We present global and regional correlations between whole-rock values of Sr/Y and La/Yb and crustal thickness for intermediate rocks from modern subduction-related magmatic arcs formed around the Pacific. These correlations bolster earlier ideas that various geochemical parameters can be used to track changes of crustal thickness through time in ancient subduction systems. Inferred crustal thicknesses using our proposed empirical fits are consistent with independent geologic constraints for the Cenozoic evolution of the central Andes, as well as various Mesozoic magmatic arc segments currently exposed in the Coast Mountains, British Columbia, and the Sierra Nevada and Mojave-Transverse Range regions of California. We propose that these geochemical parameters can be used, when averaged over the typical lifetimes and spatial footprints of composite volcanoes and their intrusive equivalents to infer crustal thickness changes over time in ancient orogens.

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Global compilations indicate that the geochemistry of arc magmatism is sensitive to Moho depth. Magmatic products are prevalent throughout the history of Cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes. We investigate temporal variations in crustal thickness in the United States Cordillera using Sr/Y from intermediate continental arc magmas. Our results suggest that crustal thickening began during the Late Jurassic to Early Cretaceous and culminated with 55-65-km-thick crust at 85-95 Ma. Crustal thicknesses remained elevated until the mid-Eocene to Oligocene, after which time crustal thicknesses decreased to 30-40 km in the Miocene. The results are consistent with independent geologic constraints and suggest that Sr/Y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems. 1 GSA Data Repository item 2015308, Figure DR1 (unfiltered Great Basin rock analyses with proposed data filters); Table DR1 (Great Basin geochemical data); Table DR2 (accepted Great Basin data subsets by area); Table DR3 (discarded Great Basin data subsets by area); Table DR4 (global geochemical data for Quaternary rock analyses); and Table DR5 (compiled global geochemical data by arc),

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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 Pamir was largely amagmatic until the emplacement of the intermediate (SiO 2 > 60 wt. %), calcalkaline, and isotopically evolved (-13 to-5 zircon εHf (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 ~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 ~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 (SiO 2 < 60 wt. %), alkaline, and isotopically juvenile (-2 to +2 zircon εHf (t)) than the South Pamir batholith. Late Cretaceous magmatism in the Pamir is interpreted here to be the result of extension *Manuscript Click here to view linked References 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. Bangong suture zone (Fig. 1) (Yin and Harrison, 2000). The Qiangtang terrane is laterally equivalent to (from north to south) the Central Pamir terrane, the South Pamir terrane, and the Karakoram terrane, whereas there is no direct equivalent of the Lhasa terrane in the Pamir (Fig. 1 and 2) (Robinson et al., 2012). The Central Pamir terrane was accreted to the Triassic Karakul-Mazar arc-accretionary complex along the Tanymas suture (Fig. 2) (Burtman and Molnar, 1993) and the Qiangtang terrane was accreted to the Triassic Songpan-Ganzi turbidite complex along the Jinsha suture in Tibet during Late Triassic-Early Jurassic time (Yin and Harrison, 2000). The Karakul-Mazar complex in the Pamir consists of relatively undeformed Late Triassic intermediate intrusive rocks that were emplaced into a Triassic accretionary complex (Schwab et al., 2004; Robinson et al., 2012). The Karakul-Mazar magmatic rocks are believed to have originated above a north-dipping subduction zone (Schwab et al...

A regional, balanced cross-section is presented for the thinskinned Tajik fold and thrust belt (TFTB), 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 is 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 TFTB. Thermokinematic modeling indicates that deformation in the TFTB began during the Miocene (≥ ~17 Ma) and continues to the near present with long-term shortening rates of ~4 to 6 mm/yr and Pliocene to present rates of ~6 to 8 mm/yr. The TFTB 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 it entirely into a composite fold-thrust belt. The western TFTB records at least 35-40 km of total shortening and is part of the greater Tian Shan orogenic system. The eastern TFTB records ~30 km of shortening that is linked to the Pamir Mountains. The amount of shortening in the TFTB is significantly less than predicted by models of intracontinental subduction that call for subduction of an ~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 subducted beneath the Pamir. The Paleozoic-Proterozoic (?) meta-sedimentary 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 south-dipping zone of deep seismicity beneath the Pamir, which is the basis for the intracontinental 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 processes may need to be reassessed.

We present compiled geochemical data of young (mostly Pliocene-present) intermediate magmatic rocks from continental collisional belts and correlations between their whole-rock Sr/Y and La/Yb ratios and modern crustal thickness. These correlations, which are similar to those obtained from subduction-related magmatic arcs, confirm that geochemistry can be used to track changes of crustal thickness changes in ancient collisional belts. Using these results, we investigate temporal variations of crustal thickness in the Qinling Orogenic Belt in mainland China. Our results suggest that crustal thickness remained constant in the North Qinling Belt (~45–55 km) during the Triassic to Jurassic but fluctuates in the South Qinling Belt, corresponding to independently determined tectonic changes. In the South Qinling Belt, crustal thickening began at ~240 Ma and culminated with 60–70-km-thick crust at ~215 Ma. Then crustal thickness decreased to ~45 km at ~200 Ma and remained the same to the present. We propose that coupled use of Sr/Y and La/Yb is a feasible method for reconstructing crustal thickness through time in continental collisional belts. The combination of the empirical relationship in this study with that from subduction-related arcs can provide the crustal thickness evolution of an orogen from oceanic subduction to continental collision.

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. K E Y W O R D S basin subsidence, foreland basins, geodynamics, stratigraphy, subduction-related basins, tectonics and sedimentation 526 | EAGE CHAPMAN et Al.