Studies of fluid flow indicators, Pacific margin of Costa Rica

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:425867

Silver, E; McAdoo, B ^[1]; Langseth, M ^[2]; Orange, D ^[3]

---

1. Univ. of California, Santa Cruz, CA (United States)
2. Lamont-Doherty Earth Observatory, Palisades, NY (United States)
3. Monterey Bay Aquarium Research Institute, CA (United States)

Seismic reflection profiles off Costa Rica image a decrease in thickness of the underthrust sedimentary section from the Middle America Trench, implying a significant reduction of porosity in the outer 3-5 km from the trench and a source of vent water through the wedge. We encountered no evidence of discrete fluid venting over the outer 3-5 km of this margin from dives using the ALVIN submersible or from heat flow measurements (based on absence of chemosynthetic vent communities and heat flow anomalies in this zone). Vent communities occur farther upslope, associated with a series of out-of-sequence thrusts, with two mud diapirs, and a mid-slope canyon. We infer that fracture permeability dominates in the out-of-sequence thrusts, upflow of fluid-rich muds in the diapir, and focusing of fluid flow in the canyon. Over 100 heat flow observations on the wedge and incoming COCOS plate showed a broad area of anomalously low heat flow (13 mW/m{sup 2}) seaward of the frontal thrust, whereas the expected heat flow for ocean crust of early Miocene age is seven times greater. The very low regional heat flow may reflect refrigeration by vigorous sea water flow through the upper crust pillow basalts. Heat flow increases to about 30 mW/m{sup 2} throughout the lower slope to mid-slope, implying a combination of widespread fluid venting, reheating of the cooled crust and frictional heating at the base of the wedge. The lack of discrete vents over the outer 3-5 km of the margin indicates diffuse flow and likely temporal episodicity, as this region has been aseismic since 1950.

• OSTI ID: 425867
• Report Number(s): CONF-960527--
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
COSTA RICA
FLUID FLOW
HEAT FLUX
OCEANIC CRUST
OFFSHORE SITES
SEA BED
SEISMIC SURVEYS