Lower Cretaceous clastic dykes in southern Tibet: Characteristics and palaeogeographic significance

A clastic dyke is a stratigraphically vertical, wall-like body of clastic material that fills open fissures across strata, and may provide important information on regional depositional processes and tectonic activity. Clastic dyke swarms composed of greyish to brown, fine- to medium-grained lithic-rich volcaniclastic sandstone are widely exposed in the lower Aptian shales in the Gucuo and Wölong areas of the Tethyan Himalaya in southern Tibet, where they are distributed along a series of normal faults. The length and width of clastic dykes range from 5 to 50 m and from 20 to 150 cm, respectively. The U–Pb age spectra of zircon grains contained in clastic dykes are similar to those in Lower Cretaceous sandstones exposed in the studied areas (i.e., Wölong volcaniclastics) and display a youngest peak at 124.3± 2 Ma. Petrographic analyses show that the underlying strata and dykes contain more volcanic lithic fragments and less feldspar grains than the overlying strata, indicating that the dykes were derived from underlying strata and injected upward along faults. Their formation may be explained by a fault-induced clastic-dyke model: normal faults provided conduits for clastic material injected from the underlying unlithified sandstones which were undergoing refluidization owing to over-pressure. We infer that the liquefaction of underlying strata was induced by earthquakes associated with the extensive magmatic activities and regional extension that affected the Indian passive margin of Neotethys during the Early Cretaceous.