No abstract

The northeast Cathaysia area is characterized by an archetypical, transpressional system with widespread strike‐slip shear zones whose geometries, kinematics, and ages are critical for deciphering the Phanerozoic tectonic evolution of South China. We present new structural, geochronological and thermochronological data from the shear zones in the east Wuyishan and Chencai domains, which record two phases of deformation. The first phase corresponds to sinistral oblique shearing along arrays of NNE oriented, steep‐dipping zones under amphibolite facies conditions. The sinistral oblique shearing commenced at ~451 Ma, concurrently with regional NW/SE directed thrust shearing and folding; the coexistence of sinistral and thrust structures indicates NW‐SE transpressive shortening deformation. Dating by 40Ar/39Ar shows that such deformation terminated before 400 Ma and was followed by cooling through ~450–350°C at ~400–370 Ma. Our results, merged with published data, aid in tracing an Early Paleozoic orogen that extends through the Jiangnan domain into the northeast Cathaysia, with the southeast Yangtze acting as a foreland belt. The synorogenic shortening was interpreted as resulting from underthrusting of the Cathaysia beneath the east Yangtze. The second phase involved dextral oblique shearing associated with NNE‐SSW transpression under greenschist to amphibolite facies conditions at 245–228 Ma, which was followed by postkinematic magmatism and cooling at ~221–200 Ma. In the Cathaysia, similar Middle Triassic dextral shear zones were widespread and operated with approximately east striking thrusts as mutually complementary structures; their kinematic coupling can be explained by a contractional termination model. Geodynamically, we attributed Middle Triassic dextral transpression to the collisions of South China with North China and Indochina.

Based on field investigations, interpretations of high‐resolution UAV images, and analyses of available InSAR data, we mapped the fault geometry and surface ruptures of the 2021 Mw 7.4 Maduo earthquake that occurred on a low‐activity strike‐slip fault within the Tibetan Plateau. The results indicate that (a) the earthquake activated a fault that is ∼161 km long and has complicated structural geometry; (b) the surface rupture occurs over a distance of 148 km, but is separated into three distinct segments by two large gaps (38 and 20 km, respectively); (c) within the surface‐rupture segments, the horizontal and vertical displacements are typically 0.2–2.6 m (much lower than the InSAR‐based slip maximum of 2–6 m at depth) and ≤0.4 m, respectively. The two large gaps of the Maduo surface rupture represent the two largest surface‐rupture discontinuities of strike‐slip earthquakes ever documented, and coincide with structurally complicated fault portions and near‐surface soft sediments.

Some hydrocarbon reservoirs are trapped beneath salt bodies, where seismic imaging is greatly challenged due to poor illumination. Multiple reflections have different propagation wave paths from primary reflections and thus can be used to complement the illuminations where primary reflections from beneath the salt are not acquired. Consequently, migration of multiples can sometimes provide better subsalt images compared to conventional migration which uses primary reflections only. In this paper, we propose to modify conventional reverse time migration so that multiples can be used as constructive reflection energy for subsalt imaging. This new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data instead of primary reflections. In the reverse time migration process, multiples recorded on the surface are extrapolated backward in time to each depth level, and the observed data with both primaries and multiples are extrapolated forward in time to the same depth levels, followed by a crosscorrelation imaging condition. A numerical test on the Sigsbee2B data set shows that a wider coverage and a more balanced illumination of the subsurface can be achieved by migration of multiples compared with conventional migration of primary reflections. This example demonstrates that reverse time migration of multiples might be a promising method for complex subsalt imaging.