At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (∼19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-medium-sized prisms have formed (∼16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (∼8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (∼43,500 km) is calculated to be 1.5 km³/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km³/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km³/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km³/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km³/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km³/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km³/yr to a total of 1.1 km³/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km³/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted material is estimated to range from 1.3 to 1.8 km³/yr. Subcrustally subducted material is either returned to the terrestrial crust by arc-related igneous processes or crustal underplating or is lost from the crust by mantle absorption. Geochemical and isotopic data support the notion that upper mantle melting returns only a small percent of the subducted material to the terrestrial crust as arc igneous rocks. Limited areal exposures of terrestrial rocks metamorphosed at deep (>20–30 km) subcrustal pressures and temperatures imply that only a small fraction of subducted material is reattached via deep crustal underplating. Possibly, therefore much of the subducted terrestrial material is recycled to the mantle at a rate near 1.6 km³/yr, which is effectively equivalent to the commonly estimated rate at which the mantle adds juvenile igneous material to the Earth's layer of terrestrial rock.

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Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones

The fold-and-thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the Earth's upper crust. Although there is considerable natural variation among the many fold-and-thrust belts and accretionary wedges that have been recognized and explored, several features appear to be universal . In cross section, fold-and-thrust belts and accretionary wedges occupy a wedge-shaped deformed region overlying a basal detachment or decollement fault; the rocks or sediments beneath this fault show very little deformation. The decollement fault characteristically dips toward the interior of the mountain belt or, in the case of a submarine wedge, toward the island arc; the topography, in contrast, slopes toward the toe or deformation front of the wedge. Deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault-bend folding. Two North American fold-and-thrust belts that exhibit these features are shown in Figure 1. Neither of these two examples is tectonically active today; the southern Canadian fold-and-thrust belt was active during the late Jurassic and Cretaceous (150-100 Ma), whereas the southern Appa­ lachians were deformed during the late Carboniferous to Permian Alle­ ghenian orogeny (300-250 Ma). Figure 2 shows two examples that are currently active: the Taiwan fold-and-thrust belt, produced by the sub­ duction of the Eurasian plate beneath the Philippine Sea plate (Suppe 1981 , 1987); and the Barbados accretionary wedge, produced by the sub-

Critical taper model of fold-and-thrust belts and accretionary wedges

[1] Mud volcanism and diapirism have puzzled geoscientists for ∼2 centuries. They have been described onshore and offshore in many places on Earth, and although they occur in various tectonic settings, the majority of the features known to date are located in compressional tectonic scenarios. This paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described. Mud volcanoes show variable geometry (up to tens of kilometers in diameter and several hundred meters in height) and a great diversity regarding the origin of the fluid and solid phases. Gas (predominantly methane), water, and mud may be mobilized at subbottom depth of only a few meters but, in places, can originate from several kilometers depth (with minor crustal or mantle input). The possible contribution of mud extrusion to global budgets, both from quiescent fluid emission and from the extrusive processes themselves, is important. In regions where mud volcanoes are abundant, such as the collision zones between Africa and Eurasia, fluid flux through mud extrusion exceeds the compaction-driven pore fluid expulsion of the accretionary wedge. Also, quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and, hence, to greenhouse climate.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000RG000093

Significance of mud volcanism

The permeability of argillaceous formations, although rarely measured and poorly understood, is commonly a critical parameter in analyses of subsurface flow. Data now available suggest a regular relation between permeability and porosity in clays and shales and permeabilities that, even at large scales, are significantly lower than usually assumed. Permeabilities between 10 -23 and 10 -17 m 2 have been obtained at porosities between 0.1 and 0.4 in both laboratory and regional studies. Although it is clear that transmissive fractures or other heterogeneities control the large-scale hydraulic behavior of certain argillaceous units, the permeability of many others is apparently scale independent. These results have significant implications for understanding fluid transport rates and abnormal pressure generation in basins, and could prove important for waste isolation efforts.

/pdf/how-permeable-are-clays-and-shales-54e8ay58zq.pdf

How permeable are clays and shales

Recent measurements from Ocean Drilling Program leg 110 and Deep Sea Drilling Project leg 78a indicate that pore pressures near the toe of the Barbados accretionary prism may be close to lithostatic and that the decollement is a zone with relatively high rates of fluid flow and methane transport. We used a numerical model of fluid flow to estimate intrinsic permeabilities, pore pressures, and flow velocities that are consistent with these observations. Model results suggest that the permeability of the decollement may be 3–5 orders of magnitude greater than that of adjacent prism sediments. If permeabilities in the prism vary with depth in a manner similar to those in sedimentary basins, the average intrinsic permeability of the decollement, kd, must be about 10−14 m2. When kd is 10−13 m2, high pore pressures do not develop near the deformation front in the model. If kd is 10−15 m2, simulated pressures are unrealistically high in both the prism and underthrust sediments arcward of the deformation front. Water originating from compaction in the decollement and underthrust sediments flows laterally seaward, while water expelled from prism sediments flows upward to the ocean floor. However, flow velocities are small, and the net motion of pore water in prism and underthrust sediments is arcward relative to the deformation front because of tectonic transport. Pore water migrates seaward in spite of tectonic transport only in discrete zones with higher permeability, in this case the decollement.

Permeabilities, fluid pressures, and flow rates in the Barbados Ridge Complex

The mechanisms for the development of accretionary prisms in subduction complexes is numerically simulated using finite element plasticity analysis and the Barbados problem as an example. The accreted and subducting sediments are modeled as elastoplastic continua separated by rigid-plastic contact elements to simulate the thrusting mechanism. Results of the analyses demonstrate the importance of both lateral compression and topographic slope in the development and propagation of accretionary prisms in the Barbados subduction complex, affirming the analytical theory on the mechanics of thin-skinned fold-and-thrust belts presented by Chapple [1978]. Numerical analyses also reveal that the shape of the accretionary prism requires a critical balance in material characteristics such as the frictional strength of the basal layer, the modulus of the decollement, and the flow laws governing the yielding of the elastoplastic accretionary prism itself. Finally, it is shown that, unless the high normal stress at the base of the overriding plate is counteracted by a high fluid pressure localized in the decollement region, the frictional component of the basal traction will make the horizontal propagation of accretionary prisms difficult.

Numerical modeling of accretionary wedge mechanics: Application to the Barbados subduction problem

Sediment compaction at convergent margins expels pore fluids, which in turn influence many aspects of subduction zone geology. Drilling in the Barbados Ridge complex during ODP Leg 110 and DSDP Leg 78A provided information about sediment types, porosities, and the geometry of the complex. In this paper, we use these observations to estimate the rates of sediment porosity loss and accompanying fluid expulsion from the prism, the decollement, and the underthrust sediments. Rates of porosity loss depend on how rapidly the sediments move arcward through the complex. We compute rates of sediment movement in the prism and decollement as a function of distance from the deformation front. This calculation assumes that the rates of sediment movement decrease arcward as the result of porosity loss in the prism and decollement and thickening of the prism. According to computed rates of sediment movement for the prism, sediment deceleration is greatest within the first 3 to 5 km arcward from the deformation front. Similarly, most dewatering of the prism sediments takes place in this region. Beyond 5 km, rates of sediment dewatering become great­ est in the underthrust sediments, assuming these sediments are carried downward at a uniform rate with the oceanic plate.

http://www-odp.tamu.edu/publications/110_SR/VOLUME/CHAPTERS/sr110_21.pdf

21. fluid flow within the barbados ridge complex, part i: dewatering within the toe of the prism 1

Measurements of sediment physical properties and pore-water chemistry gathered during ODP Leg 110 and DSDP Leg 78A suggest that (1) fluid flow in the decollement is predominantly updip, and (2) near-lithostatic pore pressures may exist just above the decollement at Site 542. We use these observations to constrain a numerical model of fluid flow in the toe of the complex. Gravitational and tectonic forces drive flow within the complex and are incorporated in the numerical model by esti­ mating the rate and distribution of fluid generation from sediment compaction. Modeling results reveal that near-litho­ static fluid pressures could form at Site 542 if the equivalent prism permeability is between I0 -18 and I0 -19 m2 (hydrau­ lic conductivity of I0 -9 to I0 -10 cm/s). Predominantly horizontal flow in the decollement can occur if the decollement permeability exceeds the equivalent prism permeability by three to four orders of magnitude. Under these conditions, average linear fluid velocities in the decollement range between 3 and 10 cm/yr (1.5 to 5 times the convergence rate). Fluid velocities in the prism are about 1000 times slower. Depending on the permeability contrast between the decolle­ ment and the underthrust sediments, between 65% and 90% of the fluids expelled beneath the accretionary prism flow out of the complex through the decollement.

http://www-odp.tamu.edu/publications/110_SR/VOLUME/CHAPTERS/sr110_22.pdf

Shirley J. Dreiss

Papers

Permeabilities, fluid pressures, and flow rates in the Barbados Ridge Complex

Numerical modeling of accretionary wedge mechanics: Application to the Barbados subduction problem

21. fluid flow within the barbados ridge complex, part i: dewatering within the toe of the prism 1

22. fluid flow within the barbados ridge complex, part ii: permeability estimates and numerical simulations of flow velocities and pore pressures 1