High density of structurally controlled, shallow to deep water fluid seep indicators imaged offshore Costa Rica

Abstract

[1] We used high-resolution mapping to document 161 sites of potential fluid seepage on the shelf and slope regions where no geophysical seep indicators had been reported. Identified potential seabed seepage sites show both high-backscatter anomalies and bathymetric expressions, such as pockmarks, mounds, and ridges. Almost all identified seabed features are associated with bright spots and flat spots beneath, as mapped within the 3-D seismic grid. We obtained EM122 multi-beam data using closely spaced receiver beams and 4-5 times overlapping multi-beam swaths, which greatly improved the sounding density and geologic resolvability of the data. At least one location shows an acoustic plume in the water column on a 3.5 kHz profile, and this plume is located along a fault trace and above surface and subsurface seepage indicators. Fluid indicators are largely associated with folds and faults within the sediment section, and many of the faults continue into and offset the reflective basement. A dense pattern of normal faults is seen on the outer shelf in the multi-beam bathymetry, backscatter, and 3-D seismic data, and the majority of fluid seepage indicators lie along mapped fault traces. Furthermore, linear mounds, ridges, and pockmark chains are found on the upper, middle, and lower slope regions. The arcuate shape of the shelf edge, projection of the Quepos Ridge, and high density of potential seep sites suggest that this area may be a zone of former seamount/ridge subduction. These results demonstrate a much greater potential seep density and distribution than previously reported across the Costa Rican margin.Components: 12,500 words, 15 figures.Keywords: fluid seepage; fluid flow; subduction zones; Marine Geology and Geophysics; Costa Rica.Index Terms: 3060 Marine Geology and Geophysics: Subduction zone processes (1031, 3613, 8170, 8413)

Normal faulting and mass movement during ridge subduction inferred from porosity transition and zeolitization in the Costa Rica subduction zone

“…With the Structures From the 2-D Seismic Profile BGR99-7 at Site U1380 Normal faulting is supported by the structure of the fault zones and normal fault stress regime observed from the drilled cores [Harris et al, 2013b] and is also consistent with the dense pattern of interpreted normal faults in the outer shelf based on multibeam bathymetry, backscatter, and 3-D seismic data [Kluesner et al, 2013;Bangs et al, 2015Bangs et al, , 2016. The missing material (800 6 70 m) across the MSR (section 5.1) could possibly all be due to normal fault offset (250 6 70 m) with no mass movement.…”

Horizontal principal stress orientation in the Costa Rica Seismogenesis Project (CRISP) transect from borehole breakouts

“…A fault zone near the base of the sediment cover would mechanically detach the sediments from the deeper margin wedge, and the principal stress directions could be dominantly controlled by the pattern of normal faults in the sediment cover. Decoupling of the sediment cover from the deep stress field is also supported by the observation that the S Hmax azimuth in U1413 is the same as that above 865 mbsf in U1379, although the stress field beneath U1413 should be affected by the subduction of a seamount [ Kluesner et al ., ]. If a network of faults with different orientations dissects the sediment cover in a number of separate blocks, principal stress directions can be different in different blocks and may rotate near faults within a block.…”

Hydrogeological responses to incoming materials at the erosional subduction margin, offshore Osa Peninsula, Costa Rica

“…Drilling and seismic surveys along the margin suggest active and long‐term tectonic erosion [ Ranero and von Huene , ; Ranero et al ., ; Vannucchi et al ., , , ]. Active fluid venting documented by ubiquitous mud mounds and seepages is widespread along strike [ Sahling et al ., ; Kluesner et al ., ]. Several geochemical signatures (e.g., low salinity, isotopic features and entrainment of thermogenic methane) indicate that discharged fluids originate from dehydration reactions at temperatures between 85° and 150°C [ Hensen et al ., ; Schmidt et al ., ].…”