Role of the 3-D distributions of load and lithospheric strength in orogenic arcs: polystage subsidence in the Carpathians foredeep

摘要

It has been widely documented that the depth of foredeeps does not always reflect the topography of the neighboring orogens. In many cases, the topographic load is insufficient to explain basin subsidence. Such is the case of the SE Carpathians where an anomalously deep (almost 13 km) foreland basin has evolved since the Middle Miocene (Badenian). A peculiar feature of this basin is its position relative to the orogen. In contrast to typical foredeeps, which deepen towards the belt, the maximum depth of this basin is 10–20 km out of the orogen. The subsidence in the Carpathians Bend foreland is characterized by two stages: the first is Middle Miocene (Badenian) in age and is related to NE–SW extension when fault-bounded basins were formed. Modeling shows that the foreland underwent small pre-orogenic uniform thinning. The modeling also predicts <100 m post-rift subsidence in accordance with the regional unconformity observed at the Middle/Upper Miocene (Badenian/Sarmatian) boundary. The second subsidence stage follows rifting and is caused by flexural loading of the Carpathians nappes. According to the planform flexural modeling results, the location of depocenter in front of the Carpathians Bend in the latter contractional stage can be accounted for by the present topography if lateral variations in lithospheric strength are taken into account. Since the depth of the predicted basin is half of what is actually observed, an extra-load is required, which is equivalent to 500–800 m in terms of extra topography. In this case, the predicted basin corresponds with the observed geometry in terms of position/shape and depth, the latter depending on the magnitude of intraplate stresses as well. The modeling also suggests that the flexural fore-bulge of the Carpathians system is represented by the uplifted Dobrogea. Our explanation for the large subsidence recorded by the SE Carpathians foredeep highlights the control exerted by lateral changes in lithospheric strength on 3-D subsidence patterns in arcuate orogenic belts.