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High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the highpressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta.

High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the highpressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta.

40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.

40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.

Highlights • The South Tibetan detachment system played a major role in Himalayan evolution. • Near Mt Everest, the detachment system accommodated large displacements under both brittle and ductile conditions. • Rapid cooling of footwall rocks reflected tectonic denudation by brittle slip from ca. 15.6 to at least 13.0 Ma. • Thermal–kinematic modeling suggests displacement on the detachment to be at least 61 km. Abstract North-dipping, low-angle normal faults of the South Tibetan detachment system (STDS) are tectonically important features of the Himalayan–Tibetan orogenic system. The STDS is best exposed in the N–S-trending Rongbuk Valley in southern Tibet, where the primary strand of the system – the Qomolangma detachment – can be traced down dip from the summit of Everest for a distance of over 30 km. The metamorphic discontinuity across this detachment implies a large net displacement, with previous studies suggesting >200 km of slip. Here we refine those estimates through thermal–kinematic modeling of new (U–Th)/He and 40Ar/39Ar data from deformed footwall leucogranites. While previous studies focused on the early ductile history of deformation along the detachment, our data provide new insights regarding the brittle–ductile to brittle slip history. Thermal modeling results generated with the program QTQt indicate rapid, monotonic cooling from muscovite 40Ar/39Ar closure (ca. 15.4–14.4 Ma at ca. 490 °C) to zircon (U–Th)/He closure (ca. 14.3–11.0 Ma at ca. 200 °C), followed by slower cooling to apatite (U–Th)/He closure at ca. 9–8 Ma (at ca. 70 °C). Although previous work has suggested that ductile slip on the detachment lasted only until ca. 15.6 Ma, thermal–kinematic modeling of our new data suggests that rapid (ca. 3–4 km/Ma) tectonic exhumation by brittle–ductile to brittle fault slip continued to at least ca. 13.0 Ma. Much lower modeled exhumation rates (≤0.5 km/Ma) after ca. 13 Ma are interpreted to reflect erosional denudation rather than tectonic exhumat

Highlights • The South Tibetan detachment system played a major role in Himalayan evolution. • Near Mt Everest, the detachment system accommodated large displacements under both brittle and ductile conditions. • Rapid cooling of footwall rocks reflected tectonic denudation by brittle slip from ca. 15.6 to at least 13.0 Ma. • Thermal–kinematic modeling suggests displacement on the detachment to be at least 61 km. Abstract North-dipping, low-angle normal faults of the South Tibetan detachment system (STDS) are tectonically important features of the Himalayan–Tibetan orogenic system. The STDS is best exposed in the N–S-trending Rongbuk Valley in southern Tibet, where the primary strand of the system – the Qomolangma detachment – can be traced down dip from the summit of Everest for a distance of over 30 km. The metamorphic discontinuity across this detachment implies a large net displacement, with previous studies suggesting >200 km of slip. Here we refine those estimates through thermal–kinematic modeling of new (U–Th)/He and 40Ar/39Ar data from deformed footwall leucogranites. While previous studies focused on the early ductile history of deformation along the detachment, our data provide new insights regarding the brittle–ductile to brittle slip history. Thermal modeling results generated with the program QTQt indicate rapid, monotonic cooling from muscovite 40Ar/39Ar closure (ca. 15.4–14.4 Ma at ca. 490 °C) to zircon (U–Th)/He closure (ca. 14.3–11.0 Ma at ca. 200 °C), followed by slower cooling to apatite (U–Th)/He closure at ca. 9–8 Ma (at ca. 70 °C). Although previous work has suggested that ductile slip on the detachment lasted only until ca. 15.6 Ma, thermal–kinematic modeling of our new data suggests that rapid (ca. 3–4 km/Ma) tectonic exhumation by brittle–ductile to brittle fault slip continued to at least ca. 13.0 Ma. Much lower modeled exhumation rates (≤0.5 km/Ma) after ca. 13 Ma are interpreted to reflect erosional denudation rather than tectonic exhumat

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 118 (2013): 1333–1341, doi:10.1002/jgrf.20091., Detrital mineral thermochronology of modern sediments is a valuable tool for interrogating landscape evolution. Detrital zircon (U-Th)/He thermochronology is of particular interest because zircons are durable and withstand transport in glacial and fluvial systems far better than, for example, apatite. However, because of the time-intensive nature of conventional zircon (U-Th)/He thermochronology, most previous studies of this kind have relied on data for a few tens of grains, even though conventional wisdom holds that a substantially larger number is necessary for a robust characterization of the population of cooling ages in a sample. Here, we introduce a microanalytical approach to detrital zircon (U-Th)/He thermochronology that addresses many factors that can complicate the interpretation of conventional zircon (U-Th)/He data, particularly with respect to alpha ejection and injection and U + Th zoning. In addition, this technique permits the effective dating of naturally abraded and broken grains, and, therefore, lessens the potential for sampling bias. We apply both conventional and laser microprobe techniques to a detrital sample from the Ladakh Range in the northwestern Indian Himalaya, showing that the two yield very similar principal modes of apparent ages. However, the laser microprobe data yield a broader spectrum of ages than that of the conventional data set, which we interpret to be caused by bias related to the selection requirements for zircons used for conventional dating. This method thus provides a time-efficient route to obtaining a higher-resolution distribution of dates from a single sample, which will, in turn, yield higher-fidelity constraints regarding catchment-wide erosion rates for surface process studies., Funding for this project was provided by NSF EAR-0642731, awarded to KVH and a Lewis and Clark Grant awarded to AT-L., 2014-01-26

Detrital mineral thermochronology of modern sediments is a valuable tool for interrogating landscape evolution. Detrital zircon (U-Th)/He thermochronology is of particular interest because zircons are durable and withstand transport in glacial and fluvial systems far better than, for example, apatite. However, because of the time-intensive nature of conventional zircon (U-Th)/He thermochronology, most previous studies of this kind have relied on data for a few tens of grains, even though conventional wisdom holds that a substantially larger number is necessary for a robust characterization of the population of cooling ages in a sample. Here, we introduce a microanalytical approach to detrital zircon (U-Th)/He thermochronology that addresses many factors that can complicate the interpretation of conventional zircon (U-Th)/He data, particularly with respect to alpha ejection and injection and U + Th zoning. In addition, this technique permits the effective dating of naturally abraded and broken grains, and, therefore, lessens the potential for sampling bias. We apply both conventional and laser microprobe techniques to a detrital sample from the Ladakh Range in the northwestern Indian Himalaya, showing that the two yield very similar principal modes of apparent ages. However, the laser microprobe data yield a broader spectrum of ages than that of the conventional data set, which we interpret to be caused by bias related to the selection requirements for zircons used for conventional dating. This method thus provides a time-efficient route to obtaining a higher-resolution distribution of dates from a single sample, which will, in turn, yield higher-fidelity constraints regarding catchment-wide erosion rates for surface process studies.

We present data on the burial, displacement and exhumation history of the Himalayan fold-thrust belt in eastern Bhutan. These data document the magnitude and timing of displacement of large, discrete structures and highlight temporal variability in shortening rates. Eight new40Ar/39Ar ages from white mica, 32 new zircon (U-Th)/He ages, 7 new apatite fission track ages, and 1 new U-Pb zircon (LA-MC-ICP-MS) metamorphic rim growth age are combined with published cooling ages and deformation temperatures, and incremental shortening magnitudes from restorations of two published balanced cross sections, to illustrate the kinematic and temporal development of the Bhutan thrust belt. Integrating these data from ∼23 Ma to the present illustrates rapid horizontal shortening rates (28–35 mm/yr) between 23–20 Ma and 15–10 Ma, separated by more moderate rates (10–23 mm/yr). Shortening rates decrease significantly to 7–10 mm/yr (and possibly as low as 3–4 mm/yr) from 10 to 0 Ma. This decrease is interpreted to represent the onset of strain partitioning in the eastern part of the Himalayan-Tibetan orogenic system, between shortening in the Bhutan thrust belt, uplift of the Shillong Plateau, and deformation and outward growth of the northern and eastern Tibetan Plateau. Within estimated error, horizontal shortening rates during emplacement of the Main Central thrust sheet and during construction of the upper Lesser Himalayan duplex approached India-Asia tectonic velocities. Thus, for periods of time between ∼23–20 Ma and ∼15–10 Ma, the Bhutan thrust belt may have absorbed nearly all India-Asian convergence at this longitude.

We present data on the burial, displacement and exhumation history of the Himalayan fold-thrust belt in eastern Bhutan. These data document the magnitude and timing of displacement of large, discrete structures and highlight temporal variability in shortening rates. Eight new40Ar/39Ar ages from white mica, 32 new zircon (U-Th)/He ages, 7 new apatite fission track ages, and 1 new U-Pb zircon (LA-MC-ICP-MS) metamorphic rim growth age are combined with published cooling ages and deformation temperatures, and incremental shortening magnitudes from restorations of two published balanced cross sections, to illustrate the kinematic and temporal development of the Bhutan thrust belt. Integrating these data from ∼23 Ma to the present illustrates rapid horizontal shortening rates (28–35 mm/yr) between 23–20 Ma and 15–10 Ma, separated by more moderate rates (10–23 mm/yr). Shortening rates decrease significantly to 7–10 mm/yr (and possibly as low as 3–4 mm/yr) from 10 to 0 Ma. This decrease is interpreted to represent the onset of strain partitioning in the eastern part of the Himalayan-Tibetan orogenic system, between shortening in the Bhutan thrust belt, uplift of the Shillong Plateau, and deformation and outward growth of the northern and eastern Tibetan Plateau. Within estimated error, horizontal shortening rates during emplacement of the Main Central thrust sheet and during construction of the upper Lesser Himalayan duplex approached India-Asia tectonic velocities. Thus, for periods of time between ∼23–20 Ma and ∼15–10 Ma, the Bhutan thrust belt may have absorbed nearly all India-Asian convergence at this longitude.

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Tectonics 23 (2004): TC5013, doi:10.1029/2003TC001559., The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ~5% of the eroding crust has been generated since India-Asia collision. The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The 39Ar/40Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0–15 Ma. Modern average exhumation rates are estimated at ~0.6 km/m.y. or less, and have slowed progressively since the early Miocene (~20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (~1.0 km/m.y.) driven by climate change. The 39Ar/40Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record., Scottish Universities Environmental Research Centre (SUERC) is supported by the Scottish Universities and the British National Environmental Research Council (NERC). P. C. wishes to thank the Joint Oceanographic Institutions (JOI) for partial support of this work.

Although most data suggest that the India-Eurasia continental collision began 45-55 Myr ago, the architecture of the Himalayan-Tibetan orogen is dominated by deformational structures developed in the Neogene period (<23 Myr ago). The stratigraphic record

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U‐Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze Craton and during the Triassic Indosinian Orogeny. Indosinian grains in the Mekong are younger (210–240 Ma) than those in the Red River (230–290 Ma), suggesting preferential erosion of the Qiangtang Block of Tibet into the Mekong. The Red River has a higher proportion of 700–800 Ma grains originally derived from the Yangtze Craton. 40Ar/39Ar dating of muscovite grains demonstrates that rocks cooled during the Indosinian Orogeny are dominant in both rivers, although the Mekong also shows a grain population cooling at 150–200 Ma that is not seen in the Red River and which is probably of original Qiangtang Block origin. Conversely, the Red River contains a significant mica population (350–500 Ma) eroded from the Yangtze Craton. High‐grade metamorphic rocks exposed in the Cenozoic shear zones of southeast Tibet‐Yunnan are minority sources to the rivers. However, apatite and zircon fission track ages show evidence for the dominant sources, especially in the Red River, only being exhumed through the shallowest 5–3 km of the crust since ∼25 Ma. The thermochronology data are consistent with erosion of recycled sediment from the inverted Simao and Chuxiong Basins, from gorges that incise the eastern flank of the plateau. Average Neogene exhumation rates are 104–191 m/Myr in the Red River basin, which is within error of the 178 ± 35 m/Myr estimated from Pleistocene sediment volumes. Sparse fission track data from the Mekong River support the Ar‐Ar and U‐Pb ages in favoring tectonically driven rock uplift and gorge incision as the dominant control on erosion, with precipitation being an important secondary influence.

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U-Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze

The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ~5% of the eroding crust has been generated since India-Asia collision. The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The 39Ar/40Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0–15 Ma. Modern average exhumation rates are estimated at ~0.6 km/m.y. or less, and have slowed progressively since the early Miocene (~20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (~1.0 km/m.y.) driven by climate change. The 39Ar/40Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record.

The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U‐Pb ages of zircons spanning 3 Gyr show that only ∼5% of the eroding crust has been generated since India‐Asia collision. The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The 39Ar/40Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0–15 Ma. Modern average exhumation rates are estimated at ∼0.6 km/m.y. or less, and have slowed progressively since the early Miocene (∼20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (∼1.0 km/m.y.) driven by climate change. The 39Ar/40Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record.

The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ∼5% of the eroding crust has been generated since India-Asia collision. The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The 39Ar/40Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0-15 Ma. Modern average exhumation rates are estimated at ∼0.6 km/m.y. or less, and have slowed progressively since the early Miocene (∼20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (∼1.0 km/m.y.) driven by climate change. The 39Ar/40Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record.