Seismic structure of the Rivera plate beneath the Jalisco block, western Mexico, from the PESCADOR experiment Award: 01-12152, 01-12149, 01-12058, 01-11983, 01-11738 (April 2001) H. Brown1, S. Holbrook1, P. Paramo1, D. Lizarralde2, G.J. Axen3, J. Fletcher4, A. González-Fernández4, A. Harding5, G. Kent5, P. Umhoefer6 1University of Wyoming ; 2 Woods Hole Oceanographic Institution; 3New Mexico Tech; 4CICESE; 5Scripps Institution of Oceanography; 6Northern Arizona University Wide-angle seismic reflection/refraction data from a transect crossing the Rivera/North America plate boundary (Transect 5PE, Fig. 1), collected as part of the 2002 PESCADOR active-source seismic experiment, reveal dramatic crustal deformation during a rapid transition from rifted-margin formation to subduction initiation at the northern end of the Middle American Trench (Brown et al., 2006; Brown, 2007). These data indicate that the overriding plate is composed of a 30-km-thick, two-layer extended continental crust, which was thinned in response to rifting dur- ing the opening of the Gulf of California. Velocities in the upper crust (6.0-6.5 km/s) correspond to a bulk granitic composition and those of the lower crust (6.6-6.8 km/s) are consistent with bulk diorite, similar to that found on the conjugate rifted margin at the tip of Baja California. The newly formed rifted margin was then deformed as sub- duction initiated here, presumably as a northward propagation of the Middle America Trench. Active subduction is indicated by deformation within the modern trench, and crustal-scale deformation is indicated by the uplifted and eroded crust of the outer shelf and the back tilted, deformed, syn-rift sediments of San Blas Basin (Fig. 1). The wide-angle seismic data show that the northern oceanic Rivera plate subducts beneath the Jalisco block of west- ern Mexico, with reflections and refractions from a continuous slab observed from the trench to 75 km east. The slab dips at ~26.5 degrees down to a depth of at least 30 km, which is steeper than results from earlier teleseismic studies and indicates a lateral variation in plate dip across 21 degrees N. These results push the boundary of the Rivera plate under North America at least as far north as the southern San Blas basin, and thus they have implica- tions for sub-lithospheric mantle flow and volcanism within the Trans-Mexican volcanic belt. Figure (top left) The southern end of the Gulf of California. Transect 5PE with ocean-bottom and land seismometer locations indicated as red dots. (lower left) Migrated multi-channel seismic (MCS) data from the northernmost Middle America Trench showing an active deformation front and a bottom-simulating reflector within the nascent accretion- ary prism. (right) Migrated MCS data with enlargements showing de- formation and prominent unconformity associated with the transition from extension to compression and subduction. Brown, H. E., P. Paramo, W. S. Holbrook, D. Lizarralde, G. Kent, A. Harding, A. Gonzalez, J. M. Fletcher, P. Umhoefer, and G. Axen, Crustal structure of the southern Gulf of California and subducting Rivera plate, T41D-1607 pp., American Geophysical Union, 2006. Brown, H. E., “Crustal rupture, creation, and subduction in the southern Gulf of California, Mexico and the role of gas hydrate in the Storegga slide, Norway”, Ph.D. Thesis, University of Wyoming, Laramie, 2007. 5.