ABSTRACT The formation of the “expansion breccia” observed in the Lower Cretaceous Maiolica limestone in the Umbria-Marches region of Italy is attributable to a fluid-assisted brecciation process that occurred during the late Miocene exhumation of the Northern Apennines. The hydrothermal fluids probably originated as brine solutions trapped in the Burano anhydrite while it was in a plastic state. The migration of the Burano from the plastic to the brittle domain during unroofing resulted in liberation and injection of over-pressured hydrothermal fluids into the overlying limestone, causing hydraulic fracturing. Mapping of breccia morphology along a 400-m transect showed structures produced by different flow regimes, with chaotic and mosaic breccia characterizing the core parts of the section and mineral-filled fractures and veins in the margins. Based on the clast size in the chaotic breccia, the estimated velocities for fluidizing the aggregates of clasts and sustaining the clasts in suspension are, respectively, 15 cm/s and 65 cm/s. Crack growth was probably the main mechanism for the fragmentation of the limestone. Explosion fracturing patterns were only sporadically observed in the breccia, indicating substantial heat loss of the over-pressured fluids during their ascent to the Earth’s surface.

ABSTRACT The structural style at depth of the Umbria-Marche fold-and-thrust belt, which occupies the outer province of the Northern Apennines of peninsular Italy, has long been debated and interpreted in terms of thin-skinned or thick-skinned deformation models, respectively. Thin-skinned models predict that the Mesozoic–Tertiary sedimentary cover was detached along Upper Triassic evaporites and translated northeastward along stepped thrust faults above a relatively undeformed basement. On the other hand, thick-skinned models predict the direct involvement of conspicuous basement slices within thrust-related folds. A description of selected examples in the southeastern part of the Umbria-Marche belt reveals that some compressional structures are indeed thin-skinned, their style being controlled by rheological properties of a mechanically heterogeneous stratigraphy containing multiple décollements, whereas other structures are genuinely thick-skinned, their style being dominated by the reverse-reactivation of pre-orogenic normal faults deeply rooted within the basement. Therefore, the contrast of thin- versus thick-skinned structural styles, an issue that has generated a long-lasting debate, is only apparent, since both styles are documented to coexist and to have concurred in controlling the final compressional geometry of the fold-and-thrust belt.

ABSTRACT The Messinian Salinity Crisis (5.85–5.35 Ma) represents a nearly unprecedented unloading and loading event. During the Messinian Salinity Crisis, two important things happened in terms of surface load changes—the accumulation of thick evaporites represent a load addition, while the desiccation of the Mediterranean represents a load subtraction. The desiccation and evaporite deposition were followed by the rapid addition of water, refilling the Mediterranean during the Zanclean. The calculated flexural response to load changes imposed significant changes on the horizontal stresses in the upper crust of the Northern Apennines, an active orogenic system characterized by simultaneous zones of extension and compression that migrated eastward over time. We show that these flexural stresses (approaching ± 50 MPa), added to the preexisting stresses across the Northern Apennines, were large enough to have caused some areas in compressional stress regimes to flip to extensional regimes, and vice versa. Among other things, our model predicts that the Marches Apennines region, which was beginning to undergo compression at the leading edge of the orogen, should have experienced a brief interval of extensional deformation. Previous structural studies of this region have shown that there was, in fact, a brief period of extensional faulting near the onset of the Messinian Salinity Crisis, which was then followed by a return to compressional deformation, just as our model predicts. The hypothesis presented here provides a novel explanation for this extension occurring in an area that should have been undergoing compression, one in which the unique events of the Messinian Salinity Crisis generated significant flexural stresses that changed the deformational regime during the relatively brief time of the Messinian Salinity Crisis. We further suggest that this hypothesis may provide insight into similar unexpected tectonic episodes during this time period in other parts of the Apennines.

Abstract The Caledonian and Variscan orogens in northern Europe and the Alpine-age Apennine range in Italy are classic examples of thrust belts that were developed at the expense of formerly rifted, passive continental margins that subsequently experienced various degrees of post-orogenic collapse and extension. The outer zones of orogenic belts, and their adjoining foreland domains and regions, where the effects of superposed deformations are mild to very mild make it possible to recognize and separate structures produced at different times and to correctly establish their chronology and relationships. In this paper we integrate subsurface data (2D and 3D seismic reflection and well logs), mainly from the North Sea, and structural field evidence, mainly from the Apennines, with the aim of reconstructing and refining the structural evolution of these two provinces which, in spite of their different ages and present-day structural framework, share repeated pulses of alternating extension and compression. The main outcome of this investigation is that in both scenarios, during repeated episodes of inversion that are a characteristic feature of the Wilson cycle, inherited basement structures were effective in controlling stress localization along faults affecting younger sedimentary cover rocks.

ABSTRACT The geometry of collisional mountain belts, which were formed at the expense of passive continental margins, is often complex because orogenic structures, such as thrusts and related folds, commonly interfere with pre-orogenic extensional structures, namely, normal faults, resulting in kinematically complex, composite structural assemblages. In these settings, analysis of the relationships between depositional and structural features may provide very useful tools to correctly unravel the local sedimentary and deformational history and relative ages of structures. Analysis of the relationships between minor normal faults and slumps near Frontale in the Umbria-Marche Apennines of Italy made it possible to correctly unravel the local chronology of events and hence to infer the depositional and deformation history of a part of the Upper Cretaceous–Paleogene Scaglia Rossa Formation pelagic basin. The results of this investigation made it possible to ascribe the normal faults to events that predate the construction of the Umbria-Marche mountain belt. Therefore, the normal faults at Frontale are distinct from those that overprint the main compressional structures responsible for the present-day seismicity of central Italy.

ABSTRACT The Umbria-Marche Apennine range, part of the Northern Apennines of Italy, is a classic example of a fold-and-thrust belt developed at the expense of a formerly rifted, passive continental margin that experienced various degrees of postorogenic extension and/or collapse. This setting comprises the outer zones of the Northern Apennines, a collisional orogen, and their adjacent Adriatic foreland domain, where the effects of superposed deformations are mild to very mild, making it possible to recognize and separate structures produced at different times and to correctly establish their relative chronology and time-space relationships. In this paper, we integrated subsurface data (seismic reflection profiles and well logs) and surface structural field evidence with the aim to reconstruct and refine the structural evolution of these two provinces, the Umbria-Marche Apennine range and adjacent Adriatic foreland, which were subject to repeated pulses of alternating extension and compression. The main outcome of this investigation is that the tectonic evolution of the study area may be effectively described in terms of a deformation history characterized by structural inheritance, where structures emanating from the basement and developed during the pre-orogenic rifting stage were effective in controlling stress localization along faults affecting younger sedimentary cover rocks during the subsequent orogenic and postorogenic events.

ABSTRACT Breccias affecting the pelagic Lower Cretaceous Maiolica limestone of the Umbria-Marche Apennines of central Italy contain 10-cm-diameter to submillimeter angular clasts of white pelagic limestone and black chert, separated by a filling of sparry calcite. The clasts can often be seen to have originally fitted together, indicating extension without shear, and this is the case in all three dimensions, arguing for roughly isotropic volumetric expansion. Breccia fragments are separated by sparry calcite bodies comparable in width to the fragments; this shows that the breccias were not formed by collapse, or by a single large explosion, after either of which the fragments would surely have fallen to the bottom of the cavity, but probably by multiple small expansion events, each followed by calcite deposition in the small voids that opened up. The breccia sometimes occurs in dramatic topographic walls, a few tens of meters in both width and height, although there is not a one-to-one correspondence between breccia and walls. The sparry-calcite fill indicates that water with dissolved CO 2 was involved in formation of the breccias, presumably providing the high fluid pressure that forced the fragments apart. The breccia is bounded stratigraphically above by the middle Cretaceous Marne a Fucoidi (Fucoid marls), which appears to represent an aquiclude that limited the volume of high fluid pressure ( P F ). Although the mechanism of formation of the expansion breccias is not yet clear, we list observations that need to be accounted for by such a mechanism and discuss how these observations might be explained.

Abstract The advances in the field of inversion tectonics, pioneered by Mike Coward and co-workers, have shown that many thrust belts originated at the expense of pre-orogenic rift basins that originally had complex extensional architectures. These architectures are reflected by significant lateral thickness and facies changes within the deformed stratigraphic sequences. Although these changes are widely documented from the restoration of balanced sections across thrust fans, they are significantly less well documented from restoration of duplex structures. As a consequence, most available duplex models assume layer-cake stratigraphic sequences. In this contribution a peculiar mesoscopic duplex is described. This structure developed across a single irregular, previously extended quartzite layer of the Arenarie di Poggio al Carpino Formation (Upper Permian-Lower Triassic), during the Late Tertiary orogenic event that led to development of the Apennine chain. Through comparison of the analysed duplex with macroscopic analogues, it is proposed that similar structures may occur on a wide variety of scales, in the Apennines as well as in other orogenic belts.