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The Khumbu region of Nepal is host to numerous large leucogranite bodies that comprise some of the most iconic peaks in the Himalaya. These leucogranites occur throughout the high-metamorphic grade rocks exposed there in the immediate footwall of the orogen-scale South Tibetan detachment system. Previous work on leucogranites in the area has been limited spatially but has yielded U-Th/Pb ages between ~23 and 16 Ma. The current study expands upon that work with 522 new individual zircon, monazite and xenotime U-Th/Pb analyses across 10 leucogranite specimens collected from across the Khumbu. The dates returned outline the episodic crystallization of leucogranites at ~19 and 17 Ma, both of which contain significant inherited age components presumably from their source rocks. 40Ar/39Ar dating of muscovite from the same leucogranite bodies indicate rapid cooling through Ar closure (~510 °C) on the scale of ~500 kyr. The weighted mean of all 40Ar/39Ar dates, 16.48 ± 0.05 Ma, coincides with the timing of motion along the normal-sense Qomolangma detachment indicating a potential genetic relationship between the leucogranite cooling and extension. Finally, U(-Th)/He dating of apatite and zircon show that cooling slowed significantly post argon closure and that the rapid cooling may have been ephemeral. [ABSTRACT FROM AUTHOR]

The Andean Plateau of NW Argentina is a prominent example of a high‐elevation orogenic plateau characterized by internal drainage, arid to hyper‐arid climatic conditions and a compressional basin‐and‐range morphology comprising thick sedimentary basins. However, the development of the plateau as a geomorphic entity is not well understood. Enhanced orographic rainout along the eastern, windward plateau flank causes reduced fluvial run‐off and thus subdued surface‐process rates in the arid hinterland. Despite this, many Puna basins document a complex history of fluvial processes that have transformed the landscape from aggrading basins with coalescing alluvial fans to the formation of multiple fluvial terraces that are now abandoned. Here, we present data from the San Antonio de los Cobres (SAC) area, a sub‐catchment of the Salinas Grandes Basin located on the eastern Puna Plateau bordering the externally drained Eastern Cordillera. Our data include: (a) new radiometric U‐Pb zircon data from intercalated volcanic ash layers and detrital zircons from sedimentary key horizons; (b) sedimentary and geochemical provenance indicators; (c) river profile analysis; and (d) palaeo‐landscape reconstruction to assess aggradation, incision and basin connectivity. Our results suggest that the eastern Puna margin evolved from a structurally controlled intermontane basin during the Middle Miocene, similar to intermontane basins in the Mio‐Pliocene Eastern Cordillera and the broken Andean foreland. Our refined basin stratigraphy implies that sedimentation continued during the Late Mio‐Pliocene and the Quaternary, after which the SAC area was subjected to basin incision and excavation of the sedimentary fill. Because this incision is unrelated to baselevel changes and tectonic processes, and is similar in timing to the onset of basin fill and excavation cycles of intermontane basins in the adjacent Eastern Cordillera, we suspect a regional climatic driver, triggered by the Mid‐Pleistocene Climate Transition, caused the present‐day morphology. Our observations suggest that lateral orogenic growth, aridification of orogenic interiors, and protracted plateau sedimentation are all part of a complex process chain necessary to establish and maintain geomorphic characteristics of orogenic plateaus in tectonically active mountain belts. [ABSTRACT FROM AUTHOR]

Extensive exposure of intrusive igneous rocks along the Ross orogen of Antarctica—an ancient accretionary orogen on the margin of East Gondwana—provides an exceptional opportunity to study continental arc magmatism. There is significant petrologic and geochemical variability in igneous rocks within a ~ 500-km-long segment of the arc in southern Victoria Land. The conspicuous occurrence of carbonatite and alkaline silicate rocks (nepheline syenite, A-type granite, and alkaline mafic rocks) adjacent to large complexes of subalkaline granitoids is not adequately explained by traditional models for continental arc magmatism. Extensive geochemical analysis (> 100 samples) and zircon U–Pb geochronology ( n = 70) confirms that alkaline and carbonatitic magmatism was partially contemporaneous with the emplacement of large subduction-related igneous complexes in adjacent areas. Major pulses of subalkaline magmatism were compositionally distinct and occurred at different times along the arc. Large bodies of subalkaline orthogneiss and granite (sensu lato ) were emplaced over similar time intervals (ca. 25 Myr) to the north (ca. 515–492 Ma) and south (ca. 550–525 Ma) of the alkaline magmatic province, although the initiation of these major pulses of magmatism was offset by ca. 35 Myr. Alkaline and carbonatitic magmatism spanned at least ca. 550–509 Ma, overlapping with voluminous subalkaline magmatism in adjacent areas. The most primitive rocks from each area have similarly enriched trace element compositions, indicating some common characteristics of the magma sources along the arc. The samples from the older subalkaline complex have invariably low Sr/Y ratios (< 40), consistent with relatively shallow magma generation and differentiation. The younger subalkaline complex and subalkaline rocks within the area of the alkaline province extend to higher Sr/Y ratios (up to ~ 300), indicative of generation and differentiation at deeper levels. The significant spatial and temporal diversity in magmatism can be explained by a tectono-magmatic model that invokes alternating phases of extension and contraction in the overriding plate. The low Sr/Y magmatism of the earlier subalkaline complex and the largely contemporaneous alkaline and carbonatitic magmatism occurred during a phase of extension in the overriding plate ca. 550–525 Ma. The differences in composition of this early phase of magmatism can be explained by variable lithospheric thickness along the margin, with the alkaline magmas derived from partial melts of thick metasomatized lithosphere. The younger subalkaline complex was emplaced during a phase of contraction in the overlying plate ca. 515–505 Ma. The latest phase of (post-orogenic) magmatism occurred during extension in the overriding plate ca. 505–492 Ma. [ABSTRACT FROM AUTHOR]

The presence of hot, weak crust is a central component of recent hypotheses that seek to explain the evolution of continent-continent collisions, and in particular may play an important role in accommodating the >3000 km of convergence within the Himalaya–Tibetan collision over the last ∼55 Myr. Models that implicate flow of semi-viscous midcrustal rocks south toward the front of the Himalayan orogen, ‘channel flow’, are able to account for many geologic observations in the Himalaya, while alternative models of collision, particularly ‘thrust-wedge taper’, demonstrate that much of the observed geology could have formed in the absence of a low-viscosity mid-crustal layer. Several recent studies, synthesized here, have prompted a shift from initial assumptions that channel flow and thrust-wedge taper processes are by definition mutually exclusive. These new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel- and wedge-type processes operate in spatially and temporally distinct domains within the orogen, and further, that the system may migrate back and forth between these types of behavior. This continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the Himalayan orogen and, hence, for understanding the evolution of Earth's many continent-continent collision zones. [ABSTRACT FROM AUTHOR]

The article analyzes how strain partitioning developed within the Ama Drime Massif and the Leo Pargil Dome in order to further understand the kinematic evolution of the orogen in the transitional zone between crustal shortening in the foreland and orogen-parallel extension in southern Tibet. It observes that the Leo Pargil dome was exhumed from depths near the Tethyan Sedimentary Sequence while the Ama Drime Massif was exhumed from some of the deepest positions exposed in the central Himalayas.

A study is presented that aims to calculate the dating constraints for the onset of the cooling history of the Greater Himalayan Sequence (GHS) in northwest Bhutan. It relates that thermochronologic data derived from two numerical diffusion models suggest that high-grade rocks from three structural levels of the GHS experienced rapid cooling following high temperature metamorphism at different times, implying a progressive eastward change in the exhumation mechanism across the GHS orogen.

Abstract: The Ama Drime Massif is a north–south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [c] axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ∼400 and 650°C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least ∼21–42km of displacement on the Ama Drime detachment. Samples from the Nyönno Ri detachment were exhumed from similar depths. We propose that exhumation on the Nyönno Ri detachment during initiation of orogen-parallel extension (11–13Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment. [ABSTRACT FROM AUTHOR]

In the Caledonides of northern Scotland temperatures of metamorphism ( T m) and deformation ( T d) progressively increase structurally up section in the Moine thrust sheet at the foreland edge of the Scandian (mid Silurian) orogenic wedge. However, the thermal history of the structurally overlying, more hinterland positioned thrust sheets is less well known. This study focuses on determining T d and T m for both the central/upper part of the Moine thrust sheet and the lower part of the overlying Sgurr Beag thrust sheets in the middle of the Northern Highlands Terrane. Preserved microstructures and quartz c-axis fabric opening angles in the Moine and Sgurr Beag thrust sheets imply T d of 460 °C to 605 °C ± 50 °C. Thermobarometry and pseudosection-based P-T constraints, indicate T m of ∼550–680 °C at 4.8–7.2 kbar in the Moine thrust sheet and T m of ∼620 °C at 5.6–7.7 kbar in the Sgurr Beag thrust sheet. Together, T d and T m in the Moine and Sgurr Beag thrust sheets indicate that deformation continued after peak metamorphic conditions in the Sgurr Beag thrust sheet. Monazite and xenotime petrochronology show that T m, and possibly T d, record Precambrian metamorphism. Peak metamorphism is associated with the Knoydartian orogenic event (840-720 Ma), with possible reworking during Scandian thrusting (430-425 Ma). [ABSTRACT FROM AUTHOR]

Geologic mapping combined with microstructural and geochronologic analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style despite variability in the timing of metamorphism and deformation. The Main Central thrust zone at the base of the Himalayan metamorphic core varies in thickness, tectonostratigraphy, and metamorphic gradient along the 250 km of strike length studied. In situ U-Th/Pb geochronology of monazite sampled from an along-strike transect at the top of the high-strain zone records Eocene--Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The timing of prograde and retrograde metamorphism and the muscovite 40Ar/39Ar dates gradually decrease along strike from northwest to southeast. This age trend is punctuated by an abrupt ~3-8 m.y. decrease in the age of prograde and retrograde metamorphism and muscovite 40Ar/39Ar dates near the Marsyandi River in central Nepal. Quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high-strain zone document similar flow style at ~440 °C in central Nepal. Muscovite 40Ar/39Ar ages, interpreted to approximate the age of deformation at this structural level, decrease from ca. 7 to 4 Ma along strike from northwest to southeast. Diachronous deformation and metamorphism along strike in west-central Nepal demonstrate the necessity of incorporating more than a single transect into tectonic models. Along-strike tectonometamorphic variability in west-central Nepal spatially corresponds to faults in the Indian basement bounding the subsurface Faizabad ridge, highlighting the possible influence of inherited basement faults on the geometry of the basal Himalayan detachment, the Main Himalayan thrust, as well as the tectonic evolution of the structurally overlying Himalayan metamorphic core. This study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along-strike diachroneity of deformation in the Himalaya. [ABSTRACT FROM AUTHOR]

The occurrence of thrust-sense tectonometamorphic discontinuities within the exhumed Himalayan metamorphic core can be explained as part of the Main Central thrust system. This imbricate thrust structure, which significantly thickened the orogenic midcrustal core, comprises a series of thrust-sense faults that all merge into a single detachment. The existence of these various structures, and their potential for complex overprinting along the main detachment, may help explain the contention surrounding the definition, mapping, and interpretation of the Main Central thrust. The unique evolution of specific segments of the Main Central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position, and structural level of exposure of the mid-crust. This helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt. [ABSTRACT FROM AUTHOR]

The spatial extent and dynamic evolution of orogen-scale shear zones renders the determination of the timing and duration of ductile deformation and displacement challenging. The Himalayan Main Central thrust (MCT) is one of the most intensively studied thrust-sense shear zones in the world, particularly regarding its timing and duration of displacement. Yet, available timing constraints vary greatly along the length of orogen such that 'when was the MCT active' remains unresolved. From the Himalayan hinterland to the Karnali klippe, the MCT is exposed multiple times in western Nepal along its transport direction. Pressure-temperature-deformation-time (P-T-d-t) data from three comparable structural sections of the MCT, which together represent > 100 km in the direction of tectonic transport illuminate how spatio-temporal variations in recorded shear complicate attempts to decipher the timing of deformation. Metamorphic P-T data, quartz microstructures and quartz crystallographic preferred orientation data suggest that all three segments of the MCT underwent similar shear conditions and kinematic behaviour. However, each segment reached peak T conditions during different windows of time. Based on U-Th/Pb monazite petrochronology, U/Pb zircon geochronology, 40Ar/39Ar thermochronology, P-T, and quartz microstructure data, the hinterland-most MCT segment reached peak metamorphic conditions of ~650C and 0.8 GPa from 14 to 12 Ma and records a minimum temperature of deformation of 500C ± 50C; this MCT segment cooled below 450C by 6 Ma. The MCT segment on the north flank of the Karnali klippe records peak metamorphic conditions of ~700C and 1.0 GPa at ca. 30 Ma and deformation temperatures of at least 500C ± 50C; this segment cooled below 415C by 14 Ma. The foreland-most MCT segment, exposed on the south flank of the Karnali klippe, records deformation prior to 17 Ma, possibly beginning as early as 33–30 Ma with peak temperatures of 500–600C and deformation temperatures of at least 425C ± 50C; this MCT segment cooled below 315C by ca. 17 Ma. Our data indicate that the apparent record of ductile deformation on the MCT is diachronous in the transport direction. In western Nepal, a single shear zone segment does not record the full timing and duration of deformation on the MCT, even for relatively continuous down-dip structural sections. These results point to the need to study the temporal evolution of orogen-scale shear zones over significant distances in the direction of tectonic transport, acknowledge potential gaps in the petrochronological record, and incorporate P-T-d data with the variable timing of shearing when developing regional geodynamic models. [ABSTRACT FROM AUTHOR]