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1
Ririe, G. Todd. "A comparison of alteration assemblages associated with Archean gold deposits in Western Australia and Paleozoic gold deposits in the southeast United States." Canadian Journal of Earth Sciences 27, no. 12 (December 1, 1990): 1560–76. http://dx.doi.org/10.1139/e90-168.
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A comparison of high-alumina mineral assemblages spatially associated with the Haile and Brewer gold deposits in the Paleozoic Carolina slate belt (CSB) of the southeast United States and the Sons of Gwalia and Mount Celia gold deposits in the Archean Norseman–Wiluna greenstone belt (NWGB) in Western Australia suggests a similar hydrothermal origin and subsequent metamorphic and deformational history. A common hydrothermal origin is supported by the striking similarity in whole-rock chemistry, even though there were probably significant variations in original unaltered protoliths. An analysis of rocks from each of the deposits that contain high-alumina minerals demonstrates that the protoliths were leached of alkalis with respect to aluminum and have a ratio of Al2O3/Na2O + CaO + K2O greater than three. Although the rocks contain abundant high-alumina minerals, the aluminum content in these rocks is not unusually high, and it does not appear that there has been any significant transport of aluminum either into or out of the rocks that were altered.The most common high-alumina minerals found in rocks affected by the feldspar destructive alteration event include andalusite, kyanite, pyrophyllite, kaolinite, and sericite. Other minerals present in variable amounts include diaspore, chloritoid, alunite, natroalunite, paragonite, and topaz. It is possible that some of these minerals formed during the early hydrothermal alteration event and were recrystallized during metamorphism. Regardless of when the high-alumina minerals formed, textural relations suggest that the rocks had been hydrothermally altered by the time the metamorphic minerals formed. A comparison of alteration from the deposits studied in the CSB and the NWGB suggests there are many similarities to acid-sulfate alteration associated with geothermal areas, such as Yellowstone Park, Wyoming, and with acid-sulfate gold deposits, such as Goldfield, Nevada. Thus, it is possible that the protolith of the metamorphosed rocks in the CSB and NWGB contained an alteration assemblage that included alunite, pyrophyllite, and kaolinite.A generalized paragenetic sequence determined from petrographic and field observations, beginning with regional metamorphism, follows: (i) formation of andalusite, kyanite, chloritoid, and topaz during prograde metamorphism, depending on whole rock chemistry, (ii) formation of pyrophyllite and quartz-rich pods during silicification of aluminosilicate-bearing rocks, (iii) bedding parallel schistosity and fracturing produced by a deformational event, (iv) fractures filled by quartz, sericite, pyrophyllite, or calcite, (v) folding of early layering in the rocks to form a crenulation cleavage accompanied by introduction of quartz veins at high angles to foliation; and (vi) retrograde metamorphism of andalusite–kyanite-pyrophyllite to produce an assemblage of kaolinite ± diaspore.The spatial association of the acid-sulfate alteration with gold mineralization, together with comparison of analogous alteration associated with younger unmetamorphosed acid-sulfate gold deposits, suggests that at least some of the gold was introduced during the early premetamorphic alteration event. The present location of gold in each deposit is a result of local changes brought about by later metamorphic and deformational events.
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Blackford, Nolan R., Sean P. Long, Austin Stout, David W. Rodgers, C. M. Cooper, Kimberly Kramer, Russell V. Di Fiori, and Emmanuel Soignard. "Late Cretaceous upper-crustal thermal structure of the Sevier hinterland: Implications for the geodynamics of the Nevadaplano." Geosphere 18, no. 1 (November 22, 2021): 183–210. http://dx.doi.org/10.1130/ges02386.1.
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Abstract Crustal temperature conditions can strongly influence the evolution of deformation during orogenesis. The Sevier hinterland plateau in Nevada and western Utah (“Nevadaplano”) experienced a Late Cretaceous episode of shallow-crustal metamorphism and granitic magmatism. Here, we investigate the thermal history of the Nevadaplano by measuring peak thermal field gradients attained in the upper 10–20 km of the crust along an east-west transect through nine ranges in eastern Nevada and western Utah, by integrating Raman spectroscopy of carbonaceous material thermometry and published conodont alteration indices with reconstructed cross sections. Thermal field gradients of 29 ± 3 °C/km were obtained in the House and Confusion Ranges in westernmost Utah. The Deep Creek, Schell Creek, and Egan Ranges in easternmost Nevada yielded elevated gradients of 49 ± 7 °C/km, 36 ± 3 °C/km, and 32 ± 6 °C/km, respectively. Moving westward, the White Pine, Butte, Pancake, and Fish Creek Ranges exhibit gradients typically between ~20–30 °C/km. The elevated thermal gradients in easternmost Nevada are interpreted to have been attained during ca. 70–90 Ma granitic magmatism and metamorphism and imply possible partial melting at ~18 km depths. Our data are compatible with published interpretations of Late Cretaceous lithospheric mantle delamination under the Sevier hinterland, which triggered lower-crustal anatexis and the resulting rise of granitic melts. The lack of evidence for structures that could have accommodated deep burial of rocks in the nearby Northern Snake Range metamorphic core complex, combined with thermal gradients from adjacent ranges that are ~1.5–3 times higher than those implied by thermobarometry in the Northern Snake Range, further highlights the debate over possible tectonic overpressure in Cordilleran core complexes. Cross-section retro-deformation defines 73.4 ± 4.6 km (76 ± 8%) of extension across eastern Nevada and 15 km of shortening in the Eastern Nevada fold belt.
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Long, Sean P., and Matthew J. Kohn. "Distributed ductile thinning during thrust emplacement: A commonly overlooked exhumation mechanism." Geology 48, no. 4 (January 31, 2020): 368–73. http://dx.doi.org/10.1130/g47022.1.
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Abstract Quantifying the processes that control exhumation is essential for understanding the evolution of mountain belts. In the Cordilleran orogen in Nevada (western United States), rocks exhumed in the Ruby–East Humboldt metamorphic core complex underwent 4 ± 2 kbar of decompression between 85 and 60 Ma, which has been interpreted as a consequence of synorogenic extension. However, evidence for significant normal faulting in this region prior to 45 Ma is lacking. Here, we present an alternative interpretation: that this decompression can be attributed to distributed ductile thinning (DDT) of mid-crustal metamorphic rocks above the basal Cordilleran décollement during eastward translation. Such a process has been documented within the Himalayan Main Central thrust sheet, which locally accommodated up to 15 km of DDT during Miocene translation. Other examples of DDT have been documented in the Alpine and Caledonian orogens (Europe), and the Sanbagawa belt (Japan). DDT may represent a widespread exhumation process that can account for a significant portion of the decompression path of deeply exhumed rocks. As a condition of strain compatibility, thrust-parallel stretching accompanying DDT is expected to enhance displacement magnitude in the transport direction, and is therefore an important component of the deformation field that must be considered for accurate assessment of mass balance in thrust systems.
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Friedman, G. M., and D. J. Schultz. "Precipitation of vaterite (CaCO3) during oil field drilling." Mineralogical Magazine 58, no. 392 (September 1994): 401–8. http://dx.doi.org/10.1180/minmag.1994.058.392.05.
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AbstractVaterite, a CaCO3 polymorph, is a rare mineral that is said to be metastable under all known conditions. According to the literature, vaterite precipitated from carbonate solution recrystallizes spontaneously to calcite or aragonite. Yet vaterite has been identified in hard tissues of organisms, in gallstones, in contact metamorphic aureoles, in zones of thermal metamorphism, in a meteorite, and in cone-in-cone concretions. Newly precipitated vaterite has formed at the expense of carbonate rock in drilling fluids in wells of New York, Michigan, Nevada, Texas, and New Zealand. Petrographic examination reveals a light brown core of Ca3SiO5 surrounded by a colourless rim of vaterite. The δ18OPDB of New York vaterite is −12.4‰ and that of the Michigan vaterite is −17.6‰, which reflect the oxygen isotopic composition of meteoric freshwater used in drilling. The δ13CPDB value of −19.2‰ for New York vaterite and that of −17.6‰ for Michigan vaterite suggest that natural gas dissolved original carbonate in the subsurface. Drilling records from both wells indicate that natural gas was released into the drilling muds from the formations exposed at the time vaterite was encountered. Crossplots of the oxygen and carbon isotopic ratios overlap those of spurrite rocks in thermal metamorphic zones in Israel. A C-14 radiocarbon analysis of the Michigan vaterite gave an age of 953±39 yr. BP. 88.8±0.6% is modern carbon and 11.2% is dead carbon. Hence this carbon, and therefore the vaterite, is essentially modern. A sample of the New York vaterite yielded a modern age.
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Cooper, Frances J., John P. Platt, and Whitney M. Behr. "Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes." Solid Earth 8, no. 1 (February 21, 2017): 199–215. http://dx.doi.org/10.5194/se-8-199-2017.
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Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.
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Affinati, Suzanne Craddock, Thomas D. Hoisch, Michael L. Wells, and Jeffrey D. Vervoort. "Pressure-temperature-time paths from the Funeral Mountains, California, reveal Jurassic retroarc underthrusting during early Sevier orogenesis." GSA Bulletin 132, no. 5-6 (September 17, 2019): 1047–65. http://dx.doi.org/10.1130/b35095.1.
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Abstract New metamorphic pressure-temperature (P-T) paths and Lu-Hf garnet ages reveal a temporal correlation between Middle to Late Jurassic retroarc underthrusting and arc magmatism in southwestern North America. P-T paths were determined for 12 garnet porphyroblasts from six samples from the Chloride Cliff area of the Funeral Mountains in southeastern California. The composite path shows a pressure increase from 4.2 to 6.5 kbar as temperature increased from 550 to 575 °C, followed by a pressure decrease to 5.1 kbar during a further increase in temperature to 590 °C. Lu-Hf garnet ages from a pelitic schist (167.3 ± 0.7 Ma) and a garnet amphibolite (165.1 ± 9.2 Ma) place these P-T paths in the Middle Jurassic. We interpret the near-isothermal pressure increase portion of the P-T path to have developed during thrust-related burial, similar to lower grade rocks at Indian Pass, 8 km to the southeast, where garnet P-T paths show a pressure increase dated by the Lu-Hf method at 158.2 ± 2.6 Ma. We interpret the pressure decrease portion of the composite P-T path from the Chloride Cliff area to reflect exhumation contemporaneous with cooling in the Indian Pass area documented from muscovite 40Ar/39Ar step-heating ages of 152.6 ± 1.4 and 146 ± 1.1 Ma. The conditions and timing of metamorphism determined for the Indian Pass and Chloride Cliff areas, and isogradic surfaces that cut across stratigraphy, support the interpretation that the strata were dipping moderately NW during metamorphism, parallel to the thrust ramp that buried the rocks. Burial likely resulted from top-SE motion along the Funeral thrust, which was later reactivated as a low-angle normal fault with opposite motion to become the currently exposed Boundary Canyon detachment that was responsible for Miocene and possibly older exhumation. The part of the burial history captured by garnet growth occurred ∼6 m.y. before the 161 Ma peak of high-flux magmatism in the arc. Burial was contemporaneous with metamorphic ages from the western Sierra Nevada metamorphic belt, with the possible timing of accretion of arc terranes in northern California, and with the initiation of Franciscan subduction. Burial ages are also similar in timing with generally E-W crustal shortening in the retroarc that produced the East Sierra thrust system, the Luning-Fencemaker fold and thrust belt, the possible early history of the Central Nevada thrust belt, and the western thrusts of the southern Sevier belt. The timing of tectonic burial documented in this study and of high-flux magmatism in the arc supports the interpretation that the development of a coherent arc-trench system in the Early Jurassic resulted in the underthrusting of melt-fertile material beneath the arc along west- to northwest-dipping faults such as the Funeral thrust in the Jurassic, which penetrated the basement to the west as well as the roots of the magmatic arc, leading to increased magmatism.
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MILLER, DAVID M., WENDY C. HILLHOUSE, ROBERT E. ZARTMAN, and MARVIN A. LANPHERE. "Geochronology of intrusive and metamorphic rocks in the Pilot Range, Utah and Nevada, and comparison with regional patterns." Geological Society of America Bulletin 99, no. 6 (1987): 866. http://dx.doi.org/10.1130/0016-7606(1987)99<866:goiamr>2.0.co;2.
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Zuza, Andrew V., Christopher D. Henry, Seth Dee, Charles H. Thorman, and Matthew T. Heizler. "Jurassic–Cenozoic tectonics of the Pequop Mountains, NE Nevada, in the North American Cordillera hinterland." Geosphere 17, no. 6 (October 27, 2021): 2078–122. http://dx.doi.org/10.1130/ges02307.1.
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Abstract The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel 40Ar/39Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ∼1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ∼100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration.
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Gabet, Emmanuel J. "Lithological and structural controls on river profiles and networks in the northern Sierra Nevada (California, USA)." GSA Bulletin 132, no. 3-4 (July 15, 2019): 655–67. http://dx.doi.org/10.1130/b35128.1.
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Abstract In this study, the strong lithological heterogeneity of the northern Sierra Nevada (California, USA) is exploited to elucidate the role of lithology on river profiles and patterns at the mountain-range scale. The analyses indicate that plutonic, metavolcanic, and quartzite bedrock generally host the steepest river reaches, whereas gentle reaches flow across non-quartzite metasedimentary rocks and fault zones. In addition, the largest immobile boulders are often in the steepest reaches, suggesting that wide joint spacing plays a role in creating steep channels, and a positive relationship between boulder size and hillslope angle highlights the coupling of the hillslope and fluvial systems. With respect to river network configurations, dendritic patterns dominate in the plutonic bedrock, with channels aligned down the slope of the range; in contrast, river reaches in the metamorphic belts are mainly longitudinal and parallel to the structural grain. River profiles and patterns in the northern Sierra Nevada, therefore, bear a strong lithological imprint related to differential erosion. These observations indicate that attempts to infer uplift and tilting of the range based on the gradients and orientations of paleochannel remnants should first account for the effect of bedrock erodibility. Indeed, the differences in gradients of Tertiary paleochannel remnants used to argue for late Cenozoic uplift of the range can be wholly explained by differences in lithology.
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Percival, John A. "Orthopyroxene–poikilitic tonalites of the Desliens igneous suite, Ashuanipi granulite complex, Labrador–Quebec, Canada." Canadian Journal of Earth Sciences 28, no. 5 (May 1, 1991): 743–53. http://dx.doi.org/10.1139/e91-064.
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The Ashuanipi complex of the Archean Superior Province consists of an older sequence of sedimentary rocks cut by sills and goblet-shaped intrusions of the Desliens igneous suite (DIS). The complex was regionally metamorphosed to granulite facies and subsequently intruded by peraluminous granodiorite batholiths. Tonalites of the DIS contain characteristic < 2 cm inclusion-filled orthopyroxene crystals of probable igneous origin, variably recrystallized and overgrown by garnet, biotite and blocky orthopyroxene that suggest metamorphic P–T conditions of 0.5–0.62 GPa and 750–850 °C. Andesine in tonalite contains abundant drop-like quartz inclusions. Inclusion-filled pyroxene and plagioclase are both inferred to have nucleated on partly resorbed quartz and plagioclase grains within a melt. Similar poikilitic textures characterize subordinate leucotonalite, diorite, gabbro, and pyroxenite, which together with tonalite make up the DIS. Some of the petrological characteristics of the suite are unusual, but known from other regions: (i) some felsic magmatic rocks in the southern Sierra Nevada and Barrington Tops (Australia) batholiths contain primary orthopyroxene; (ii) goblet-shaped plutons are common in orogenic belts, and tonalites with this geometry occur within a sedimentary succession in the Trinity Alps of California; (iii) orthopyroxene oikocrysts characterize parts of the Bjerkreim–Sokndal lopolith of southwestern Norway.
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Canada, Andrew S., Elizabeth J. Cassel, Daniel F. Stockli, M. Elliot Smith, Brian R. Jicha, and Brad S. Singer. "Accelerating exhumation in the Eocene North American Cordilleran hinterland: Implications from detrital zircon (U-Th)/(He-Pb) double dating." GSA Bulletin 132, no. 1-2 (May 16, 2019): 198–214. http://dx.doi.org/10.1130/b35160.1.
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AbstractBasins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve records of deformation that are unavailable in footwall rocks. Following prolonged late Mesozoic–early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth’s best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermochronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105–740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (<4–6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to <100 m.y. in 45–43 Ma strata and to <10 m.y. in 43–41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1–20 m.y. lag times. These data reflect acceleration of basement exhumation rates by >1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere.
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Wannamaker, Philip E., Jeffery M. Johnston, John A. Stodt, and John R. Booker. "Anatomy of the southern Cordilleran hingeline, Utah and Nevada, from deep electrical resistivity profiling." GEOPHYSICS 62, no. 4 (July 1997): 1069–86. http://dx.doi.org/10.1190/1.1444208.
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To address outstanding questions in Mesozoic‐Cenozoic structure and present‐day deep physicochemical state in the region of the southern Cordilleran hingeline, a detailed, east‐west profile of magnetotelluric (MT) soundings 155 km in length was acquired. From these soundings, a resistivity interpretation was produced using an inversion algorithm based on a structural parameterization. In the upper ten kilometers of the transect, the interpretation shows two segments of low resistivity lying beneath allochthonous rocks of the Late Mesozoic, Sevier thrust sheet. Subsequent industry drilling motivated in part by our surveying confirms the existence and position of the eastern subthrust conductor and, more spectacularly, identifies the presence of yet deeper, autochthonous Mesozoic rocks. The conductors cannot be specified uniquely with present public data, because their electrical characteristics appear consistent with Paleozoic, pyrolized graphitic strata of either Late Devonian‐Mississippian or Middle Ordovician age. However, the drilling results show that Late Paleozoic and younger rocks lie underthrust much farther west than recognized previously, and perhaps as far west as the Utah‐Nevada border. A simple structural interpretation is offered where one underthrust segment of low‐resistivity sediments was created originally, but this segment was broken later into two major ones during higher‐angle Tertiary extension. For the middle and lower crust, the MT data imply a nearly 1-D resistivity structure of remarkable uniformity across the entire transect. In particular, there occurs a deep low‐resistivity layer most pronounced (about 8 ohm-m) in the nominal depth interval of 17.5 to 40 km. The MT data indicate that the layer cannot be confined to a single thin layer in the lower crust but instead represents vertically distributed low resistivity. With temperatures estimated from surface heat flow to range from 550°C to 1050°C with depth in the layer, and with a metaigneous mineralogy of high metamorphic grade assumed, mechanisms to produce the low resistivity can be constrained. The deep layer is thus consistent with [Formula: see text] brines at its upper levels, fluids of lower [Formula: see text] activity toward middle levels, and [Formula: see text] melting below about 30 km. The marked uniformity of the deep conductive layer across the transect suggests a similar uniformity of deep physicochemical state. However, this is not at odds with recent analyses of heat flow, Curie depth, Quaternary extension, and basaltic volcanism. Pre‐existing structural fabrics have had no measureable influence on localizing regions of high temperature, fluids and melting in the lower crust, at least averaged over the scale of tens of kilometers. Given its uniformity over a distance of 155 km or more, the depth to the regional deep conductor does not appear related to the distribution of high‐temperature geothermal resources.
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Skalbeck, John D., Robert E. Karlin, Lisa Shevenell, and Michael C. Widmer. "Gravity and aeromagnetic modeling of alluvial basins in the southern Truckee Meadows adjacent to the Steamboat Hills geothermal area, Washoe County, Nevada." GEOPHYSICS 70, no. 3 (May 2005): B1—B9. http://dx.doi.org/10.1190/1.1925739.
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The concurrent development of the Steamboat Hills geothermal area for power production and the adjacent alluvial aquifers for drinking water in Washoe County, Nevada, necessitates a good understanding of the hydrogeologic connection between these water resources. The problem is that adequate characterization of the subsurface geologic structure is not possible with existing geologic data. This need prompted us to construct a detailed 3D representation of the subsurface geologic structure based on 2.75D forward modeling of 11 gravity and aeromagnetic profiles constrained by geologic data and physical (density, magnetic susceptibility, remanent magnetic) properties. Potential-fields modeling results provided greater definition of the alluvial basins, and when combined with well-log data, yield an overall basin volume surrounding Steamboat Hills that is 64% greater than the volume derived from well-log data alone. A representation of the geothermal reservoir, consisting of altered granodiorite and metamorphic rocks, illustrates that the flow of thermal water is fault controlled. The model also suggests that thermal water may upflow along an unexplored fault flanking western Steamboat Hills. North-trending faults that conduct thermal water from the geothermal system to the alluvial aquifer appear to be zones of altered volcanics that produce subtle aeromagnetic anomalies.
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Canada, Andrew S., Elizabeth J. Cassel, Allen J. McGrew, M. Elliot Smith, Daniel F. Stockli, Kenneth A. Foland, Brian R. Jicha, and Brad S. Singer. "Eocene exhumation and extensional basin formation in the Copper Mountains, Nevada, USA." Geosphere 15, no. 5 (July 16, 2019): 1577–97. http://dx.doi.org/10.1130/ges02101.1.
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Abstract Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.
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Petronis, Michael S., John W. Geissman, John S. Oldow, and William C. McIntosh. "Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the structural evolution of the Silver Peak extensional complex, west-central Nevada." GSA Bulletin 114, no. 9 (September 1, 2002): 1108–30. http://dx.doi.org/10.1130/0016-7606(2002)114<1108:paaagd>2.0.co;2.
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Abstract The Silver Peak extensional complex, located in the Silver Peak Range of west- central Nevada, is a displacement-transfer system linking the Furnace Creek–Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene–Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal- axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual- polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329°, inclination I = 37°, α95 = 4.3°, number N = 30 sites; in situ) (α95 = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation (D = 021°, I = 57°, α95 = 4.3°, N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation (D = 032°, I = 53°, α95 = 8.8°, N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight 40Ar/39Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ∼250 °C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The 40Ar/39Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 °C to 100 °C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system.
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Santamaría-López, Ángel, Isabel Abad, Fernando Nieto, and Carlos Sanz de Galdeano. "Early Mylonitization in the Nevado-Filábride Complex (Betic Cordillera) during the High-Pressure Episode: Petrological, Geochemical and Thermobarometric Data." Minerals 13, no. 1 (December 23, 2022): 24. http://dx.doi.org/10.3390/min13010024.
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In the western part of the Sierra de los Filabres area, there are fine-grained metamorphic rock bands, showing a field aspect simitar to slates, as previously described in the geological literature of the studied region. They are variable in thickness, from millimeters to tens of meters and appear intercalated in the schist succession. The geochemical resemblance between both types of rocks (major, minor and trace elements), determined by a statistical approach and the comparison of depositional condition indices, points to a similar sedimentary origin of the protolith but different clay content. Mineral facies and illite “crystallinity” indices in the so-called slates indicate that they followed the same metamorphic path and reached the same metamorphic grade than schists. According to compositional zoning detected in micas and garnets present in both lithologies and the P-T conditions deduced from garnets, the mineral nucleation and growth episode of the main mineral paragenesis in these fine-grained schists was more remarkable during the high-pressure event, with a no significant effect of the latter low-pressure–high-temperature episode. In contrast, the coarse-grained schists developed higher size minerals during the low-pressure–high-temperature episode. A differential mylonitization process during the metamorphism is proposed to justify the discrepant field appearance and the contrasting response of both types of metapelitic rocks to the latter metamorphic event.
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Torres-Ruiz, J., A. Pesquera, and V. López Sánchez-Vizcaíno. "Chromian tourmaline and associated Cr-bearing minerals from the Nevado-Fildbride Complex (Betic Cordilleras, SE Spain)." Mineralogical Magazine 67, no. 3 (June 2003): 517–33. http://dx.doi.org/10.1180/0026461036730114.
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AbstractChromian tourmaline, in association with other Cr-bearing minerals (amphibole, mica, epidote, chlorite, titanite, rutile and chromian spinel), occurs in fine calc-schist levels within metacarbonate rocks from the Nevado-Filabride Complex, SE Spain. Electron microprobe analyses of tourmaline and coexisting minerals document both chemical differences dependent on the host-rock type and an irregular distribution of Cr at grain scale. Tourmaline is Na-rich dravite, with average Mg/(Mg+Fe) ratios of 0.83 and 0.63 a.p.f.u. and Cr contents of 0.32 and 0.18 a.p.f.u., in dolomitic and ankeritic marbles, respectively. Tourmaline contains small but significant concentrations of Zn (av. 0.01 a.p.f.u.) and in ankeritic marble it also contains Ni (av. 0.04 a.p.f.u.). Zn-rich chromian spinel appears as small relict inclusions in silicates, with average Cr, Fe, Al and Zn contents of 1.201, 1.241, 0.411 and 0.107 a.p.f.u., respectively. Amphibole, epidote, mica and chlorite show average Cr contents of 0.088, 0.138, 0.115 and 0.267 a.p.f.u., respectively, in dolomitic marbles, and 0.103, 0.078, 0.065 and 0.185 a.p.f.u., respectively, in ankeritic marbles. Cr-silicates formed through metamorphic reactions involving detrital Cr-rich spinel, in addition to clay minerals and carbonates. The B necessary to form tourmaline was probably derived from the leaching of underlying evaporitic rocks.
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Augier, Romain, Guillermo Booth-Rea, Phillipe Agard, José Miguel Martínez-Martínez, Laurent Jolivet, and José Miguel Azañón. "Exhumation constraints for the lower Nevado-Filabride Complex (Betic Cordillera, SE Spain): a Raman thermometry and Tweequ multiequilibrium thermobarometry approach." Bulletin de la Société Géologique de France 176, no. 5 (September 1, 2005): 403–16. http://dx.doi.org/10.2113/176.5.403.
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Abstract The HP/LT rocks of the Nevado-Filabride complex (eastern Betic Cordillera) were exhumed during the Serravallian but knowledge of their retrograde P-T evolution remains fragmentary and not established for all its tectonic units. The present paper places detailed constraints on the P-T evolution of the two deeper units of the Nevado-Filabride complex, namely the Ragua and the Calar Alto units in order to constrain their exhumation and the role of the km-thick Dos Picos shear zone separating them. Our approach uses both TWEEQU software multiequilibrium thermobarometry and Raman spectrometry thermometry. The study enables to [i] estimate the peak-temperature P-T conditions (c. 520°C) and then to establish the first P-T path of the Ragua unit, [ii] conclude that the Ragua and the Calar Alto units suffered comparable metamorphic evolutions with [iii] a well constrained HT excursion following a strong decompression characterised by limited heating. The study also enables to infer that the major Dos Picos shear zone was a post-metamorphic thrust occurring during the final retrogression stages. These results point to exhumation processes intermediate between those of syn– and post– orogenic contexts during the late evolution of the Betics.
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RODRÍGUEZ-TOVAR, FRANCISCO J., ALFRED UCHMAN, and ÁNGEL PUGA-BERNABÉU. "Borings in gneiss boulders in the Miocene (Upper Tortonian) of the Sorbas Basin, SE Spain." Geological Magazine 152, no. 2 (July 28, 2014): 287–97. http://dx.doi.org/10.1017/s0016756814000302.
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AbstractMarine invertebrate borings are very rare in crystalline rocks, providing evidence of particular strategies producers use to colonise these unfavourable substrates. In the Sorbas Basin (Almería, southern Spain), Upper Miocene transgressive successions contain blocks of metamorphic and igneous rocks of the Nevado–Filabride Complex of the Betic Cordillera. Ichnological analysis of the embedded blocks shows the presence of two types of macroborings located in gneiss boulders, revealed to be an extraordinary case worldwide. The most abundant are regular hemispherical depressions ascribed reservedly to the well-known, mostly bivalve boringGastrochaenolites. The second one is a pouch-like depression, tapering downward, elliptical in outline, and clearly different to other non-circular-in-outline, pouch-shaped macroborings. Thus, a new ichnogenus and ichnospeciesCuenulites sorbasensishas been defined. According to the overall shape, an endolithic or semi-endolithic bivalve using chemical means to bore is suggested as the tracemaker. Colonisation could be determined by sea-level coastal dynamics, with decreasing energy during advancing transgression allowing boring, which was then stopped due to supply of fine-grained sediment that killed the borers.
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Laborda-López, Casto, Julio Aguirre, Stephen K. Donovan, Pilar Navas-Parejo, and Sergio Rodríguez. "Fossil assemblages and biostratigraphy of metamorphic rocks of the Nevado-Filábride Complex from the Águilar tectonic arc (SE Spain)." Spanish Journal of Palaeontology 30, no. 2 (May 9, 2020): 275. http://dx.doi.org/10.7203/sjp.30.2.17257.
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Sánchez-Vizcaíno, V. López, J. A. D. Connolly, and M. T. Gómez-Pugnaire. "Metamorphism and phase relations in carbonate rocks from the Nevado-Filábride Complex (Cordilleras Béticas, Spain): application of the Ttn + Rt + Cal + Qtz + Gr buffer." Contributions to Mineralogy and Petrology 126, no. 3 (January 23, 1997): 292–302. http://dx.doi.org/10.1007/s004100050251.
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Bessière, Eloïse, Laurent Jolivet, Romain Augier, Stéphane Scaillet, Jacques Précigout, José-Miguel Azañón, Ana Crespo-Blanc, Emmanuel Masini, and Damien Do Couto. "Lateral variations of pressure-temperature evolution in non-cylindrical orogens and 3-D subduction dynamics: the Betic-Rif Cordillera example." BSGF - Earth Sciences Bulletin 192 (2021): 8. http://dx.doi.org/10.1051/bsgf/2021007.
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The long-term Pressure-Temperature-time-deformation (P-T-t-d) evolution of the internal zones of orogens results from complex interactions between the subducting lithosphere, the overriding plate and the intervening asthenosphere. 2-D numerical models successfully reproduce natural P-T-t-d paths, but most orogens are non-cylindrical and the situation is far more complex due to 3-D pre-orogenic inheritance and 3-D subduction dynamics. The Mediterranean orogens are intrinsically non-cylindrical. Their 3-D geometry results from the complex shape of the Eurasian and African margins before convergence and from the dynamics of slab retreat and tearing leading to strongly arcuate belts. More than many other segments, the Betic-Rif belt is archetypal of this behavior. A synthesis of the tectonometamorphic evolution of the Internal Zones, also based on recent findings by our group in the framework of the Orogen Project (Alboran domain, including the Alpujárride-Sebtide and Nevado-Filábride complexes) shows the relations in space and time between tectonic and P-T evolutions. The reinterpretation of the contact between peridotite massifs and Mesozoic sediments as an extensional detachment leads to a discussion of the geodynamic setting and timing of mantle exhumation. Based on new 40Ar/39Ar ages in the Alpujárride-Sebtide complex and a discussion of published ages in the Nevado-Filábride complex, we conclude that the age of the HP-LT metamorphism is Eocene in all complexes. A first-order observation is the contrast between the well-preserved Eocene HP-LT blueschists-facies rocks of the eastern Alpujárride-Sebtide Complex and the younger HT-LP conditions reaching partial melting recorded in the Western Alpujárride. We propose a model where the large longitudinal variations in the P-T evolution are mainly due to (i) differences in the timing of subduction and exhumation, (ii) the nature of the subducting lithosphere and (iii) a major change in subduction dynamics at ∼20 Ma associated with a slab-tearing event. The clustering of radiometric ages obtained with different methods around 20 Ma results from a regional exhumation episode coeval with slab tearing, westward migration of the trench, back-arc extension and thrusting of the whole orogen onto the African and Iberian margins.
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SALEEBY, JASON B., Division of Geol. "Paleogeographic and Tectonic Setting of Axial and Western Metamorphic Framework Rocks of the Southern Sierra Nevada, California." AAPG Bulletin 77 (1993). http://dx.doi.org/10.1306/bdff7296-1718-11d7-8645000102c1865d.
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SCHWEICKERT, R. A., and M. M. LAHRE. "Age and Tectonic Significance of Metamorphic Rocks Along the Axis of the Sierra Nevada Batholith: A Critical Reappraisal." AAPG Bulletin 75 (1991). http://dx.doi.org/10.1306/20b23967-170d-11d7-8645000102c1865d.
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