An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

The Longmenshan Thrust Belt in Eastern Tibet resulted from a Mesozoic orogeny and Cenozoic reworking. It is generally believed that the Cenozoic tectonics along the Longmenshan Thrust Belt are mostly inherited from the Mesozoic. Reconstructing the Mesozoic tectonics of the Longmenshan Thrust Belt is therefore important for understanding its evolutionary history. On the basis of detailed structural analysis, we recognized a Main Central Boundary that divides the Longmenshan Thrust Belt into a Southeastern Zone and a Northwestern Zone. Both zones underwent a main D1 event characterized by D1E top-to-the-SE thrusting in the Southeastern Zone and D1W top-to-the-NW/N thrusting in the Northwestern Zone. In the Southeastern Zone, a D2 top-to-the-NW/N normal faulting that cuts the D1E structures is developed along the NW boundary of the basement complexes. Newly obtained and previous geochronological data indicate that the D1E and D1W events occurred synchronously at ca. 224−219 Ma, and the D2 top-to-the-NW/N normal faulting was episodically activated at ca. 166−160 Ma, 141−120 Ma, 81−47 Ma, and 27−25 Ma. Episodic and synchronously activated top-to-the-NW normal faulting and top-to-the-SE thrusting along the northwestern and southeastern boundaries of the basement complexes, respectively, leads us to propose that the basement slices were episodically imbricated to the SE during the Late Jurassic−Early Cretaceous and Late Cretaceous−earliest Paleocene. The D1 amphibolite facies metamorphic rocks above the basement complexes recorded fast exhumation during the Late Jurassic−Early Cretaceous. We propose that the early Mesozoic northwestward basement underthrusting along a crustal “weak zone” was responsible for the D1 double-vergent thrusting and amphibolite facies metamorphism. Subsequent basement-slice imbrications reworked the Longmenshan Thrust Belt and exhumed the amphibolite facies rocks. Our results highlight the importance of basement underthrusting and imbrication in the formation and reworking of the intracontinental Longmenshan Thrust Belt in Eastern Tibet.

Supplemental Material: An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

Three figures (Figs. S1–S3) and two tables (Tables S1–S2). Figure S1: Images and related Energy Dispersive X-ray spectra to show that quartz melts infilled in feldspar crack; Figure S2: Features of samples selected for 40Ar-39Ar dating; Figure S3: Inverse isochron lines that yield false initial 40Ar/39Ar ratio; Table S1: Detailed SIMS U-Pb zircon results of sample CX48; Table S2: Detailed 40Ar/39Ar VG3600 Furnace Step-Heating Analytical Results.

Supplemental Material: An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

Three figures (Figs. S1–S3) and two tables (Tables S1–S2). Figure S1: Images and related Energy Dispersive X-ray spectra to show that quartz melts infilled in feldspar crack; Figure S2: Features of samples selected for 40Ar-39Ar dating; Figure S3: Inverse isochron lines that yield false initial 40Ar/39Ar ratio; Table S1: Detailed SIMS U-Pb zircon results of sample CX48; Table S2: Detailed 40Ar/39Ar VG3600 Furnace Step-Heating Analytical Results.

Supplemental Material: An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

Three figures (Figs. S1–S3) and two tables (Tables S1–S2). Figure S1: Images and related Energy Dispersive X-ray spectra to show that quartz melts infilled in feldspar crack; Figure S2: Features of samples selected for 40Ar-39Ar dating; Figure S3: Inverse isochron lines that yield false initial 40Ar/39Ar ratio; Table S1: Detailed SIMS U-Pb zircon results of sample CX48; Table S2: Detailed 40Ar/39Ar VG3600 Furnace Step-Heating Analytical Results.

Supplemental Material: An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

Three figures (Figs. S1–S3) and two tables (Tables S1–S2). Figure S1: Images and related Energy Dispersive X-ray spectra to show that quartz melts infilled in feldspar crack; Figure S2: Features of samples selected for 40Ar-39Ar dating; Figure S3: Inverse isochron lines that yield false initial 40Ar/39Ar ratio; Table S1: Detailed SIMS U-Pb zircon results of sample CX48; Table S2: Detailed 40Ar/39Ar VG3600 Furnace Step-Heating Analytical Results.

Supplemental Material: An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

Three figures (Figs. S1–S3) and two tables (Tables S1–S2). Figure S1: Images and related Energy Dispersive X-ray spectra to show that quartz melts infilled in feldspar crack; Figure S2: Features of samples selected for 40Ar-39Ar dating; Figure S3: Inverse isochron lines that yield false initial 40Ar/39Ar ratio; Table S1: Detailed SIMS U-Pb zircon results of sample CX48; Table S2: Detailed 40Ar/39Ar VG3600 Furnace Step-Heating Analytical Results.

A possible precursor prior to the Lushan earthquake from GPS observations in the southern Longmenshan

Global Positioning System (GPS) stations installed in and around the epicenter of the Lushan earthquake (Mw 6.7), which occurred almost 5 years after the 2008 Wenchuan earthquake, recorded preseismic deformation corresponding to the Lushan earthquake within the southern Longmenshan thrust belt. A half-space dislocation model is used to simulate the theoretical values of the postseismic displacements caused by the 2008 Wenchuan earthquake, and after transforming the reference frame and filtering the GPS displacement time series, the theoretical and observed GPS values are compared to identify the geodetic anomaly preceding the Lushan earthquake. The abnormal extent of this geodetic anomaly decreases with increasing epicentral distance for each GPS site. This geodetic signal reflects preslip along a locked section of the 2013 seismogenic fault, which caused the accumulation of elastic strain energy until the faulting strength was overcome, thereby generating the Lushan earthquake. Hence, this anomaly might be used as an observable and identifiable precursor to forecast an impending earthquake within a period of less than two and half years before its occurrence.