Holocene-late Pleistocene non-tropical carbonate sediments and tectonic history of the western rift basin margin of the southern Gulf of California

Using high-resolution seismic reflection profiling and dating of (1) shallo w marine vibracores and (2) sediments collected from uplifted marine terrac es we reconstruct the tectonic history and sediment accumulation patterns o f Holocene to late Pleistocene warm-temperate to subtropical carbonates in the southern Gulf of California, Mexico. The study was conducted in the vic inity of La Paz where carbonates form along the fault bounded narrow wester n shelf of the tectonically active Gulf of California rift basin. The non-t ropical nature of the setting is responsible for (1) poor cementation of th e bioclastic carbonates, and (2) a composition which is dominated by rhodol iths (coralline red algae), corals and mollusks. Unrimmed carbonate flats f orming in small pocket bays and a rhodolith bioherm, which has a surface ar ea of more than 20 km(2) and is up to 16 m thick, constitute the major carb onate factories. Holocene carbonate accumulation rates were deduced from se ismic and core data and are highest on the rhodolith bioherm (260 cm/ka) an d in subtidal zones of pocket bays (210 cm/ka), and lowest on the inner and middle shelf (100 cm/ka). Taken together, rates of carbonate accumulation are intermediate in magnitude between higher rates recorded in fully tropic al carbonate settings and lower rates typical of cool-water carbonates. Sei smic reflection profiles demonstrate that Isla Espiritu Santo in the center of the study area is a west dipping fault block, which is tectonically inf luenced by two distinct faults., the La Paz and Espiritu Santo faults. The latter fault accommodates at least 700 rn of east-side down normal offset. and forms a steep eastern escarpment leading into the La Paz slope basin. S ome of the sediments produced in the shallow carbonate factories of the nar row La Paz shelf are transported across this escarpment and are redeposited in the slope basin at a water depth of 750 m. Uranium-series dates of mari ne terraces exposed on Isla Espiritu Santo indicate that late Pleistocene u plift along the eastern side of the island could be as high as 310 mm/ka wh ereas downdropping along the western side of the island occurred at a rate of up to 15 mm/ka. Isla Espiritu Santo therefore constitutes part of the up lifted shoulder of the western margin of the Gulf of California rift basin. Patterns of vertical tectonic movements of Isla Espiritu Santo together wi th fault offsets on the surrounding seafloor and slump features on steep su bmarine slopes, point to continuous activity of the La Paz and Espiritu San to faults. Results of this study will assist in recognizing and interpretin g similar settings along ancient non-tropical rift basin margins worldwide and especially in the Neogene of the Gulf of California. (C) 2001 Elsevier Science B.V. All rights reserved.