High-pressure rocks from subduction complexes are key records of the physical and chemical processes that operate on the subduction interface, but interpretation of these records requires accurate structural understanding of where they formed in the subduction zone and the mechanisms by which they were exhumed. We present new geologic mapping, outcrop-scale observations, and geochronology from subduction-zone assemblages at Punta Prieta Ridge on Cedros Island, Baja California (Mexico), to investigate the history of subduction, exhumation, and structural assembly of these rocks. The rocks of Punta Prieta Ridge are exposed in the footwalls of high- and low-angle normal faults that carry Cretaceous forearc basin strata and attenuated mantle sections of the Cedros Island Ophiolite in their hangingwalls. The footwall rocks are subduction-zone assemblages organized into distinct nappes that decrease in metamorphic grade and degree of strain structurally downward. Garnet-amphibolite and blueschist blocks within the subduction complex yield 40Ar/39Ar cooling ages between 172 and 144 Ma and are hosted in siliciclastic rocks which yield detrital zircon maximum depositional ages between 92 and 72 Ma. Based on field evidence and age relationships, we interpret the occurrence of older higher-grade blocks in younger clastic matrix to be depositional rather than the result of tectonic mixing. We then present a model for their exhumation and assembly via multiple cycles of extensional unroofing of the subduction complex and sedimentary recycling of high-pressure rocks back into the subduction trench. We conclude by comparing the record of subduction on Cedros Island to other parts of the Cordillera.

High-elevation, low-relief continental plateaus are major topographic features and profoundly influence atmospheric circulation, sediment transport and storage, and biodiversity. Although orogenic surface-uplift mechanisms for modern continental plateaus near known plate margins like Tibet are well-characterized, they cannot account for examples in intracontinental settings like the Colorado Plateau. In contrast to canonical plate-tectonic uplift mechanisms, broad-scale hydration-induced metasomatism of the lower crust has been suggested to reduce its density and increase its buoyancy sufficiently to contribute to isostatic uplift. However, the relationships between key petrophysical properties in these environments are not fully quantified, which limits application of this model. Here, we develop a series of petrological models that describe the petrological and topographic effects of fluid-rock interaction in non-deforming continental crust of varying composition. We apply an open-system petrological modelling framework that utilizes reactive-transport calculations to determine the spatial and temporal scales over which mineralogic transformations take place compared with the magnitude of infiltration of aqueous fluids derived from devolatilization of subducting oceanic lithosphere. The buoyancy effect of hydration-induced de-densification is most significant for metabasic lower crust, intermediate for metapelitic crust, and minimal for granodioritic crust. We apply these results to a case study of the similar to 2 km-high Colorado Plateau and demonstrate that under ideal conditions, hydration of its lower-middle crust by infiltrating aqueous fluids released by the Farallon slab during Cenozoic low-angle subduction could have uplifted the plateau surface by a maximum of similar to 1 km over 16 Myr. However, realistically, although hydration likely has a measurable effect on surface tectonics, the uplift of orogenic plateaus is likely dominantly controlled by other factors, such as lithospheric delamination.

Abstract Fe‐ and Mn‐oxides are common secondary minerals in faults, fractures, and veins and potentially record information about the timing of fluid movement through their host rocks. These phases are difficult to date by most radioisotopic techniques, but relatively high concentrations of U and Th make the (U‐Th)/He system a promising approach. We present new petrographic, geochronologic and thermochronologic analyses of secondary oxides and associated minerals from fault zones and fractures in southeastern Arizona. We use these phases in attempt to constrain the timing of fluid flow and their relationship to magmatic, tectonic, or other regional processes. In the shallowly exhumed Galiuro Mountains, Fe‐oxide (U‐Th)/He dates correspond to host‐rock crystallization and magmatic intrusions from ca. 1.6 to 1.1 Ga. Step‐heating 4He/3He experiments and polydomain diffusion modeling of 3He release spectra on these samples are consistent with a crystallite size control on He diffusivity, and little fractional loss of radiogenic He since formation in coarse‐grained hematite, but large losses from fine‐grained Mn‐oxide. In contrast to Proterozoic dates, Fe‐ and Mn‐oxides from the Catalina‐Rincon and Pinaleño metamorphic core complexes are exclusively Cenozoic, with dates clustering at ca. 24, 15, and 9 Ma, which represent distinct cooling or fluid‐flow episodes during punctuated periods of normal faulting. Finally, a subset of Fe‐oxides yield dates of ca. 5 Ma to 6 ka and display either pseudomorphic cubic forms consistent with oxidative retrogression of original pyrite or magnetite, or fine‐grained botryoidal morphologies that we interpret to represent approximate ages of recrystallization or pseudomorphic replacement at shallow depths.

Neogene, syn-collisional extensional exhumation of Asian lower-middle crust produced the Shakhdara-Alichur gneiss-dome complex in the South Pamir. The <1 km-thick, mylonitic-brittle, top-NNE, normal-sense Alichur shear zone (ASZ) bounds the 125 x 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 km-thick, mylonitic-brittle, top-SSE South Pamir normal-sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22-17 Ma Alichur-dome-injection-complex leucogranites transition from foliation-parallel, centimeter- to meter-thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization-temperature constraints suggest Alichur-dome exhumation from 10-15 km depth. Thermochronologic dates bracket footwall cooling between 410-130 degrees C from 16-4 Ma; tectonic cooling/exhumation rates (42 degrees C/Myr, 1.1 km/Myr) contrast with erosion-dominated rates in the hanging wall (2 degrees C/Myr, <0.1 km/Myr). Dome-scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E-W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between 20-4 Ma to: (i) Mostly northward, foreland-directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non-underthrust Central Pamir.

Investigation of a >6‐km‐thick succession of Cretaceous to Cenozoic sedimentary rocks in the Tajik Basin reveals that this depocentre consists of three stacked basin systems that are interpreted to reflect different mechanisms of subsidence associated with tectonics in the Pamir Mountains: a Lower to mid‐Cretaceous succession, an Upper Cretaceous–Lower Eocene succession and an Eocene–Neogene succession. The Lower to mid‐Cretaceous succession consists of fluvial deposits that were primarily derived from the Triassic Karakul–Mazar subduction–accretion complex in the northern Pamir. This succession is characterized by a convex‐up (accelerating) subsidence curve, thickens towards the Pamir and is interpreted as a retroarc foreland basin system associated with northward subduction of Tethyan oceanic lithosphere. The Upper Cretaceous to early Eocene succession consists of fine‐grained, marginal marine and sabkha deposits. The succession is characterized by a concave‐up subsidence curve. Regionally extensive limestone beds in the succession are consistent with late stage thermal relaxation and relative sea‐level rise following lithospheric extension, potentially in response to Tethyan slab rollback/foundering. The Upper Cretaceous–early Eocene succession is capped by a middle Eocene to early Oligocene (ca. 50–30 Ma) disconformity, which is interpreted to record the passage of a flexural forebulge. The disconformity is represented by a depositional hiatus, which is 10–30 Myr younger than estimates for the initiation of India–Asia collision and overlaps in age with the start of prograde metamorphism recorded in the Pamir gneiss domes. Overlying the disconformity, a >4‐km‐thick upper Eocene–Neogene succession displays a classic, coarsening upward unroofing sequence characterized by accelerating subsidence, which is interpreted as a retro‐foreland basin associated with crustal thickening of the Pamir during India–Asia collision. Thus, the Tajik Basin provides an example of a long‐lived composite basin in a retrowedge position that displays a sensitivity to plate margin processes. Subsidence, sediment accumulation and basin‐forming mechanisms are influenced by subduction dynamics, including periods of slab‐shallowing and retreat.

The timing of closure of the Paleotethys and Rushan ocean basins and suturing of Gondwanan crustal fragments in the Pamir is not well resolved. Whereas the Central Pamir terrane is generally interpreted to have collided with the Northern Pamir terrane at the end of the Triassic, closure of the Rushan ocean and collision of the Southern Pamir terrane has been interpreted to be either broadly coeval (i.e., Late Triassic–Early Jurassic ~200 Ma) or have occurred significantly after closure of the Paleotethys in the Late Jurassic. New petrographic analyses and detrital zircon U-Pb data of the Lokzun Group Flysch and the terrigenous Darbasatash Group of the Southern Pamir terrane advocate for coeval terrane accretion of the Central and Southern Pamir terranes by the Latest Triassic. The Rhaetian Lokzun Group Flysch and the Early Jurassic Darbasatash Group have well-constrained stratigraphic ages and lie directly below and above a post-Cimmerian orogeny angular unconformity, respectively. Petrographic results for both successions indicate a recycled orogenic belt source, and detrital zircon age spectrum for both contain two prominent Phanerozoic age populations: an Early Silurian–Early Devonian (~440–408 Ma) and a Carboniferous Late Triassic (~350–222 Ma). These results suggest sediment was sourced from the Karakul-Mazar terrane, located in the northern Pamir, which requires closure of the Paleotethys and Rushan ocean basins and initial collision with the Central and Southern Pamir terranes by the end of the Late Triassic. Furthermore, these results suggest that the Northern Pamir terrane was topographically elevated relative to the surrounding terranes throughout the early Mesozoic.

We present a new interpretation of the Bashgumbaz Complex (BSC), a fragment of a large mafic-ultramafic nappe exposed along the Alichur Valley in the Pamir, whose significance is of paramount importance for the understanding of the Cimmerian orogeny in South Pamir. The BSC is exposed along the contact between the SE and SW Pamir and consists of a low-grade metamorphic association dominated by gabbros and serpentinized harzburgites with minor bodies of quartzdiorite and plagiogranite tectonically coupled to basaltic and rhyolitic volcanic products. Tectonic slices of metasedimentary rocks are interleaved within the complex. The BSC is in contact with a sedimentary unit rich in olistostostromes with exotic blocks containing a Lower Permian fauna showing affinity with Central Pamir. The BSC is intruded by a Lower Cretaceous granitic body with a U-Pb zircon age of 117 Ma, crosscutting the whole complex. Petrographic, mineralogical and geochemical data suggest a supra-subduction-zone affinity for the gabbros and diorites. Significant enrichment in LILE and LREE compared to HREE, coupled with negative anomalies of Nb, Ti, Zr and other HFSE support this interpretation. U-Th-Pb dating of zircon from a diorite provides a lower Norian (ca. 222 Ma) youngest-crystallization-age constraint, suggesting that the BSC formed in a supra-subduction setting on the South Pamir terrane prior to the collision between Central and South Pamir blocks. It was later underthrust and then obducted onto the southern margin of the closing Rushan-Pshart Ocean, which separated these two Gondwanan terranes during the Permian–Triassic. The obduction of the Bashgumbaz ophiolites can be considered as a time constraint for the collision between the South and Central Pamir terranes, which occurred within the framework of the Cimmerian orogeny that shaped the southern margin of Eurasia.

We present a new interpretation of the Bashgumbaz Complex (BSC), a fragment of a large mafic-ultramafic nappe exposed along the Alichur Valley in the Pamir, whose significance is of paramount importance for the understanding of the Cimmerian orogeny in South Pamir. The BSC is exposed along the contact between the SE and SW Pamir and consists of a low-grade metamorphic association dominated by gabbros and serpentinized harzburgites with minor bodies of quartzdiorite and plagiogranite tectonically coupled to basaltic and rhyolitic volcanic products. Tectonic slices of metasedimentary rocks are interleaved within the complex. The BSC is in contact with a sedimentary unit rich in olistostostromes with exotic blocks containing a Lower Permian fauna showing affinity with Central Pamir. The BSC is intruded by a Lower Cretaceous granitic body with a U-Pb zircon age of 117Ma, crosscutting the whole complex. Petrographic, mineralogical and geochemical data suggest a supra-subduction-zone affinity for the gabbros and diorites. Significant enrichment in LILE and LREE compared to HREE, coupled with negative anomalies of Nb, Ti, Zr and other HFSE support this interpretation. U-Th-Pb dating of zircon from a diorite provides a lower Norian (ca. 222Ma) youngest-crystallization-age constraint, suggesting that the BSC formed in a supra-subduction setting on the South Pamir terrane prior to the collision between Central and South Pamir blocks. It was later underthrust and then obducted onto the southern margin of the closing Rushan-Pshart Ocean, which separated these two Gondwanan terranes during the Permian–Triassic. The obduction of the Bashgumbaz ophiolites can be considered as a time constraint for the collision between the South and Central Pamir terranes, which occurred within the framework of the Cimmerian orogeny that shaped the southern margin of Eurasia.

Despite Miocene extension and exhumation of middle to lower crust in a series of gneiss domes and interpreted Cenozoic delamination of the lower crust, the crust in the modern Pamir Mountains is among the thickest in the world. Cenozoic shortening, crustal thickening, and prograde metamorphism in the Pamir have been associated with India-Asia collision. However, new mapping in the South Pamir terrane indicates relatively minor, distributed shortening since the Jurassic, which occurs in a thrust belt overprinted by late Cenozoic transpression. The thrust belt connects with the Rushan-Pshart suture zone, a Mesozoic terrane boundary. New detrital zircon U-Pb and detrital zircon fission track ages of synorogemc clastic rocks exposed in the footwall of thrust faults in the South Pamir thrust belt provide maximum deposition ages (76-112 Ma), which are interpreted to document Cretaceous shortening prior to India-Asia collision. Furthermore, zircon (U-Th)/He and apatite (U-Th)/He data from the South Pamir terrane generally record cooling ages of ca. 102-44 Ma, suggesting limited Cenozoic exhumation. These results (1) are consistent with widespread Cretaceous deformation throughout the Pamir-Tibet orogen with limited Cenozoic upper crustal shortening in the South Pamir terrane, (2) together with previous studies, allow for the possibility that the upper crust of the Pamir orogen was characterized by net extension during the Cenozoic rather than net shortening, and (3) are consistent with models that relate Cenozoic crustal thickening to the insertion of Indian lower crust beneath the Pamir. Lower crustal thickening of the South Pamir terrane is difficult to reconcile with the prograde metamorphic history of gneiss domes in the South Pamir terrane and may require a relatively shallow (<15-20 km) shear zone separating lower crustal contraction from upper crustal extension.

The amalgamation of the Central Asian Orogenic Belt in the southwestern Tian Shan in Tajikistan is represented by tectono‐magmatic‐metamorphic processes that accompanied late Paleozoic ocean closure and collision between the Karakum‐Tarim and Kazakh‐Kyrgyz terranes. Integrated U‐Pb geochronology, thermobarometry, pseudosection modeling, and Hf geochemistry constrain the timing and petro‐tectonic nature of these processes. The Gissar batholith and the Garm massif represent an eastward, along‐strike increase in paleodepth from upper‐batholith (~21–7 km) to arc‐root (~36–19 km) levels of the Andean–syn‐collisional Gissar arc, which developed from ~323–288 Ma in two stages: (i) Andean, I‐type granitoid magmatism from ~323–306 Ma due to northward subduction of the Gissar back‐arc ocean basin under the Gissar microcontinent, which was immediately followed by (ii) syn‐collisional, I‐S‐type granitoid magmatism in the Gissar batholith and the Garm massif from ~304–288 Ma due to northward subduction/underthrusting of Karakum marginal‐continental crust under the Gissar microcontinent. A rapid isotopic pull‐up from ~288–286 Ma signals the onset of juvenile, alkaline‐syenitic, post‐collisional magmatism by ~280 Ma, which was driven by delamination of the Gissar arclogite root and consequent convective asthenospheric upwelling. Whereas M–HT/LP prograde metamorphism in the Garm massif (650–750°C/6–7 kbar) from ~310–288 Ma was associated with subduction‐magma inundation and crustal thickening, HT/LP heating and decompression to peak‐metamorphic temperatures (~800–820°C/6–4 kbar) at ~288 ± 6 Ma was driven by the transmission of a post‐collisional, mantle‐derived heat wave through the Garm‐massif crust.

New geochronologic, geochemical, and isotopic data for Mesozoic to Cenozoic igneous rocks and detrital minerals from the Pamir Mountains help to distinguish major regional magmatic episodes and constrain the tectonic evolution of the Pamir orogenic system. After final accretion of the Central and South Pamir terranes during the Late Triassic to Early Jurassic, the P amir was largely amagmatic until the emplacement of the intermediate (SiO2 > 60 wt.%), talc-alkaline, and isotopically evolved (-13 to -5 zircon epsilon Hf-(t)) South Pamir batholith between 120-100 Ma, which is the most volumetrically significant magmatic complex in the Pamir and includes a high flux magmatic event at similar to 105 Ma. The South Pamir batholith is interpreted as the northern (inboard) equivalent of the Cretaceous Karakoram batholith and the along-strike equivalent of an Early Cretaceous magmatic belt in the northern Lhasa terrane in Tibet. The northern Lhasa terrane is characterized by a similar high-flux event at similar to 110 Ma. Migration of continental arc magmatism into the South Pamir terrane during the mid-Cretaceous is interpreted to reflect northward directed, low-angle to flat-slab subduction of the Neo-Tethyan oceanic lithosphere. Late Cretaceous magmatism (80-70 Ma) in the Pamir is scarce, but concentrated in the Central and northern South Pamir terranes where it is comparatively more mafic (SiO2 < 60 wt.%), alkaline, and isotopically juvenile (-2 to +2 zircon epsilon Hf-(t)) than the South Pamir batholith. Late Cretaceous magmatism in the Pamir is interpreted here to be the result of extension associated with roll-back of the Neotethyan oceanic slab, which is consistent with similarly aged extension-related magmatism in the Karakoram terrane and Kohistan. There is an additional pulse of magmatism in the Pamir at 42-36 Ma that is geographically restricted (similar to 150 km diameter ellipsoidal area) and referred to as the Vanj magmatic complex. The Vanj complex comprises metaluminous, high-K talc-alkaline to shoshonitic monzonite, syenite, and granite that is adakitic (La/Yb-N = 13 to 57) with low Mg# (35-41). The Vanj complex displays a range of SiO2 (54-75 wt.%) and isotopic compositions (-7 to -3 epsilon Nd-(i), 0.706 to 0.710 Sr-87/Sr-86((i)), -3 to +1 zircon epsilon Hf-(i), 6.0 to 7.6%o zircon delta O-18(VSMOW)), which reflects some juvenile mantle input and subsequent assimilation or mixing with the Central/South Pamir terrane lower crust. The Vanj complex is speculatively interpreted to be the consequence of a mantle drip or small delamination event that was induced by India-Asia collision. The age, geochemistry, outcrop pattern, and tectonic position of the Vanj magmatic complex suggest that it is part of a series of magmatic complexes that extend for >2500 km across the Pamir and northern Qiangtang terrane in Tibet. All of these complexes are located directly south of the Tanymas-Jinsha suture zone, an important lithospheric and rheological boundary that focused mantle lithosphere deformation after India-Asia collision. Miocene magmatism (20-10 Ma) in the Pamir includes: 1) isotopically evolved migmatite and leucogranite related to crustal anataxis and decompression melting within extensional gneiss domes, and; 2) localized intra-continental magmatism in the Dunkeldik/Taxkorgan complex. (C) 2017 Elsevier B.V. All rights reserved.

A regional, balanced cross section is presented for the thin-skinned Tajik fold-and-thrust belt, constrained by new structural and stratigraphic data, industrial well-log data, flexural modeling, and existing geologic and geophysical mapping. A sequential restoration of the section was calibrated with 15 new apatite (U-Th)/He ages and 7 new apatite fission-track ages from samples of the major thrust sheets within the Tajik fold-and-thrust belt. Thermokinematic modeling indicates that deformation in the Tajik fold-and-thrust belt began during the Miocene (prior to or ca. 17 Ma) and continues to near present, with long-term shortening rates of similar to 4-6 mm/yr and Pliocene to present rates of similar to 6-8 mm/yr. The Tajik fold-and-thrust belt can be characterized as two distinct, oppositely verging thrust belts. Deformation initiated at opposite margins of the Tajik foreland basin, adjacent the southwest Tian Shan and northwest Pamir Mountains, and propagated toward the center of the basin, eventually incorporating the foreland basin entirely into a composite fold-and-thrust belt. The western Tajik fold-and-thrust belt records at least 35-40 km of total shortening and is part of the greater Tian Shan orogenic system. The eastern Tajik fold-and-thrust belt records similar to 30 km of shortening linked to the Pamir Mountains. The amount of shortening in the Tajik fold-and-thrust belt is significantly less than predicted by models of intracontinental subduction, which call for subduction of an similar to 300-km-long slab of continental Tajik-Tarim lithosphere beneath the Pamir. Field observations and structural relationships suggest that the Mesozoic and younger sedimentary rocks of the Tajik Basin were deposited on and across the Northern Pamir terrane and then subsequently uplifted and eroded during orogenic growth, rather than undergoing subduction beneath the Pamir. The Paleozoic-Proterozoic(?) metasedimentary and igneous rocks exposed in the Northern Pamir terrane are equivalent to the middle-lower crust of the Tajik Basin, which has become incorporated into the Pamir orogen. We propose that the southdipping zone of deep seismicity beneath the Pamir, which is the basis for the intra-continental subduction model, is related to gravitational foundering (by delamination or large-scale dripping) of Pamir lower crust and mantle lithosphere. This contrasts with previous models that related the Pamir seismic zone to subduction with or without roll-back of Asian lithosphere. Delamination may explain the initiation of extension in the Pamir gneiss domes and does not require a change in plate boundary forces to switch between compressional and extensional regimes. Because the Pamir is the archetype for active subduction of continental lithosphere in the interior of continental plates (intracontinental subduction), the viability of this particular tectonic process may need to be reassessed.