Complex sentence: The downthrown block slips downward and basinward relative to from upthrown block thanks to the deferential load of the overlying sediments and the high mobility of the lowermost low density layer.
Split into simpler sentences: The downthrown block slips downward and basinward relative to the upthrown block. This is caused due to the deferential load of the overlying sediments and the high mobility of the lowermost low density layer.