Microstructures in Accreted Sediments of the Cascadia Margin

Clennell, Ben; Maltman, Alex

doi:10.2973/odp.proc.sr.146-1.215.1995

Cited by 7 publications

(5 citation statements)

References 34 publications

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“…Grain size analyses of material from Site 889 suggested 50–60% clay fraction in the top 50 m, increasing to 70–75% at 130 m depth and falling again to ∼50% close to the BSR [ Camerlenghi et al , 1995]. However, a high proportion of the clay size fraction appears to consist of angular rock and mineral fragments rather than clay minerals [ Clennell and Maltman , 1995]. On the basis of XRD and XRF analyses, Kopf et al [1995] suggest that clay minerals do not exceed 35% by volume, with 15–20% quartz and the remaining fraction consisting of mainly plagioclase feldspar with minor mica and calcite.…”

Section: Resultsmentioning

confidence: 99%

A three‐dimensional seismic tomographic study of the gas hydrate stability zone, offshore Vancouver Island

Hobro¹,

Minshull

Singh

et al. 2005

J. Geophys. Res.

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[1] Methane hydrate bottom-simulating reflectors (BSRs) are widespread on the northern Cascadia margin offshore Vancouver Island. We conducted a three-dimensional tomographic seismic study of the hydrate stability zone in an area around Ocean Drilling Program Site 889 using two deployments of five ocean bottom hydrophones and air gun shots along a series of closely spaced profiles in various orientations. Further constraints on reflector geometry come from coincident single-channel reflection profiles. Travel times of reflected and refracted phases were inverted with a regularized threedimensional inversion using perturbation ray tracing through smooth isotropic media for the forward step. The seismic data allow us to constrain the velocity structure in a $6 km 2 area around the drill site. Mean velocities range from 1.50 km s À1 at the seabed to 1.84 km s À1 at the BSR, and velocities at Site 889 match well those measured using a vertical seismic profile. At equivalent depths below the seafloor, velocities vary laterally by typically $0.15 km s À1. Close to the seafloor, velocities may be controlled primarily by lithology, but close to the BSR we infer hydrate contents of up to 15% of the pore space from effective medium modeling. The mean hydrate saturation in the wellconstrained volume of the velocity model is estimated to be 2.2%. There is no correlation between the seismic velocity above the BSR and the reflection coefficient at the BSR, so the latter is likely controlled primarily by the distribution of free gas beneath the hydrate stability zone.Citation: Hobro, J. W. D., T. A. Minshull, S. C. Singh, and S. Chand (2005), A three-dimensional seismic tomographic study of the gas hydrate stability zone, offshore Vancouver Island,

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Section: Resultsmentioning

confidence: 99%

A three‐dimensional seismic tomographic study of the gas hydrate stability zone, offshore Vancouver Island

Hobro¹,

Minshull

Singh

et al. 2005

J. Geophys. Res.

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“…48, this volume). Study of cores from the Cascadia margin has revealed similar structures including anastomosing veins, shear bands, incipient scaly fabrics and blocky mosaic structures that allow fluid flow when held open by high pore pressures (Clennell and Maltman, 1995). Anastomosing scaly clay was also observed in the toe region off Barbados (Prior and Behrmann, 1990).…”

Section: Evidence Of Overpressuring At Depthmentioning

confidence: 99%

Tectonic setting and processes of mud volcanism on the Mediterranean Ridge accretionary complex: evidence from Leg 160

Robertson¹,

Kopf²

1998

Proceedings of the Ocean Drilling Program

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Mud volcanism was initiated when overpressured muds rose through the Mediterranean Ridge accretionary prism. Early mud volcanism was marked by eruption of coarse clastic sediments forming small cones of debris flow deposits and turbidites, followed by eruption of large volumes of clast-rich matrix-supported debris flows. Eruption was accompanied by progressive subsidence to form moat-like features. Later, clast-poor mud flows spread laterally and built up a flat-topped cone (Napoli mud volcano). Clasts in the mud volcano sediments are mainly angular and up to 0.5 m in size. These clasts are dominated by claystone, sandstone, and limestone of early-middle Miocene age that were previously accreted. Matrix material of the mud breccias probably originated from Messinian evaporite-rich sediments located within the décollement zone beneath the accretionary wedge, at an estimated depth of 5−7 km. Pressure release triggered hydrofracturing of poorly consolidated mud near the seafloor. Eruption was accompanied by release of large volumes of hydrocarbon gas. Conditions were suitable for gas hydrate genesis at shallow depths beneath the seafloor at the Milano mud volcano. The mud volcanism is probably related to backthrusting concentrated along the rear of the accretionary wedge near a backstop of continental crust to the north. Clasts within the mud breccias were mainly derived from the North African passive margin, but subordinate lithoclastic ophiolite-related material was also derived from the north, probably from now largely obliterated higher thrust sheets of Crete. Comparisons show that in contrast to other mud volcanoes both on the seafloor (Barbados) and on land (Trinidad), which are usually relatively transient features, mud volcanism on the Mediterranean Ridge has persisted for >1 m.y. A revised hypothesis of mud volcanism at the Ocean Drilling Program sites is proposed, in relation to progressive tectonic evolution of the Mediterranean Ridge accretionary complex.

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“…Scaly foliation has also become a typical fabric nomenclature in sheared mudstones drilled in submarine subduction complex Maltman and Vannucchi 2004). Barbados (Cowan et al 1984;Labaume et al 1997;Takizawa and Ogawa 1999), Nankai (Taira et al 1992;Maltman et al 1993), Costa Rica (Vannucchi and Tobin 2000), and Cascadia (Clennell and Maltman 1995) all contain subduction complexes where scaly fabric is commonly associated with shearing. Scaly fabric has been also described in cores of basalts and boninites from the Mariana Trough (Hussong and Uyeda 1981) and from serpentinites in Guatemala (von Huene et al 1985).…”

Section: From Scaly Clays To Scaly Cleavagementioning

confidence: 99%

Myths and recent progress regarding the Argille Scagliose, Northern Apennines, Italy

Vannucchi

Bettelli

2010

International Geology Review

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Argille scagliose (scaly clay) is a geological term first used in 1840 to describe rocks in the Northern Apennines of Italy. The term was originally created to stress the mesoscopic scaliness of a type of rock that commonly outcrops in this area. The rock is also typified by a chaotic assemblage of blocky components that are embedded within the scaly matrix. Before the advent of plate tectonic concepts, the extreme complexity of these rocks posed an extreme challenge to interpret with then-standard concepts of deposition processes in sedimentary basins. Similar rocks were recognized in many other mountain belts, thus the term became widely used. At the same time, the emphasis of the term changed from a description of the matrix to a term with multiple, intensely debated, genetic associations. Only after the discovery of plate tectonics was it accepted that these rocks are formed at subduction boundaries, and that the multiple types of embedded blocks can have an origin from both slope-instabilities within an accretionary prism, and from tectonic reworking/deformation processes near the base of an active accretionary prism. This paper reconstructs the scientific evolution of thinking about argille scagliose during the years when it was one of the supreme challenges to sedimentary geologists. The often strong debate between competing hypotheses for the origin of these rocks led to many myths that still outcrop in the geological literature. We explore why this happened, the apparent future of modern research on chaotic rocks in the Apennines and other fossil accretionary prisms, and what is and should productively remain from the long geological controversy over argille scagliose.

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Microstructures in Accreted Sediments of the Cascadia Margin

Cited by 7 publications

References 34 publications

A three‐dimensional seismic tomographic study of the gas hydrate stability zone, offshore Vancouver Island

A three‐dimensional seismic tomographic study of the gas hydrate stability zone, offshore Vancouver Island

Tectonic setting and processes of mud volcanism on the Mediterranean Ridge accretionary complex: evidence from Leg 160

Myths and recent progress regarding the Argille Scagliose, Northern Apennines, Italy

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