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[1] The crustal structure of the northern Gulf of California transtensional margin has been investigated by a 280-km-long NW-SE profile, including deep multichannel seismic reflection and densely sampled refraction/wide-angle reflection seismic information combined with gravity modeling. The seismic and gravity modeling constrains two thinned crustal areas, corresponding to the upper Delfín and the upper Tiburón basins. On both sides of the profile, toward the Baja California Peninsula and the Mexico mainland, a progressive thickening of the continental crust is observed. Our results indicate that the crustal thickness is 19 km below the coastline, and it decreases to 14 and 17 km below the upper Delfín and upper Tiburón basins, respectively. In the area between both basins, the crust thickens to 19.5 km. There are significant lateral thickness variations for the different levels of the crust. The interpreted structure is consistent with the existence of an aborted rift below the upper Tiburón basin. Prominent dipping reflections in the multichannel data under upper Tiburón basin and the ridge between upper Tiburón and upper Delfín basins can be explained as a mylonite like zone related to a detachment fault. This interpretation suggests that the structural evolution of upper Tiburón basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust. The mode and locus of extension have evolved from a core complex in upper Tiburón to a narrow rift mode in upper Delfín basin.

[1] Oblique rifting began synchronously along the length of the Gulf of California at 6 Ma, yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern Gulf. Instead, multichannel seismic data show a broad shallow depression, $70 Â 200 km, marked by active distributed deformation and six $10-km-wide segmented basins lacking well-defined transform faults. We present detailed images of faulting and magmatism based on the high resolution and quality of these data. The northern Gulf crust contains a dense (up to 18 faults in 5 km) complex network of mainly oblique-normal faults, with small offsets, dips of 60-80°and strikes of N-N30°E. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfín Basins. These subparallel basins developed along splays from a transtensional zone at the NW end of the Ballenas Transform Fault. Twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone. A structural connection between the two Upper Delfín Basins is evident in the switching of the center of extension along axis. Sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material, atypical of mid-ocean ridges. On the basis of the near-surface manifestations of active faulting and magmatism, seafloor spreading will likely first occur in the Lower Delfín Basin. We suggest the transition to seafloor spreading is delayed by the lack of strain-partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover.

A refraction and wide‐angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17–18 km thick and approximately one‐dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from ~3 to ~8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher‐grade metamorphosed sediment. The lower crust below ~12 km depth is interpreted to be gabbro emplaced by rift‐related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12–18 km depth as it does to the south, and a weak reflection suggests Moho at ~28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

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.

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Subduction of the Farallon plate beneath northwestern Mexico stalled by ∼12 Ma when the Pacific‐Farallon spreading‐ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active‐source seismic reflection and wide‐angle seismic refraction profiles across southwestern BC (∼24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ∼40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave‐base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ∼8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes.

Examples of natural folds growing in a homogenous mechanical stratigraphy of alternating competent and incompetent thin layers of fine‐ and coarse‐grained sediments are examined, and the fold growth process is quantified. Our analysis reveals that the overall response to loading of siliciclastic sequences corresponds to that of flexural flow and parallel‐to‐bedding heterogeneous pure shear. Folds start out as low‐amplitude sinusoidal disturbances that rapidly become finite‐amplitude folds of heterogeneous strain. We also derive the following scaling relations: (i) degree of amplification scales with both the height above the detachment and strain, (ii) wavelength selectivity broadens with increasing strain, and (iii) deposition of syn‐sedimentary geometries is function of strain. These relations are a natural consequence of idealized area‐preserving laws of fold growth. From these results we devise a method to estimate fold strain by means of an amplitude versus depth diagram. We are also able to define a progression of fold shape change as a function of the fundamental parameter strain. Initially, structures grow by limb rotation and the selective amplification of a single dominant wavelength giving rise to sinusoidal folds. When strain reaches ~8%, softening/plastic yielding around hinges results in the development of sharp fold profiles. Limbs lock their dips at 35°–45°, suggesting that growth in this stage is permitted by hinge mobility along ramps and blind faults. Moreover, hinge migration causes fold development to accelerate spontaneously. These findings suggest that conclusions relating periods of accelerated erosion/uplift in contractional structures to tectonic processes should be treated with caution.