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The Thakkhola graben is a large-scale N-S striking, E-W extensional structure located in west-central Nepal that was actively extending ca. 17 Ma. New multi-system geochronological data from the immediate footwall of the Dangardzong fault, the main graben-forming structure in the Thakkhola graben, outline decelerating cooling paths. The cooling rate of the footwall progressively decreases from 55 ± 10°C/Myr in the early Miocene (~22–13 Ma, monazite U-Th/Pb, mica 40Ar/39Ar and zircon U-Th/He), to 23 ± 8°C/Myr in the middle to late Miocene (~13-8 Ma, zircon and apatite U-Th/He), and finally to 10 ± 2°C/Myr from 8 Ma to present day (cooling post apatite U-Th/He closure). The deceleration in cooling rate is interpreted to reflect the widespread development of N-S striking graben structures in the Tibetan plateau in the middle Miocene and the progressive partitioning of strain away from the Thakkhola graben into other, younger, extensional features. The shift in the locus of strain may reflect the progressive under-thrusting of India beneath the Tibetan plateau and/or the eastward flow of mid-to-lower crustal material away from the plateau. [ABSTRACT FROM AUTHOR]

Quartz‐inclusion geothermobarometry of texturally and chemically zoned garnet, together with U–(Th)–Pb accessory phase petrochronology, phase equilibria modelling, and major element thermobarometry delineates two metamorphic events within the Kathmandu Complex in central Nepal. Combined titanium‐in‐quartz thermometry (TitaniQ) and quartz‐in‐garnet barometry (QuiG) of quartz inclusions in garnet cores yields temperatures and pressures (555–580°C, 0.89–0.91 GPa) statistically higher than those estimated for both garnet rims and secondary garnet neoblasts (535–545°C, 0.81–0.84 GPa). Comparison of these temperatures and pressures with the results from phase equilibria modelling indicates that the garnet cores nucleated after overstepping the equilibrium garnet isograd. Estimation of the degree to which overstepping may have occurred, however, is complicated by uncertainty in the estimated reactive bulk compositions. Major element thermobarometry of the garnet rims returns temperatures and pressures lower than those preserved by the garnet cores but are consistent with previous estimates of Cenozoic metamorphism in the frontal portion of Kathmandu thrust sheet. Two generations of titanite are recognized on the basis of mineral morphology, chemical zoning, and U–Pb date (c. 500 Ma versus 29 ± 5 Ma). Trace‐element systematics of the two titanite populations indicate varying co‐stability with allanite and are consistent with Th–Pb dates of allanite growth (c. 489 Ma) and breakdown to clinozoisite (c. 30 Ma). Inclusion patterns within titanite indicate that the younger group formed following breakdown of rutile that originally coexisted with the older generation of titanite at pressures and temperatures similar to those determined from the garnet cores. Application of the Zr‐in‐titanite and Zr‐in‐rutile thermometers yields temperatures 100–150°C in excess of those indicated by both TitaniQ‐QuiG and major element thermobarometry. This discrepancy is interpreted to reflect disequilibrium partitioning of Zr and limited zircon reactivity during metamorphism. Collectively, these observations are consistent with previous estimates of the timing and conditions of metamorphism associated with the Palaeozoic Bhimphedian and Cenozoic Himalayan orogenies in central Nepal. The estimated temperatures and pressures for growth of the garnet cores are consistent with rapid burial during thrust stacking. [ABSTRACT FROM AUTHOR]

The structural and topographic evolution of orogenic plateaus is an important research topic because of its impact on atmospheric circulation patterns, the amount and distribution of rainfall, and resulting changes in surface processes. The Puna region in the north-western Argentina (between 13°S and 27°S) is part of the Andean Plateau, which is the world's second largest orogenic plateau. In order to investigate the deformational events responsible for the initial growth of this part of the Andean plateau, we carried out structural and stratigraphic investigations within the presentday transition zone between the northern Puna and the adjacent Eastern Cordillera to the east. This transition zone is characterized by ubiquitous exposures of continental middle Eocene redbeds of the Casa Grande Formation. Our structural mapping, together with a sedimentological analysis of these units and their relationships with the adjacent mountain ranges, has revealed growth structures and unconformities that are indicative of syntectonic deposition. These findings support the notion that tectonic shortening in this part of the Central Andes was already active during the middle Paleogene, and that early Cenozoic deformation in the region that now constitutes the Puna occurred in a spatially irregular manner. The patterns of Paleogene deformation and uplift along the eastern margin of the present-day plateau correspond to an approximately north-south oriented swath of reactivated basement heterogeneities (i.e. zones of mechanical weakness) stemming from regional Paleozoic mountain building that may have led to local concentration of deformation belts. [ABSTRACT FROM AUTHOR]

The Butcher Ridge igneous complex, Antarctica, is an ~6000 km3 hypabyssal silicic intrusion containing rhythmically layered glassy rocks. Baddeleyite U-Pb geochronologic analysis on a sample of the Butcher Ridge igneous complex yielded an age of ca. 182.4 Ma, which confirms that it was emplaced synchronously with the Ferrar large igneous province. Rocks of the Butcher Ridge igneous complex vary from basaltic andesite to rhyolite, and so the inferred volume of the Butcher Ridge igneous complex makes it the most voluminous silicic component of the Ferrar large igneous province. Major-element, trace-element, and isotopic data combined with binary mixing, assimilation- fractional crystallization (AFC), and energy-constrained AFC models are consistent with formation of Butcher Ridge igneous complex silicic rocks by contamination of mafic Ferrar parental magma(s) with local Paleozoic plutonic basement rocks. Field and petrographic observations and evidence for alkali ion exchange suggest that the kilometer- long, meter-thick enigmatic rhythmic layering formed as a result of secondary hydration and devitrification of volcanic glass along parallel fracture networks. The regularity and scale of fracturing/layering imply a thermally driven process that occurred during shallow emplacement and supercooling of the intrusion in the upper crust. We suggest that layering observed in the Butcher Ridge igneous complex is analogous to that reported from terrestrial and Martian cryptodomes, and therefore it is an ideal locality at which to study layering processes in igneous bodies. [ABSTRACT FROM AUTHOR]

The Tibetan plateau is host to numerous ~N‐S striking graben that have accommodated E‐W directed extension. The development of these structures has been interpreted to reflect a variety of different geological processes including plateau collapse, oroclinal bending or mid‐to‐lower crustal flow. New 40Ar/39Ar thermochronology and quartz c‐axis data from the Thakkhola graben of west‐central Nepal show that E‐W extension was ongoing at least locally by the early Miocene (ca. 17 Ma). Our new, and previously published chronologic information on the initiation of graben across the orogen shows that they typically developed immediately after cessation of the South Tibetan detachment system, a structural network that facilitated differential southward movement of the upper and middle crust. We interpret this fundamental switch in orogen kinematics to reflect recoupling of the middle and upper Himalayan crust such that the subsequent widespread flow of the mid‐to‐lower crust out of the system to the east forced brittle accommodation in the upper crust. [ABSTRACT FROM AUTHOR]

John M. Cottle, a professor of earth science at the University of California, Santa Barbara, has won the 2018 Emerging Leader in Atomic Spectroscopy Award, which is presented by Spectroscopy [...]

Combining high‐spatial resolution geochronology and geochemistry of zircon provides constraints on the geodynamic evolution of the Zagros orogen. This study underlines the significance of hitherto unknown magmatic/geological events in the central Urumieh–Dokhtar magmatic arc (UDMA) of Iran at ca. 35–22 and 10 Ma. Recently reported U–Pb zircon ages of 25–24 Ma for the granodioritic rocks from the south Ardestan area were interpreted to constrain the crystallization age. Newly determined U–Pb zircon ages of ca. 35 to 22 Ma from south Ardestan gabbroic to granitic rocks and related dikes indicate that the melt evolution continued from the Oligocene into Early Miocene time. In addition, a ca. 10 Ma U–Pb age for the zircons from a south Ardestan diorite provides evidence for a so far unknown, igneous episode. Representing the youngest magmatic stage recognized in the central part of the UDMA, the 10 Ma age is interpreted to document magmatic activity related to the Arabia–Eurasia collision, as recognized elsewhere in the Zagros orogen. South Ardestan zircons with geochemical signatures indicating continental affinity likely recrystallized in the course of a major event (asthenospheric mantle melt formation) that has affected crustal rocks. The significant lower LREEs of these zircons compared to hydrothermal zircons, moderate Th/U (>0.4), and high (Sm/La)N (>10) ratios associated with low [La] (<1 ppm) suggest that these crystals have an igneous origin and are not related to hydrothermal or metamorphic processes. These findings bear important implications for the geodynamic evolution of the Zagros orogen and support a model suggesting that magmatic activity still affected the UDMA in Late Miocene time. [ABSTRACT FROM AUTHOR]

New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust. [ABSTRACT FROM AUTHOR]

Regarding the article, 'Medicaid study: 5% of enrollees account for half of all spending' (June 10, 2015): Did the authors exclude long-term care spending? The Hon's share of LTC is [...]

Models of uplift, rifting, and heat transfer in the Transantarctic Mountains (TAM) rely on knowledge of the thermal evolution of the lower crust, but such information has remained elusive. Granulite xenoliths entrained in <2 Ma rift basalts at the TAM front chronicle the long-term thermal history of the deep crust and yield insight into the Cenozoic evolution of the TAM. Major-element thermobarometry of the xenoliths record elevated temperatures of 860–920 °C at ∼0.8 GPa. In situ coupled U-Pb and trace element zircon and titanite data reveal that the deep crust experienced intense heating at least twice: once at 850–900 °C ca. 500 Ma and again at 740–900 °C starting ca. 37 Ma. The Cenozoic temperature-time path indicates a high geothermal gradient beneath the TAM front—with 800–900 °C at 20–30 km depth and 900–1000 °C temperatures at the Moho (30–35 km depth)—and is interpreted to reflect heating by the adjacent West Antarctic Rift System. Despite elevated temperatures in the lower crust, minimal melting beneath the present-day TAM is inferred given that the crust is refractory, having been previously depleted during Ordovician magmatism. The identification of hot crust beneath the TAM forces revisions to estimates of the elastic thickness of the lithosphere, which underpin models invoking a flexural origin for the high elevations of the TAM. The observed Cenozoic heating trend supports models that suggest uplift was driven by a buoyant thermal anomaly beneath the TAM front. The finding of dry and strong deep crust is also in line with models wherein the TAM have maintained high elevations since the Mesozoic. • Xenoliths from the Transantarctic Mountains (TAM) record multiple two of high-temperature metamorphism in the lower crust. • Cenozoic rift-related heating to >800 °C suggested by zircon, rutile, apatite, and titanite petrochronology. • Data require reevaluation of elastic properties of lithosphere, with implications for the origins of high TAM topography. [ABSTRACT FROM AUTHOR]