In the Southern Gulf of California: the Role of the Southern Baja California Microplate and Its Eastern Boundary Zone
RESEARCH Quantifying rates of “rifting while drifting” in the southern Gulf of California: The role of the southern Baja California microplate and its eastern boundary zone Paul J. Umhoefer1,*, C. Plattner2, and R. Malservisi3 1SCHOOL OF EARTH AND SUSTAINABILITY, NORTHERN ARIZONA UNIVERSITY, 624 S. KNOLES DRIVE, ASHURST BUILDING, ROOM A108, FLAGSTAFF, ARIZONA 86011, USA 2DEPARTMENT FÜR UMWELT- UND GEOWISSENSCHAFTEN, LUDWIG-MAXIMILIANS-UNIVERSITÄT MÜNCHEN, SEKTION GEOLOGIE, LUISENSTRASSE 37, 80333 MÜNCHEN, GERMANY 3SCHOOL OF GEOSCIENCES, UNIVERSITY OF SOUTH FLORIDA, 4202 E. FOWLER AVENUE, NES107, TAMPA, FLORIDA 33620, USA ABSTRACT The southern Baja California (Mexico) microplate has been rapidly moving away from the North America plate since ca. 12 Ma. This relative motion toward the northwest developed an oblique-divergent plate boundary that formed the Gulf of California. The rift-drift hypothesis postulates that when a continent ruptures and seafloor spreading commences, rifting on the plate margins ceases, and the margins start to drift, subside, and accumulate postrift sediments, eventually becoming a passive margin. In contrast to this hypothesis, the southern part of the Baja California microplate (BCM), and in particular its actively deforming eastern boundary zone, has continued significant rifting for millions of years after seafloor spreading initiated within the southern Gulf of California at 6–2.5 Ma. This is a process we call “rifting-while-drifting.” Global positioning system (GPS)–based data collected from 1998 to 2011 show relative motion across the eastern boundary zone up to ~2–3.2 mm/yr with respect to a stable BCM. Furthermore, the velocity directions are compatible with normal fault- ing across the eastern boundary zone nearly perpendicular to the trend of the plate boundary at the latitude of La Paz and therefore a highly strain partitioned domain.
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