Vertical and wide-angle seismic exploration of crustal structure, and the active evolution of the North Aegean Trough between the Sea of Marmara and Gulf of Corinth
Abstract
The North Aegean Trough (NAT), which is the deepest among Aegean marine troughs, is bordered by the termination of the North Anatolian Fault, and thus marks the interaction of this strike-slip fault with mainland Greece and the extensional Aegean domain. In the development of academic exploration of active regions at the scale of the whole crust, with marine multichannel seismics (MCS), the STREAMERS acquisition almost ten years ago provided a first hint of the feasibility there and in other parts of the Aegean and Ionian seas (Sachpazi et al., Tectonophysics 1996), and a template for later MCS and coincident wide-angle reflection surveying. These profiles were acquired by the French N/O Nadir, with an only 96-channel streamer 2.4 km length, and with an only 840 cu. in. generator capacity of a 8 gun array, but for the first time shot in the "single-bubble" mode that was developed in this survey. With respect to the present standard set by 2001 cruises in the Gulf of Corinth (US R/V Maurice Ewing, Taylor et al. this meeting) and in the Sea of Marmara (French N/O Nadir, Hirn et al., EGS 2002 and this meeting) this was a 2 to 3 times shorter streamer cable, a 2 to 4 times smaller number of hydrophone groups, and a 10 to 3 times smaller source. The SEISGREECE survey, with a source 3 times that of STREAMERS but other parameters as modest, explored the Gulf of Corinth and the Cyclades and added profiles in the North Aegean to this early attempt. A first result of merging the two surveys was to lend credence to possible structures detected by the first single profile. This revealed an active, recent normal-fault imaged down to 10 km depth, that cuts at a N 110°E strike the northern side of the NAT (Laigle et al., Geology, 2000). Indeed although processing has been hampered by the modest streamer length and only 16 or 24-fold coverage, the data now resolve clearly the sedimentary structure, image the basement, detect intra-basement faults, an upper crustal reflective zone and reflectors interpreted as the top and the base of the lower crust. Based on the consistency of seismic evidence over the different profiles, though this is faint on each taken by itself, clear basement-involving faults that appear tosole into an upper crustal reflective domain which is interpreted as a shallow dipping detachment. These profiles reveal also uppercrustal structures consistent with the evolution in time and space of such a detachment. The latter has low angle dip grossly northeastward, from very shallow under the base of sediments in the SW corner of the NAT (N of Sporadhes Islands) to the 10 km depth where the active normal fault on the northern slope has been imaged. It is imaged in a joint consideration of the pres-stack depth migrated line and of the stack with multiple-suppression. Beneath, the top of the lower crust and the Moho are imaged in places. In-line and broadside wide-angle recordings by stations along the Magnesian peninsula and Evvia detect an abnormaly shallow position and facies of the lower crust under the southwestern edge of the NAT. Its extent is limited by the southern and western edges of the NAT and it can be viewed as marking the location of largest finite thinning of the upper crust. Striction and transport along the detachment are suggested to occur by the drag of the SW transported southern limb to the North Anatolian fault at the North Sporadhes escarpment.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....12495S