bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of the rate and amount of absolute lateral shortening in an orogen using diachronism and strike slipped segments
Authors: Phillips W. E. A.
Published: May 2008

By reference to the Caledonide Orogen, a method is illustrated by which quantitive estimates may be obtained of lateral movements between opposing forelands of a collision orogen. The procedure involves: 1: Identification of a rigid foreland frame (2 plates) bounding the collision orogen and establishing the orientation of the main movement direction between forelands, 2: Measurement along strike of diachroneity in the ending of arc volcanism and of cooling ages for granites; relating this to a rate of migration of collision along strike of the collision suture, and calculation of the rate of plate convergence from this, knowing the angle between the collision suture and the plate movement direction, 3: Recognition of swings in strike which predate collision, and 4: Measurement of lateral offset of dated collision points in the strike slip sector of the curved orogen and calculation from this of the components of lateral shortening around the curve. Application of these methods to the Caledonide Orogen of the British Isles, Scandinavia and E Greenland, gives values for foreland convergence on a trend of 115–295° of 0.88 cm/year between the Lower Caradoc (430 Ma) and Lower Ludlow (
c.
400 Ma). Convergence slowed to 0.75 cm/year on this trend during the Caledonian orogenesis between Lower Ludlow (
c.
400 Ma) and the end of the Middle Devonian (
c.
365 Ma), giving lateral convergences of about 267 km in the E Greenland W Norway region (
c.
22%) and 216 km (
c.
31%) in the paratectonic Caledonides of the British Isles.
Strain within thrust sheets
Authors: Coward M. P. , Kim J. H.
Published: May 2008

The finite strains within the thrust sheets of the Moine Thrust Zone in NW Scotland have been factorized into components of simple shear and longitudinal strain. Variations in these strains have been examined in the Cambrian sediments in the Eriboll and Assynt areas and strain maps produced of the Glencoul Nappe, Assynt. There are heterogeneous simple shear strains parallel to the layering resulting from bending of the nappe over a step in the thrust plane and to frictional drag at the base of the thrust. Layer parallel shortening and associated layer parallel thickening occur in the front of the thrust zone at Eriboll, above a decoupling plane in the lower part of the Cambrian sequence. Within the Cambrian rocks of the Glencoul Nappe in Assynt, there is a steady eastward increase in intensity of layer parallel shortening togeather with local anomalous zones characterized by more intense shortening or extension. These anomalous zones overlie shear zones in the Lewisian basement. There are variations in shear strain on the plane which contains the normal to bedding resulting from differential movement of the nappe. These shear strains distort and fold the bedding planes. Combinations of these shear strains and the longitudinal strains result in a wide range of ellipsoid shapes from oblate to prolate and also explain the pattern and sequences of folds in the nappes and mylonite belts. The folds are formed by local variation in shear and longitudinal stress and, hence, there may be no simple correlation of fold phases along, or across, a thrust zone.
Tectonics of the Helvetic Nappes
Authors: Ramsay J. G.
Published: May 2008

This paper gives a general account of the principal major nappes, fold and fault styles, and minor structural features of the Helvetic Nappes with particular emphasis on research results over the past few years. The principal stratigraphic variations are summarized and their influence on development of tectonic structures described. Measurements of strain variations throughout the nappe pile can be used to evaluate the significance of cleavage and linear fabrics. Pressure solution is a particularly important rock deformation process throughout the whole region and provides the source for the fibrous vein fillings which characterize much of this region.
Fold-and-thrust tectonics in the Helvetic Nappes (E Switzerland)
Authors: Pfiffner O. A.
Published: May 2008

The Helvetic Nappes in E Switzerland form an example of décollement nappes. The Säntis Thrust represents both an upper and a lower detachment. In the units underlying this detachment shortening is accomplished by a combination of folding and thrusting and is strongly influenced by the occurrence of 2nd order décollement horizons. Nappe-internal deformation resulted in the reorientation of folds (development of a fold arc) and smoothing out of ramps and flats in thrust surfaces. Two types of folds can be recognized: true buckle folds and folds due to ramps and flats.

The movement direction of the thrust sheets was S-N and fold axes oblique to this direction developed probably due to initially ENE striking ramps. Transverse faults with a strike slip component allow changes in displacement along individual thrust surfaces.
Some observations on the development of thrust faults in the Ultradauphinois Zone, French Alps
Authors: Beach A.
Published: May 2008

The Ultradauphinois Thrust Nappe involved a westerly movement of external autochthonous Jurassic and Eocene sedimentary sequences by décollement on the Trias. A complex thrust belt evolved at its southern end as the movement was hindered by the upfaulted Pelvoux basement massif. All of the Jurassic rocks exhibit strong, enetrative ductile strains—in the lowermost thrust slices (most westerly part of the sheet) forward spreading strains (NE-SW stretching) involving a combination of simple and pure shear, and in the uppermost thrust slices a NW-SE stretching and rearward compression as the main nappe continued to move past to the N of the hindrance. As the resistance to movement increased in the S, the completely distributed ductile strain became localized in numerous zones, characterized by the formation of many veins and strong local deformation of these veins. This marks the onset of stratigraphic imbrication and repetition. The progress from penetrative ductile to localized ductile to discrete brittle strain is recorded by the sequence of structures observed in the field. The main discrete, brittle thrust surfaces represent the last major stage in a continuous sequence of deformation.
Gravity sliding in the Maritime Alps
Authors: Graham R. H.
Published: May 2008

Fallot’s ‘intercutaneous’ Tinée Nappes are interpreted as gravity slides initiated by uplift of the Argentera Massif and emplaced partly by the ductile deformation of slate horizons, and partly by movement on discrete shear planes. Together, the structures stack between 8 and 11 km of Jurassic and Lower Cretaceous strata—the telescoped cover of the Argentera Massif. The lowest nappe is the furthest travelled.

Sliding on a larger scale may have contributed to shortening in the more external parts of Provence.
Sutures, thrusts and nappes in the Variscan Arc of western Europe: plate tectonic implications
Authors: Matte Ph. , Burg J. P.
Published: May 2008

The Variscan Arc of western Europe, convex to the W, links the Iberian and the middle European Variscides. It is characterized by nappes and thrusts with converging vergences towards the inner part of the arc. This is illustrated by two cross-sections, one roughly E-W in northwestern Spain, the other N-S in the French Massif Central. This segment of the Variscan belt appears to be a typical collision belt whose plate tectonic evolution may be explained in the following stages: (i) consumption of a Lower Palaeozoic ocean by an intraoceanic subduction towards the NW, (ii) choking of subduction by underthrusting of continental crust beneath the oceanic lithosphere (obduction), (iii) intracontinental deformation during which the arcuate form results from the progressive impingement of a southern continental promontory (part of Africa?) into the northern Laurasian continent.
Thrust and strike slip fault interaction along the Chaman transform zone, Pakistan
Authors: Lawrence R. D. , Yeats R. S. , Khan S. H. , Farah A. , DeJong K. A.
Published: May 2008

The interaction between thrust and strike slip fault systems is well detailed in Pakistan where the Chaman transform zone connects the Makran and Himalayan convergence zones and contains an internal convergence zone in the Zhob district. The transform zone contains numerous strike slip faults of which the Chaman fault proper is the westernmost. We can demonstrate at least 200 km of left lateral displacement along the Chaman fault alone. In the Zhob belt N-S shortening by folds and a major thrust fault amounts to several dozen kilometres. The 400 km wide Makran convergence zone is now being shortened by E-W oriented folds, thrust faults, and reverse faults. As these faults in the Makran zone approach the transform zone, their traces bend to the N and motion on each of them becomes oblique, combining reverse and left lateral slip. They merge continuously with the strike slip faults of the Chaman transform zone. The Makran thrust system and the Chaman transform zone first became active in the late Oligocene or early Miocene. Later (Pliocene?), a component of left lateral shear occurred across the entire Makran Zone in association with the opening of the newly identified Haman-i-Mashkel fault trough S of the Chagai Hills and W of the Ras Koh. The total displacement and displacement rate across the Chaman transform zone varies in response to the rates of convergence in the plates E and W of the zone.
Active thrusting and the evolution of the Zagros fold belt
Authors: Jackson J. A. , Fitch T. J. , McKenzie D. P.
Published: May 2008

Earthquake locations and fault-plane solutions reveal the spatial distribution and kinematics of strain release by faulting; information that is difficult to infer from geological evidence alone. In the Alpine-Himalayan belt, thrust faulting can be classified into one of three distinct types, each of which illustrates a particular aspect of continental deformation. Some thrusts are mechanically linked to strike-slip faults. Low angle thrusting is observed in regions where intermediate depth earthquakes and volcanism show that subduction is or was occurring. High angle reverse faulting at shallow depths is seen in the Zagros and it is suggested that this is the result of rejuvenation of old normal faults which stretched and thinned the basement of a Mesozoic continental margin.
Geometrical problems and implications of large scale over-thrusting in the Banda Arc -Australian margin collision zone
Authors: Audley-Charles M. G.
Published: May 2008

Inversion of stratigraphical sequences, the presence of high grade metamorphic rocks sitting on serpentinite rafts bounded by unmetamorphosed sedimentary rocks, and zones of imbrication have been cited as evidence of large scale overthrusting in the Outer Banda Arc. Earthquake data reveal a Benioff zone implying intermediate depth and deep focus underthrusting beneath the Banda Arc but there is a paucity of shallow focus shocks with underthrusting mechanisms. Multichannel seismic reflection surfeys between the Outer Banda Arc and the Australian-New Guinea shelf have been interpreted as showing imbrication in the north wall of the Timor Trough and the S wall of the Seram Trough. Quaternary steep angled block faulting post-dates and thus obscures much of the thrust faulting in the Banda Arc.

Many geometrical problems arise in trying to interpret the structural history of this collision zone between Australia and the volcanic (Inner) Banda Arc. They include:- (1) the apparent absence of a subduction trench and accretionary arc-trench gap in the Banda Arc; (2) the location of the surface trace of the Benioff zone before collision; (3) the history of the Benioff zone after the Pliocene oceanic trench was destroyed; (4) the relationship of the developing fold and thrust belt to the pre-collision geometry of the Australia-New Guinea continental margin; (5) the apparent absence of the continental slope and rise in the northern Australia collision zone; (6) the relationship of the crystalline basement of the Outer Banda Arc to the cover rocks and (7) the tectonic significance of the apparent continuity of the stratigraphically and structurally very different Sunda and Banda Arcs.

Continent-arc collision resulted from the convergence of Australia and the Banda volcanic arc. The associated deformation, represented by the folding and local imbrication of the Australian continental rise sediments of the Outer Banda Arcs with the emplacement of overthrust exotic sheets, was accomplished in 2 Ma. It appears to have been caused by the failure of the Benioff zone to accommodate the thick sediment prism of Australian proximal continental rise. Geometrical considerations suggest that, as a consequence, the Benioff zone and most of the approximately 200 km wide arc-trench gap were overridden by the Australian lithospheric plate during the continued plate convergence of the last 3 Ma. An important conclusion is that the Banda Arc fold and thrust belt developed in proximal continental rise deposits at the foot of the Australian continental slope
Neogene thrust emplacement from a frontal arc in New Guinea
Authors: Milsom J.
Published: May 2008

In northern New Guinea there is clear evidence that at some stage in the Neogene the northern margin of the Australian continent was in collision with a southwards facing island arc. There is still active volcanism along the whole length of this arc and a deep oceanic trench opposite the segment E of the collision zone. The Finisterre Range, a part of the former frontal arc and now part of northern New Guinea, lies up to 50 km S of its expected position with respect to the segments to E and W. Geological and geophysical evidence combine to suggest that this offset is not a deep seated feature, but is the result of movement on a very shallow thrust. Detachment of the thrust sheet from the remainder of the upper crust evidently took place along the line of weakness represented by the volcanic arc.

The mobile segment may have been the part of the frontal arc that first collided with the continental margin; the adjacent segments, which were probably, at the time of collision, opposite largely oceanic, and therefore much deeper, trenches, have not moved in this way.

Since its initial formation, the thrust sheet, or some parts of it, has evidently been involved in movements parallel to the strike of the arc.
The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains
Authors: Price R. A.
Published: May 2008

The thick (∼40 km) slab of Hudsonian (>1750 Ma) continental crust that extends under western Canada from the Canadian Shield can be followed westward, on the basis of its distinctive magnetic anomalies, its influence on the Bouguer gravity values, the results of deep seismic refraction experiments, and the results of geomagnetic depth sounding of the deep electrical conductivity structure, to the Kootenay Arc. The Kootenay Arc is basically a W-facing monocline of crustal dimensions, across which the change in structural level involves an aggregate stratigraphic thickness of up to 20 km. It marks the western edge of the continental craton over which the displaced supracrustal rocks have been draped.

Balanced structure sections of the thrust and fold belt, which take into consideration the deep crustal structure, as constrained by the geophysical data, show that: (i) in early Campanian time the continental crust that now lies beneath the western Rocky Mountains and the Purcell anticlinorium was covered with the platformal Palaeozoic to Upper Jurassic rocks and the exogeoclinal Mesozoic rocks that now form the northeasterly verging imbricate thrust slices of the eastern Rocky Mountains; (ii) the Cordilleran miogeocline developed outboard from the edge of the continental craton, on tectonically attenuated continental crust, or oceanic crust; and (iii) tectonic shortening of about 200 km in the supracrustal rocks in the Rocky Mountains must be balanced at a deeper level, W of the Kootenay Arc, by the shortening of the oceanic or attenuated continental crust.

The net convergence between the Cordilleran magmatic arc and the autochthonous cover on the continental craton is a type of intra-plate subduction that was antithetic to the SW verging subduction zone marking the boundary of the North American Plate. The basement of the back-arc or marginal basin, in which the miogeocline formed, was consumed; but the adjoining continental margin was not. The foreland thrust and fold belt is a shallow subduction complex that was tectonically prograded over the margin of the continental craton, as the supracrustal cover scraped off the down-going slab was piled up against the overriding mass, and spread laterally eastward under its own weight.
The nature and significance of large ‘blind’ thrusts within the northern Rocky Mountains of Canada
Authors: Thompson R. I.
Published: May 2008

The northern Canadian Rocky Mountains comprise a rugged, structurally complex Foothills subprovince of large amplitude box and chevron folds, and a structurally diverse Rocky Mountain subprovince in which large mappable thrusts are rare. The boundary between them is, in some regions, defined by the unfaulted E-dipping limbs of an
en echelon
sequence of large mountain-front anticlines. The lack of thrusts, especially along the mountain front, contrasts with the well exposed linearly continuous thrusts of the Front Ranges structural subprovince within the southern Rocky Mountains, and leads to the impression that little lateral displacement has occurred.

Where deep cross-cutting valley erosion combines with increased fold plunge, it is apparent that the frontal anticlines are, in reality, large allochthonous sheets displaced many kilometres eastward relative to the craton on flat thrusts that separate Ordovician shales from underlying Devonian and Mississippian shales. The faults can be traced, in some places, eastward to the mountain front where they cut abruptly through the thick hanging wall successions of carbonate rocks; however, they cannot be mapped further eastward into surface exposures because they terminate within a décollement zone of Devonian and Mississippian shales, where the displacement on them is transformed into disharmonic folds and tectonic thickening of overlying units.

The Devonian and Mississippian shale succession is interpreted as a fundamental décollement zone of regional extent that separates a lower structural level of thrustfaulted carbonate rocks from an upper structural level characterized by folded late Palaeozoic and Mesozoic units. The shortening represented by Foothills folds is interpreted to equal the amount of shortening on ‘blind’ thrusts beneath the western margin of the Foothills structural subprovince.

A structural reinterpretation across the Muskwa Anticlinorium using the blind thrust interpretation demonstrates that the mountain-front Tuchodi Anticline may represent a large allochthonous thrust sheet folded over a large step in the blind thrust on which it was transported.

The northern Rocky Moutains, narrower and less foreshortened than the southern Rocky Mountains, are interpreted as a thin-skinned tectonic regime similar to but orogenically less mature than the southern Rocky Mountains.
Metamorphic complex of SE Canadian Cordillera and relationship to foreland thrusting
Authors: Brown R. L.
Published: May 2008

The Rocky Mountain miogeocline thickens westward into a basin that was established during the Proterozoic. The western margin of the basin is flanked by a platform that was developed upon Aphebian continental crust. These Aphebian rocks comprise the basement gneisses of the Shuswap Complex that have telescoped onto the cratonic margin of North America. The floor of the basin is assumed to have been transitional or oceanic crust that was subducted westward beneath the Aphebian continental crust of the Shuswap Complex.

Deformation of the platform and western margin of the basin was initiated in the Palaeozoic and culminated in the Jurassic. Development of easterly overturned folds in the basin and thrust faults along its eastern margin occurred during the final stages of ductile shortening of the basin. Thrust faults developed in the western margin of the miogeocline and deformation migrated eastward as the Shuswap Complex encroached upon the cratonic margin of the North American plate.

Eastward displacements of allochthonous cover rocks of the Shuswap Complex are post metamorphic events that are most likely interlocked with thrust faulting in the western margin of the Rocky Mountain miogeocline. Westward underthrusting of the basement of the miogeocline probably ceased by late Eocene and a regime of NE-SW extension was established. Most recent movements of allochthonous cover rocks and late doming of the metamorphic complex are related to this period of Tertiary extension.
Thrusts and nappes in the North American Appalachian Orogen
Authors: Hatcher R. D.
Published: May 2008

The Appalachian Orogen in North America was subjected to three major deformational-thermal events: the Taconic (Ordovician-Silurian), Acadian (late Devonian), and Alleghanian (Permian). Each event involved large-scale horizontal transport of thrust-nappes ranging from tens to hundreds of kilometres in different parts of the orogen. Transport was dominantly westward although major Acadian-generated eastward transport occurred in southern New England.

There is a direct relationship between chronological proximity to a thermal peak and numbers of thrusts. Thrusts were produced during the Taconic and Acadian events which pre- and post-dated the thermal peak, as well as being synchronous with it. Transport of the Bay of Islands ophiolites and other large masses along the western margin of the orogen occurred before Taconic metamorphism, but probably only immediately before. Many large thrusts of the Appalachians were active during two or even three of the deformational-thermal events, and more than once within a single event. This is particularly true for the Blue-Ridge-Inner Piedmont mega-nappe, which involves at least 225 km of horizontal transport.

Compressional tectonics were probably the dominant process responsible for all thrusts in the Appalachians, except the Taconic klippes. The Alleghanian décollement Valley and Ridge thrusts are overridden by crystalline Alleghanian and older thrusts of the Blue Ridge in the southern Appalachians. The same mechanism must apply to the central Appalachians.

Thrusting and formation of crystalline thrust-nappes in the Appalachians and other mountain chains may be an adiabatic process which functions to dissipate much of the thermal energy produced during subduction (both A and B types) and collision events.
COCORP seismic reflection profiling across thrust faults
Authors: Brewer J. A. , Cook F. A. , Brown L. D. , Oliver J. E. , Kaufman S. , Albaugh D. S.
Published: May 2008

It is very important to have good subsurface data in order to understand the nature and behaviour of thrust faults. Deep crustal seismic reflection profiling is the best technique currently available to make detailed subsurface studies of such important problems as the attitude and extent at depth of major faults, and hence deduce the mode of deformation and tectonic forces producing them. The Consortium for Continental Reflection Profiling (COCORP) is collecting large quantities of seismic reflection data from the deep crust and upper mantle in many parts of the U.S.A. Areas of major thrusting which have been profiled so far by COCORP include the Wind River Mountains in Wyoming and the Southern Appalachians of Georgia and Tennessee. Seismic profiles have been very successful in delineating a major thrust fault of moderate dip underlying the Wind River Mountains, thus demonstrating that compressional tectonics were dominant in their formation. In Georgia and Tennessee the seismic profiles demonstrate that the major tectonic feature of the Southern Appalachians is a relatively thin overthrust sheet, which may have moved at least 260 km. Deep crustal seismic reflection profiling thus appears to be an indispensable tool for the study of areas in which thrusting and nappe formation have occurred.
The Shimanto Belt of Japan: Cretaceous-lower Miocene active-margin sedimentation
Authors: Taira A. , Okada H. , Whitaker J. H. , Smith A. J.
Published: May 2008

South-east of the two paired metamorphic belts of Kyushu, Shikoku and Honshu, and separated from them by the Chichibu and Sambosan belts and the Butsuzo Tectonic Line, lies a belt of mildly metamorphosed sedimentary rocks called the Shimanto Supergroup. Their area, extending from the Nansei Islands through Kyushu, Shikoku, Kii Peninsula, Akaishi and Kanto Mountains to the Boso Peninsula, is comparable in size with the combined Franciscan Formation and the Great Valley sequence in California. A Cretaceous lower group is flanked on the Pacific side by an upper group of Palaeogene to early Miocene age. The Shimanto sediments, mostly sandstones and mudstones, were deformed by cyclic subduction into open and isoclinal folds which close south-eastwards, and thrust slices which in places incorporate basaltic pillow lavas and radiolarian cherts.

Sandstones from Kyushu and Shikoku show marked changes of composition with stratigraphic position and the feldspar content can be used to distinguish Cretaceous from Tertiary sandstones. Data on sandstone petrography and palaeocurrents indicate that sediments of the Shimanto Supergroup were probably derived from the NW. Precambrian gneisses and older Mesozoic granitic rocks of the Korean Peninsula may have contributed sediments in Early Cretaceous times, when the Shimanto Terrane was located to the east of southern Korea. The unroofing of granites in the Inner Zone of SW Japan may also have contributed to the Cretaceous and later sedimentation. Quartz-rich, well-sorted sands in younger Shimanto sediments may have been recycled from older Shimanto formations.

The thick Shimanto sedimentary sequences were laid down in a variety of environments within a forearc basin on accretionary complex, trench-slope break, trench inner slope and trench settings; first in the Cretaceous and again, in a more southerly position, in the Palaeogene and early Miocene. In the shallower waters on the inner (arc) side of the Palaeogene forearc basin, coarsening- and thickening-upward deltaic sequences were deposited. In deeper water further offshore, submarine channel and fan complexes, base-of-slope slump deposits and red shales with cherts accumulated. Mélanges with basalts and slump olistostromes occupied the trench-slope break, while inner trench wall perched basins on accretionary basement were filled with coarsening-upward flysch, slump-olistostrome facies and mélanges with metabasalts: similar lithologies probably occupied a trench fill.

Sedimentation and deformation were controlled by intermittent cyclic subduction of the Kula Plate towards the north. Phases of tension and down-faulting, forming long, narrow intra-arc basins, alternated with those of compression. Comparable sedimentary environments are found today off SE Japan on the submarine terraces, trench-slope break, inner slope (including perched basins) and fill of the Nankai Trough.
Tectonics of some forearc fold belts in and around the arc-arc crossing area in central Japan
Authors: Ogawa Yujiro
Published: May 2008

The zonal structure of the old Honshu arc in central Japan is deflected and cut by the colliding younger Izu-Bonin arc causing peculiar types of accretionary fold belts. The Shimanto Fold Belt (s.s.), made up of Cretaceous flysch and pre-flysch sediments, shows a collisional fold pattern: arcward vergence in the inner side and oceanward vergence on the outer side. The minor structures are also different, with shear folds on the inner side of the belt and lens folds on the outer side. Structures in the inner side grew by flattening and those in the outer side by simple shearing. The divergent development of structural characteristics is explained by the differences in geothermal gradient and stress fields across the belt. The Izu-Bonin arc has a forearc fold belt situated between the Honshu and Izu-Bonin arcs. The stratigraphical, palaeogeographical and structural history of the area shows that it developed in a strike-slip regime associated with the transform fault between the Eurasian and Philippine Sea plates.
Forearc geological structure of the Japanese Islands
Authors: Shiki Tsunemasa , Misawa Yoshibumi
Published: May 2008

Under the forearc region off the Japanese Islands a conspicuous basal acoustic reflection plane can be traced at the surface of the acoustic basement from the oceanic plate into the bottom of the accretionary prism. Recent studies indicate this plane cannot be the upper boundary plane of the subducting oceanic plate, but must be the upper layer of the oceanic basaltic basement (or chert layer).

The tectonic activity of the Wadati-Benioff zone appears to be a thrust movement which cuts the boundary between the oceanic basement and the overlying sediments. Thrusts cut the basal reflector and extend into the accretionary prism. Large earthquakes which occur at intermediate and shallow depths under the continental shelf and slope are a direct manifestation of these thrust movements.

Thrust faulting also occurred in past geological time. Acidic and basic magma, possibly generated along the earthquake zone at intermediate depths, ascended along the thrust faults or associated normal faults. Intrusions occurred 15–16 Ma ago in the forearc region of Southwest Japan and 22 Ma ago in that of Northeast Japan; subsequently basins subsided landward of the intrusions. In some cases the forearc igneous rocks carried continental and oceanic crustal xenoliths. Good examples of such forearc igneous rocks, including xenoliths, occur in the Permian and Triassic structural belts of the Japanese Islands.

The model described here is specific to the forearc regions of the Japanese Islands. We envisage that different tectonic parameters such as convergence rate and sediment supply may engender different structural processes in other forearcs.
Facies belts of the Middle America Trench and forearc region, southern Mexico: results from Leg 66 DSDP
Authors: Moore J. Casey , Watkins Joel S. , McMillen Kenneth J. , Bachman Stephen B. , Leggett Jeremy K. , Lundberg Neil , Shipley Thomas H. , Stephan Jean-Francois , Beghtel Floyd W. , Butt Arif , Didyk Borys M. , Niitsuma Nobuaki , Shephard Les E. , Stradner Herbert
Published: May 2008

The Middle America Trench SE of Acapulco is flanked by a steep canyon-incised slope and narrow shelf, showing one of a variety of sedimentary facies patterns possible at convergent margins. Piston and drill cores from this region define eight facies belts including: (1) a pelagic facies of brown clay, (2) an outer slope mud facies, (3) a trench sand facies, (4) a foraminiferan-free facies on the lower slope, (5) a foraminiferan-bearing facies on the mid-slope, (6) a laminated mud facies on the upper slope, (7) a shelf facies of sand and mud, and (8) a canyon facies of sand and gravel. The superposition of trench and lower slope sediment during accretion results in a fining upward sequence reflecting a gradual uplift of the seafloor through the trench sediment-plume. The lower limit of the foraminiferan-bearing facies is defined by the absence of
in situ
calcareous foraminiferans and is controlled by the calcite compensation depth. The upper slope laminated mud facies probably reflects the depth range of the oxygen minimum zone.

In the Leg 66 area sedimentation rates are high in the trench and on the outer and lower slope, decrease on the mid-slope, and increase again on the shelf. On the inner shelf, waves and currents concentrate sand which funnels through a prominent submarine canyon, bypassing the mud-dominated slope and accumulating in the trench. A terrigenous sediment-plume generated by trench turbidity flow causes accelerated sediment accumulation to about 500 m above and 40 km seaward of the trench. The volume of material transported by the trench sediment-plume is five or six times greater than that moved by the shelf sediment-plume which supplies detritus to the shelf, upper slope and mid-slope environments.
Tectonic processes along the Middle America Trench inner slope
Authors: Shipley Thomas H. , Ladd John W. , Buffler Richard T. , Watkins Joel S.
Published: May 2008

The relationship between sedimentation and structural variations observed in geophysical data provides insight into tectonic processes on the convergent Middle America Trench margin. We observe that some of the hemipelagic and pelagic oceanic sediments seem to continue relatively undeformed beneath the lowermost slope. Trench sediment bodies restricted to ponds in the vicinity of submarine canyons are incorporated into the lower slope by folding and thrust faulting. This offscraping (shallow accretion) at the base of the Middle America Trench slope appears at least partly dependent on the presence of trench fill. The internal structure of the so-called accretionary zone, the top of which is usually identified on seismic reflection profiles by a prominent zone of diffractions, is rarely resolved in the seismic reflection data. The shallow part of the accretionary zone along much of the margin may consist of deformed slope sediments since down-section increase in both folding and other deformation are observed in the seismic reflection and drill data. Beneath the undeformed and deformed slope sediments, offscraped trench fill may occur if trench fill was present in the ancient trench. The deeper part of the accretionary zone representing the bulk of the volume is not resolved in the reflection data but may consist of other sediments or crustal rocks added by an underplating process at deeper structural levels.
Sedimentation in different tectonic environments of the Middle America Trench, southern Mexico and Guatemala
Authors: McMillen Kenneth J. , Enkeboll Robert H. , Moore J. Casey , Shipley Thomas H. , Ladd John W.
Published: May 2008

Late Pleistocene to Holocene sediment facies and composition within and bordering the Middle America Trench offshore southern Mexico and Guatemala reflect two distinctly different tectonic provinces. The truncated Mexican margin with crystalline rocks onshore and a narrow shelf exhibits a locally thick sandy trench fill. Sand mineralogy of quartz, feldspar, and biotite matches the onshore source terrane. The Guatemalan margin with a volcanic terrane onshore and a wide forearc basin show smaller amounts of predominantly muddy trench fill. Sand composition of volcanic rock fragments, plagioclase, and heavy minerals accurately reflects the volcanic source. Trench fill in both areas corresponds to submarine canyon location, and extensive bypassing of the slope occurs. The outer shelf contributes little sediment to the Mexican slope and trench, the main source being in the littoral zone and inner shelf. Conversely, most Guatemalan slope and trench sand has come from the shelf. Trench fill correlates best with onshore geology, with similar slope sediments in both areas. The Mexican margin with locally thick trench fill displays clear evidence of accretion, the Guatemalan margin with meagre trench fill apparently has not accreted lower plate sediments recently. Possibly, larger volumes of trench fill encourage accretion.
A summary of Deep Sea Drilling Project Leg 67 shipboard results from the Mid-America Trench transect off Guatemala
Authors: von Huene Roland , Aubouin Jean , Azema Jacques , Blackinton Grant , Carter Jerry A. , Coulbourn William T. , Cowan Darrel S. , Curiale Joseph A. , Dengo Carlos A. , Faas Richard W. , Harrison William , Hesse Reinhard , Hussong Donald M. , Ladd John W. , Muzylov Nikita , Shiki Tsunemasa , Thompson Peter R. , Westberg Jean
Published: May 2008

The Middle America Trench off Guatemala was transected by 24-channel seismic reflection surveys, seismic-refraction surveys, and drilling with the
Glomar Challenger
. The drilling was done at three sites on the oceanic Cocos plate and four sites on the Caribbean plate. These plates converge at about 10 cm yr
−1
. At all drill sites sediment of upper Miocene to Quaternary age is almost entirely hemipelagic mud with interbedded thin volcanic ash, except in the trench where mud and fine sand turbidites less than 400 000 yr old are ponded. However, the underlying rocks are very different. On the oceanic Cocos plate a basal chalk sequence of lower and middle Miocene age is overlain by a thin section of abyssal clay. At a site only 3 km landward of the trench axis where drilling penetrated the slope deposits we recovered a Cretaceous to lower Miocene claystone sequence resting on a section containing igneous rock of continental affinity. A large net subduction of sediment along with ocean crust has occurred during the present (Miocene-Quaternary) episode of subduction and perhaps parts of the continental framework have been subducted as well. However, no current model satisfactorily explains the surprising occurrence of Cretaceous-Miocene claystone at the foot of the trench slope.
Cenozoic structure, stratigraphy and tectonics of the central Peru forearc
Authors: Kulm L. D. , Thornburg T. M. , Schrader H.-J. , Resig J. M.
Published: May 2008

The central Peru forearc (6°–14°S) is characterized by a variety of structural and tectonic styles. Continental shelf basins, Sechura, Salaverry, and East Pisco, are floored by a block faulted Precambrian to Palaeozoic massif and accumulated as much as 3 km of Cenozoic and probably Mesozoic sediment. The massif rises to form an Outer Shelf High (OSH) covered with disrupted Neogene strata. Structural and stratigraphic relationships suggest several pulses of uplift and subsidence for the massif with the most recent uplift detected on the positive massif features.

Upper slope basins are cradled between the OSH and a seaward Upper Slope Ridge (USR) of deformed sediment. Late Eocene and younger clastic sediments, with minor limestone beds, occupy the eastern Trujillo Basin, whereas late Miocene to Pleistocene dolomicrite, micrite and glauconitic micrite are prominent in the western part of the basin. Both the Trujillo and Yaquina Basins (7°–9°30′S) exhibit severe internal disruption caused by compressional stresses. To the south, the Lima Basin (9°30′–13°S) contains similar carbonate lithologies, but this basin and underlying massif have subsided 275 to 500 m Ma−1since Pliocene-Pleistocene time, indicating tensional stresses.

Geological and geophysical data show the subducting plate is rupturing by thrust faulting of layer 2 basalts which may form a sediment-basalt mélange in the overlying subduction complex. Because the seaward boundary of the arc massif is not well-defined, several forearc models are presented which allow the subduction complex to range in width from 15 to 60 km.

The Peru forearc off Lima (12°S) exhibits many of the tectonic characteristics of the Japan forearc off Honshu, especially the subsiding slope basin and contemporaneous accreting subduction complex.
Forearc and other basins, continental margin of northern and southern Peru and adjacent Ecuador and Chile
Authors: Moberly R. , Shepherd G. L. , Coulbourn W. T.
Published: May 2008

The continental margin between the Peru-Chile Trench and the west coast of South America is the type example of the Andean margin, where one of the two converging blocks of lithosphere is oceanic and the other is continental. Along Peru and Chile, basement at the coast and under the landward side of forearc basins is dominantly metamorphic and plutonic. Apparently, the edge of continental crust has been stoped away during subduction. The seaward side of basins may be structurally high dams of sediments accreted and deformed by subduction, or they may be horsts of continental basement. Subsurface mapping in the on-shore and off-shore oilfields of the forearc basin of north-western Peru has shown a mosaic of faulted blocks of tensional origin, overlain by low-angle gravity glides. Details of this faulting, however, cannot be seen on single- or multi-channel seismic records of forearc basins, even where they cross the fields. Off northern and southern Peru, forearc basins are separated from one another along strike by culminations of the basement; across strike there may be more than one forearc basin. The pattern of culminations and depressions may be inherited from structures of the oceanic crust and trench. Deeper seismic reflectors of the basins generally dip landward and converge seaward, and show progressive migration shoreward of the centres of deposition. Sediment movement into the basins and down the slopes is by turbidity currents and slumps. Subduction is an efficient process, removing about four-fifths of the trench sediments.

The Progreso Basin under the Gulf of Guayaquil owes its origin to a transform fault, the Dolores-Guayaquil megashear, oblique to the trench. It is a pull-apart basin or rhombochasm opened in the wake of the northward movement of a small plate. A forearc basin at Talara and the adjacent Progreso Basin have substantial hydrocarbon accumulations.
The geology of the western part of the Borbón Basin, North-west Ecuador
Authors: Evans C. D. R. , Whittaker J. E.
Published: May 2008

The western part of the Borbón Basin, North-west Ecuador, contains some 5 km of predominantly thin-bedded Tertiary mudstones with rare shallow-water sand-stones, resting on a basement of Cretaceous basalts. The basin is floored by thin, discontinuous, turbiditic carbonates of middle Eocene age, which pass up into a 3 km thick sequence of mudstones, cherty at the base, extending into the mid-Miocene. Shallow-water sandstones accumulated during the late Miocene and early Pliocene and were succeeded by deeper water mudstones, which in the Pliocene, are over 1 km thick and contain resedimented sandstone units. The strata are folded into NE-SW-trending structures, parallel to the mid-slope basement high centred on the Río Verde area. Stratigraphic evidence suggests that much of this folding, and the emergence of the high, post-dates the late Pliocene (
c.
2.9 Ma).

Since the middle Eocene the area has occupied the arc-trench gap between the Ecuador-Colombia oceanic trench, and the volcanic arc of the Andes; it now lies on the oceanward side of the mid-slope basement high.
Ancient plate boundaries in the Bering Sea region
Authors: Marlow M. S. , Cooper A. K. , Scholl D. W. , McLean H.
Published: May 2008

Plate tectonic models of the Bering Sea suggest that the abyssal Bering Sea Basin is underlain by oceanic crust, a supposition supported by refraction and magnetic data. The oceanic crust is thought to be a remnant of the Kula(?) plate that was isolated within what is now the Bering Sea when the proto-Aleutian arc began to form between the Alaska Peninsula and Kamchatka in late Mesozoic or earliest Tertiary times. Prior to the formation of the Aleutian arc, the Kula(?) plate moved NW, directly underthrusting eastern Siberia; the plate’s eastern edge either obliquely underthrust or slid past the Bering Sea margin along a transform boundary.

The Koryak Range in eastern Siberia is composed in part of mélange units that include Palaeozoic and Mesozoic allochthonous blocks juxtaposed within a matrix of Cretaceous sedimentary rocks. Structural trends suggest that these blocks were accreted into the Koryak area from the south along an ancient subduction zone formed by underthrusting of the Kula(?) plate.

The base of the Bering Sea continental margin that extends from eastern Siberia to the Alaska Peninsula—the so-called Beringian margin—is underlain by a thick (7–10 km) sedimentary section along the base of the slope. Rocks dredged from the basement exposed farther up the slope (1500–2000 m deep) include shallow-water Upper Jurassic sandstone that is unconformably overlain by shallow-water Eocene to Miocene diatomaceous mudstone. Fauna in the dredge samples indicate that the shelf edge has subsided several kilometres since late Palaeogene time, perhaps in response to the cessation of motion relative to the adjacent oceanic plate and subsequent sediment loading of the oceanic plate.

Uplift of the former plate boundary exposed in the Koryak Range occurred principally in late Cenozoic time, and collapse of the adjacent plate boundary, the Beringian margin, began in earliest Tertiary time and has continued to the present. Both tectonic events occurred after the site of active plate collision shifted south to near the present Aleutian Trench. We are uncertain as to why these two ancient, yet adjacent former plate boundaries should behave so differently, i.e. why one area was folded and uplifted while the other was extensionally deformed and subsided, both apparently in response to the cessation of convergent or strike-slip plate motion.
The Chugach Terrane, a Cretaceous trench-fill deposit, southern Alaska
Authors: Nilsen Tor H. , Zuffa Gian G.
Published: May 2008

The Chugach terrane of southern Alaska extends for approximately 2000 km along the margin of the Gulf of Alaska. A seaward flysch facies of the terrane, the Chugach flysch terrane, represents the fill of a Late Cretaceous trench and consists of structurally deformed turbidites with some mafic volcanic rocks. It is intruded by anatectic granitic plutons of early Tertiary age. The Chugach flysch terrane in most places is bounded to the north by a landward-dipping thrust fault separating it from mélange of the Chugach terrane at least as young as Late Cretaceous. To the south the flysch terrane contains oceanic volcanic rocks and is bounded by faults that separate it from Palaeogene turbidites or upper Mesozoic metavolcanic rocks.

Interpretations of folds and faults in SW Alaska suggest that the Chugach terrane was deformed during NW-directed subduction. Palaeocurrents indicate primarily westward flow along the axis of the outcrop belt, and secondary southward transport. Turbidite facies associations indicate an east-to-west progression from inner-fan to middle-fan, outer-fan, fan-fringe and basin-plain deposits down the axis of the outcrop belt, and a bounding slope facies association to the north. Rock-fragment petrography of sandstone samples from the Chugach terrane indicates derivation from a magmatic arc that was increasingly dissected eastward and from an older subduction complex. The magmatic arc and adjacent shallow-marine forearc basin deposits are located to the north in southern Alaska. Palaeomagnetic data from the Chugach terrane and adjacent deposits indicate original deposition far to the south of the present latitude of Alaska. The entire magmatic arc-forearc basin-trench complex migrated northward in early Tertiary time, when it was probably oroclinally bent and accreted as a mesoplate to Alaska prior to the middle Miocene.
The Barbados Ridge Complex: tectonics of a mature forearc system
Authors: Westbrook G. K.
Published: May 2008

The Barbados Ridge Complex is the wide accretionary sediment pile associated with the Lesser Antilles island arc. Its width is so great (>200 km) that a trench no longer exists on the oceanward side of it. This development is a product of the age of the system (>50 Ma) and the thickness of sediment on the ocean floor (0.8 km in the north, >4 km in the south). The northward decrease in elevation of the sediment pile and the variation in the style of initial deformation at the leading edge of the pile are related to the northward change in sediment thickness and type. The region in which deformation is prevalent has a westward limit just west of the axis of the minimum negative Bouguer gravity anomaly. In the south the axis of this minimum is coincident with the Barbados Ridge (an outer ‘sedimentary arc’). The deformed rocks of the accretionary pile are overlain by later sediments, which show varying degrees of deformation, and often occupy small basins.

In the northern part of the complex, the relief and structure of the accretionary pile are complicated by ridges and troughs running east-west across the general trend of structures. These appear to be related to variations in the relief of the subducted oceanic basement.