Paul J. Umhoefer¹*

1 Geology Program, School of Earth Sciences & Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA

Abstract

Rifts in the interior of continents that evolve to form large oceans typically last for 30 to 80 m.y. and longer before complete rupture of the continent and onset of sea-floor spreading. A distinct style of rifts form along the active tectonic margins of continents, and these rifts more commonly form marginal seas and terranes or continental blocks or slivers that are ruptured away from their home continent. The Gulf of California and the Baja California microplate make up one of the best examples of the latter setting and processes. In the southern Gulf of California, sea-floor spreading commenced only ~6–10 m.y. after the formation of the oblique-divergent plate boundary at ca. 12.5 Ma. Three main factors caused this rapid rupture: (1) an inherited long, narrow belt of hot, weak crust from a volcanic arc that was active immediately before formation of the oblique-divergent plate boundary and that lay between two strong batholith belts; (2) relatively rapid plate motion resulting in high strain rates; and (3) a dominant role of strike-slip faulting in the highly oblique-divergent setting that formed large pull-apart basins with rapid and focused crustal thinning in a linked en-echelon system. Accentuating factor 1 is that the formation of slab windows associated with microplate capture west of the Baja California peninsula may have further weakened the crust. These causes of rapid rupture of continental lithosphere are mostly linked to the fact that the Gulf of California developed along a long-lived tectonically active margin of a continent with a convergent or oblique-convergent setting since at least the Jurassic, but not a margin that was thickened in a major contractional orogen. This combination of causes and factors suggests that rifts that form at active margins are fundamentally different than continental interior rifts, and that these differences can produce vastly different rifting histories. The formation and northwestward motion of the Baja California microplate also show that “terranes” formed in an obliquedivergent setting can form and move long distances over relatively short geologic time intervals.

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Manuscript received 11 June 2011; accepted 6 Sept. 2011.

DOI: 10.1130/G133A.1