: Implications for the timing and mechanisms of high-velocity lower crust formation (2012)
    Geological Society of America (GSA)
    In: Geosphere
    Details
    Publication Date: 2012-12-04
    Description: Integration of petrologic, chronologic and petrophysical xenolith data with geophysical observations can offer fundamental insights into understanding the evolution of continental crust. We present the results of a deep crustal xenolith study from the northern Rocky Mountain region of the western U.S., where seismic experiments reveal an anomalously thick (10–30 km), high seismic velocity (compressional body wave, Vp 〉 7.0 km/s) lower crustal layer, herein referred to as the 7.x layer. Xenoliths exhumed by Eocene minettes from the Bearpaw Mountains of central Montana, within the Great Falls tectonic zone, include mafic and intermediate garnet granulites, mafic hornblende eclogite, and felsic granulites. Calculated pressures of 0.6–1.5 GPa are consistent with derivation from 23–54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories including prograde burial and episodes of decompression. Samples with barometrically determined depths consistent with residence within the seismically defined 7.x layer have calculated bulk P-wave velocities of 6.9–7.8 km/s, indicating heterogeneity in the layer. Shallower samples have markedly slower velocities consistent with seismic models. New monazite total U-Th-Pb data and a variety of additional published geochronology indicate a prolonged and episodic metamorphic history, beginning with protolith ages as old as Archean and followed by metamorphic and deep crustal fluid-flow events ca. 2.1 Ga, 1.8–1.7 Ga, and 1.5–1.3 Ga. We suggest that the 7.x layer in this region owes its character to a variety of processes, including magmatic underplating and intraplating, associated with multiple tectonic events from the Neoarchean to the Mesoproterozoic.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 9
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    Formation of pluton roofs, floors, and walls by crack opening at Split Mountain, Sierra Nevada, California (2012)
    Geological Society of America (GSA)
    In: Geosphere
    Details
    Publication Date: 2012-10-01
    Description: Spectacularly exposed contact relations of the Split Mountain interpluton screen, Sierra Nevada batholith, indicate that its bounding plutons grew by injection of dikes. The screen is composed of Cambrian metasedimentary rocks and ranges from a few tens of meters to ~500 m thick. On Split Mountain, the screen is bounded by gently dipping intrusive contacts with two Jurassic plutons, the overlying Tinemaha Granodiorite and underlying leucogranite of Red Mountain Creek. Field relations indicate that both plutons grew mainly by opening of subhorizontal crack systems and that stoping was at most a minor process. The intrusive contacts are sharp and cut across wall-rock structures that reflect pre–165 Ma tectonic shortening. Wall-rock xenoliths are extremely rare in both plutons. Contrary to previous descriptions, neither pluton has an exposed steep wall, but later tectonic deformation locally produced steep contacts. Westward, the Split Mountain screen enters the subvertical Sawmill Lake mylonitic shear zone, which sheared it downward to a subvertical dip. On the north side of the Red Mountain Creek pluton, the primary intrusive contact dips gently, but a zone of high-angle faults steps the contact incrementally downward toward the north and east from near the range crest to the eastern range front. These faults were not previously recognized and the resulting map pattern has been misconstrued to reflect northward steepening of the contact. The Cretaceous Lamarck Granodiorite intrudes the western contact of the vertical section of the screen along a locally concordant contact. However, elsewhere the intrusive contact dips more gently and sharply cuts across the screen and the shear zone. Concordant segments of the intrusive contact reflect opening of fractures that were guided by the shear zone fabric. The Split Mountain interpluton screen thus owes its shape and its location between several plutons to the opening of fractures that admitted the bounding intrusions. The screen originally formed in the Jurassic as a subhorizontal body between the vertically stacked, broadly laccolithic Tinemaha and Red Mountain Creek plutons. After part of the screen was tectonically sheared into a subvertical orientation, the Lamarck Granodiorite invaded steep fractures concordant with the shear zone.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 10
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    Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA (2015)
    Geological Society of America (GSA)
    Details
    Publication Date: 2015-11-18
    Description: The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8–1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky–related deformation are preserved, both nearly coeval with peak metamorphic conditions of ~0.9–0.8 GPa and 〉700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750–1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (~45–40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (~30–25 km) in the central-southeast area. The age of high-grade tectonism appears to become younger southeastward away from the core of the orogen, from ca. 1810–1780 Ma in the Highland Mountains, to ca. 1780–1750 Ma in the Ruby Range, Tobacco Root Mountains, and northwesternmost Northern Madison Range, and 1750–1720 Ma in the central Northern Madison Range. These spatial and temporal patterns of lateral growth and propagation of the orogen are similar to those observed in other collisional orogenic systems, and they may reflect multiple collision phases, protracted collision, and/or postcollisional collapse.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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