Toe Of Wedge Research Articles

Two-sided orogen: Collision and erosion from the sandbox to the Southern Alps, New Zealand

In continental collision zones, the indentor is of approximately the same vertical dimension as the material being deformed, thereby allowing material to move over the indentor and out of the deforming orogen. This simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite-dipping topographic slopes. These two wedges are mechanically coupled, and the mechanics are strongly influenced by the erosion conditions imposed on the wedge. Where the growing mountains perturb prevailing, moisture-laden winds, the erosion pattern is highly asymmetric, erosional work being concentrated at the toe of the steep wedge facing the indentor (the inboard wedge). Concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision. These include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity. The outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the Southern Alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units, as in the western Alps of Europe. The other wedge (the outboard wedge) undergoes relatively little erosion, is broad with a more gentle surface slope, and maintains an outcrop pattern consisting of upper crustal material.

Sediment compaction and fluid migration in the Makran Accretionary Prism

The Makran continental margin in the Gulf of Oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland. A recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region. Velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge, as seen in previous sonobuoy wide‐angle data. Accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism; this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment. A prominent “bottom simulating reflector” appears 500–800 m beneath the sea bed. Several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas, which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile. The hydrate reflector appears to shallow in the region of some faults, suggesting a temperature anomaly due to the presence of warm pore fluids. A heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens. Simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly. The inferred presence of overpressured pore fluids in the Makran suggests that accreted sediments have a low permeability. The seismic evidence suggests a two‐stage compaction process, with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones.

Franciscan Complex olistostrome at Crescent City, northern California

ABSTRACTAn olistostrome and bounding turbidites are exposed within the late Mesozoic Franciscan Complex along the Crescent City (California) coastline. Facies grade in character from Mutti & Ricci Lucchi (1978) mixed facies B, C and D, to F (the olistostrome), to mixed A, B and E, progressing upwards within the Franciscan stratigraphic section. The facies F unit outcrop is up to 600 m thick and extends 12 km along strike. It consists of oblate to tabular blocks, up to 200 m in maximum dimension, of greenstone, tonalite, radiolarian chert, limestone, phyllite and greywacke dispersed in a scaly argillite matrix. The olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites, by the presence of abundant recycled sedimentary clasts within the unit, and by the presence of sedimentary breccias and associated dismembered, slump‐folded turbidites both within the olistostrome and among subjacent turbidites. Sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected, late Mesozoic Sierran‐Klamath magmatic are.Franciscan rocks record an early pervasive, layer‐parallel flattening strain in such features as extensional faults, necking and pinch‐and‐swell structures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges, and the formation of scaly foliation in the olistostrome matrix. The latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay. These were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome. The presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix, and of sandstone dykes that intrude fractures and associated drag‐folded turbidite beds indicate that Franciscan sediments were not lithified during their early deformation. These sediments were deposited in either a trench or trench slope basin, and were first deformed either by gravity collapse of the trench slope cover or, less likely, by vertical loading beneath the toe of the accretionary wedge. They later were folded during internal shortening of the growing Franciscan accretionary prism.

Formation of forearc basins by collision between seamounts and accretionary wedges: An example from the New Hebrides subduction zone

Seabeam data reveal two deep subcircular reentrants in the lower are slope of the New Hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin. The Malekula reentrant lies just south of the partly subducted Bougainville seamount. This proximity, as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off Japan, suggests that the Malekula reentrant formed by the collision of a seamount with the arc. An arcuate fold-thrust belt has formed across the mouth of the reentrant, forming the toe of a new accretionary wedge. The Efate reentrant may show the next stage in basin development. This reentrant lies landward of a lower-slope ridge that may have begun to form as an arcuate fold-thrust belt across the mouth of a reentrant. This belt may have grown by continued accretion at the toe of the wedge, by underplating beneath the reentrant, and by trapping of sediment shed from the island arc. These processes could result in a roughly circular forearc basin. Basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the Aleutian trenches.

Dewatering and extensional deformation of the Shikoku Basin hemipelagic sediments in the Nankai Trough

The Shikoku Basin hemipelagic sequence, which underlies the Nankai Trough wedge, S.W. Japan, thins by 50% between the outer edge of the trench wedge and DSDP Site 582, 14 km arcward. A sedimentation model, which incorporates changes in sedimentation rates with time and with distance from the trench wedge toe, indicates that 57% of the total thinning occurs as a result of temporally varying sedimentation rates and a time transgressive facies boundary between the trench wedge turbidites and the underlying hemipelagites. Burial-induced consolidation beneath the wedgeshaped turbiditic overburden, accounts for the remaining 43% of arcward thinning within the hemipelagic unit. Rapid dewatering, modeled as one-dimensional consolidation suggests that the excess pore water pressures are quite low during this progressive dewatering. Thus, high pore water pressures should not be assumed to occur universally in convergent margin settings. Normal faults and vein structures in the hemipelagites suggest near-horizontal extension in addition to vertical consolidation. The estimates of excess pore water pressures together with fault geometries and horizontal extensional strains, determined from the geometry of the subducting oceanic plate, can be used to constrain the stress conditions at failure. The expulsion of hot water from the rapidly dewatering sediments in the Nankai Trough may help to explain anomalously high heat flow in the central part of the trough.

Estimating interface advance due to abrupt changes in replenishment in unconfined coastal aquifers

Dimensional analysis is used to establish a functional relationship between the interface toe position and the other parameters for a one-dimensional, unconfined, coastal aquifer subjected to a rapid decrease in replenishment. Non-dimensional curves for the motion of the toe of the salt-water wedge for some cases are presented. The curves were developed from accurate numerical solutions of the equations for motion of the interface and can be used for reliable estimates of the response of interfaces under the conditions assumed for the analyses. An approximate formula for the rate of movement of the toe is derived for the special case when replenishment reduces to zero.

SEDIMENT BUDGET OF THE LOWER FRASER RIVER

The Lower Fraser River, being continually developed for better navigation and larger port- facilities, is an area of active sedimentation. The dynamic processes influenced by river discharge, tides and winds are probably the most important factors in the transport and deposition of sediments in the estuary. The main part of the Fraser Estuary is vertically homogeneous (non-stratified) and the sediments are transported progressively seaward and are accumulated near the limit of net landward flow. The lower estuary is stratified and coarse sediments are trapped near the toe of the salt wedge while the fine sediments are carried seaward with the outflowing river water. The hydrometric and sediment survey of the lower Fraser River are described. Survey results are used in determining the sediment balance of the river reach. In the budget analysis, the river and estuary are divided into four consecutive reaches, the sediment discharges are subdivided to represent about 10 particle size ranges and the balance is then determined for each reach and particle size range. Conclusions are drawn with respect to the sediment transport and depositional characteristics, annual variations, and the agreement between the sediment entering the estuary and the sediment dredged.

Seawater Intrusion in Layered Aquifers

Seawater intrusion beneath a semipervious layer in a two‐layer coastal aquifer system is described. Freshwater can occur beneath the aquitard because of the head losses induced by upward flow through the semipervious layer. A steady state Dupuit model is formulated to predict the piezometric head variation landward of and above the intruding seawater. Exact solutions are given for the landward head variation, but a numerical solution is necessary for the zone above the intruding seawater. The mathematical model is compared to experimental results of a Hele‐Shaw model study of Long Island, New York. The analysis provides reasonable predictions of the shape of the interface between freshwater and seawater, but the location of the toe of the seawater wedge is inadequately described, apparently because of regional flow conditions on the island that are not simulated in the analysis.

Intruded Salt-Water Wedge in Porous Media

A laboratory model of a two-dimensional, confined aquifer containing an isotropic homogeneous, porous medium was used to investigate the gravitational convection and dispersion of salt water. An equation for the rate of movement of the intruding wedge toe is presented and experimentally verified for the case of zero net fresh ground-water flow to the ocean. The earlier work of Henry in defining the equilibrium position of the intruded wedge is experimentally confirmed. The extent of the zone of dispersion at the salt-fresh interface due to the steady fresh-water flow to the ocean is treated analytically, and reasonable agreement is found experimentally. The additional interfacial mixing due to tidal effects is shown to be governed by a longitudinal and a lateral coefficient of dispersion that are functions of the pore system geometry of the medium and the tidally-induced seepage velocities.