The Klamath Mountains in northern California and southern Oregon are thought to record 200+ m.y. of subduction and terrane accretion, whereas the outboard Franciscan Complex records classic ocean-continent subduction along the North American margin. Unraveling the Klamaths’ late history could help constrain this transition in subduction style. Key is the Mesozoic Condrey Mountain Schist (CMS), comprising, in part, a subduction complex that occupies a structural window through older, overlying central Klamath thrust sheets but with otherwise uncertain relationships to other, more outboard Klamath or Franciscan terranes. The CMS consists of two units (upper and lower), which could be correlated with 1) other Klamath terranes, 2) the Franciscan, or 3) neither based on regional structures and limited extant age data. Upper CMS protolith and metamorphic dates overlap with other Klamath terranes, but the lower CMS remains enigmatic. We used multiple geochronometers to constrain the timing of lower CMS deposition and metamorphism. Maximum depositional ages (MDAs) derived from detrital zircon geochronology of metasedimentary rocks are 153-135 Ma. Metamorphic ages from white mica K-Ar and Rb-Sr multi-mineral isochrons from intercalated and coherently deformed metamafic lenses are 133-116 Ma. Lower CMS MDAs (<153 Ma) predominantly postdate the age of other Klamath terranes, but subduction metamorphism appears to predate the earliest coherent Franciscan underplating (ca. 123 Ma). The lower CMS thus occupies a spatial and temporal position between the Klamaths and Franciscan and preserves a non-retrogressed record of the Franciscan Complex’s early history (>123 Ma), otherwise only partially preserved in retrogressed Franciscan high grade blocks.

Our understanding of the impact of melt generation and the interplay between magmatism and mechanical stretching during progressive rifting leading to seafloor spreading remains rudimentary. The Eastern North American Margin (ENAM) provides an excellent location to study the influence of rift magmatism on continental break-up considering the preservation of ~30 Myr of syn-rift strata and voluminous basaltic dikes, sills, and flows. Previous studies mainly focused on magmatism preserved in ENAM rift basins, emphasizing Central Atlantic Magmatic Province (CAMP) activity. Aeromagnetic datasets show pervasive magmatism across the ENAM proximal domain in the form of dikes that largely remain undated. We present in-situ apatite U-Pb geochronology and whole-rock geochemical data from diabase dikes along the ENAM to determine the temporal and chemical evolution of Mesozoic dike emplacement and evaluate whether these magmas were emplaced rapidly at ~201 Ma or in episodic pulses during rifting and break-up. New in-situ apatite U-Pb analyses collectively indicate multiple magmatism pulses along the proximal domain of the ENAM, clustering around ~201 Ma, ~180 Ma, and ~150 Ma. A first pulse at the Triassic/Jurassic boundary is likely due to decompression melting of an enriched mantle, a second smaller pulse in the Early Jurassic potentially correlative to the Blake Spur Magnetic Anomaly and lithospheric breakup, and a third small pulse in the Early Jurassic potentially correlative to the transition to symmetric seafloor spreading. These results indicate prolonged off-axis magmatism is likely due to slow spreading rates driving delocalization of extension away from the rift axis into the proximal domain.

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25 to 40 km by reaction with CO2-rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400-500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites – perhaps facilitated by fluid-rock reaction – in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, perhaps mainly at a serpentinization front.

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20-70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit (CBU) preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution associated with Eocene subduction. Despite decades of research, the pressure-temperature-deformation history (P-T-D), and timing of subduction and exhumation, are matters of ongoing discussion. Here we show that the CBU on Syros comprises three coherent tectonic slices, and each one underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and return flow are distinguished by stretching lineations and ductile fold axis orientations: top-to-the-S (prograde-to-peak subduction), top-to-the-NE (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole chemical zonations record cooling during decompression, indicating return flow along the top of a cold subducting slab. New multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology suggest that three nappes record distinct stages of peak subduction (53-52 Ma, ~50 Ma (?), and 47-45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50-40 Ma and~43-20 Ma, respectively, and also young with structural depth. The datasets support a revised tectonic framework for the CBU, involving subduction of structurally distinct nappes and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of ~1.5-5 mm/yr, and accommodated ~80% of the total exhumation of this HP/LT complex.

Between 81º30’ E and 83ºE the Himalayan range’s “perfect” arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at mid-lower crustal depths plays a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three-dimensional geometry and kinematics of the megathrust at mid-lower crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high-slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at mid-lower crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along-strike variation in Himalayan-plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central-western Himalaya evolve in response to crustal accretion at depth and the three-dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic-wedge growth and that the mid-lower crustal accretion determines the location of plateau edge.

The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program (IODP) provides an erosional record from the Indus River drainage dating back to 10.8 Ma, and with a single sample from 15.5 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1200 Ma and 1500–2300 Ma show increasing preferential erosion of the Himalaya relative to the Karakoram at 7.99–7.78 Ma and more consistently starting by 5.87 Ma. An increase in the contribution of 1500–2300 Ma zircons starting by 1.56 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon U-Pb age populations is consistent with bulk sediment Nd isotope data implies greater zircon fertility in Himalayan bedrock compared to the Karakoram and Transhimalaya. The initial change in spatial erosion patterns at 7.0–5.87 Ma occurred during a time of drying climate in the Indus foreland. The increase in ILH erosion postdates the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the Lesser Himalayan thrust duplex, especially during the Pliocene may also be important. The influence of the Nanga Parbat Massif to the bulk sediment flux is modest, in contrast to the situation in the eastern Himalaya syntaxis.

Slovakia is located within the Central Western Carpathians (CWC), one of many connected curved mountain belts prominent throughout the Mediterranean area and Europe. It is divided into tectonic domains considered “superunits,” termed the Gemeric, Veporic, and Tatric that correlate to the lower, middle, and upper Austoalpine nappes. For example, granite bodies exposed in the unit (termed apophyses) yield a wide range of zircon ages from 310±21 Ma to 87±4 Ma. This range of ages leads to problems in deciphering where the Gemeric unit was located in global plate reconstructions of eastern Europe and the western Carpathians specifically. This case study involves U-Pb dating of magmatic and detrital zircons from the Gemeric tectonic unit. This area records the Variscan orogeny that formed the CWC, rifting, and opening of the Meliata Ocean. This ocean was created due to the formation of a back-arc basin during closing/subduction of the Paleo-Tethys Ocean. We aim to constrain the timing of rifting and identify the provenance of Meliata Ocean radiolarian sediments collected from an obducted Meliata ophiolite suite (Dobsina, Slovakia). The relative age of the Variscan orogeny extends from the late Devonian to early Permian and was followed by rifting throughout the Mesozoic within the CWC. Eventually, the Meliata Ocean closed during the Cretaceous. Zircons from several S-type granites were collected throughout the Gemeric tectonic unit; they were dated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry and imaged using cathodoluminescence. Rim crystallization ages from the granites are 295.8±3.4 Ma (2σ, 238U-206Pb) to 213.1±4.4 Ma. Ages from the detrital zircons are 346.4±4.5 Ma to 263.9±2.7 Ma, indicating that sediments overlying the Meliata Ocean ophiolite contain remnants of both the Variscan orogeny and Gemeric granites.

The thermal structure of the subduction interface changes drastically within the first few million years following subduction initiation (i.e. subduction infancy), resulting in changing metamorphic conditions and degree of mechanical coupling. Metamorphic soles beneath ophiolites record snapshots of subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high-temperature (HT) and lower low-temperature (LT) units. This apparent inverted metamorphic gradient has been attributed to metamorphism under different Pressure-Temperature (P-T) conditions along the interface. However, peak P-T and timing of LT sole subduction are poorly constrained. Samples from Oman Drilling Project core BT-1B (104 m of metamorphic rocks) reveal that the LT sole subducted to similar peak P as the HT sole, but experienced ~300˚C lower peak T. Prograde fabrics in meta-sedimentary and meta-mafic rocks record Si-in-phengite values and amphibole chemistries consistent with peak P-T of ~8-12 kbar and ~450-550˚C in the epidote-amphibolite facies. Retrograde fabrics record a transition from near pervasive ductile to localized brittle strain under greenschist facies conditions. Titanite U-Pb ages (two samples) constrain timing of peak LT sole subduction to 95.7 ± 1.1 Ma, which may post-date the HT sole by ~6-8 Myr. In light of previous HT sole thermobarometry and geochronology, these new results support a model of protracted subduction and accretion while the infant subduction zone cooled at rates of ~100˚C/Myr for ~1-5 Myr. Temporal overlap of LT sole metamorphism and ophiolite crust formation suggests that underthrusting and cooling may lead to interface weakening, facilitating upper plate extension and forearc spreading.

The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program (IODP) provides an erosional record from the Indus River drainage dating back to 10.8 Ma, and with a single sample from 15.5 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1200 Ma and 1500–2300 Ma show increasing preferential erosion of the Himalaya relative to the Karakoram at 7.99–7.78 Ma and more consistently starting by 5.87 Ma. An increase in the contribution of 1500–2300 Ma zircons starting by 1.56 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon U-Pb age populations is consistent with bulk sediment Nd isotope data implies greater zircon fertility in Himalayan bedrock compared to the Karakoram and Transhimalaya. The initial change in spatial erosion patterns at 7.0–5.87 Ma occurred during a time of drying climate in the Indus foreland. The increase in ILH erosion postdates the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the Lesser Himalayan thrust duplex, especially during the Pliocene may also be important. The influence of the Nanga Parbat Massif to the bulk sediment flux is modest, in contrast to the situation in the eastern Himalaya syntaxis.

Dob’s Linn (Scotland) is a location that has significantly influenced our understanding of how life evolved over the Ordovician to early Silurian. The current chronostratigraphic boundary between the Ordovician and Silurian periods is a Global Boundary Stratotype Section and Point (GSSP) at Dob’s Linn calibrated to 443.8±1.5 Ma, partly based on biostratigraphic markers, radiometric ages, and statistical modeling. Graptolites are used here as relative dating markers. We dated hundreds of zircon grains extracted from defined metabentonites from six horizons exposed at Dob’s Linn using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Each zircon was imaged using cathodoluminescence, and most show igneous zoning with minimal alteration. Sample locations range from 42 meters above to 5 meters below the recognized GSSP for the Ordovician-Silurian. Samples were responsibly collected and analyzed for paleontology and geochemistry in other work. Overall, many 238U-206Pb zircon ages from the section are significantly younger than expected. The youngest zircon in sample DL7, located 5 meters below the GSSP, yielded a 238U-206Pb age of 402±12 Ma (±2s, 5% disc). Nineteen spots on zircons from this sample are younger than the presently assigned GSSP age, including more concordant results of 426±8 Ma (0.8% disc) and 435±5 Ma (0.2% disc). The youngest zircon in sample 19DL12, < 1 m below the GSSP, is 377±8 Ma (2% disc) with a more concordant age of 443±7 Ma (0.6% disc). A sample located directly on the GSSP (19DL09) yields 327±5 Ma (0.8% disc). Eight spots on zircons from this sample are also younger than the presently assigned GSSP age. We also dated two samples (DL24 and BRS23) 8 meters above the GSSP, and the youngest, most concordant zircon ages in these samples are 400±11 Ma (5% disc) and 421±9 Ma (0.4% disc), respectively. Overall, the U-Pb ages would re-assign the Dob’s Linn chronostratigraphic section to Silurian-Devonian. The young age results could be attributed to Pb loss due to hydrothermal alteration during the Acadian and Alleghenian orogenies. Future work will implement Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) to obtain accurate U-Pb dating and evaluate the potential effects of Pb loss.