Cordilleran Section - 111th Annual Meeting (11–13 May 2015)

Paper No. 3
Presentation Time: 2:15 PM


BRADLEY, Dwight C., U.S. Geological Survey, 11 Cold Brook Rd, Randolph, NH 03593 and MACDONALD, Francis A., Department of Earth and Planetary Sciences, Harvard University, 2, Cambridge, MA 02138,

Giant promontories are widespread but commonly overlooked features of the present-day passive margins. A giant promontory is defined here as a peninsula of continental crust that protrudes hundreds of kilometers from a passive continental margin, or from the corner of a continent where two passive margins meet. These are not to be confused with the low-amplitude irregularities that occur at intervals along most passive margins. Among the eight best modern examples are the Gunnerus Plateau off Antarctica, the South Tasman Rise off Australia, and the Malvinas Plateau off South America. These eight have average dimensions of 750 by 350 km. They show considerable diversity relating to the shapes and trajectories of diverging plates during breakup, degree of jostling of subsidiary fault-bounded blocks, rotational opening of sphenochasms, amount of stretching, and amount and relative age of magmatism.

Giant promontories might have formed during the breakup of the supercontinents Rodinia and Nuna but have not been widely recognized. Properties of the modern examples suggest some identifying criteria. They are cored by continental crust that was created or last reworked during the previous collisional cycle. The early histories of the two flanks of a promontory will have different ages and kinematics because separate continents or microcontinents drift away in different directions at different times. The shape of Laurentia and gaps and age jumps along its Paleozoic margins together suggest that promontories dating from breakup of Rodinia may have existed in NE Greenland (potentially displaced to the North Slope of Arctic Alaska), Rockall, Alabama, the Mojave, and the Yukon.

During ocean closure (typically, arc-passive margin collision), a promontory may be exposed to earlier and more intense tectonism than elsewhere along the margin. Unique events are also possible (e.g., south Florida's Paleogene collision with Cuba). Giant promontories are unlikely to have deep lithospheric keels and may be prone to being dislodged and rotated during collision. Thus, what starts as a promontory may end up as a microcontinent in an orogen. Such an origin should be evaluated for microcontinental fragments with passive margin successions in the circum-Laurentian orogenic belts (e.g., Farewell, Cassiar, and Arctic Alaska).