Multi-stage evolution of deformed peridotites in the Yarlung-Tsangpo Suture Zone: A link to mantle shear initiation at H2O-deficient ultraslow-spreading ridges

Earth-like plate activation, especially fault nucleation at mid-ocean ridges (MOR), commonly needs prerequisite shear zone development. However, initiation of high-temperature mantle deformation beneath MOR are not systematically constrained and poorly understood. As remnants of the Neo-Tethyan oceanic lithosphere formed at ultraslow spreading ridges, the Zhongba ophiolitic mantle in the Yarlung-Tsangpo Suture Zone, southern Tibet, provides an excellent example for deciphering the initial strain localization processes during the dynamic transition of the oceanic mantle. The petrogenesis and rheologic evolution of deformed ultramafic rocks were unveiled through a comprehensive analysis of the petrology, g-eochemistry, mineral water contents, and deformation microstructures in this study. The harzburgites studied are classified into two groups: porphyroclastic and mylonitic harzburgites. These ultramafic rocks are highly refractory and water-depleted (48-107 wt ppm in orthopyroxene), a consequence of multiple melting events. Initially, 10%-18% partial melting occurred in the spinel facies, followed by an additional 2%-8% melting in garnet phases under anhydrous conditions. Late-stage melt-rock interactions resulted oscillation in depletion and the formation of interstitial clinopyroxene with elevated H2O content. The peridotites exhibit a crystallographic preferred orientation transition in minerals, from mantle flow (Ol: A- and E-type; Opx: AC-type) to shear (Ol: D-type; Opx: CA-type), following a progressive stress gradient (5.8-99.5 MPa) during subsolidus cooling (porphyroclasts: 1239-1122 °C; neoblasts: 864-923 °C) under a similarly reducing environment with Δlog(fO2)FMQ ranging from -1.74 to -0.69. Deformation mechanisms switch from dislocation creep to dislocation-accommodated grain-boundary sliding, and finally to diffusion creep as grain size reduction. The inferred stress-temperature evolutionary path and a two-stage model are proposed to elucidate incipient mantle shear behaviors under an H2O-deficient ultraslow-spreading ridge environment. This scenario bridges the gap between sub-ridge mantle flow and oceanic detachment fault nucleation, and provides new insights into lithospheric-scale shear zone development and deep-seated microearthquake seismicity formation at slow-to-ultraslow spreading ridges.