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 geochemical signatures of sparry calcite-sealing expansion breccias, calcite veins, and host clasts were analyzed for their strontium ( 87 Sr/ 86 Sr) and oxygen and carbon (δ 18 O, δ 13 C) stable isotopic signatures. The breccias occur within the Lower Cretaceous Maiolica Formation. Related but different breccias are found in a few places in the Upper Cretaceous to Eocene Scaglia Rossa Formation of the Umbria-Marche Apennines fold-and-thrust belt (Italy). We propose hydraulic fracturing by fluid overpressure as a possible mechanism for generation of the breccias in these formations. Our data are compatible with the hypothesis of a hydraulically fractured breccia formed by cyclic buildup and rapid decompression of CO 2 -rich fluids, with overpressures generated by entrapment of CO 2 by structural and stratigraphic seals. Strontium and oxygen isotope ratio data suggest that the CO 2 -rich fluids may have originated from carbonate metasomatism of the mantle, resulting from subduction of carbonate-rich lithologies constituting the downgoing slab. This is consistent with previous conceptual models inferring that in the central part of the Northern Apennines, which is characterized by thick continental crust, CO 2 -rich fluids derived from mantle metasomatism would become trapped in structural seals, creating high fluid overpressures.

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.