Fletcher John M.

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John M.

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  • Preprint
    Variation in styles of rifting in the Gulf of California
    ( 2007-06-18) Lizarralde, Daniel ; Axen, Gary J. ; Brown, Hillary E. ; Fletcher, John M. ; Gonzalez-Fernandez, Antonio ; Harding, Alistair J. ; Holbrook, W. Steven ; Kent, Graham M. ; Paramo, Pedro ; Sutherland, Fiona ; Umhoefer, Paul J.
    The rifting of continental lithosphere is a fundamental solid-earth process that leads to the formation of rifted continental margins and ocean basins. Understanding of this process comes from observations of the geometry of rifted margins and the magmatism resulting from rifting, which inform us about the strength of the lithosphere, the state of the underlying mantle, and the transition from rifting to seafloor spreading. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present the first crustal-scale images across conjugate margins of multiple segments within a single rift that has reached the stage of oceanic spreading. A surprisingly large variation in rifting style and magmatism is observed between these segments, from wide rifting with minor syn-rift magmatism to narrow rifting in magmatically robust segments. These differences encompass much of the variation observed across nearly all other non-end-member continental margins. The characteristics of magmatic endmember margins are typically explained in terms of mantle temperature. Our explanations for the variation in the Gulf of California, in contrast, invoke mantle depletion to account for wide, magma-poor rifting and mantle fertility and possibly the influence of sediments to account for robust rift and post-rift magmatism in the Gulf of California. These factors may vary laterally over small distances in regions that have transitioned from convergence to extension, as is the case for the Gulf of California and many other rifts.
  • Article
    Seismic structure of the southern Gulf of California from Los Cabos block to the East Pacific Rise
    (American Geophysical Union, 2008-03-15) Paramo, Pedro ; Holbrook, W. Steven ; Brown, Hillary E. ; Lizarralde, Daniel ; Fletcher, John M. ; Umhoefer, Paul J. ; Kent, Graham M. ; Harding, Alistair J. ; Gonzalez, A. ; Axen, Gary J.
    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 ∼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 ∼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°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 ∼35 km. Following crustal extension, new oceanic crust ∼6.4-km-thick was formed at a rate of ∼48 mm a−1 to accommodate plate separation.
  • Article
    Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California
    (American Geophysical Union, 2012-05-08) Brothers, Daniel S. ; Harding, Alistair J. ; Gonzalez-Fernandez, Antonio ; Holbrook, W. Steven ; Kent, Graham M. ; Driscoll, Neal W. ; Fletcher, John M. ; Lizarralde, Daniel ; Umhoefer, Paul J. ; Axen, Gary J.
    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.