Crustal structure and the Tectonic Evolution of the Northern Antartic Peninsula From the TENAP Seismic Experiment.

DELLA VEDOVA, B.; ACCAINO, F.; CERNOBORI, L.; PELLIS, G.; PETRONIO, L.; RINALDI, C.; ROMANELLI, M.; TASSONE, A.

California

Encuentro; Fall Meeting; 1998

AGU

New refraction and iiiultichannel seismic reflection (MCS) data were collected on the northern tip of the Antarctic Peninsula (AP) during the austral summer 1996-97, as part of the joint Italy-Argentina geo­physical project TENAP (Cenozoic Tectonic Evolution of the Northern Antarctic Peninsula). The major target was to investigate the changes along the prescnt/paleo- plate boundary of the western AP, in partic­ular the limited extent of the Bransfield back-arc basin and the role of the Hero Fracture Zone (HFZ), and the Cenozoic crustal deformation and processes associated with these changes. The field investigations include active and passive seismology, gravity and magnetic measure­ments and field geology; they were undertaken in two 400 km long corridors running parallel on the sides of the HFZ alignment and cross­ing the AP, and along a 200 km long tie-line intersecting the corridors on the pacific margin. Three near vertical MCS profiles (700 km in total) were acquired offshore the west coast of the AP, using a 3 km long streamer and a 60.5 litres air-gun array operated in single bubble mode, with a shooting interval of 50 m. A second run was performed along the same lines shooting every 250 m into 16 ocean-bottom seis­mographs (OBS) on the lines, and into 15 portable seismic stations, deployed on the islands and coastal areas around the AP. The energy from both runs was recorded up to a source-receiver distance of 240 km. Preliminary ray-tracing models, integrated with MCS data and gravity modelling, indicate a crustal thickness of 35-40 km and upper man­tle Vp velocities of 8.1 km/s beneath the AP cordillera and differing crustal structures and geometries on its pacific margin. To the NE of the HFZ, the margin is characterized by: active subduction at the South Shetland trench, an accretionary prism of limited extent, a back­stop interested by rifting and block faulting in correspondence of the Bransfield basin, and a Moho depth of 16-18 km (7.9 km/s Vp veloc­ities). In contrast, the margin to the SW of the HFZ has a sutured ocean-continent boundary and Moho depths of 25-26 km (8.0 km/s Vp velocities). The relevant tectonic feature, imaged as a sudden 10 km Moho jump along the tie-line, seems to be the crustal expression of the drastic lateral changes in the upper mantle structure across the HFZ and constitutes a limit to the southwestward propagation of the Bransfield rifting. at three spatial scales: 1) Long bi features most expected to show r above. Simultaneous tracking tim< Short baselines (10-15 km) crossi neous tracking time about 2 hour km) at each site to test for local n action. Simultaneous tracking tin of 1996-99 will establish the netwc lines. About 8 dual-frequency rec used at one time. Monuments are 0.05 m above the rock. Antenna r for forced centering and fixed heif of field campaigns will be needed.