Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

Abstract

[1] Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by ''bright spots'' and ''gas curtains'' scattered throughout the sedimentary basin to the south, and by d C and d18 O isotopes of carbonates, which are similar to those recovered from other hydrate-bearing regions. The BSR reflection coefficient of À0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organicrich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate-bearing sediments are uplifted because of current triple junction tectonics.

Geochemical and age constraints on the formation of the Gorda Escarpment and Mendocino Ridge of the Mendocino transform fault in the NE Pacific

“…Multichannel seismic data (Trehu et al, 1995(Trehu et al, , 2003 and bathymetric data characterize the Mendocino transform fault as a series of east-west crustal slivers with signifi cant vertical relief between the Gorda and Pacifi c plates. The Mendocino Ridge is thought to be an uplifted transverse ridge of oceanic basalt that formed at the Gorda Ridge (Fisk et al, 1993), with lithology and age relationships characterized by previous ROV and submersible investigation results (Fisk et al, 1996;Duncan et al, 1994).…”

The Morphometry of the Deep-Water Sinuous Mendocino Channel and the Immediate Environs, Northeastern Pacific Ocean

“…At this point, there is a marked decrease in both north-side and south-side levee heights where the channel has moved beyond the immediate margin and trends out of the zone of landslide and mass A side channel heads on the eastern summit region of Gorda Escarpment (Figures 8 and 9). The channel was first described by Tréhu et al [25] and is clearly shown in their seismic data ( Figure 4, lines 3, 16 and 24). The new MBES bathymetry shows that the upper side channel does not head in a canyon or even on the margin, but simply appears as a broad shallow swale at the 750 m isobath on a broad high.…”

“…Although, the cores do not provide any evidence for how long the channel has existed, the single-channel seismic-reflection profiles from Cacchione et al [5] and multichannel seismic-reflection profiles from Tréhu et al [25] (Figure 4) show that the maximum thickness of the Mendocino Channel ranges from~200-~300 m, using the formula for the conversion of travel time to subbottom depth for the western U.S. margin [34]. If the assumption used above that the turbidites and sands represent events of only days or perhaps months duration, then subtracting the turbidites and sands, and using the occurrences of turbidites in the cores as representative of the turbidites in the seismic section, the hemipelagic sedimentation rate from BX-1 can be used as typical for the entire thickness of the Mendocino Channel (admittedly these assumptions could be challenged, but that is all the data available).…”

“…Tréhu et al [25] reported on a series of N-S multichannel seismic profiles that cross a section of Gorda Escarpment that includes Mendocino Channel. The profiles (Figure 4) show that Mendocino Channel is perched against the north wall of Gorda Escarpment and has constructed a levee to the north, overlapped in places by landslide deposits and buried in other places by distal sediments of Eel Fan.…”

“…Conventional 14 C ages of organic debris within the sands date the latest turbidite at 970 ± 80 yBP [5], but no turbidite was recovered that would correlate to the 1906 M w 7.8 San Francisco earthquake. However, the occurrence of numerous large landslides in the immediate area of Mendocino Channel (e.g., [25]), coupled with the local seismicity makes earthquakes prime candidates for the initiation of sediment failures that evolved into turbidity currents that coursed down the margin and onto the basin floor.…”