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ABSTRACT: Tectonic Controls on Sedimentation in Mesozoic Convergent Margin Basin of Baja California (Mexico)

C. J. Busby-Spera, D. P. Smith, W. R. Morris

Mesozoic rocks of the Baja California peninsula form one of the most extensive, best exposed, oldest (160 m.y.), and least-tectonized and metamorphosed convergent margin basin complexes in the world. Much of the fill of these basins consists of coarse-grained volcaniclastic and epiclastic sequences that directly reflect the tectonic evolution of the region.

The early history of the convergent margin was dominated by sedimentation in small, steep-sided basins within oceanic island arc systems. The Triassic and Jurassic convergent margin basins probably represent proto-Pacific terranes that traveled from another area. These terranes were assembled by the Late Jurassic to Early Cretaceous, and underlie the forearc region of a medial Cretaceous oceanic island arc system. This system fringed the Mesoamerican continental margin, and underwent regional-scale extension during subduction of old, dense lithosphere. The latest phases of sedimentation in the convergent margin occurred in broad, relatively stable forearc basins of a mature continental

arc, during the Late Cretaceous to Paleocene. Nonetheless, intrabasinal faults provided some controls on depositional systems and bathymetry. We speculate that these faults formed in response to oblique convergence which ultimately resulted in 10-19° northward displacement of Baja California relative to the North American craton, from the latitude of Central America to northern Mexico.

The fill of oceanic island arc basins in Baja California is dominated by coarse-grained marine wedges including (1) arc apron deposits, consisting of pyroclastic and/or volcanic epiclastic debris deposited in intra-arc or back-arc basins, and (2) slope apron deposits, consisting of epiclastic debris shed from local fault scarps and more distally derived arc volcaniclastics, deposited in forearc basins. The fill of the continental forearc basins in Baja California is dominated by fan-delta through shelfal to deep-sea fan deposits. Magnetostratigraphic correlations in the continental forearc basins will enable us to distinguish tectonic (local or regional) vs. eustatic (global) controls on sedimentation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990