3D Cascadia Basin - Middle Valley (2000)

Middle Valley is a sedimented spreading center at the northern end of the Juan de Fuca Ridge, formed by simultaneous volcanism, extensional faulting, and sedimentation [Rohr and Schmidt, 1994]. The valley is filled with Pleistocene turbidites deposited when sea level was lower and terrigeneous sediments were transported down the continental slope and into the Cascadia Basin [Davis and Villinger, 1992].

The relatively impermeable sediment cover over very young and hot crust limits the recharge and discharge of hydrothermal fluids and creates a thermal regime, in which high formation temperatures can occur at relatively shallow levels in the crust. Furthermore, the sediments allow the thermal and fluid regimes to be characterized through coring, heatflow measurements, and drilling.

ODP Leg 139 was designed to investigate hydrothermal processes and products at Middle Valley. Four sites were drilled in four distinct hydrologic environments: Holes at Site 855 penetrated a zone of fluid recharge along a normal fault, drilling at Site 856 sampled a large polymetallic sulfide deposit created by hydrothermal fluid discharge that is no longer active (Bent Hill), Site 857 was drilled into a hydrothermal reservoir zone, where high temperature fluids were suspected to reside in the upper oceanic crust beneath a complete sediment seal, and Holes at Site 858 were located in and ajacent to a hydrothermal vent field (Dead Dog). Two more sites were later added during ODP Leg 169 (Site 1035 at Bent Hill and Site 1036 at Dead Dog) [Shipboard Scientific Party, 1998].

Due to the high fluid temperatures >200 °C, driving forces controlling fluid upflow through the sediments are expected to be stronger than in the Cascadia Basin. Furthermore, migrating fluids are known to affect the physical properties of the sediments significantly. However, the resolution of previously collected seismic data was not sufficient to provide detailed images of sub-surface structures near the seafloor.

High resolution seismic surveys are required to provide an overview over local and regional multi-stage crustal tectonics and their related features, which can be imaged in the sediment column. Furthermore, variations in physical properties due to intense fluid flow (e.g. a local decrease or increase of porosity) can only be identified by variations in seismic reflection amplitudes if the source frequencies are sufficiently high. A 3D multi-frequency seismic survey with small line spacing of only 25 m in the vicinity of the ODP drill sites was intended to investigate the relation between tectonic features and fluid flow as well as between sediment alteration and seismic reflection amplitude in greater detail.


The sites of ODP Legs 139 and 189 are located in a small area at the eastern rim of Middle Valley. In seismic data, the Bent Hill sulfide deposit is expressed as a prominent seafloor elevation characterized by an almost complete loss of coherent reflection energy at greater depth (Line GeoB00-532). Furthermore, the very high quality GI-Gun records clearly image small scale features like growth-faults, so far providing the best available data set to get insight into the local tectonic history of the area. Pronounced lateral and vertical variations in reflection amplitudes, which are in some places not related to visible faults or structural anomalies within the sediment section, suggest that sediments my reveal different stages of alteration and consequently different physical properties.

 The whole of the data collected at Middle Valley and in the vicinity of the ODP sites suggest that various processes like fluid flow and tectonics significantly affect both internal structure and physical properties of the sediments on very small scales.




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