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Identifcation of sediment–basement structure in West Papua province, Indonesia, using gravity and magnetic data inversion as an Earth’s crust stress indicator

Lewerissa Richard, Sismanto, Lapono Laura A. S.

Acta Geophysica

2023

Vol. 71, no. 1

209--226

West Papua province in eastern Indonesia is positioned in a dynamic tectonic zone along with the collision of the Australian, Pacific, and Eurasian plates. The interaction resulted in the formation of strike-slip faults such as Koor, Sorong, Ransiki, and Yapen, that are prone to earthquakes in the region. The rocks of West Papua in the northern part are a contribution to the Pacific Ocean plate consisting of ophiolite and volcanic arcs of the archipelago, even while the rocks of the Australian plate in the southern part are dominated by quaternary and siliciclastic sedimentary. It has a wide variety of resources, including oil and gas. This study combines the interpretation of regional gravity and magnetic data derived from satellite observations to identify the subsurface structure of West Papua. This is performed since most studies were conducted on the surface and did not significantly focus on the subsurface. The composition of subsurface is determined through three-dimensional (3-D) unconstrained inversion modeling using the iterative reweighting inversion of regional gravity and magnetic anomalies as a function of density contrast and magnetic susceptibility of rocks. In depth variations, gravity inversion produces density contrast ranging from −0.348 to 0.451 gr/cm3 , whereas magnetic inversion provides rock susceptibility varying between −0.363 and 0.223 SI. Gravity and magnetic inversions characterize the subduction of the Pacific Ocean plate in the north, extensive intrusion of igneous rocks, and low density-susceptibility contrast in the Bintuni basin as a source of oil and natural gas. The boundary between the sediment layer and the basement is believed to be 15–20 km deep, with rocks from the uplifted mantle in the north and a Silur-Devon aged Kemum formation in the south.

Models of the Earth's crust structure in the East Carpathian Region determined from inversion of farfied P-waveforms

Starodub G., Gnyp A.

Acta Geophysica Polonica

1999

Vol. 47, nr 4

375-400

P-waveform inversion study of local crustal structure beneath the three broadband seismic stations of the East Carpathian Regional Seismic Network has been carried out. Our inversion technique involves inverting a set of radial components of observed seismograms in the frequency domain to estimate layer thicknesses, Vp, Vs, Qp, Qs. Initial four- and five-layer models of the Earth's crust under the station Uzhgorod (SU, east-northern fringe of the Pannonian deep) and Mizhgirya (SM, Folded Carpathians) were derived from deep seismic sounding data (International Geotraverse II). Final models show comparably moderate changes in the positions of the main seismic boundaries except for the disappeared boundary between two upper layers of the SM model where Vp contrast dropped from 0.9 km/s (in the initial model) to 0.15 km/s (in the final one). The total crustal thickness at SU site changed from 27.0 to 28.7 km and at SM site - from 55.0 to 57.1 km. Low Vs layer at midcrustal depths (7.2-21.0 km) is obtained for the SU model. A significant increase of Vp is observed in the crust-mantle transition layer (38.2-57.1 km) of the SM model. The most unusual result is obtained for the station Kosiv (SK, Frontcarpathian foredeep). SK site is characterized by a complex geological structure. The tectonic boundaries are running very closely there. The initial four layer model was built using common depth point data. Its thickness was 43.0 km and increased to 51.3 km in the final model due to the thickening of three upper layers. The fairly abrupt crust-mantle boundary is found with Vs contrast of 1.2 km/s (against 0.6 km/s initially).