The Baja California peninsula borderland: structural and sedimentological characteristics

Structural and sedimentological data from three oceanographic cruises defin e the peninsular margin of the Gulf of California as a borderland similar t o the California Continental Borderland. Bathymetric and high resolution se ismic profiles show some active normal faults with a lateral strike slip co mponent, which are parallel and oblique at low angle to the peninsular coas t, and delimit horst and graben structures, Preliminary conclusions are tha t, active faults are related to the movement of the peninsular block toward s the northwest. As part of the opening of the Gulf (circa 6.5 my ago), grabens and half-gra bens, separated by highlands, islands and banks have formed. Some of the ba sins are now inland, such as Santa Rosalia, Loreto and San Jose del Cabo. a nd are filled with Pliocene and possibly Miocene marine rocks. Other basins are modem shallow bays, like Concepcion (30 m deep); others are slope basi ns with a range of depths. such as Alfonso and Cochimie basins at about 400 m; and La Giganta and La Paz basins at 500 and 700 m, respectively. Sedimentation in these basins is varied. Inland basins (Pliocene) are fille d with shallow marine sedimentary rocks, clastics and evaporites; Concepcio n is a starved basin filled with some terrigenous. but mainly biogenic sedi ments; the modem deep basins (Alfonso. La Giganta and La Paz) are filled wi th terrigenous sediments on their peninsular margin, biogenic if they are l imited by islands, and laminated sediments if they are stagnant or have slo pes or sills intersecting the oxygen minimum layer of the water column. (C) 2001 Elsevier Science B.V. All rights reserved.