45387-B2
Neogene Seep Structures of the Central California Region and Relationships between Fluid Flow and Tectonic Deformation in an Active Transform Margin
The study of some of the best-preserved examples of ancient seep precipitates exposed in coastal outcrops of the northern Santa Cruz County is offering new and important insights on the relationships between stratigraphy, tectonic deformation and flow of fluids and gases in active transform continental margins. The field and analytical work conducted during the last year was aimed mainly at addressing two of the main goals formulated in the initial proposal: 1) Reconstructing the three dimensional (3D) relationships between geometry/distribution of the seep precipitates and the stratigraphic and structural strucutres of the host formation. 2) Asses the larger-scale regional controls on occurrence, timing and distribution of both fossil and modern seepages in Monterey Bay and their relationships with the transform tectonics of the San Andreas Fault System (SAFS). Structural geology combined with high resolution geospatial analysis using Terrestrial Laser Scanning (TLS) techniques have been used for the first time to reconstruct the 3D geometric relationships between stratigraphy (bedding, unconformities), geologic structures (fractures, faults, folds) and geometry/morphology of seep carbonates such as chimneys and carbonate pavements and their conduits in folded and fractured biosiliceous rocks of the late Miocene Santa Cruz Mudstone. This pioneer approach allows quantitative, 3D modeling of the complex geomorphology of the seep precipitates and their spatial distribution, including their spatial variability as a function of geological structure. Dr. Aiello and an undergraduate student assistant are reconstructing high-resolution 3D geospatial/structural models of two outcrops of fossil seep precipitates. One outcrop (approx. area of survey 300x50m) is exposed along seacliffs in the city of Santa Cruz. The other seep locality is exposed at Waddel Bluff near Año Nuevo north of Santa Cruz (approx. area of survey 250x30m). The 3D surveys were focused on the portions of the outcrops where the seep precipitates (low-Mg calcite) have been already sampled and partially analyzed for both petrographic and stable isotope analyses. Prior to conducting the geospatial surveys, a local geodetic framework of topographic benchmarks was first created using differential GPS technology (10 to 30 cm accuracy). The geospatial surveys were carried out with a state-of-the-art TLS, a Trimble VX Spatial Station. The laser scanner is equipped with Direct Reflex (DR) technology, a direct drive system with robotic servo-mechanisms and a built-in digital camera. Using this new technologic and methodological approach the complex geological surfaces representing the faulted and bedded rocks of the host formation and the fossil seeps were accurately measured and reproduced to create 3D models of the outcrops. Post-processing operations and interpretation of the geospatial data is currently underway and it includes editing and merging of point clouds (surveyed points), interpolation and contouring, creation of surface meshes, and photographic rendering of three-dimensional (3D) surface models. Methods are also being developed for the analysis/parameterization of the surface scans to obtain the orientation of faults and fractures and distribution, elongation and volume of the seep precipitates. These methods imply a phase of in situ structural measurements and the extrapolation of these measurements across the whole surveyed areas throughout automatic analysis of geospatial data using Trimble's Real Work Advanced 3D software. The field and post-processing techniques and methods developed for field collection of geospatial and structural data for this research project are a powerful tool that can be used for the exploration of these and other fossil and modern hydrocarbon seeps exposed elsewhere in the world. The regional-scale implications of modern and past fluid flow in the Monterey Bay and its relationships with the transform “wrench” tectonics of the San Andreas Fault System (SAFS) have been also further investigated by analyzing and interpreting seismic reflection profiles and aeromagnetic and gravity anomaly data available for the region. The results of these reconstructions confirm our preliminary hypothesis concerning that parallelism between orientations of the plumbing system recorded by the seep structures and NE-trending structures that segment the continental crust of the Monterey Bay into smaller tectonic blocks. In a publication that the PI is preparing for submission to the journal “Geology”, we suggest that these NE-trending structures are damage zones produced during the eastward migration of the SAFS since its inception (about 25 Ma). Damage zones could have acted as conduits for hydrocarbon-rich fluids from offshore basin depocenters to the seep localities. The results of this research have been fully integrated in the educational activities of Dr. Aiello at Moss Landing Marine Laboratories. For instance the methods developed to produce geospatial models of the seep outcrops are used in the class “Habitat Maopping” that the PI is teaching during the current fall semester 2008. Undergraduate students attending the class “Geological Oceanography” are also greatly benefiting from the advancements done with this research and are periodically involved in field trips and field activities at the fossil seep localities.
