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Abstract Magnetite-apatite (MtAp) ore deposits are broadly distributed within Mesoproterozoic rocks of the New Jersey Highlands (USA); however, the age and origins of the ores, and relationships to other Fe-P-(Ti) ore deposits in the Grenvillian Orogen are presently unknown. We use zircon U-Pb geochronology and isotope geochemistry to (1) constrain the timing of MtAp mineralization in the New Jersey Highlands, (2) evaluate the relationship between the MtAp ores and associated lithologies, (3) develop a magmatic model for ore genesis, and (4) place the New Jersey ores within the context of Grenville tectonics and regional mineralization. Ore-body zircon crystals exhibit complex, magmatic textures that record episodic or prolonged crystallization over an extended duration from ca. 1074 Ma to 905 Ma, suggesting that MtAp mineralization occurred during Ottawan to post-Ottawan orogenesis. Cumulate textures of MtAp ores and a close spatial and temporal association among ores, pegmatitic granitoids (ca. 1058 Ma to ca. 986 Ma regionally), and clinopyroxene syenites (most dates ca. 1097 Ma to ca. 926 Ma at one mine) suggest that MtAp mineralization and intrusive magmatism reflect co-magmatic processes. Zircon O isotope data indicate that the MtAp deposits and associated lithologies formed from crustal melts (δ18O = 7.66‰–9.24‰ for the orthogneiss-hosted deposits and δ18O = 5.40‰–6.29‰ for the amphibolite-hosted deposit). Zircon Hf isotope data are more complex. Some MtAp deposits and associated lithologies record Hf signatures consistent with a crustal melt source (εHfi = 0.6–11.1). Whereas other MtAp deposits have extremely radiogenic signatures (εHfi = 29–542) due to the high modal abundance of apatite and monazite in the ores and, consequently, high Lu/Hf ratios. MtAp mineralization in the New Jersey Highlands is broadly coeval with MtAp mineralization in the Adirondack Highlands of New York (USA), rare earth element (REE) mineralization in the Hudson Highlands of New York, and nelsonite mineralization at Lac à l’Orignal and Lac Mirepoix in Québec (Canada). Collectively, these results indicate that orogen-scale, Fe-P-(Ti) and REE mineralization processes operated during Ottawan to post-Ottawan orogenesis throughout the Grenville orogen. 

Mélange (or block-in-matrix structures) exerts a first-order control on both the mechanical and chemical evolution of subduction megathrusts. However, the timing and mechanisms that form mélanges are variable and debated. Field observations and (micro-) structural analyses from a metasedimentary mélange in the lawsonite blueschist unit of the Catalina Schist (Santa Catalina Island, California, USA) reveal that syn-subduction deformation and fluid-mediated processes led to mélange formation at the plate interface. Deposited as turbidites, early shear occurred parallel to bedding planes (S1 foliation). At near peak subduction conditions, at the base of the subduction seismogenic zone (∼1.0 GPa, 320 °C), the rocks were intensely deformed in recumbent open to tight folds (F2) with axial planar cleavages (S2). Fracturing, fluid flow, and quartz precipitation are preserved as extensional vein mesh networks in fold noses. Continued shearing led to boudinage of these strengthened noses and transformation into strong blocks within the weaker less-veined matrix composed of high-strain fold limbs (S1−2). Microstructures reveal viscous deformation in the high-strain fold limbs occurred by pressure-solution creep of fine-grained quartz ± albite. In contrast, the fold noses and/or blocks contain coarse-grained quartz veins with little evidence of deformation. These rocks record the development of syn-subduction block-in-matrix mélange structures through the interaction of deformation and mineral precipitation; pressure solution weakened fold limbs-turned-matrix and veining strengthened fold noses-turned-blocks. Although mélange structure is often invoked to explain tremor and slow slip, rheological analysis indicates that these metasedimentary rocks can host tectonic creeping but cannot accommodate slow-slip strain rates by the deformation mechanisms preserved in their microstructures. 

Abstract The Himalaya‐Karakoram‐Tibet (HKT) orogen provides an unrivaled opportunity to study the dynamic linkages between deep and surface processes during collisional orogenesis. However, these efforts are hindered by conflicting interpretations on the number and timing of collisional events, and the timing of crustal thickening and associated surface uplift. Here, we resolve this with quantitative paleo‐crustal thickness estimates in the northwestern HKT orogen. We show that: (a) the paleo‐Asian margin had thick crust (50–60 km) at least 65 Ma prior to terminal collision, consistent with a continental arc setting, (b) crustal thickening to 60 km or more occurred at ca. 60–50 Ma in the Kohistan‐Ladakh arc and by 40–25 Ma in the paleo‐Asian margin, indicating a multi‐stage Himalayan collision, and (c) modern crustal thicknesses in the northwestern HKT have been sustained since ca. 40–25 Ma suggesting an orogenic steady‐state in which crustal thickening, crustal flow, and surface uplift have been balanced by erosion. 

Theory suggests the possibility for significant deviations between total pressure (or dynamic pressure) and lithostatic pressure during crustal metamorphism. If such deviations exist, the implications for orogenic reconstruction would be profound. Whether such non-lithostatic pressure conditions during crustal metamorphism are recorded and preserved in the rock record remains unresolved, as direct field evidence for this phenomenon is limited. Here, we investigate the Paleogene Tethyan Himalaya fold-thrust belt in Himachal Pradesh, northwestern India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin stratigraphic section. Field-based kinematic studies demonstrate relatively moderate shortening strain across the Tethyan Himalaya. However, basal Tethyan strata consistently yield elevated pressure-temperature-time (P-T-t) estimates of 7–8 kbar and ~650°C, indicative of deep burial during Himalayan orogeny (ca. 20–45 Ma, 25–30 km depths). These P-T-t conditions can be reconciled by: (1) deep Cenozoic burial along cryptic structures and/or significant flattening of the Tethyan strata; (2) basal Tethyan strata recording metamorphism and deformation related to pre-Himalayan tectonism; or (3) non-lithostatic pressure conditions (i.e., tectonic overpressure). To test these models, we systematically mapped the Tethyan fold-thrust belt along the Pin Valley transect in northwestern India, a classic site for stratigraphic, paleontological, paleoenvironmental, and structural reconstructions. The Pin Valley region provides an opportunity to study a structurally continuous metamorphic field gradient from the near-surface to structural depths between 10–15 km, which should reflect P conditions ≤4 kbar if lithostatic. We integrate a multi-method approach combining detailed geologic mapping with quantitative analytical techniques (e.g., thermometry, finite strain analyses, thermo/geochronology, and thermobarometry) to quantify the magnitude, kinematics, thermal architecture, and timing of regional deformation, metamorphism, and subsequent exhumation. Results show: (1) throw on shortening structures is moderate to low (≤4 km); (2) temperature-depth relationships record a continuous, but regionally elevated, upper-crustal geothermal gradient of ≥40 °C/km, which is inconsistent with deep burial models (≤25 °C/km); (3) minimal flattening of basal Tethyan strata; (4) upper Tethyan strata yield pre-Himalayan low-temperature thermochronology dates, further refuting deep Cenozoic burial; and (5) basal Tethyan P-T-t estimates confirm elevated mid-crustal conditions of ~7 kbar, 630°C at 10–15 km depths during the Cenozoic. Preliminary volume expansion calculations are minimal; therefore, mechanisms involving non-hydrostatic thermodynamics, deviatoric stresses, rock strength contrasts, and tectonic mode switching are being explored.