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 Middle Paleozoic section of the Appalachian Plateau exhibits a mechanical stratigraphy defined by layers that emit seismic energy with unique signatures in response to a strain energy accumulated on time scales associated with local, regional, and plate-scale processes. The Earth is in a state of frictional equilibrium, which means that even small changes in effective stress cause brittle failure and the concomitant release of ambient seismic energy. Stress changes as low as 0.001 MPa, the level of stress changes during Earth tides or the transmission of a fluid pressure wave, can activate failure on critically oriented fractures. These phenomena lead to a release of ambient seismic energy, which can be mapped using seismic emission tomography (SET) methods to image fracture networks emitting coherent seismic waves. We used a buried array of 54 sondes to identify active fracture networks over a contiguous volume of 3.76 km 3 within Middle Paleozoic rocks hosting two Marcellus gas shale wells drilled under the Appalachian Plateau of Lycoming County, Pennsylvania, USA. We sampled ambient seismic emissions before and after two stimulations and found that the pattern was repeatable. The fracture patterns illuminated by ambient seismic emissions defined a mechanical stratigraphy populated by clouds of seismic activity separated by packages of beds emitting relatively less seismic energy. The unique attribute of the beds emitting less seismic energy is a lower least horizontal stress (S hmin ) relative to adjacent mechanical units in the section. These low stress beds include the bottom portion of both the Marcellus and Burket/Geneseo black shales. There are three thicker mechanical units carrying clouds of higher energy emissions. These three units include siltstones of the Brallier above the Burket/Geneseo package, silty shale beds of the Mahantango between the Marcellus and Burket/Geneseo packages, and Silurian-Devonian carbonates below the Marcellus package. In map view, emission patterns in the Brallier follow Alleghanian J2 joints. Patterns in the Mahantango are consistent with slip along columnar joint zones like those cutting upward in outcrops of shale on the Appalachian Plateau. In sum, SET reveals a mechanical stratigraphy based on the release of strain energy from three major units of the Middle Paleozoic section.

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 Sections in strongly to moderately curved foreland fold-thrust belts may be restorable, but retrodeformation of two-dimensional (2D) serial sections produces overlap of their hinterland ends, making the incompatibility problem obvious. Outward-radiating displacement vectors predict along-strike stretching that increases toward the outer portions of arcuate foreland fold-thrust belts. Balancing of curved foreland fold-thrust belts becomes a three-dimensional (3D) material balance problem, involving non-plane strain. We propose a technique that will solve the 3D balancing problem if: (1) boundary pin lines, consisting of strike-boundary and normal-boundary lines, are present and identifiable; (2) a set of constant-length lines can be defined; (3) a set of section lines is constructed normal to tectonic strike; and (4) a set of boundary and internal constant-area regions is defined. This technique recognizes as false the unstated assumption that the deformed and undeformed state coordinate frames are identical. Accordingly, the displacement vector field of curved orogens does not parallel the deformed state strike normals. Instead, physically possible displacement vector fields require body rotations about a set of vertical axes during deformation, indicating that, during deformation, out-of-plane motions occur normal to the deformed-state strike normals relative to the external deformed-state coordinate frame. The final position of the plane strain surfaces parallels the deformed-state position of the strike normals, because they rotate into this position.

Location The effects of the Alleghanian Orogeny on marine sedimentsof the Catskill Delta may be examined at two locations in the Finger Lakes District of New York State (Fig. 1). The firstlocation lies at the junction of New York 14 and 414 south of Watkins Glen, New York. Here, a 100-ft (30-m)-thick section of the upper Genesee Group maybe followed for 1,300 ft (400 m) along the New York 414 roadcut southwest of the intersection. Park in town near the intersection and walk uphill from theintersection. Many benches may be reached by climbing over theguard rail of New York 414. The second location is at Taughannock Falls State Park on New York 896.6 mi ( 11 km) north of Ithaca, New York, wherea 200-ft (60-m)-deep gorge contains a continuous outcrop of the Tully Limestone and the overlying Genesee Group. Park at theentrance along New York 89. Outcrops may be viewed by walkingwestward from the entrance along the trail on the south sideof Taughannock Creek. The pavement of the creek bed may beexamined by climbing down into the creek bed at many pointsalong the trail. The trail also permits excellent views of the rockwalls of the gorge.

This paper presents a structural interpretation of a part of the central and northern Appalachian foreland using the correlation in orientation of such deformation features as mechanical twins, solution cleavage, crenulation cleavage, pencils, joints, and deformed fossils. Such a correlation suggests that, within the central Appalachians, the Alleghanian orogeny consists of two major phases: a deformation possibly as old as Pennsylvanian, herein called the Lackawanna phase, and a second deformation, termed the Main phase of Permian or younger age. Effects of the Lackawanna phase deformation are found mainly in the Hudson River Valley and Pocono plateau, while effects in the Main phase deformation are found throughout the Valley and Ridge and Alleghany Plateau. The Lackawanna phase is interpreted as the product of strike-slip motion, possibly between the Avalon microcontinent and North America. The Main phase may record the final convergence of Africa against North America and accreted terranes.