Seismic Structure of Middle Valley near Sites 855–858, Leg 139, Juan De Fuca Ridge

Abstract

Geologic structure around holes drilled during Ocean Drilling Program Leg 139 is interpreted from single-and multichannel seismic reflection data. The holes are in a fault block of a sedimented rift valley (Middle Valley) of the Juan de Fuca ridge. This block is bound by two west-facing normal faults that separate crustal blocks with significantly different sedimentary sequences. Sediments in the drilled block are 200 to 500 ms thick and are underlain by nearly flat, high-amplitude reflectors. These events are 500-1000 m in width and appear to step down to the west towards younger crust. The events are interpreted to be the top of a sediment-sill complex that formed during simultaneous volcanism, extension, and sedimentation. Four groups of these bright reflectors can be correlated along strike in the fault block. Because they are elongated along strike, we infer that they are sills that were intruded as magma welled up along extensional faults. Several previously unmapped west-facing normal faults have been interpreted on the west side of the fault block; their offsets increase with depth and range from 10 to 100 ms. Contrary to a previous interpretation, we show that the character of sediments and basement across the eastern block-bounding fault near Site 855 is different and that this fault has been active for some time. Site 856 and another hydrothermal mound to the north are associated with shallow basalt reflectors significantly smaller in their lateral extent than the elongated sills within the fault block. Sites 857 and 858 are near the western block-bounding fault; Holes 857A and 857C are separated by a small offset fault. A region no more than 2 km along strike around Site 858 appears to have been uplifted by tens of meters. This uplift is presumably related to late-stage igneous activity documented by drilling.

Temperature dependence of the strontium distribution coefficient in calcite; an experimental study from 408 degrees to 2008 degrees C and application to natural diagenetic calcites

“…The site is characterized by high heat flow and is considered to contain a hydrothermal reservoir beneath the thick sediment fill (ϳ 900 m), where high-temperature fluids interact with the upper igneous crust and supply fluids to nearby seafloor vents . It is apparent from observed interstitial water and Sr isotopic gradients Baker et al 1994b) that there is some basement-derived advection of hydrothermal pore fluids at this site, probably along an intersected normal fault (Rohr and Schmidt 1994). Age constraints are limited at Site 857, principally because of the high geothermal gradient and the resulting rapid alteration of biogenic components.…”

Multiple scales of hydrothermal circulation in Middle Valley, northern Juan de Fuca Ridge: Physical constraints and geologic models

“…Party, 1992a; Rohr and Schmidt, 1994], the distance between two-way travel times 3.25 and 3.30 s is 37.5 m. Vertical exaggeration is -32x. Vent field (indicated by arrow) is seismically transparent and located directly above a structure (fault?)…”

Clam distribution and subsurface hydrothermal processes at Chowder Hill (Middle Valley), Juan de Fuca Ridge

“…Seismic profiles across the Dead Dog vent field in Middle Valley indicated the presence of a prominent basement fault, approximately 2 km to the east of the vent field (Rohr & Schmidt 1994). Fluids presently venting at the Dead Dog field showed chemical signatures of interaction with both basaltic and sedimentary rock (Butterfield et al 1994, Goodfellow & Peter 1994 and were believed to originate within a sill-sediment complex that formed the transition to oceanic crust in the area of the fault block .…”