Patrick J. Barosh

Vast early Miocene lakes of the central Tibetan Plateau

Lacustrine strata of the Wudaoliang Group demonstrate that two vast lake complexes covered the central Tibetan Plateau during early Miocene time. The Wudaoliang Group, which is characterized by dolostone, limestone, and marl as thick as 310–350 m, lies horizontally or with a slight dip above Oligocene reddish-brown sandstone, and is covered by upper Miocene and Pliocene mudstone and sandstone. The extensive distribution of this group outlines interconnected lake basins of 5000–15,000 km 2 or larger in the north-central Tibetan Plateau, such as the Wudaoliang, Beiluhe, Tuotuohe, Tongtianhe, and East Wenquan Basins, and lake basins 2000 km 2 or larger in the south-central plateau, such as the Ando, Naqu, Bangoin, Lunpola, and Shuanghu Basins. These lake basins, separated by mountain ranges and islands, were linked by water passages in early Miocene time, when they apparently formed a huge lake complex that covered as much as 100,000 km 2 in the north-central Tibetan Plateau and another one >50,000 km 2 in the south-central part of the plateau. Such immense lakes existed for several million years between ca. 23.5 and ca 13.5 Ma after the Tibetan Plateau rose, and their interconnected basins define the topography during early Miocene time. The fossils contained in these basins indicate a change from the warm, dry climate of the Oligocene to moderately cool and wet conditions at the beginning of the early Miocene, followed by progressive cooling and drying, which suggests continuous uplift during deposition of the Wudaoliang Group.

Tectonics and Topography of the Tibetan Plateau in Early Miocene

: Early Miocene stratigraphy, major structural systems, magmatic emplacement, volcanic eruption, vegetation change and paleo-elevation were analyzed for the Tibetan Plateau after regional geological mapping at a scale of 1:250,000 and related researches, revealing much more information for tectonic evolution and topographic change of the high plateau caused by Indian-Asian continental collision. Lacustrine deposits of dolostone, dolomite limestone, limestone, marl, sandstone and conglomerate of weak deformation formed extensively in the central Tibetan Plateau, indicating that vast lake complexes as large as 100,000–120,000 km2 existed in the central plateau during Early Miocene. Sporopollen assemblages contained in the lacustrine strata indicate the disappearance of most tropical-subtropical broad-leaved trees since Early Miocene and the flourishing of dark needle-leaved trees during Early Miocene. Such vegetation changes adjusted for latitude and global climate variations demonstrate that the central Tibetan Plateau rose to ca. 4,000–4,500 m and the northeastern plateau uplifted to ca. 3,500–4,000 m before the Early Miocene. Intensive thrust and crustal thickening occurred in the areas surrounding central Tibetan Plateau in Early Miocene, formed Gangdise Thrust System (GTS) in the southern Lhasa block, Zedong-Renbu Thrust (ZRT) in the northern Himalaya block, Main Central Thrust (MCT) and Main Boundary Thrust (MBT) in the southern Himalaya block, and regional thrust systems in the Qaidam, Qilian, West Kunlun and Songpan-Ganzi blocks. Foreland basins formed in Early Miocene along major thrust systems, e.g. the Siwalik basin along MCT, Yalung-Zangbu Basin along GTS and ZRT, southwestern Tarim depression along West Kunlun Thrust, and large foreland basins along major thrust systems in the northeastern margin of the plateau. Intensive volcanic eruptions formed in the Qiangtang, Hoh-Xil and Kunlun blocks, porphyry granites and volcanic eruptions formed in the Nainqentanglha and Gangdise Mts., and leucogranites and granites formed in the Himalaya and Longmenshan Mts. in Early Miocene. The K2O weight percentages of Early Miocene magmatic rocks in the Gangdise and Himlayan Mts. are found to increase with distance from the MBT, indicating the genetic relationship between regional magmatism and subduction of Indian continental plate in Early Miocene.

Early Cenozoic Multiple Thrust in the Tibetan Plateau

Recently completed regional geological mapping at a scale of 1 : 250,000 or larger across all of the Tibetan Plateau coupled with deep seismic surveys reveals for the first time a comprehensive depiction of the major early Cenozoic thrust systems resulting from the northward subduction of the Indian Continental Plate. These systems define a series of overlapping north-dipping thrust sheets that thickened the Tibetan crust and lead to the rise of the plateau. The few south-dipping thrusts present apparently developed within a sheet when the back moved faster than the toe. Many of the thrusts are shown to extend to the middle-lower crustal depths by seismic data. The regional thrust systems are the Main Central, Renbu-Zedong, Gangdese, Central Gangdese, North Gangdese, Bangoin-Nujiang, Qiangtang, Hohxil, and South Kunlun Thrusts. The minimal southward displacements of the South Kunlun, Hohxil, South Qiangtang, and Central Gangdese Thrusts are estimated to be 30 km, 25 km, 150 km and 50 km, respectively. Deep thrusting began in the Himalaya-Tibetan region soon after India-Eurasia continental collision and led to crustal thickening and subsequent uplift of the Tibetan Plateau during Late Eocene-Early Miocene when the systems were mainly active. The major thrust systems ceased moving in Early Miocene and many were soon covered by lacustrine strata. This activity succeeded in the late Cenozoic to crustal extension and strike-slip movement in the central Tibetan Plateau. The revelation of the full array of the early Cenozoic thrust systems provides a much more complete understanding of the tectonic framework of the Tibetan Plateau.

Frostquakes in New England

Frostquakes are generated by tensional fracturing of the ground due to relatively rapid freezing. They are often accompanied by explosive noise and may reach Intensity V effects, yet they are only felt in neighborhoods and are not recorded on seismic networks. Frostquakes may crack macadam, concrete slabs, shallow soil pipe and some foundation materials along the line of the fracture. They occur in the early hours of the morning in mid-winter during major cold snaps. The ones investigated occurred in glacial sand and silt and none were found on till or bedrock. The recognition of frostquakes is important in correcting the records for fault-caused earthquakes. Frostquakes in the historic record complicate the difficulty of predicting large earthquakes from small ones.