Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.
[1] The Costa Rica convergent margin, investigated during Ocean Drilling Program Leg 170, offers a unique opportunity to compare the tectonic effects of rapid subduction on incoming oceanic sediments to their laterally equivalent underthrust counterparts and to terrestrially derived wedge materials. Elevated pressure laboratory measurements of ultrasonic compressional and shear wave velocity, and porosity, are used to examine the importance of tectonic, lithologic, and diagenetic controls on physical and elastic properties of sediments in these three key tectonic domains. Depositional and stress path histories of the three domains, for example, can be distinguished by (1) trends of in situ velocity-porosity (and derived measurements) correspondence and (2) the mechanical response of representative materials to isotropic consolidation. A compressional wave velocity-porosity model, critical for the application of seismic imaging to margin-wide physical property and mass balance estimates, is developed from the laboratory measurements and shown to be consistent with the information derived from LWD bulk density and migrated seismic reflection data. This consistency of the velocityporosity model over the large range of both frequency and length measurement scales is a key result, supporting the assertion that core and borehole physical property measurements can be extrapolated to larger domains. Finally, dewatering and overpressure effects, critical factors in subduction zone and fault process dynamics and increasingly common multiphase/converted wave imaging targets, are discussed in terms of laboratoryestimated in situ compressional and shear wave velocity relationships. INDEX TERMS:5114 Physical Properties of Rocks: Permeability and porosity; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 3022 Marine Geology and Geophysics: Marine sediments-processes and transport; KEYWORDS: subduction zones, elastic properties, dewatering Citation: Gettemy, G. L., and H. J. Tobin, Tectonic signatures in centimeter-scale velocity-porosity relationships of Costa Rica convergent margin sediments,
Understanding fault architecture at multiple scales is crucial to delineate in situ fault zone physical properties and rupture dynamics through modeling and geophysical imaging/monitoring. An exposure of the active large‐offset, strike‐slip San Gregorio Fault at Moss Beach, CA provides a unique field site to relate the well‐mapped fault zone architecture with compressional wave velocity (Vp) structure measured at centimeter to meter scales. Laboratory ultrasonic velocities of fault zone samples, adjusted for fluid‐related frequency and structural dispersion, indicate that (i) a seismic velocity reduction of ∼30% characterizes the central smectite‐rich clay gouge relative to the rocks 100 m away in the relatively undeformed host rocks, and (ii) the across‐fault velocity profile trends for the seismic to ultrasonic bandwidth correlate almost exactly to the previously mapped macroscale fault zone structure. These results highlight the value of conducting multiscaled investigations when measuring fault zone properties defined by physical elements at multiple scale lengths.
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