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

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

Trieste

Encuentro; Broad-Band seismic observation Meeting; 1999

GNGS-OGS

During the austral summer 1996-97, new refraction and multichannel seismic reflection data were collected along three transects crossing the northern Antarctic Peninsula (AP), as part of the joint Italy-Argentina geophysical project TENAP (Cenozoic Tectonic Evolution of the Northern Antarctic Peninsula). The major aim was to investigate changes along the present/paleo-plate boundary of the western AP, in particular the limited extent of the Bransfield back-arc basin, the role of the Hero Fracture Zone (HFZ) and the associated Cenozoic crustal deformation and processes. The field investigations included 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 lineament and crossing the AP, and along a 200 km long tie-line intersecting the corridors on the Pacific margin. The seismic energy was generated using a 60.5 litre air-gun array, operating in single bubble mode to provide deeper penetration. Two shooting passages were performed along the profiles: one for near-vertical-incidence multichannel seismic acquisition (50 m shooting interval) and one for wide-angle reflection and refraction profiling (250 m shooting interval). 16 Ocean Bottom Seismometer deployments and 14 land portable seismic stations were used to record refraction and wide-angle reflection data along the three transects. Three near-vertical multichannel seismic profiles (700 km in total) were acquired offshore the west coast of the AP, using a 3 km long streamer. Ray-tracing models of the more distinct refraction and wide-angle reflection arrivals, integrated with multichannel seismic data and gravity modelling, indicate a crustal thickness of 34-38 km and upper mantle Vp of 8.0-8.1 km/s beneath the AP and quite different crustal structures to the NE and SW of the HFZ lineament, on its Pacific margin. To the NE of the HFZ, the AP margin is characterized by active subduction at the South Shetland Trench, an accretionary prism of limited extent and a margin dissected by rifting and block faulting, mainly in correspondence of the Bransfield Basin, where the Moho is at 15-17 km depth (upper-mantle Vp of 7.9 km/s). The crystalline crust beneath the southwestern end of the South Shetland volcanic arc and of the adjacent Bransfield Basin is 10-13 km thick, with a Vp of 6.0-6.6 km/s. The top of the faulted crust deepens from about 3 km depth beneath the eastern Bransfield basin to about 8 km beneath the South Shetland Is. In contrast, the margin to the SW of the HFZ shows a crust characterized by a sharp ocean-continent boundary, a Vp of 6.0 km/s at the top and 7.0 km/s at the bottom, without major intracrustal discontinuities, and a flat Moho at about 25 km depth. The transition on the Pacific margin, from a 25 km thick crust (to the SW of the HFZ) to a 16 km thick crust (in the Bransfield Basin), is imaged as a sudden 10 km jump in the Moho depth, over a distance of about 20 km. This major tectonic disturbance lies on the landward continuation of the HFZ lineament and it stops the southwestward propagation of the rifting in the Bransfield basin.