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Han, L, Hole JA, Stock JM, Fuis GS, Kell A, Driscoll NW, Kent GM, Harding AJ, Rymer MJ, Gonzalez-Fernandez A, Lazaro-Mancilla O.  2016.  Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project. Journal of Geophysical Research-Solid Earth. 121:7469-7489.   10.1002/2016jb013139   AbstractWebsite

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 similar to 3 to similar 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 similar to 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 similar to 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.

Brothers, D, Harding A, Gonzalez-Fernandez A, Holbrook WS, Kent G, Driscoll N, Fletcher J, Lizarralde D, Umhoefer P, Axen G.  2012.  Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California. Geophysical Research Letters. 39   10.1029/2011gl050828   AbstractWebsite

Subduction of the Farallon plate beneath northwestern Mexico stalled by similar to 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 (similar to 24.5 degrees 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 similar to 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 similar to 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. Citation: Brothers, D., A. Harding, A. Gonzalez-Fernandez, W. S. Holbrook, G. Kent, N. Driscoll, J. Fletcher, D. Lizarralde, P. Umhoefer, and G. Axen (2012), Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California, Geophys. Res. Lett., 39, L09307, doi:10.1029/2011GL050828.

Chereskin, TK, Levine MD, Harding AJ, Regier LA.  1989.  Observations of Near-Inertial Waves in Acoustical Doppler Current Profiler Measurements Made During the Mixed Layer Dynamics Experiment. Journal of Geophysical Research-Oceans. 94:8135-8145.   10.1029/JC094iC06p08135   AbstractWebsite

Measurements of upper ocean shear made during the Mixed Layer Dynamics Experiment (MILDEX) provide evidence of large horizontal scale motion at near-inertial frequency. The measurements consist of shipboard acoustic Doppler current profiles. Four large-scale spatial surveys of 2–4 days duration were made by the R/V Wecoma as a set of boxes approximately 60 km per side around a drifting current meter buoy. Velocity time series from the drifting buoy and from sonar measurements made from FLIP also indicated the presence of motions at near-inertial frequency. Horizontal length and time scales of the motion are estimated from the phase of the shear vector measured during the spatial surveys. Estimates of the length scale of the waves range from 500 to 1000 km, and the frequency is approximately 1.1f. The behavior of the phase is found to be consistent with a model of narrow-band inertial waves with vertical structure such that there is a zero crossing in velocity at the base of the mixed layer (40–60 m).

Paramo, P, Holbrook WS, Brown HE, Lizarralde D, Fletcher J, Umhoefer P, Kent G, Harding A, Gonzalez A, Axen G.  2008.  Seismic structure of the southern Gulf of California from Los Cabos block to the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005113   AbstractWebsite

Multichannel reflection and coincident wide-angle seismic data collected during the 2002 Premier Experiment, Sea of Cortez, Addressing the Development of Oblique Rifting (PESCADOR) experiment provide the most detailed seismic structure to date of the southern Gulf of California. Multichannel seismic (MCS) data were recorded with a 6-km-long streamer, 480-channel, aboard the R/V Maurice Ewing, and wide-angle data was recorded by 19 instruments spaced every similar to 12 km along the transect. The MCS and wide-angle data reveal the seismic structure across the continent-ocean transition of the rifted margin. Typical continental and oceanic crust are separated by a similar to 75-km-wide zone of extended continental crust dominated by block-faulted basement. Little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust, suggesting a constant rate of magmatic productivity since seafloor spreading began. Oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature (1300 degrees C) and passive mantle upwelling at the early stages of seafloor spreading. The crustal thickness, width of extended continental crust, and predicted temperature conditions all indicate a narrow rift mode of extension. On the basis of upper and lower crust stretching factors, an excess of lower crust was found in the extended continental crust. Total extension along transect 5W is estimated to be similar to 35 km. Following crustal extension, new oceanic crust similar to 6.4-km-thick was formed at a rate of similar to 48 mm a(-1) to accommodate plate separation.

Tong, CH, Lana C, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2005.  Subsurface tectonic structure between overlapping mid-ocean ridge segments. Geology. 33:409-412.   10.1130/g21245.1   AbstractWebsite

Our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation. The significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level. The overlap region between mid-ocean ridge segments at 9 degrees N on the East Pacific Rise is characterized by aligned cracks that are structurally more closely related to the propagating-ridge segment. These aligned cracks, which are approximately parallel to the ridge segments, provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers, especially those with overlap basins covered by volcanic edifices. The aligned cracks of the 9 degrees 03'N overlapping spreading center are more similar to the ridge-parallel lineaments observed between overlapping axial-summit collapse troughs than those found at larger overlapping spreading centers. Our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related.

Lizarralde, D, Axen GJ, Brown HE, Fletcher JM, Gonzalez-Fernandez A, Harding AJ, Holbrook WS, Kent GM, Paramo P, Sutherland F, Umhoefer PJ.  2007.  Variation in styles of rifting in the Gulf of California. Nature. 448:466-469.   10.1038/nature06035   AbstractWebsite

Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width(1), with narrow rifts thought to form as necking instabilities(2) ( where extension rates outpace thermal diffusion(3)) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings(1,4). Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models(5) to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism - from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting ( extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.