Quantifying the growth of continental crust through crustal thickness and zircon Hf-O isotopic signatures: A case study from the southern Central Asian Orogenic Belt

Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ∼14−22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust.

Crustal thickness of the Grenville orogen: A Mesoproterozoic Tibet?

The Grenville Province on the eastern margin of Laurentia is a remnant of a Mesoproterozoic orogenic plateau that comprised the core of the ancient supercontinent Rodinia. As a protracted Himalayan-style orogen, its orogenic history is vital to understanding Mesoproterozoic tectonics and paleoenvironmental evolution. In this study, we compared two geochemical proxies for crustal thickness: whole-rock [La/Yb]N ratios of intermediate-to-felsic rocks and europium anomalies (Eu/Eu*) in detrital zircons. We compiled whole-rock geochemical data from 124 plutons in the Laurentian Grenville Province and collected trace-element and geochronological data from detrital zircons from the Ottawa and St. Lawrence River (Canada) watersheds. Both proxies showed several episodes of crustal thickening and thinning during Grenvillian orogenesis. The thickest crust developed in the Ottawan phase (~60 km at ca. 1080 Ma and ca. 1045 Ma), when the collision culminated, but it was still up to 20 km thinner than modern Tibet. We speculate that a hot crust and several episodes of crustal thinning prevented the Grenville hinterland from forming a high Tibet-like plateau, possibly due to enhanced asthenosphere-lithosphere interactions in response to a warm mantle beneath a long-lived supercontinent, Nuna-Rodinia.

Supplemental Material: Proterozoic–Phanerozoic tectonic evolution of the Qilian Shan and Eastern Kunlun Range, northern Tibet

Table S1: Summary of Geochronology Results of Intrusive rocks in the Qilian Shan; Table S2: Summary of Geochronology Results of Intrusive rocks in the East Kunlun Range; Table S3: LA-ICP-MS results for zircons U-Pb ages of igneous, sandstone, and metamorphic sedimentary samples in this study; Table S4: Geochemistry Data for Plots of age against crustal thickness of the Qilian Shan; Table S5: Geochemistry Data for Plots of age against crustal thickness of the Eastern Kunlun Range.

Supplemental Material: Proterozoic–Phanerozoic tectonic evolution of the Qilian Shan and Eastern Kunlun Range, northern Tibet

Table S1: Summary of Geochronology Results of Intrusive rocks in the Qilian Shan; Table S2: Summary of Geochronology Results of Intrusive rocks in the East Kunlun Range; Table S3: LA-ICP-MS results for zircons U-Pb ages of igneous, sandstone, and metamorphic sedimentary samples in this study; Table S4: Geochemistry Data for Plots of age against crustal thickness of the Qilian Shan; Table S5: Geochemistry Data for Plots of age against crustal thickness of the Eastern Kunlun Range.

Moho Geometry of the Okinawa Trough Based on Gravity Inversion and Its Implications on the Crustal Nature and Tectonic Evolution

The Okinawa Trough (OT) is an incipient back-arc basin, but its crustal nature is still controversial. Gravity inversion along with sediment and lithospheric mantle density modeling are used to map the regional Moho depth and crustal thickness variations of the OT and its adjacent areas. The gravity inversion result shows that the crustal thicknesses are 17–22 km at the northern OT, 11–19 km at the central OT, and 7–19 km at the southern OT. Because of the crust with a thickness larger than 17 km, the slow southward arc movement, and scarce contemporaneous volcanisms, the northern OT should be in the stage of early back-arc extension. All of the moderate crustal thickness, high heat flow, and intense volcanism at the central OT indicate that this region is probably in the transitional stage from the back-arc rifting to the oceanic spreading. A crust that is only 7 km thick, lithosphere strength as low as the mid-ocean ridge, and MORB-similar basalts at the southern OT demonstrate that the southern OT is at the early stage of seafloor spreading.

Supplemental Material: Crustal thickness of the Grenville orogen: A Mesoproterozoic Tibet?

Brief review of Grenville geology, analytical methods, data reduction, and datasets.<br>

Supplemental Material: Crustal thickness of the Grenville orogen: A Mesoproterozoic Tibet?

Brief review of Grenville geology, analytical methods, data reduction, and datasets.<br>