CONTINENTAL TRANSFORM FAULTS, KEY ELEMENTS FOR BREAK-UP, MAGMATISM AND MARGIN ARCHITECTURE

Continental transform faults (CTFs) represent the largest linear structures on Earth, cutting the lithosphere, with a spatial extent (in some cases) of several thousands of kilometers. In addition to transform margins, a large portion of divergent margins seem to have exploited CTFs during continental break-up, presumably as a result of an inherent mechanical weakness. CTFs can therefore be viewed as a key modifier to the Wilson Cycle. In the Atlantic-Arctic realm the following oceans seem to have opened CTFs: Eurasia Basin, NE Atlantic, Bay of Biscay, and Central Atlantic. The Amerasia Basin probably opened orthogonally along a large part of the c. 4000 km long De Geer transform. The remaining part of the De Geer Line is an undisputable transform margin. Farther south, the Labrador Sea is a further candidate for opening of a CTF.

Implications of this mode of continental break-up include the influence on magmatic production and the associated development of divergent margins. Magma-rich margins are recognized by thick and subaerial melt additions to the initial oceanic crust and continental margin. Factors such as extension rate, mantle temperature, and mantle composition have been invoked to explain the “above normal” melting of this margin type. We point out, however, that since CTFs cut the entire lithosphere, continental separation along such steep lithospheric boundaries can result in large lateral thermal differences, in turn governing edge-driven convection and melt addition. Subaerial magmatic construction effectively overfills the emerging gap between the continents, thereby preserving the steep CTF margin geometry.

Several Atlantic examples are shown where this process probably occurred, including the steep, magma-rich US Central Atlantic margin, which exploited a CTF formed during Variscan dextral movement between Gondwana and Laurentia. Furthermore, we will present ongoing work that uses 3D thermal-mechanical simulations to examine how the orientation and relative strength of CTFs impact the processes leading to continental break-up and magmatic production.

The connection between steep continental edges, convection, and magmatism was first made 30 years ago. Our line of thinking on CTFs suggests that this concept is overdue for rejuvenation.