Paper No. 12
Presentation Time: 11:45 AM


DOBBS, Stephen C., Department of Geosciences, Depauw University, 9678 N. Glen Drive, Moorseville, IN 46158, KUNKEL, Forrest G., Department of Geosciences, Depauw University, 11122 West Oak Tree Road, Brookville, IN 47012, JOSEPH, William T., Department of Geosciences, Depauw University, 60 Revere Court, Deerfield, IL 60015 and COPE, Tim D., Geosciences, DePauw Univ, 602 S. College Ave, Greencastle, IN 46135,

The Yanshan fold-thrust belt is located north and east of Beijing in Hebei Province, China. It houses the finest exposed record of tectonism in the North China block. The occurrence of thick accumulations of conglomerate within Triassic-Jurassic basin systems in the Yanshan indicate that the northern North China block underwent at least two episodes of orogenesis during the Mesozoic. Our group has identified a series of unconformities within these basins that outline a more detailed stratigraphic framework for the Yanshan region.

The Triassic-Middle Jurassic Xiabancheng basin contains syntectonic conglomerate in the Lower Jurassic Xingshikou Formation and the Middle Jurassic (ca. 174 Ma) Xiahuayuan Formation. An unconformity between Xingshikou redbeds and the coal-bearing Xiahuayuan Formation marks a change in both tectonic setting and climate between Early and Middle Jurassic time. The Xiahuayuan Formation contains a bimodal suite of volcanics, absent within rocks beneath the basal unconformity, that suggests crustal extension during Middle Jurassic time.

The Middle-Upper Jurassic Gubeikou basin contains extensional arc andesites of the Tiaojishan Formation (160-152 Ma) that are overlain by Upper Jurassic (ca. 152-146 Ma) conglomerate of the Tuchengzi Formation. Our work in the Gubeikou basin suggests that the Tuchengzi Formation unconformably overlies the Tiaojishan andesites and is syndepositional with shortening. In our map area, the south-vergent Gubeikou thrust both cuts and is overlapped by the Tuchengzi Formation. Tuchengzi Formation conglomeratic strata in the footwall of the Gubeikou thrust are folded into a tight growth syncline. These data imply that the Gubeikou Thrust developed at the same time as the Tuchengzi Formation, and that N-S contractile tectonism followed intra-arc extension at ca. 152 Ma. The Tuchengzi Formation was deposited synchronously within separate basins that formed during north-south compression. In the Gubeikou basin, conglomerate clasts and detrital zircons were derived not only from Proterozoic strata in the upper plate of the Gubeikou thrust, but were also recycled from overlying Lower and Middle Jurassic strata. These findings imply that the Yanshan fold-thrust belt may have a detailed, complex history involving both extensional and contractile tectonics.