ABSTRACT Aseismic ridge subduction is common along modern convergent margins. We enumerate six criteria that can be used to recognize aseismic ridge subduction in orogens, including a magmatic gap with uplift followed by bimodal volcanism, which commonly includes explosive, voluminous rhyodacitic volcanism that erupts far from the trench. Features temporally linked with the explosive volcanism include retroarc thrusts and consequent thrust-loaded retroarc foreland basin development. Using these criteria to examine features of the Taconic orogen, together with new stratigraphic and structural data from the Utica basin that constrain the basin subsidence architecture and thrust timing, we propose that at least the older units of the 456–435 Ma Oliverian Plutonic Suite in New England were generated during steepening of the downgoing slab after passage of a subducting aseismic ridge. Weakened crust from delamination and decompression melting promoted westerly directed thrusts (present-day coordinates) that loaded the Taconic retroarc foreland. The resulting Utica basin subsided rapidly and nearly synchronously over an ~150-km-wide region and contains interbedded 453–451 Ma ash layers from the Oliverian Plutonic Suite or coeval plutons to the south. This history of basin subsidence indicates that the major thrust loads that drove development of the Utica basin were emplaced over a similarly brief interval beginning ca. 455 Ma. Thus, the Taconic thrusts, the Utica basin, the volcanic ashes, and the early Oliverian felsic magmatic units could all be related to an aseismic ridge subduction event. Because of the ubiquity of seamount chains, we expect that aseismic ridge subduction affected other segments of the Taconic orogen.

A field of stress-release bedrock structural features occurs on the floor of western Lake Ontario south of Toronto, Canada. These features were investigated using side-scan and multibeam sonars, high-resolution seismic profiling, and submersible dive observations. The study region was mostly stripped of its glacial drift in late glacial time, and the region has since accumulated only a relatively thin, discontinuous cover (1–2 m) of lacustrine sediment. The stress-release features affect the flat to gently dipping interbedded shales and calcareous siltstones of the Upper Ordovician Georgian Bay Formation. The features consist of sub-lakefloor buckles, about 50–100 m wide with structural relief of 5+ m, and surface bedrock popups, 10–15 m wide with a general relief of 1–2 m. Deeper bedrock faults are possibly associated with some of the sub-lakefloor buckles. Trends of the popups and buckles can be grouped into six modes from 7.5° to 347.5°. Abutting and sediment onlap relationships suggest that the pop-ups formed throughout late and postglacial time following the Last Glacial Maximum ∼20,000 yr ago. The earliest set of popups is estimated to have formed before 9500 B.P.; they trend WNW, collinear with isobases of glacial rebound, and do not parallel major geophysical or structural linear zones in the region. These and other factors suggest that this set developed in response to glacial rebound-induced stress. Later popups form an irregular pattern with several orientations of axes, suggesting that the horizontal principal stress vectors were of similar magnitude. The decrease of rebound strain with time and clockwise rotation of modern contours of basin tilting relative to glacial lake isobases suggest that popups today are likely a response to reduced glacial stress combined with far-field tectonic stress.

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Abstract Weaponry can be conveniently and safely concealed in enemy underground bedrock facilities (UGF). The bedrock environment surrounding UGF offers a high degree of protection for the assets contained within. Physical characteristics of the surrounding bedrock constrain the effects of conventional and even nuclear weapons. Brittle structures in the bedrock such as fracture systems have anisotropic characteristics and present a formidable obstacle to the survival of penetrating weapons. Knowledge of the three-dimensional (3-D) characteristics of bedrock fracture systems in enemy UGF, which may be covered by soil or vegetation, is of paramount importance to the weapons development community in its quest to penetrate anisotropic environments. We utilize rigorous methodologies to predict fracture characteristics in overburden-covered regions from outcrop, core, borehole, and remote sensing data. We have established digital scanline and scangrid methodologies to characterize fracture geometries. The digital data allow us to easily analyze the fractures in terms of fractal and more advanced geostatistical techniques. We have developed theoretical and practical guidelines for determining the two-dimentional (2-D) density of fractures from one-dimentional (1-D) (scanline) data. Additionally, we have developed theoretical relationships between 2-D and 3-D fracture densities. Integration of digital field data with density and spatial structure of the fracture networks allows us to predict the distribution of fractures in areas removed from the outcrop. These methodologies, once refined, fully tested, and verified, will allow us to characterize three-dimensional fracture systems in potential target areas worldwide by remote sensing means alone.

Modern submarine sediment slides produce two features: a slide scar that delineates a zone of removal and a deposit of slide material. The upturned edges of the slide scar form prominent scarps with relief of generally less than 100 m. The zone of deposition includes “hummocky,” “blocky,” and “debris flow” high-resolution (3.5–12 kHz) seismic fades. These echo types probably represent olistoliths, piles and/or folds of deformed sediment, and debris flow deposits, respectively. Cores and bottom photographs exhibit deformed, chaotic material typical of debris flow deposits. Sediment slides are common, not only on active margins, but also on passive margins. Slide complexes are not restricted to base-of-slope sites; rather, a single slide on a passive margin can stretch over 700 km from the continental shelf break to the abyssal plain. Giant submarine sediment slides have implications for studies of melange. Sediment slides provide an extremely important mechanism for generating the internal chaos characteristic of melange. Because sediment slides are not restricted to convergence zones, the presence of a chaotic unit in the rock record does not imply, of itself, a paleosubduction zone. Other characteristics of the melange must be observed and studied before proposing a paleotectonic site of formation. These characteristics include clast lithology and structural fabric history. The Cambro-Ordovician Dunnage Formation is a melange that crops out in north-central Newfoundland in the Dunnage tectono-stratigraphic zone. The Dunnage melange exhibits soft-sediment deformation features similar to those observed in cores raised from submarine sediment slides. These features are consistent with the interpretation that the chaotic nature of the Dunnage Formation formed initially by sediment sliding. The deformational features include pebbly mudstones with no cleavage and isoclinal folds that are overprinted at high angles by non-axial-planar cleavage. The original site of deposition of the Dunnage Formation is equivocal, but angular clasts in the Dunnage were derived from both an island arc and oceanic crust. This observation suggests that the heads of the slides were very close to subaerial exposures of a combined island arc and ophiolite terrane (perhaps similar to present-day Luzon). By Silurian times, the Dunnage Formation was involved in overthrusting which, considering regional relationships, was probably forearc thrusting.