Detachments and normal faulting in, the Marche fold-and-thrust belt (central Apennines, Italy): inferences on fluid migration paths

In the outermost domains of the central Apennines fold-and-thrust belt, the structural architecture of the late Miocene-early Pliocene contractional e difice was controlled by competence contrasts in the Calcarlous-Marly seque nces of Mesozoic-Tertiary age, and by a different state of lithification of the rock units at the onset of deformation. Field data on relative,chronol ogy of outcrop-scale structures (cleavage, veins, faults, folds) are presen ted for the three largest thrust-ramp anticlines of the Marche fold-and-thr ust belt: Monte Gorzano, Acquasanta and Montagna dei Fiori-Montagnone. The data show that the timing and geometry of deformation structures differ for : (1) the lower Calcareous interval of late Triassic-early Cretaceous age ( LCI) bounded on top by the intermediate detachment (ID) of the Fucoidi Marl s; 2) the upper Calcareous-Marly interval (UCMI) of late Cretaceous-Oligoce ne age; (3) the uppermost detachment zone (UDZ) of lower-middle Miocene age ; (4) the topmost Messinian Flysch sequence (FS). In the UDZ early episodes of deformation are manifested by compaction of a poorly lithified sequence followed by pervasive development of layer-parallel pressure-solution clea vage. Reverse faults ramp obliquely across the stratigraphic sequence, and are coated by multiple overgrowths of calcite fibers. These structures are deformed by large, eastward-verging asymmetric folds with N-S axial trends, and are cut by late generations of reverse faults. Normal faults started t o develop in the fold backlimbs during the final stages of shortening, in m iddle-late Pliocene times. These early normal faults were reactivated durin g episodes of late Pliocene-Pleistocene extensional downfaulting, and are n ow superposed on the compressional edifice. The UDZ is interpreted to have temporarily sealed the upward escape of fluids during the initial episodes of shortening. Pervasive interlayer flow in the poorly lithified sequence w as responsible for development of broken beds and scaly fabrics, similar to those observed in accretionary prisms. Only in the latest stages of deform ation did propagation of discrete faults provide an interconnected pathway for fluid migration, until the final offset of the UDZ. The structural rela tionships suggest that fluids trapped within the fold cores and sealed by t he UDZ were finally driven upwards due to progressive disruption of the thr ust belt by late normal faults of late Pliocene to Pleistocene and Holocene age. Large-scale fluid migration along structurally-controlled pathways wa s enhanced by the strong components of uplift consequent to the final stage s of deformation in the Marche fold-and-thrust belt, and was eventually ass ociated with episodes of normal seismic faulting. (C) 1999 Elsevier Science Ltd. All rights reserved.