THE MIDDLE JURASSIC MARGIN OF LAURASIA—A RECORD OF FAULTING ACCOMMODATING PLATE ROTATION

Geologic structures, stratigraphic relationships and tectonic settings at the margin of the Laurasia plate at ca. 170 Ma, are compatible with abrupt, concurrent, initiation of sinistral fault movement accommodating clockwise rotation. Not only did the oceanic crust of the central Atlantic basin form at 170 Ma but also the floors of the Mediterranean Sea, the Gulf of Mexico (GOM), the Great Valley of California and the southern Caspian Sea.

The beginning of sinistral movement began in the Bajocian at about 169 Ma, abruptly following the Siskiyou orogeny in the Klamath Mountains of western North America, the initial pulse of the Yanshanian orogeny in China, and a late stage of the Cimmerian orogeny in Eurasia. A working hypothesis is that the oceanic domains, e.g. GOM, formed in response to transtension at regional releasing steps that are linked by segments of a circum-Laurasia sinistral fault. Notable contractional domains that are attributed to transpression at restraining bends include the Blue Mountains and contemporaneous Elko orogenic belt (northwestern USA), the Angayucham terrane (western Alaska), Chersky collision belt (Siberia, Northeast Russia), and domains of contractional structures in eastern China. Nineteen articles in GSA Special Paper 513 provide background for assessment of the postulated fault-bounded Laurasia margin. Opening of the Bajocian oceanic basins was the culminating event of the break-up of Pangea. Formation of oceanic–driven crust in the Middle Jurassic was preceded by pulses of rifting that affected the margin of eastern North America at ca. 235-230, 201-198, and 170-165 Ma as recorded by clastic units in fault-controlled basins.

During clockwise rotation of Laurasia, adjacent microplates record counterclockwise movement suggesting ball-bearing-like motion. Modest rotations of large plates present challenging problems for global plate circuit models in that plate movement does not require formation of oceanic crust.