Abstract. Conflicting interpretations of the > 500 km long, east-west-trending Qiangtang metamorphic belt have led to very different and contradicting models for the PermoTriassic tectonic evolution of central Tibet. We define two metamorphic events, one that only affected pre-Ordovician basement rocks and one subduction-related Triassic highpressure metamorphism event. Detailed mapping and structural analysis allowed us to define three main units that were juxtaposed due to collision of the north and south Qiangtang terranes after closure of the Ordovician-Triassic ocean that separated them. The base is formed by the Precambrian-Carboniferous basement, followed by nonmetamorphic ophiolitic mélange containing mafic rocks that range in age from the Ordovician to Middle Triassic. The top of the sequence is formed by strongly deformed sedimentary mélange that contains up to > 10 km size rafts of both unmetamorphosed Permian sediments and highpressure blueschists. We propose that the high-pressure rocks were exhumed from underneath the south Qiangtang terrane in an extensional setting caused by the pull of the northward subducting slab of the Shuanghu-Tethys. Highpressure rocks, sedimentary mélange and margin sediments were thrust on top of the ophiolitic mélange that was scraped off the subducting plate. Both units were subsequently thrust on top of the south Qiantang terrane continental basement. Onset of Late Triassic sedimentation marked the end of the amalgamation of both Qiangtang terranes and the beginning of spreading between Qiantang and north Lhasa to the south, leading to the deposition of thick flysch deposits in the Jurassic.

Quantitative evaluation of earthquake‐induced permeability changes is important for understanding key geological processes, such as advective transport of heat and solute and the generation of elevated fluid pressure. Many studies have independently documented permeability changes in either an aquifer or an aquitard, but the effects of an earthquake on both the aquifer and aquitard of the same aquifer system are still poorly understood. In this study, we use the well water‐level response to earth tides and atmospheric pressure to study the changes in hydraulic properties in an aquifer and an overlying confining layer in Beijing, China, following the 11 March 2011 Tohoku earthquake in Japan. Our results show that both the tidal response amplitude and the phase shift increased and that the phase shift changed from negative to positive after the earthquake. We identified increased permeability in both the aquifer and aquitard by the barometric response function method. The horizontal transmissivity of the aquifer increased by a factor of 6, and the vertical diffusivity of the aquitard doubled.