Supplemental Material: Middle-lower crustal flow in response to the India-Eurasia collision: Structural evidence from the southern Chong Shan belt within the Sundaland block, southeastern Tibetan Plateau

S1: Analytical Methods; Table S1: Summary of Mineral assemblages, microstructures and temperature data; Table S2: Zircon U-Pb LA-ICP-MS data of the granitic rocks from the Chong Shan structural belt.

Supplemental Material: Middle-lower crustal flow in response to the India-Eurasia collision: Structural evidence from the southern Chong Shan belt within the Sundaland block, southeastern Tibetan Plateau

S1: Analytical Methods; Table S1: Summary of Mineral assemblages, microstructures and temperature data; Table S2: Zircon U-Pb LA-ICP-MS data of the granitic rocks from the Chong Shan structural belt.

Utilization of continental transforms in break-up: observations, models, and a potential link to magmatism

Reactivation of continental transform faults (hereafter; transforms) is identified herein as a significant factor in continental break-up, based on a global review of divergent margins and numerical modelling. Divergent margins that have reactivated transforms are characterized by linear and abrupt terminations of thick continental crust. Transforms represent some of the largest structures on Earth, and these megastructures represent major lithospheric weaknesses and are therefore prone to reactivation upon changes in the stress field, which typically occur during plate break-up. The blunt termination of the margins is consistent with observations of very limited pre-breakup lithospheric thinning of such margins. This mode of break-up appears to occur abruptly, and contrasts notably with highly tapered and slowly extended divergent margins. Magma leakage along transforms is well-known worldwide where divergence occurs across such features. This leakage may evolve to dike injections, further reducing the plate strength. We observe that many of the blunt margins we attribute to transform reactivation have been prone to above-normal magmatism and are marked by seaward dipping reflectors underlain by high-velocity lower crustal intrusions. The magmatism may be directly related to the separation of abruptly terminated margins, whereby the large resulting lateral thermal gradients trigger edge-driven convection and melt addition.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5756724

Hafnium isotopes in zircons track the changing sources for the Late Cretaceous magmatism in Eastern Srednogorie, Bulgaria

We present new Hf isotopic data of magmatic zircons from the Eastern Srednogorie zone. The data outline two clear temporal trends: rising initial εHf from the initiation of the magmatism at ~95 Ma to 81 Ma, followed by a rapid decline in the initial εHf in the 81–78 Ma time period. The first trend highlights the increasing participation of mantle melts in the formation of magmatic products in the East Balkan and Strandzha regions, which is likely dictated by the southward retreat of the subducting slab. This trend is also evident in published Hf isotopic data on zircons from Central Srednogorie zone. The second trend of rapidly decreasing initial εHf of zircons is interpreted to reflect increased proportion of lower crustal melts in an intra-arc rift extensional environment (the Yambol-Burgas region) between 81 and 78 Ma; this trend is not observed in the Central Srednogorie zone.

U-Pb Age and Hf Isotope Systematics of Zircon from Eclogite Xenoliths in Devonian Kimberlites: Preliminary Data on the Archaean Roots in the Junction Zone between the Sarmatian and Fennoscandian Segments of the East European Platform

The results of a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating and a Hf isotope study of zircon crystals separated from small eclogite xenoliths found in Devonian kimberlites within the Prypyat horst, Ukraine, have been reported. The studied area is located in the junction zone between the Sarmatian and Fennoscandian segments of the East European Platform. Four laser ablation sites on two zircon grains yielded Paleoproterozoic U–Pb ages between 1954 ± 24 and 1735 ± 54 Ma. In contrast, three of four Hf sites revealed negative εHf values and Paleoarchean to Mesoarchean model ages, excluding the possibility that the eclogite xenoliths represented segments of a juvenile Paleoproterozoic subducted slab or younger mafic melts crystallized in the subcontinental lithospheric mantle. A single laser ablation Hf spot yielded a positive εHf value (+3) and a Paleoproterozoic model age. Two models for eclogite origin can be proposed. The first foresees the extension of the Archean lower-crustal or lithospheric roots beneath the Sarmatia–Fennoscandia junction zone for over 200 km from the nearest known outcrop of Archean rocks in the Ukrainian Shield. The second model is that the Central Belarus Suture Zone represents a rifted-out fragment of the Kola–Karelian craton that was accreted to Sarmatia before the actual collision of these two segments of Baltica.

Late Cretaceous upper-crustal thermal structure of the Sevier hinterland: Implications for the geodynamics of the Nevadaplano

Crustal temperature conditions can strongly influence the evolution of deformation during orogenesis. The Sevier hinterland plateau in Nevada and western Utah (“Nevadaplano”) experienced a Late Cretaceous episode of shallow-crustal metamorphism and granitic magmatism. Here, we investigate the thermal history of the Nevadaplano by measuring peak thermal field gradients attained in the upper 10–20 km of the crust along an east-west transect through nine ranges in eastern Nevada and western Utah, by integrating Raman spectroscopy of carbonaceous material thermometry and published conodont alteration indices with reconstructed cross sections. Thermal field gradients of 29 ± 3 °C/km were obtained in the House and Confusion Ranges in westernmost Utah. The Deep Creek, Schell Creek, and Egan Ranges in easternmost Nevada yielded elevated gradients of 49 ± 7 °C/km, 36 ± 3 °C/km, and 32 ± 6 °C/km, respectively. Moving westward, the White Pine, Butte, Pancake, and Fish Creek Ranges exhibit gradients typically between ~20–30 °C/km. The elevated thermal gradients in easternmost Nevada are interpreted to have been attained during ca. 70–90 Ma granitic magmatism and metamorphism and imply possible partial melting at ~18 km depths. Our data are compatible with published interpretations of Late Cretaceous lithospheric mantle delamination under the Sevier hinterland, which triggered lower-crustal anatexis and the resulting rise of granitic melts. The lack of evidence for structures that could have accommodated deep burial of rocks in the nearby Northern Snake Range metamorphic core complex, combined with thermal gradients from adjacent ranges that are ~1.5–3 times higher than those implied by thermobarometry in the Northern Snake Range, further highlights the debate over possible tectonic overpressure in Cordilleran core complexes. Cross-section retro-deformation defines 73.4 ± 4.6 km (76 ± 8%) of extension across eastern Nevada and 15 km of shortening in the Eastern Nevada fold belt.

Timing and kinematics of the Variscan orogenic cycle at the Moldanubian periphery of the central Bohemian Massif

The high-grade complexes along the northern Moldanubian periphery of the central Bohemian Massif provide an outstanding structural record of all episodes of the Variscan collisional evolution. Kinematics and timing of orogenic processes have been examined by structural and microstructural study of middle and lower crustal rocks combined with xenotime and monazite geochronology. Four distinct tectonic events have been identified in the studied units. A first relict sub-horizontal fabric S1 associated with the HP/HT metamorphism is developed only in the lower crustal rocks and was related to back-arc extension or lower crustal flow in a supra-subduction domain. This fabric was at c. 340 Ma completely reworked to the sub-vertical foliation S2 by the major collisional thickening leading to the lower and middle crust juxtaposition. Thereafter, the extensional collapse of thickened orogenic system caused strong refolding to the HT sub-horizontal fabric at c. 325 Ma. The region was subsequently affected by the NNE–SSW oriented horizontal shortening related to the dextral shearing and clockwise rotation of crustal blocks adjacent to the large scale dextral shear zone, the Elbe Zone. It led to the fragmentation and reorientation of the Moldanubian margin to the current position.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5708800.v1

Plagioclase archives of depleted melts in the oceanic crust

Mid-ocean ridge and ocean island basalts provide vital but incomplete insights into the chemical structure of Earth’s mantle. For example, high-anorthite plagioclase carried by these basalts is generally too primitive and incompatible-element depleted to have crystallized from them. Moreover, erupted basalts rarely preserve the strong isotopic and incompatible-element depletions found in some melt inclusions and mantle residua represented by abyssal peridotites. By integrating experimental observations with published analyses of natural crystals and glasses, we demonstrate that high-anorthite plagioclase is in equilibrium with melts generated by high-degree melting of depleted mantle sources. Although such melts seldom erupt, their imprints on crystal and melt inclusion records nonetheless suggest that high-anorthite plagioclase grows from endmember but essentially unexotic magmas. The widespread occurrence of high-anorthite plagioclase in both oceanic basalts and the oceanic crust hence indicates that depleted melts are pervasive in the upper mantle and lower crust despite rarely reaching the surface. Plagioclase archives therefore imply that depleted melts play much a greater role in lower crustal accretion than typically recognized and that the upper mantle may also be more depleted than previously thought.

Rare allanite in granulitised eclogites constrains timing of eclogite to granulite facies transition in the Bhutan Himalaya

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are determined by linking geochemical, geochronological and microstructural data from metamorphic rock-forming and accessory minerals. Exposures of lower orogenic crust provide important insights into orogenic evolution, but are rare in young continental collision belts such as the Himalaya. In NW Bhutan, eastern Himalaya, a high-grade metamorphic terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. We have collected U-Pb isotope and trace element data from allanite, zircon and garnet from metabasite boudins exposed in the Masang Kang valley in NW Bhutan. Our observations and data suggest that allanite cores record growth under eclogite facies conditions (>17 kbar ~650°C) at ca. 19 Ma, zircon inner rims and garnet cores record growth during decompression under eclogite facies conditions at ca 17-15.5. Ma, and symplectitic allanite rims, garnet rims and zircon outer rims record growth under granulite facies conditions at ~9-6 kbar; >750°C at ca. 15-14.5 Ma. Allanite is generally considered unstable under granulite-facies conditions and we think that this is the first recorded example of such preservation, likely facilitated by rapid exhumation. Our new observations and petrochronological data show that the transition from eclogite to granulite facies conditions occurred within 4-5 Ma in the Eastern Himalaya. Our data indicate that the exhumation of lower crustal rocks across the Himalaya was diachronous and may have been facilitated by different tectonic mechanisms.