Reports: DNI850792-DNI8: Active Outer Forearc Basin Formation by Syn-convergent Extension above the Hellenic Subduction Zone, Crete, Greece

Karl W. Wegmann, PhD, North Carolina State University

Outer forearc or trench-slope basins (OFBs) develop in the upper portion of accretionary wedges at convergent margins as a result of plate subduction. They are situated offshore, between an elevated outer-arc high and the subduction trench. Historically, these basins have received relatively little attention as potential hydrocarbon provinces primarily because of the perception that low heat flow in accretionary wedge settings does not necessarily support substantial petroleum generation. However, recent investigations of outer forearc environments off the coasts of the Pacific Northwest, Costa Rica, and Indonesia illustrate that deep burial of source rocks, presumably during subduction and growth of the accretionary wedge can compensate for low heat flow and that OFBs may be in general more prolific hydrocarbon producers than previously thought. Because an increasing percentage of the next generation of recoverable hydrocarbon reserve discoveries are likely to come from previously overlooked locations, OFBs may become areas of future oil and gas production. In this second year of funding, we are investigating active outer forearc basin development at the onshore-offshore transition from the linked perspective of basin evolution and regional seismic hazard analysis on the island of Crete, which is situated above the Hellenic subduction margin in the eastern Mediterranean.

The Hellenic margin is the largest, fastest and most seismically active subduction zone in the Mediterranean. Long-lived Cenozoic convergence and subduction the Nubian (Africa) plate beneath Eurasian lithosphere has resulted in the construction of a large south-facing orogenic wedge. Rollback of oceanic African lithosphere and southward retreat of the subduction zone with respect to a fixed Eurasia likely initiated sometime in the Eocene and continues today. This geodynamic setting has given rise to a forearc characterized by a series of dramatic 2 to 4 km high escarpments that bound OFBs south of the Island of Crete; one of the few subaerial forearc highs along the Hellenic margin. It is generally agreed that these escarpments represent the surface expression of large intra-crust faults, yet the kinematics of faulting remains contentious. Different geologic and geophysical datasets have been used to argue that these structures accommodate either shortening due to continued plate convergence or extension driven by processes related to slab rollback (Fig A & B). Resolving the debate over the kinematics of the large-scale structures embedded in the Hellenic forearc is paramount to our understanding of seismic hazards, the development of forearc basins, and geodynamic processes operating in this region.

We use results from field and laboratory experiments of the tectonic geomorphology and structural geology of the south-central coastline of Crete to constrain the kinematics and evolution of the structure responsible for the Ptolemy trough, a large forearc basin south of Crete (Fig. C). Field surveys and geochronology of marine terraces reveal the pattern of late Quaternary uplift along the south-central coastline. Two large south-dipping extensional faults, which extend offshore into the Ptolemy trough, are found to offset Pleistocene marine terraces and are inferred to be active with average slip rates of ca. 0.5 mm/yr. The footwalls and hanging walls of these faults are found to be uplifting at rates of 0.5-1 mm/yr and 0.2-0.3 mm/yr, respectively (Fig. D & E). Fault-scaling relationships illustrate that it is geometrically impossible for the more easterly of these two faults to be simultaneously active with a north dipping thrust fault coincident with the axis of the Ptolemy trench. Rather, this large-scale offshore intra-crustal fault is interpreted as extensional, mimicking the active on-shore fault systems. This finding implies that the seismic threat posed by this fault is not as great as some previous research suggests. These findings, coupled with results from analyses of digital topography of south-central Crete, are consistent with a model where the Ptolemy trough, and presumably other basins in the Hellenic forearc, developed through the growth and linkage of extensional faults (Fig. A). We interpret our results as supporting a geodynamic model where extension is largely driven by processes related to slab rollback and regional uplift is the result of either inflation of the subduction wedge due to the deep underplating of material above the subduction interface and/or asthenospheric wedging related to the southerly retreat of the African slab.

Year 2 funding is supporting the continued doctoral research of Sean Gallen at NC State. We are collaborating with geoscientists from the Natural History Museum of Crete, the Technical University of Crete, and the University of Arizona. We presented our research at the 2011 GSA annual meeting, and will present new results made possible with this funding at the 2012 fall AGU meeting. We have several manuscripts in preparation and intend to submit the first one before January, 2013 to the journal Tectonics.