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1. Episodic Late Eocene to Recent Extension in the vicinity of the Ruby Mountains and East Humboldt Range, Elko County, Nevada

   https://doi.org/doi: 10.1130/abs/2021CD-363293  

   McGrew, Allen J.; Metcalf, James M. (May 2021, Abstracts with programs Geological Society of America)

   null (Ed.)

   The Ruby Mountains, East Humboldt Range and Wood Hills (REHW) of Elko County Nevada, one of the classic metamorphic core complexes of the Cordillera, preserves a protracted and episodic record of both ancient and modern crustal extension that has only recently been unraveled based on its thermochronometrically constrained cooling history. Extension began during the Late Eocene synchronously with a major pulse of intermediate to felsic magmatism preserved locally by plutonic rocks intruded into the REHW and regionally by widespread Late Eocene to early Oligocene volcanism (“the ignimbrite flare-up”). The Eocene-Oligocene event accommodated at least 15 km of extension concentrated in the northern half of the complex and associated with deposition in the Elko Basin to the west, a relatively thin (~1 km), broad sequence of Late Eocene lacustrine and related strata that contrasts with the younger sedimentation patterns represented by the narrower, thicker (up to 4+km), coarse clastics of the Miocene Humboldt Basin. Though locally significant, the Eocene-Oligocene extensional phase appears not to have been associated with broadly distributed regional extension, again contrasting with Miocene and younger events. The initial phase of extension slowed or halted by the mid-Oligocene, after which extension re-accelerated in the latest Oligocene to early Miocene (~25 – 21 Ma), correlative with deposition of a coarse clastic and lacustrine sequence known as the Clover Formation. This extensional phase propagated farther south than the earlier phase along the full length of the REHW. Extension likely slowed again between ~21 Ma and ~17.5 Ma, after which it abruptly re-accelerated through the Middle Miocene to ~10 Ma, synchronous with deposition of the thick, coarse clastics of the Humboldt Formation. Middle Miocene extension likely initiated with crustal-scale heating marking the impingement of the Yellowstone hot spot in NW Nevada. Sometime after 10 Ma, the interior of the core complex was transected by east-dipping normal faults that today define the steep eastern face of the Ruby Mountains and East Humboldt Range; these face west-dipping normal faults along the west flank of the Pequop Mountains and Spruce Mountains. Extension continues today at a rate of ~1 mm/yr as represented by the 2008 MW 6.0 Wells Earthquake. 

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2. Cenozoic Basin Evolution During Alternating Extension and Shortening in the Southern Central Andes Along the Chile-Argentina Border, 37–38°S

   https://doi.org/10.2475/001c.115328  

   Encinas, Alfonso; Rosselot, Eduardo; Sagripanti, Lucía; Folguera, Andrés; Horton, Brian K; Orts, Darío; Valencia, Victor A; Arriagada, Gabriel; Butikofer, Paz; Solórzano, Andrés (April 2024, American Journal of Science)

   The south-central Chile and Argentina margin experienced a regional phase of extensional tectonics during the Oligocene–early Miocene, forming several basins across the forearc, Andean Cordillera, and retroarc regions. These basins accumulated thick successions of volcanic and sedimentary rocks. Subsequently, Neogene contractional tectonics led to the development of the current Andean Cordillera and the deposition of synorogenic clastic deposits in foreland basins. Traditionally, the Cura Mallín Formation, comprising a lower volcanic unit (CMV) and an upper sedimentary unit (CMS), has been interpreted to have formed during the Oligocene–early Miocene extensional phase. However, some studies propose deposition of the CMS in a foreland basin during the early–late Miocene. To unravel the transition from extensional to contractional tectonics in the Andes of south-central Chile and Argentina, we conducted new geochronological analyses (U-Pb, LA-ICP-MS) and integrated these results with structural, stratigraphic, and sedimentological observations in key sections within the CMS and the overlying Trapa-Trapa Formation in the Principal Cordillera along the Chile-Argentina border (37°–38°S). Our findings indicate that only the lower part of the CMS was deposited in an extensional setting, as evidenced by the presence of an inverted extensional wedge dated at ∼20 Ma. The middle-upper CMS (∼19 to 9 Ma) and contemporaneous units to the east exhibit evidence of syncontractional deformation, suggesting deposition in a foreland basin generated by shortening of the western Principal Cordillera. Around 9 Ma, uplift of the Agrio and Chos Malal fold and thrust belts, east of the Principal Cordillera, led to segmentation of the foreland basin. The Trapa Trapa Formation was deposited in a hinterland basin, with sediment sourced from the east. After ∼6.5 Ma, major contractional deformation shifted westward, resulting in intense folding of the CMS and Trapa Trapa Formation and subsequent thrusting of the western Principal Cordillera over the Central Depression. Our study suggests that deformation progressed toward the eastern foreland during the early to late Miocene and then shifted toward the western forearc during the late Miocene to Pleistocene. 

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3. Landslides in the Transantarctic Mountains: lower Jurassic and older strata displaced in late Mesozoic to late Cenozoic time

   https://doi.org/10.1080/00288306.2021.1929349  

   Elliot, David H. (June 2021, New Zealand journal of geology and geophysics)

   Transantarctic Mountains and one at Carapace Nunatak, south Victoria Land. Four consist of Kirkpatrick Basalt lavas alone, two comprise Kirkpatrick lavas with associated pyroclastic rocks, one consists of Hanson Formation beds and Kirkpatrick lavas, and one involves Fremouw Formation strata. One possible block, of uncertain origin, consists only of Hanson Formation beds. All rocks comprising the displaced blocks, except one, are Early Jurassic in age. The exception is the inferred slide involving the Triassic Fremouw beds. The locations of some landslides are consistent with emplacement on present-day topography, which has been little modified since the middle Miocene, but the time of emplacement of others is either Oligocene to pre-middle Miocene or pre-dates the onset of glaciation in Eocene/ Oligocene time. The older landslides reflect fortuitous preservation of an ancient landscape not unlike that of today, one dominated by horizontal beds consisting of resistant dolerite sills and quartz-rich sandstones alternating with intervals of weak fine-grained sedimentary beds, and capped by basalt lavas. The landslides are interpreted to document three stages in landscape evolution: a pre-glaciation semi-arid landscape, an early warm-based glacial environment, and a late cold-based glacial setting. 

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4. A Revised Cooling and Extensional Exhumation History for the Harrison Pass Pluton, Southern Ruby Mountains Metamorphic Core Complex, Elko County, Nevada

   McGrew, A.J.; Metcalf, J.M. (April 2020, Vision for Discovery: Geology and Ore Deposits of the Great Basin, Geological Society of Nevada 2020 Symposium Proceedings)

   Koutz, F.R.; Pennell, W.M. (Ed.)

   A key question in the tectonic evolution of the Sevier orogenic belt of the western U.S. Cordillera is when and why the overthickened crust of the hinterland plateau began to collapse giving rise to the modern extensional tectonic regime. Delineating the exhumation history of the Ruby Mountains, East Humboldt Range and Wood Hills metamorphic core complex (REHW) of Elko County, Nevada offers important evidence bearing on this question. Recent work from the northern REHW records a three-phase extensional history: (1) ~15–20 km of Late Eocene extension, (2) a second pulse of extension of similar rate and magnitude beginning in the late Oligocene or early Miocene (by 21 Ma) and continuing to approximately 11 Ma, and (3) the Basin-and-Range extensional regime continuing at reduced rate to today. In contrast, previous work from the Harrison Pass area in the southern REHW does not recognize an imprint from the Late Eocene phase of extension, and places the onset of the second extensional phase after ~17 Ma. New intermediate closure temperature thermochronology from the Harrison Pass pluton indicates that it remained at significant depth until at least ~25 Ma, severely limiting any possible Late Eocene to early Oligocene extension, consistent with previous interpretations. However, the new results challenge the previously proposed post-17 Ma onset for extension at Harrison Pass. New, intermediate closure temperature (U-Th)/He titanite and zircon ages from the eastern half of the pluton almost entirely predate 17 Ma and instead support an extensional onset bracketed between the Early Miocene (21 Ma) and the late Oligocene (25 Ma). Integrating potassium feldspar 40Ar/39Ar multi-diffusion domain modeling with the lower closure temperature thermochronometric systems reveals an inflection to faster cooling rates after ~25 Ma and further supports this inference. Nevertheless, all but the farthest east and structurally shallowest of the samples also show a second inflection point at ~17 Ma. We argue that previously reported apatite fission track and apatite (U-Th)/He data captured this post-17.5 Ma reacceleration event but missed the earlier, late Oligocene-early Miocene extension recorded by the higher temperature thermochronometers. The latest Oligocene to early Miocene extensional phase correlates with extensional events reported from southern Nevada and Arizona that may relate to the relaxation of contractional boundary conditions during the early evolution of the San Andreas margin. However, the post-17.5 Ma resurgence in extension probably correlates with large-scale crustal weakening across the northern Basin and Range province attending the arrival of the Yellowstone thermal plume. 

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5. A Revised Cooling and Extensional Exhumation History for the Harrison Pass Pluton, Southern Ruby Mountains Metamorphic Core Complex, Elko County, Nevada

   A.J. McGrew J.R.Metcalf (April 2000, Vision for Discovery: Geology and Ore Deposits of the Great Basin, Geological Society of Nevada 2020 Symposium Proceedings)

   F.R. Koutz W.M. Pennell (Ed.)

   A key question in the tectonic evolution of the Sevier orogenic belt of the western U.S. Cordillera is when and why the overthickened crust of the hinterland plateau began to collapse giving rise to the modern extensional tectonic regime. Delineating the exhumation history of the Ruby Mountains, East Humboldt Range and Wood Hills metamorphic core complex (REHW) of Elko County, Nevada offers important evidence bearing on this question. Recent work from the northern REHW records a three-phase extensional history: (1) ~15–20 km of Late Eocene extension, (2) a second pulse of extension of similar rate and magnitude beginning in the late Oligocene or early Miocene (by 21 Ma) and continuing to approximately 11 Ma, and (3) the Basin-and-Range extensional regime continuing at reduced rate to today. In contrast, previous work from the Harrison Pass area in the southern REHW does not recognize an imprint from the Late Eocene phase of extension, and places the onset of the second extensional phase after ~17 Ma. New intermediate closure temperature thermochronology from the Harrison Pass pluton indicates that it remained at significant depth until at least ~25 Ma, severely limiting any possible Late Eocene to early Oligocene extension, consistent with previous interpretations. However, the new results challenge the previously proposed post-17 Ma onset for extension at Harrison Pass. New, intermediate closure temperature (U-Th)/He titanite and zircon ages from the eastern half of the pluton almost entirely predate 17 Ma and instead support an extensional onset bracketed between the Early Miocene (21 Ma) and the late Oligocene (25 Ma). Integrating potassium feldspar 40Ar/39Ar multi-diffusion domain modeling with the lower closure temperature thermochronometric systems reveals an inflection to faster cooling rates after ~25 Ma and further supports this inference. Nevertheless, all but the farthest east and structurally shallowest of the samples also show a second inflection point at ~17 Ma. We argue that previously reported apatite fission track and apatite (U-Th)/He data captured this post-17.5 Ma reacceleration event but missed the earlier, late Oligocene-early Miocene extension recorded by the higher temperature thermochronometers. The latest Oligocene to early Miocene extensional phase correlates with extensional events reported from southern Nevada and Arizona that may relate to the relaxation of contractional boundary conditions during the early evolution of the San Andreas margin. However, the post-17.5 Ma resurgence in extension probably correlates with large-scale crustal weakening across the northern Basin and Range province attending the arrival of the Yellowstone thermal plume. 

   more » « less