TECTONICS,VOL. 8, NO. 1, PAGES 99-115, FEBRUARY 1989 PRE-PLIOCENE EXTENSION AROUND THE GULF OF CALIFORNIA AND THE TRANSFER OF BAJA CALIFORNIA TO THE PACIFIC PLATE J. M. Stockand K. V. Hodges Department of Earth, Atmospheric, and PlanetarySciences, Massachusetts Institute of Technology, Cambridge Abstract. Late Miocene (12-5 Ma) extension aroundthe edges of theGulf of California has been alternatively attributed to "Basin and Range" extension, back arc extension, or development of the Pacific-North Americaplate boundary. Thisextension was ENE directed and similar in structural style to extension in the Basin and Range province. Timing constraints permit nearly synchronous onset of this deformation in a belt extending SSE from northernmost Baja Californiato the mouthof the gulf. Where this extensional faulting continued throughPliocenetime to the present, synchronous with motion on the modern transform plate boundary in the Gulf of California, no change in direction of extension can be resolved. Revised constraints on Pacific- North America platemotion support the development of this late Miocene extensionas a component of Pacific-North America displacement that couldnot be accommodated by strike-slip displacement along theexisting plate boundary west of the Baja California peninsula. This scenario implies that transfer of BajaCalifornia fromthe North America plate to the Pacific plate was a gradual process, beginning about 12-10 Ma, when motion of the Pacific plate relative to North America was partitioned intoseparate regimes of strike-slip and dip-slip displacement onopposite sides of BajaCalifornia. INTRODUCTION Opening of the Gulf of Californiais oftenattributed to two sequential extensional events: middle to late Miocene "protogulf" extension [Moore and Buffington, 1968; Karig and Jensky, 1972; Moore,1973] and thePliocene development of thePacific-North America plate boundary, fromabout 5.5 Ma to thepresent [Larson et al., 1968; Curray and Moore, 1984]. Copyright1989 by theAmerican Geophysical Union. Paper number 88TC03753. 0278-7407/89/88TC-03753510.00 Originally,the protogulf concept was usedto explainan area of anomalously old oceanic crust adjacent to the Mexican margin at the mouth of the Gulf of California [Moore and Buffington, 1968]. More recently, this concepthas been expanded to include late Miocene extensional faulting and marine sediments from areas surrounding the northern and central parts of the Gulf of California [e.g.,Karig andJensky, 1972; Moore, 1973; Gastil et al., 1979]. These late Miocene extensional structures and sediments are exposed aroundthe gulf in the "Gulf Extensional Province" [Gastilet al., 1975] on the east side of Baja California and the west coast of mainlandMexico (Figure 1). The amountand directionof extensionof the pre-5.5 Ma protogulf, and its relation to Pacific-North America motion, is not well known. Several causes have been proposed for late Miocene circumgulf extension. Karig andJensky [1972] suggested that it was back arc extension, but moreprecise constraints on the timing of cessation of subduction west of Baja California [MammerickxandKlitgord, 1982;Lonsdale, 1989b] showthat it was not contemporaneous with active subduction. The geographic continuity, and similarityin extension direction, of the Gulf Extensional Province and the southern Basin and Range extensional province (in Arizona and Sonora)invited suggestions that this entire Miocene extensional belt resulted from the same process of "Basin and Range extension" [Gastil, 1968;Dokka andMerriam, 1982;Curray andMoore, 1984].In both of these scenarios, late Miocenecircumgulf extension is assumed to havebeen oblique to Pliocene extension associated with the developmentof the Pacific-North America plate boundary, and it is thought to have weakened the crustand hence facilitated thepropagation of thePacific-North America plate boundary into the gulf. By contrast, Gastil and Krummenacher [1977] viewed post-10 Ma extension in Sonora as a continuous interval of "rhombochasmic extension", implicitly related to the development of the Pacific-North America plate boundary. Spencerand Normark [1979] and Hausback [1984] proposedthat late Miocene circumgulf extensionaccommodated the component of Pacific-North America displacement perpendicular to thepreexisting strike- slip faults west of Baja California, after counterclockwise