Middle Miocene to early Pliocene oblique extension in the southern Gulf of California

Abstract

A multichannel seismic (MCS) experiment spanning 600 km across the Alarcón Rise and its conjugate rifted margins in the southern Gulf of California (western North America) provides insight into the spatial and temporal evolution of extension between Baja California and the mainland (Mexico). Stratigraphic analysis of multiple rift basins within the Alarcón spreading corridor indicates an initial stage of oblique extension starting ca. 14-12 Ma. This initial phase of extension was characterized by the formation of several large basins in the center of the gulf and on the southeast margin with negligible synrift sedimentation. A second phase of oblique extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began ca. 8-5 Ma and was characterized by the formation of smaller half-grabens distributed across the conjugate margins that contain both synrift and postrift deposits. A key feature imaged within the MCS data is a highly refl ective, ropey layer at the top of basement, interpreted to be either volcanic rocks from the 25-12 Ma Comondú Group, and/or early rifting volcanic rocks that are between 11 and 9 Ma, or younger. This volcanic layer is extensively faulted, suggesting that it predates the episode of early extension. Upper crustal extension appears to be equally distributed across conjugate margins, forming a symmetrical continental rift. Two styles of rifted basin are observed; older basins (estimated as 14-11 Ma using sedimentation rates) show distributed extension with extensive basement faulting. In contrast, the younger basins (likely post-6 Ma) are asymmetrical with synrift deposits thickening into the basin-bounding faults. The northeastsouthwest geomorphic expression of the Tamayo bank and trough and other features provides additional evidence that northwestsoutheast oblique extension began ca. 12 Ma. These new spatial and temporal constraints, when combined with a crustal thickness profi le obtained across the entire Alarcón corridor, suggest that signifi cant northwestsoutheast oblique extension within the Gulf of California started well before 6 Ma, in contrast to earlier models.

Pulling apart the Mid to Late Cenozoic magmatic record of the Gulf of California: is there a Comondú Arc?

“…3), coincident with the onset of rifting of the GoC at this time (e.g. Stock & Hodges 1989;Umhoefer et al 2001;Sutherland et al 2012).…”

“…The second outcome is that biostratigraphical studies, particularly from the offshore basins are recording a more protracted history of rift basin subsidence and sediment accumulation in the GoC and that marine sedimentation began at least in the middle Miocene (c. 12 Ma; Helenes 2012; cf. Sutherland et al 2012). This would require unrealistically rapid crustal thinning and subsidence if all extension began c. 12 Ma.…”

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“…14-12.5 Ma); i.e., extension occurred when subduction was still ongoing. However, after the infl uential work of Stock and Hodges (1989), most (e.g., Henry andArandaGómez, 1992, 2000;Umhoefer et al, 2001;Umhoefer, 2011;Fletcher et al, 2007;Lizarralde et al, 2007;Seiler et al, 2011;Sutherland et al, 2012;Miller and Lizarralde, 2013) assumed that extension in the Gulf of California region began only at the end of Middle Miocene time, when subduction ended and the transfer of Baja California to the Pacifi c plate began. In the past two decades workers have debated the onset of oblique rifting in the Gulf of California (e.g., Stock and Hodges, 1989;Gans, 1997;Oskin and Stock, 2003;Fletcher et al, 2007;Miller and Lizarralde, 2013), but a general consensus has existed where gulf opening was fundamentally a post-subduction event, controlled by the highly oblique, northwestward motion of Baja California that was able to rift the continental lithosphere in ~6-10 m.y.…”

“…Henry and Fredrikson (1987) and Henry (1989) suggested that extension in Sinaloa adjacent to the Gulf of California might be as old as Late Oligocene based on the age of a few dated north-northwest-trending dikes in the region, and a maximum age of 17 Ma was reported for extensional faulting. Subsequently, it has been assumed that the GEP in this area developed after the end of the Middle Miocene (Henry and Aranda Gómez, 2000;Ferrari et al, 2002;Aranda Gómez et al, 2003;Sutherland et al, 2012), although geologic studies and radiometric ages to support this interpretation were restricted to a few locations.…”

Chronostratigraphy of the Hottah terrane and Great Bear magmatic zone of Wopmay Orogen, Canada, and exploration of a terrane translation model

“…12 million years, Baja has been dextrally transported at least 300 km north and west of its origin (Fig. 11A: Umhoefer 2011;Sutherland et al 2012;Bennett et al 2013; also see Bryan et al 2014), while the Gulf of California evolved on top of the previous and deceased arc system (Fig. 11A;Umhoefer 2011;Bennett et al 2013).…”