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

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

Wellington

Simposio; 8th Internatonal Symposium on Antartic Earth Sciences; 1999

ISAES

New refraction and multichannel seismic reflection data were collected along three transects of die 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. Investigations included active and passive seismology, gravity and magnetic measurements, and geology. They were undertaken in two 400 km long corridors running parallel to the sides of the HFZ lineament and crossing the AP, and along a 200 km tie-line intersecting the corridors on die 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 die three transects. Three near-vertical multichannel seismic profiles (700 km in total) were acquired offshore of die west coast of die AP, using a 3 km long streamer. Ray-tracing models of die more distinct refraction and wide-angle reflection arrivals, integrated wiui multichannel seismic data and gravity modelling, indicate a crustal diickness of 34-38 km and upper mantie Vp of 8.0-8.1 km/s beneath die AP. However, quite different crustal structures are revealed to die NE and SW of die HFZ lineament, on its Pacific margin. To die NE of die HFZ, die AP margin is characterized by active subduction at die Soutii Shedand Trench. There is an accretionary prism of limited extent, and a margin dissected by rifting and^block faulting, mainly in correspondence witii die Bransfield Basin, where die Moho is at 15-17 km deptii (upper-mantie Vp of 7.9 km/s). The crystalline crust beneath die soutirwestem end of die Soutii Shetland Is. volcanic arc and beneatii die 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 beneaui the eastern Bransfield basin to about 8 km beneath the Soudi Shetland Is. In contrast, die margin to the SW of the HFZ shows a crust characterized by a sharp ocean-continent boundary, and a Vp of 6.0 km/s at die top and 7.0 km/s at uie bottom. There are no major intracrustal discontinuities, but there is a flat Moho at about 25 km deptii. The transition on the Pacific margin, from a 25 km duck 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 die Moho deptii, over a distance of about 20 km. This major tectonic disturbance lies on die landward continuation of die HFZ lineament and stops die soudiwestward propagation of die rifting in die Bransfield basin.