This paper presents results of an interdisciplinary investigation of the relation between fluids,... more This paper presents results of an interdisciplinary investigation of the relation between fluids, fluid flow, and deformation in the toe region of the Nankai accretionary prism. The techniques include thin-section petrography, SEM, TEM and microprobe analyses, and X-ray computed tomography as well as laboratory experiments. Together, the data suggest three structural/hydrologic regimes within the prism. These are: (1) the accreting sediments above the décollement zone, (2) the décollement zone, and (3) the underthrust sediments. The regime above the décollement is characterized by sediments that are progressively dewatered through both a penetrative fabric and a pervasive, but apparently poorly interconnected, set of core-scale deformation structures. The décollement is characterized by a relatively high density of structures/meter and is considered to be a regime of low stress but frequent failure. Hydrologically the décollement retards the vertical flow of fluids and enhances the potential for overpressuring in the footwall. Finally, the footwall regime contains very few tectonic structures and is structurally isolated from the stresses related to plate convergence. This regime provides an important component to the tectonics of the Nankai prism, however, because it supplies the overpressured fluids that cause the décollement to fail at relatively low shear stresses.
Proceedings of the Ocean Drilling Program, 131 Scientific Results, 1993
Structural, physical property, and magnetic data from Ocean Drilling Program Site 808 in the Nank... more Structural, physical property, and magnetic data from Ocean Drilling Program Site 808 in the Nankai Trough, Japan, indicate that both sediment loading and plate convergence have driven dewatering and consolidation in this region. Evidence for this interpretation is provided by a migrated seismic reflection profile, computed tomography of core-scale structures, magnetic susceptibility and P-wave velocity data, and experimental studies of Site 808 samples. These data also show that the strain (including volume change) resulting from plate convergence is partitioned into both penetrative deformation structures as well as more discrete, core-scale structures with finite displacements. These core-scale structures range from relatively subtle, kink-like deflections of the primary phyllosilicate fabric to sharp discontinuities with probable displacements much greater than the dimensions of the core barrel. Although all of the structures acted at least in part as dewatering conduits, evidence of concentrated fluid flow in this region of the prism is limited to a narrow interval almost 150 m above the décollement (located at between 946 and 965 mbsf). This interval correlates with the middle of a hemipelagic sequence above the décollement that appears to have thinned, apparently through dewatering, relative to a more seaward section. Thinning and dewatering appear to have been induced by deposition of a more clastic sedimentary sequence (the outer marginal trench-wedge sediments) that grades upward into a coarse-grained trench-fill sequence. Importantly, the hemipelagic sequence below the décollement appears to have thinned very little, suggesting that these sediments are underconsolidated and overpressured. This interpretation is consistent with porosity measurements from below the décollement. Microscopic and submicroscopic studies of sediments from within the décollement record a cyclic deformation sequence of displacement-brecciation-porosity collapse and compaction that may also reflect deformation of an overpressured sequence. Finally, the structural, physical property, and magnetic data also yield kinematic and geometric results consistent with the present convergent vector between the Philippine Sea Plate and Eurasia. These data indicate a shortening direction that trends between 308° and 315°, consistent with plate convergent vectors that trend between 310°a nd 314°.
Effects of sediment composition and diagenesis on deformation processes at the Nankai and Costa Rica decollements
Insights into shallow‐level processes of mountain building from the Northern Apennines, Italy
Journal of the Geological Society, 2000
Recent observations from submarine convergent margins reveal that sediments at shallow levels of ... more Recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations in physical conditions as they progressively deform and lithify—the early stages of mountain building. Similar on‐land evidence is comparatively sparse, as a result of difficulties of exposure and of early features having been obscured. Here we describe aspects of three structures from the Northern Apennines of Italy that corroborate these notions of complex shallow‐level orogenic processes. The Upper Cretaceous Castiglioncello mélange formed during the subduction phase of Apennine orogenesis; the major Cervarola thrust and the smaller, intra‐formational, Balduini thrust, both of Tortonian age, represent the ensuing collisional phase. All three structures formed at burial depths less than 2 or 3 km and are syn‐diagenetic. Although differing in detail, all three show features such as clay fabrics and mineralized veins that record complex...
In experimentally deformed clays, three parameters which influence the geometry of shear zones ar... more In experimentally deformed clays, three parameters which influence the geometry of shear zones are water content, strain rate and the orientation of the shear zones with respect to any primary fabric. The shear zones are the main microstructures induced in the clays within the experimental conditions (triaxial compression at water contents between 20 and 35% w/w and strain rates between 10-4 and 10-s s-l). Small changes in water content produce a significant change in the appearance of shear zones: structures in wetter sediments are more complex and more numerous than those in drier sediments which tend to produce a small number of discrete planar shear zones. Strain rate is a far less important influence on shear-zone geometry. The orientation of the zones with respect to a primary fabric is also significant. Shear zones which are parallel to the fabric have a simpler geometry than zones which intersect the fabric at a high angle. Knowledge of these factors may help interpret the conditions within which shear zones formed in naturally-deformed soft sediments.
Toes of accretionary prisms record the initial deformation of wet sediments during accretion at a... more Toes of accretionary prisms record the initial deformation of wet sediments during accretion at a convergent plate margin. ODP Leg 131 succeeded in coring the toe of the Nankai prism, SW Japan, together with the basal drcollement, the underthrust sediments and the basaltic ocean basement. An unprecedented structural geological inventory has been compiled. Gravitational deformation is important in the slope-apron deposits, but tectonic stresses, too, are transmitted to these levels. Core-scale deformation bands, due to heterogeneous bulkshortening of the prism, are exceptionally well-developed at Nankai, and range through kink-bands, shear zones and faults. Anomalous frequencies indicate localized high-strain zones. Stress-inversion analysis gives a principal compression close to the plate convergence vector. The frontal thrust of the prism is a 26 m thick zone of breccia/scaly fabric, with a 309 m net slip. It may well be currently active, and accommodating almost half of the present convergence strain. The basal drcollement, a 19 m thick breccia zone, separates the deforming, overconsolidated sediments of the prism from the virtually undeformed materials below. These are highly underconsolidated and probably overpressured. Anomalous fluid pressures localize several of the prism structures. Unlike some other prisms, drainage of the Nankai prism is dominantly pervasive rather than controlled by major structures.
Journal of Geophysical Research: Solid Earth, 1993
We are pleased that experimental research into fluid flow in deformed sediments has continued sin... more We are pleased that experimental research into fluid flow in deformed sediments has continued since our original results were published [Arch and Maltman, 1990] and acknowledge the helpful contribution of Brown and Moore [this issue] to this important topic. The results obtained in 1988 showed that permeability in directions close to parallelism with shear zones in clays was considerably greater than that in a perpendicular direction. In addition, we illustrated deformed natural sediments containing shear zones that bore witness, through mineralization, entrained sediment, enhanced diagenesis, etc., to preferred flow along the zones (see also Maltman [1988]). The slowness of some of these chemical processes means that the concentrated flow along a particular shear zone must have been prolonged, persisting for some time after the structure formed. The optical and electron microscope observations we presented unequivocally showed an intense margin-parallel alignment of clay flakes within the shear zones, and so we called upon the reduced tortuosity in that direction as the key to explaining the greater permeability along the zones. At the same time, we explicitly noted the concomitant porosity reduction that is now commented on by Brown and Moore. The fact remains that irrespective of the theoretical predictions drawn from Pouiseille's law, the observations clearly show that flow is preferred along the shear zones. Brown and Moore do not discuss in any detail the permeability variations within shear zones but concentrate their useful discussion on the permeability relationships between shear zones and the host material. Porosity variations are J. Arch, Aspinwall and Company, Walford Manor, Baschurch,
The main objective of Leg 131 was to provide data on the deformational processes and associated h... more The main objective of Leg 131 was to provide data on the deformational processes and associated hydrogeology of the Nankai prism toe. Drilling succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, reaching 1327 mbsf (metres below seafloor) with good core recovery (55%). Excellent correlation of the lithology and structure, including the frontal thrust and the decollement, with seismic reflection images was also determined. Bedding dips, faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to NW-SE shortening, as expected from the plate tectonic convergence vector. Below the decollement, no significant deformation features were observed, indicating that the decollement is a sharp discontinuity in stress transmission.
Jurassic, basalt, moraine, flint, alluvial, magma: what are these words and what do they have to ... more Jurassic, basalt, moraine, flint, alluvial, magma: what are these words and what do they have to do with wine? The answers are here in this book. They are geological terms that reflect a bond between wine and the land. Understanding geology, however, is tricky. Geological concepts are obscure; processes can be imperceptibly slow, invisible, and unimaginably ancient. The terminology is formidable, such that even the names of common rocks carry an air of mystery. Geology is introduced plainly, starting with basic principles, all in the context of wine. The emphasis is on the kinds of processes that shape vineyards, and on the minerals, rocks and soils that host the vines. Geological words now commonly seen in wine writings are systematically explained. You will learn the stories behind some of the names, the human face of geology. The book also explores how the geology-wine connection manifests in the finished product and evaluates its importance, particularly in the contexts of miner...
What Are Vineyards Made Of ?
Vineyards, Rocks, and Soils
What strikes you first when looking at a vineyard? Perhaps the vines themselves? Your eye may be ... more What strikes you first when looking at a vineyard? Perhaps the vines themselves? Your eye may be caught by random scatterings of gnarly old bushes or by the military neatness of rows of trained vines, luxuriant in foliage in summer and little more than gaunt woody skeletons in winter. But possibly more striking might be the land itself—the geology, or at least manifestations of it. The vines may extend across a vast, flat plain, or they may be perched on a vertiginous slope, or anywhere in between—it depends on the bedrock geology. How well the vines grow will be influenced by how that bedrock weathers into soil and how the vine roots respond. The soil may have an eye-catching color or may be astonishingly stony, consisting of little more than rock debris. This quality, too, depends on the geology. But what exactly is this vineyard ground? What are such things as bedrock, soil, and stone made of? Where do they come from? How did they get this way? The answers form the basis of under...
The Minerals that Make Rocks and Soils
Vineyards, Rocks, and Soils, 2018
This chapter is about the minerals based on silicon and oxygen, the silicates, the ones that make... more This chapter is about the minerals based on silicon and oxygen, the silicates, the ones that make siliceous rocks. And because this means most rocks, apart from limestone and the other calcareous materials, they are often referred to as the “rock- forming minerals.” Let’s be clear at the outset that with these siliceous rocks we’re talking about silicate compounds, which involve the element silicon and a subgroup of silica minerals, which we’ll come to at the end of the chapter. None of this has anything to do with silicone, the synthetic polymer of multifarious uses. The principles discussed here are the same as those developed in the previous chapter, but the silicates present special challenges. Indeed, for a long time they were very tricky things to understand at all. The early geologists had at their disposal new ways of chemically analyzing minerals, and they applied them with gusto. They made impressively rapid progress, but they were baffled by the silicates. Their analyses ...
Vineyards and the Mists of Geological Time
Vineyards, Rocks, and Soils, 2018
Geological time is much mentioned in the wine world. Many a label proclaims the geological age of... more Geological time is much mentioned in the wine world. Many a label proclaims the geological age of the rocks and soils in which the vines were growing; many a vineyard description enthuses about just how old its bedrock is. The age may be expressed as a fine-sounding technical term or as a quantity, typically, some unimaginably large number of millions of years: “The area’s best vineyards are on Turonian soils”; “Cretaceous limestone is best for our vines”; “the wine’s secret is the Devonian slate”; “our Shiraz grows in soils 500 million years old.” It’s almost as though the older the geology can be made to appear, somehow the finer the wine. I must declare my own position in all this: surely the geological age of the bedrock has little to do with viticulture? The age of the soil is certainly relevant, as it is continually changing on a human timescale, but these geological time words almost always are referring to the age of the vineyard bedrock. And almost invariably the age of th...
The Geology of Wine, Spirits and Beer
Geological maps: an introduction
Choice Reviews Online, 1991
Some fundamentals of geological maps the nature of geological maps - the 1:2,500,000 map of the U... more Some fundamentals of geological maps the nature of geological maps - the 1:2,500,000 map of the USA and the ten-mile map of the UK the three dimensional aspect - structure contours measurements in three-dimensions - strike and dip, formation thickness and depth geological cross-sections visual assessment of outcrop patterns unconformities folds faults - the fundamentals more on faults - contractions (thrust), extension, and strike-slip faults igneous and metamorphic rocks - mineral deposits geological history from maps the production of geological maps the heritage of geological maps current trends in geological maps.
Deformation Structures and Fluid Flow in the Toe Region of the Nankai Accretionary Prism
Proceedings of the Ocean Drilling Program, 131 Scientific Results, 1993
Evaluation of hydrogeologic properties of the Barbados accretionary prism: a synthesis of Leg 156 results
Proceedings of the Ocean Drilling Program, 156 Scientific Results, 1997
Proceedings of the Ocean Drilling Program, 131 Scientific Results, 1993
Drilling at Site 808 (ODP Leg 131) provided an extensive record of the discrete brittle structure... more Drilling at Site 808 (ODP Leg 131) provided an extensive record of the discrete brittle structures (small faults and shear bands) at the toe of the Nankai accretionary complex. Brittle failure is occurring throughout the hole, although most of the deformation has been observed between the frontal thrust and the décollement (from 365 to 963 mbsf). Brittle failure occurs in the turbiditic trench fill (0 to 600 mbsf) but also in the ash-bearing hemipelagites from the upper Shikoku Basin (600 to 800 mbsf) and the ash-free hemipelagites of the lower Shikoku Basin sequences, down to the décollement (963 mbsf). The geometry of the tectonic features has been recorded in a local frame related to the core liner and then corrected to an absolute frame by the use of paleomagnetic measurements. We used the resulting geometry of homogeneous populations of slickenlined faults to estimate the reduced stress tensor. The main result is to show three consistent stress patterns. Most of the fault clusters from the frontal thrust down to the décollement are in good agreement with a compressional regime with a northwest-trending σj (azimuth N305° to N315 °). The compression direction is thus roughly parallel to the local direction of the relative convergence. It is also perpendicular to the trend of the anticlinal ridges. Discrete faults, locally appearing as clusters (e.g., 30 m above the décollement) agree with a quite different compression direction (west-southwest-east-northeast) and are restricted to the hemipelagic sequences above the décollement. They could be related to the heterogeneous internal deformation of this layer. Although isolated normal faults exist, their occurrence as clusters is restricted to the Shikoku Basin hemipelagic sequences and indicate a east-west to northwest-southeast extension axis (σ 3). Finally, the analysis of Mohr diagrams related to the best-fitting tensors shows some similarities and differences between the fault populations of the turbidite and those of the hemipelagite sequences. The angle of friction corresponding to 95% of the data is 30° ± 5°, whereas including the remaining 5% in turbidites and hemipelagites would result in a lower friction angle (18° ± 8°). The major difference concerns the angle between conjugate sets which are about 60° in the turbidites and lower (35°-40°) in a 200 m thick zone above the décollement and can be explained by higher fluid pressure in the hemipelagites or by odd mechanical properties of this hemipelagic material.
Measurements in three dimensions: strike and dip, formation thickness and depth
Geological maps: An Introduction, 1990
We are beginning to see why geological maps are such a powerful and convenient means of conveying... more We are beginning to see why geological maps are such a powerful and convenient means of conveying information about the three-dimensional configuration of rocks. Nevertheless, it is often necessary to specify the arrangement in words or numbers. Geologists do this by using the concept of strike and dip. The general idea was introduced in section 2.3.1; the first part of this chapter explains it in detail. The second part of the chapter expands on methods of subsurface projection and some useful measurements that can be made from maps. These techniques are of use in applied geology where, for many purposes, the work will have to be done as accurately as possible, especially if sums of money are at risk. Some of the corrections that may have to be borne in mind for this kind of mapwork are introduced.
The heritage of geological maps
Geological maps: An Introduction, 1990
There are facets of geological maps other than those discussed in the preceding chapters, such as... more
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