Description: At DSDP Site 477, late Quaternary diatomaceous muds and delta-derived silty-sand turbidites at 2000 meters water depth have been extensively and progressively altered by a deep-seated heat source beneath a sill. Bulk petrologic and microprobe analyses have identified a crudely zoned paragenesis within 260 meters sub-bottom which ranges from unaltered to slightly altered oozes (0-50 m), anhydrite-dolomite claystones (105-125 m), illite-chlorite-pyrite claystones (125-140 m), chlorite-pyrite-calcite-carbonaceous claystones with traces of K-feldspar, albite, epidote (140-190 m), and chlorite-epidote-quartz-albite-pyrrhotite-sphene sandstone (190-260 m). Several petrologic features suggest rapid processes of ocean floor metamorphism: (1) friable and porous textures, (2) abundant relict grains with overgrowths, (3) idiomorphic habits on epidotes, feldspars, and quartz, and (4) a steep gradient in levels of alteration. Many aspects of this hydrothermal assemblage are similar to hydrothermally metamorphosed sandstones of the Cerro Prieto, Mexico, geothermal area.

Description: he three-site Leg 64 Deep Sea Drilling transect at the tip of the Peninsula of Baja California straddled the transition from continental to oceanic crust. The outer site, 474, penetrated mainly mud turbidites and bottomed in "middle" Pliocene oceanic crust about 3 m.y. old. Two sites on the lower continental slope penetrated hemipelagic muddy sediments, a thin section of low-oxygen, phosphoritic, and glauconitic sediments, and a metamorphic cobble conglomerate; one of the sites, 476, bottomed in deeply weathered granite. The oldest marine sediments at this site are early Pliocene, about 4.5 m.y. old. Depth indicators in these holes suggest that all sites were in almost 1000 meters of water by the time oceanic crust was first generated and sea-floor spreading began. Block faulting, subsidence, and deposition of marine sediments on continental crust had preceded the start of sea-floor spreading. Close examination of lineated magnetic anomalies demonstrates that the transition from continental to oceanic crust in this region is diachronous, as early as 4.9 m.y. in some places, but as young as 3.2 m.y. along the line of the transect. We propose a geological history scenario which involves termination of subduction along the western margin of Baja California at 12.5 Ma, a period of transform motion between the Pacific and North American plates along the Tosco Abreojos Transform Fault zone along the west side of Baja California, and a jump of the Pacific-North American plate edge to the alignment of the Gulf at 5.5 Ma. Between 5.5 Ma and about 3.2 Ma, separation of the blocks occurred locally by sea-floor spreading, but elsewhere by "diffuse extension", largely involving listric normal faulting and thinning of the continental crust, accompanied by subsidence and marine inundation. Thus, the plate edge system in the mouth and southern part of the Gulf evolved as early as 5.5 Ma, but the transition from rifting to drifting was diachronous, starting only 3.2 Ma along the line of the transect.