Satellite gravity fields and the identification of accreted microplates

Small-scale lithospheric terranes (microplates) are important building-blocks of continental accretion but their presence is often obscured by subsequent plate-margin deformation events and by younger volcano-sedimentary covers. The geological fabric of the eastern Anatolian-Caucasian region results from the sequential accretion of lithospheric terranes against the southwestern continental margin of the Eurasian plate. Widespread sedimentary and volcanic covers conceal some of the principal tectonic boundaries in the region, and major uncertainties persist as to the number and extent of the various terranes.

We determine whether the topographic height fits the expectance from crustal thickness, complying to the isostatic equilibrium. The input data of the study are the topography, the satellite derived gravity field, the geologic knowledge defining magmatic intrusions and tectonic terranes, arcs and sedimentary basins, the seismic Moho depth, and a seismic tomography model. We accomplish a topography-gravity regression analysis controlled by a seismic Moho model, which produces well defined positive and negative anomalies. Allowing for varying density contrast in lower crust, the topography is greatly in isostatic equilibrium and controlled by the crustal thickness, that is topographic uplift has evolved proportionally to crustal thickening. The average density contrast in lower crust is between 200 and 300 kg/m³ for the orogenic belt, with local exceptions.

The inversion of the prominent positive linear anomalies of the regional gravity field defines discrete crustal density inhomogeneities, which can be interpreted as related to specific tectonic events, thus placing cogent constraints on the accretionary history and the overall anatomy of the eastern Anatolian-Caucasian lithospheric agglomerate. Three linear belts of intracrustal increased density are found along (i) the Greater Caucasus, (ii) the Lesser Caucasus, and (iii) a previously unidentified parallel belt ca. 80 km south of the Lesser Caucasus. The latter gravity anomaly clearly delineates for the first time the southwestern margin of the South Armenian Block, a lithospheric element (microplate) whose existence has long been a matter of debate.