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.

Intraplate depth-dependent lithospheric stretching imaged by seismic reflection data

Geological and geophysical data coupled with numerical simulations have shown that lithospheric extension at passive margins may be classified into three end-member scenarios of pure shear, simple shear, and depth-dependent deformation. However, how lithospheric extension evolves in an intraplate setting remains enigmatic due to lack of reliable constraints on the deep lithospheric architecture. Here we use a seismic reflection profile across the ~800-km-wide Cretaceous intraplate extensional system of South China to illustrate depth-dependent kinematic decoupling of extension in a mechanically stratified lithosphere. The extension was initially distributed in magma-poor conditions as expressed by normal faulting in the upper crust and lower-crustal flow toward the rift axis. Necking of the crust and Moho uplift led to mantle shear-zone formation, lower-crustal flow toward the rift flanks, and deep mantle flow. We demonstrate that the extensional modes vary with decreasing mantle strength from magma-poor to magma-rich domains, as reflected in decreasing crust-mantle decoupling with increased Moho temperatures (TM), and the replacement of a two-layer (brittle vs ductile) mantle by a fully ductile mantle. These findings reveal a first-order lithospheric configuration of intraplate depth-dependent extension driven by far-field stresses attributable to slab retreat.

Interplay of near surface rift evolution and deep-seated lower crustal flow: New findings from fully quantified crustal-scale analogue models

<p>Here we present new results and findings from an analogue modelling series using an extension gradient to simulate continental rifting in rotational settings. We study the effect of a pressure-gradient driven, rift-axis parallel lower crustal flow on rift propagation. The dynamically scaled two-layer models represent a brittle upper and a ductile lower crust. To simulate different crustal set-ups, we use variable ductile/brittle ratios R<sub>DB</sub>, where increasing values indicate a hotter crust with the brittle-ductile transition at relatively shallower depth. An additional package of sand on one part of the model simulates tectonic loading to provoke a pressure-gradient driven lower crustal flow.</p><p>Several factors play a role in dynamic rift propagation such as extension rates, fault evolution and the interplay of vertical motions at the surface as well as model-internal rift-axis parallel horizontal flow. We combine surface and internal deformation analysis using stereoscopic Digital Image Correlation and Digital Volume Correlation applied on surface stereo images and XRCT images, respectively to obtain the fully quantified model deformation.</p><p>Our results show that rift propagation occurs in two consecutive stages: (i) bidirectional step-wise growth in fault length by linkage and (ii) unidirectional linear fault growth. Strain partitioning of bulk extension causes episodic alternative fault growth on conjugate rift margin faults. Over time, fault activity abandons rift boundary faults and migrates inward creating intra-rift faults. This process occurs segment-wise along the rift axis, where different fault generations are simultaneously active. We quantify increasing lower crustal flow parallel to the rift axis with increasing R<sub>DB</sub> as the result of tectonic loading. In return, such lower crustal flow causes vertical and horizontal motions at the surface expressed by dynamic topography and deformation features.</p><p>These results give insights into deformation processes of rifting and highlight the important role of extension gradients on fault growth and strain partitioning in segmented rotational rift systems. Rift-axis parallel lower crustal flow in rotational rift settings may be of relevance when dealing with restorations of 2D crustal seismic sections across rifts.</p>