ALBERT

Description: The origin of the Levantine Basin in the Southeastern Mediterranean Sea is related to the opening of the Neo-Tethys. The nature of its crust has been debated for decades. Therefore, we conducted a geophysical experiment in the Levantine Basin. We recorded two refraction seismic lines with 19 and 20 ocean bottom hydrophones, respectively, and developed velocity models. Additional seismic reflection data yield structural information about the upper layers in the first few kilometers. The crystalline basement in the Levantine Basin consists of two layers with a P-wave velocity of 6.06.4 km/s in the upper and 6.56.9 km/s in the lower crust. Towards the center of the basin, the Moho depth decreases from 27 to 22 km. Local variations of the velocity gradient can be attributed to previously postulated shear zones like the Pelusium Line, the DamiettaLatakia Line and the BaltimHecateus Line. Both layers of the crystalline crust are continuous and no indication for a transition from continental to oceanic crust is observed. These results are confirmed by gravity data. Comparison with other seismic refraction studies in prolongation of our profiles under Israel and Jordan and in the Mediterranean Sea near Greece and Sardinia reveal similarities between the crust in the Levantine Basin and thinned continental crust, which is found in that region. The presence of thinned continental crust under the Levantine Basin is therefore suggested. A β-factor of 2.33 is estimated. Based on these findings, we conclude that sea-floor spreading in the Eastern Mediterranean Sea only occurred north of the Eratosthenes Seamount, and the oceanic crust was later subducted at the Cyprus Arc.

Description: Multichannel seismic (MCS) data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity, amplitude, and signal attenuation underneath are observed, as well as the possible occurrence of paleo-bottom simulating reflectors (BSRs). Phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone (GHSZ). To constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. Heat flux values estimated on the basis of the velocity-depth functions increase with decreasing amplitude of the BSR and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is trapped under parts of the BSR, and within the GHSZ, particularly under the seafloor and under an erosional unconformity. The hypothesis of a paleo-BSR that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.

Description: MCS data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Intricate lateral variations of the reflection pattern at the assumed base of the GHSZ in terms of continuity, reflection amplitude, and signal attenuation underneath are observed, as well as the occurrence of paleo-BSR. Phase reversed reflections at an erosional unconformity above the BSR indicate free gas within the GHSZ. In order to further constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. The results from 2D-forward modelling and interactive velocity analysis show consistentresults. They exhibit a low velocity layer almost directly beneath the seafloor. Another low velocity layer with less than 1.5 km/s is present between the unconformity and the BSR.. In the vicinity and beneath prominent chemoherms, high velocities have been observed between the BSR and seafloor. Heat flux values calculated on the basis of the velocity-depth functions increase with decreasing amplitude of the bottom simulating reflector and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is present under parts of the BSR, and within the hydrate stability zone, particularly under the sea floor and under the erosional unconformity. The higher interval velocities near and beneath the chemoherms are suspected to be caused either by thick gas hydrate lenses or a significant amount of precipitated carbonate within the sediment or a combination of both. The hypothesis of a paleo-BSR that reflects the uplift of the base of the GHSZ caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.