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 first 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.
Most researchers agree and historical records concur that the Hellenic subduction interface that separates the down-going African from overriding Eurasian lithosphere represents a weakly-coupled plate boundary unlikely to generate large earthquakes. The development of outer forearc basins is believed to be controlled by upper crustal synconvergent extension above the weakly-coupled subduction interface. Faults responsible for destructive historic earthquakes are likely related to a series of prominent south-facing escarpments embedded in the forearc of the subduction zone, suggesting a genetic link between seismic hazards, the building of topographic escarpments and subsidence of adjacent basins. Discrepancies in recent literature over the vertical kinematics (compressional vs. extensional) of faults responsible for relief across these several km-high escarpments impede the reliability of estimates of their seismic potential and their role in the topographic development of the Hellenic margin; information important to both seismic hazard and hydrocarbon resource potential in this and similar settings.
By proposing a broadband geodetic experiment where we will integrate geologic processes and rates across 1 to 100,000 years, we hope to improve our understanding of the linkages between subduction processes, the style and intensity of internal deformation of outer forearc basin sedimentary packages, the potential for hydrocarbon migration and retention, and active seismic hazards along the actively uplifting south Hellenic margin. In our first field season, we focused upon (1) obtaining long-term estimates of coastal rock uplift from the dating and correlation of marine terrace deposits, and (2) utilizing fault-terrace relationships and fault-scaling parameters to constrain the structural style of forearc basin formation in south-central Crete.
Specifically, we utilized geomorphic and geochronologic datasets from Pleistocene marine terraces along the Asterousia escarpment (> 3700 m) of south-central Crete in order to reconcile competing models to assess uplift histories and vertical fault displacement amounts and rates. Flights of up to 6 terraces were mapped and elevations determined with differential GPS at 15 sites along 75 km of coastline. Radiocarbon ages and correlation of terrace elevations with the global sea level curve indicate differential along strike rates of uplift and elucidate vertical motions along a potentially important fault over the last 125,000 years.
We located and mapped a large normal fault cropping out on-shore near Tsoutsouros. Uplift rates for the Asteroussia footwall block are highest near the fault contact, decline westward, and have a mean value of ~1.0 mm/yr. The hanging wall exhibits mean uplift rates of ~0.3 mm/yr, suggesting ~0.7 mm/yr of late Pleistocene vertical fault motion. Our results imply that a large normal fault(s) bounds the Asteroussia mountain front and the genesis of large forearc escarpments and earthquake hazards are a consequence of extensional-to-transtensional processes at shallow crustal levels within the forearc high of the convergent Hellenic subduction zone. Furthermore, initial fault displacement-length scaling estimates imply that the large Cretan escarpments like the Asteroussia mountain front and paired basins like the Ptolemy trough, cannot be forming by motion on north-dipping splay thrusts above the subduction interface.
In addition to the Asterousia escarpment, we've measured terrace profiles and obtained radiocarbon dates for 10 locations along the Lefka Ori (White Mountain) escarpment of southwest coast of Crete. Reconstructed long-term rates of rock uplift are consistently about 1 to 1.5 m/ka from these survey sites, similar to those from the Asterousia Mountains. We are currently in the process of collecting continuous GPS data from two stations located in western Crete. A third station was vandalized beyond the point of repair shortly after becoming operational. To date, we have 18 months of continuous data from the two operational stations, which are managed by our collaborators at the Technical University of Crete in Chania. We will continue to collect the continuous GPS data sets, so that in another 18 months we will have a large enough dataset to enable reliable estimates of the short-term vertical uplift rate signal for these two stations. We will use the short (GPS) and long-term (marine terrace) rates of uplift to estimate the amount of frictional coupling along the plate interface beneath Crete, which we intend to relate to forearc basin formation, strength of the plate boundary, and earthquake hazards in this setting.
Year 1 funding is supporting the doctoral research of one NC State student, and I intend to recruit a second graduate student for the fall 2012 semester to conduct additional research related to this grant. We are collaborating with geoscientists from the Natural History Museum of Crete, the Technical University of Crete, and the University of Arizona. We will present our first research related to this grant at the 2011 annual meeting of the Geological Society of America in October 2011. We plan on submitting our first manuscript from this research for review by the end of the 2012 spring semester.