Tectonic Wedge Research Articles

Comparison of deep structure along three transects of the western North American Continental Margin

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver Island, and central California are now seen to be accompanied by similarities in deep crustal structure. A number of tectonic elements have been identified in the deep structure along transects in these three regions, although not all elements are present along each transect. These elements are (A) an actively subducting oceanic plate and (B) an overriding continental plate that consists of (1) a Cenozoic accretionary prism, (2) a Mesozoic accretionary prism, (3) a backstop to the Mesozoic prism, (4) a tectonically underplated body of oceanic rocks, and (5) a crustal root. The Mesozoic prism is in some cases an underthrust body (type 2a) but in other cases forms the principal component of a landward verging tectonic wedge (type 2b). The technically underplated body of oceanic rocks extends landward from the fault contact between the Cenozoic and Mesozoic prisms to a point beneath the backstop. The crustal root lies beneath the backstop and landward of the underplated body. All of these elements are interpreted to be present along the Alaskan and Vancouver Island transects. In Alaska the underplated body is interpreted to be fragments of the Kula plate; the same may be true at Vancouver Island. These two transects appear to differ in that, in Alaska, the Mesozoic prism, in one interpretation, is the principal component of a tectonic wedge (type 2b), whereas at Vancouver Island, it is an underthrust body (type 2a). Along the central California transect, active subduction is no longer taking place, and the San Andreas fault has removed the Cenozoic prism from this region of the North American plate. On the North American plate (i.e., east of the San Andreas fault), the Mesozoic prism, interpreted as the main component of a tectonic wedge (type 2b), and the backstop to the Mesozoic prism are present. There is, however, no clear evidence of tectonically underplated oceanic rocks, and the crust is thin (no root). In both Alaska and Vancouver Island, the Mesozoic prisms above the underplated bodies experienced low‐pressure/high‐temperature metamorphism at about the time of tectonic underplating; no such metamorphism is currently exposed in California. The metamorphism may have been caused by the underplating of young, hot oceanic crust, or, alternatively, by subduction of an oceanic ridge. The presence of a tectonic wedge (type 2b) in Alaska and California and the absence of such a wedge at Vancouver Island could arise either from the fact that in the former two locations the Mesozoic prisms were more voluminous, owing to either more rapid trench sedimentation or more rapid convergence, or to the possibility that at the latter location the Mesozoic prism was juxtaposed with the backstop primarily by strike‐slip faulting.

Structural and tectonic evolution of the Humber Zone, western Newfoundland, 1. Implications of balanced cross sections through the Appalachian structural front, Port Au Port Peninsula

The Appalachian structural front in western Newfoundland, which is principally a submarine feature that comes ashore on Port au Port Peninsula, is interpreted to be a structural triangle zone, or tectonic wedge. Rocks within the triangle zone, which are therefore transported, include the Taconian (Middle Ordovician) Humber Arm Allochthon, Taconian foreland clastic sediments, and the Cambro‐Ordovician platform succession. The transported clastic sediments and platformal rocks, as well as structurally involved Grenville crystalline basement exposed east of the peninsula, compose the Acadian (Siluro‐Devonian) Port au Port Allochthon, which carried the Humber Arm Allochthon as a high structural slice; Acadian transport of tens of kilometers westward is suggested. This interpretation contrasts markedly with the traditional and widely accepted interpretation of the Taconian and Acadian orogenies in western Newfoundland. Here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced, balanced cross sections across the Port au Port Peninsula. These sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone, as well as the envisioned structural linkage between exposures on Port au Port Peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast. The southeast vergent Tea Cove thrust, which formed an early upper detachment to the triangle zone, is offset and overturned by the north‐northwest vergent Round Head thrust. The Round Head thrust is interpreted to flatten into a second southeast vergent thrust, the Red Brook detachment, which formed a late upper detachment to the triangle zone. Some map‐scale structures are interpreted as fault‐bend folds above oblique ramps on the Red Brook detachment. Allochthonous crystalline basement within the triangle zone, beneath its attached platformal cover, is interpreted as a result of thrust reactivation of a preexisting basement‐cutting normal fault, which may have formed during rifting of the early Paleozoic Iapetus Ocean. This interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on Port au Port Peninsula, and is supported by the restored cross sections.

Style and magnitude of tectonic shortening normal to the San Andreas fault across Pyramid Hills and Kettleman Hills South Dome, California

Research Article| April 01, 1993 Style and magnitude of tectonic shortening normal to the San Andreas fault across Pyramid Hills and Kettleman Hills South Dome, California ROGER B. BLOCH; ROGER B. BLOCH 1Department of Applied Earth Sciences, Stanford University, Stanford, California 94305-2225 Search for other works by this author on: GSW Google Scholar ROLAND VON HUENE; ROLAND VON HUENE 2U. S. Geological Survey and GEOMAR, Kiel, Germany Search for other works by this author on: GSW Google Scholar PATRICK E. HART; PATRICK E. HART 3U S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar CARL M. WENTWORTH CARL M. WENTWORTH 3U S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information ROGER B. BLOCH 1Department of Applied Earth Sciences, Stanford University, Stanford, California 94305-2225 ROLAND VON HUENE 2U. S. Geological Survey and GEOMAR, Kiel, Germany PATRICK E. HART 3U S. Geological Survey, Menlo Park, California 94025 CARL M. WENTWORTH 3U S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (4): 464–478. https://doi.org/10.1130/0016-7606(1993)105<0464:SAMOTS>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROGER B. BLOCH, ROLAND VON HUENE, PATRICK E. HART, CARL M. WENTWORTH; Style and magnitude of tectonic shortening normal to the San Andreas fault across Pyramid Hills and Kettleman Hills South Dome, California. GSA Bulletin 1993;; 105 (4): 464–478. doi: https://doi.org/10.1130/0016-7606(1993)105<0464:SAMOTS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A blind thrust zone and the overlying fold forming the Kettleman Hills South Dome were produced by late Cenozoic shortening normal to the San Andreas fault. West of Kettleman Hills, toward the San Andreas fault, the region of Pyramid Hills experienced structural growth as early as Late Cretaceous time, and growth accelerated in Pliocene to Holocene time. These structures were imaged to a depth of 7 km by reprocessing a seismic reflection record section. Restored structural cross sections indicate a three-stage growth history for Kettleman Hills South Dome: (1) late Miocene reverse faulting, (2) Pliocene fault-propagation-type folding, and (3) late Pliocene to Holocene tectonic wedge emplacement. Late Pliocene to Holocene structural growth at Pyramid Hills probably resulted from thrust emplacement of the Franciscan Complex. A forward model demonstrates that tectonic wedging could account for post-2.5 Ma deformation of Pyramid Hills and Kettleman Hills South Dome. Horizontal shortening across Pyramid Hills and Kettle-man Hills South Dome normal to the San Andreas fault since the Cretaceous period is 3.7 km. This figure does not include shortening within the basement; adding shortening of our tectonic wedge model yields 7.9 km of shortening since Cretaceous time. Post 2.5 Ma shortening determined by our tectonic wedge model is 6.3 km. The rate of post-2.5 Ma shortening (2.5 mm/yr), when added to that on the west side of the San Andreas fault, is consistent with the values derived from plate-motion models that describe the "San Andreas" discrepancy. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Characteristic reflection patterns in the Southeast Canadian Cordillera, Northern Appalachians and Swiss Alps

There are some striking similarities and some differences between the seismic reflection sections recorded across the fold and thrust belts of the southeast Canadian Cordillera, Quebec-Maine Appalachians and Swiss Alps. In the fold and thrust belts of all three mountain ranges, seismic reflection surveys have yielded high-quality images of: (1) nappes (thin thrust sheets) stacked on top of ancient continental margins; (2) ramp anticlines in the hanging walls of faults that have ramp-flat or listric geometries; (3) back thrusts and back folds that developed during the terminal phases of orogeny; and (4) tectonic wedges and regional décollements. A principal result of the Cordilleran and Appalachian deep crustal studies has been the recognition of master décollements along which continental margin strata have been transported long distances, whereas a principal result of the Swiss Alpine deep crustal program has been the identification of the Adriatic indenter, a crustal-scale wedge that caused delamination of the European lithosphere. Significant crustal roots are observed beneath the fold and thrust belts of the Alps, southeast Canadian Cordillera and parts of the southern Appalachians, but such structures beneath the northern Appalachians have probably been removed by post-orogenic collapse and/or crustal attenuation associated with the Mesozoic opening of the Atlantic Ocean.

Structural Style in the Subandean Fold and Thrust Belt of Argentina and its Main Associated Hydrocarbon Fields

The Subandean System covers an area of 11,000 km[sup 2] in northern Argentina and it is represented by a set of structural trends orientated north-northeast-south southwest. The major oil and gas accumulations occur in its eastern trends, that is, Ramos, Aguarague, Campo Duran and Madrejones Ranges. Based on data provided by wells, seismic lines and outcrops, a series of balanced cross sections which show the structural style in each trend and the different hydrocarbon traps, was carried out. The deformation model that rules this set is represented by a Tectonic wedge which generates a triangle zone, with a lower detachment in the Kirusillas Formation (Silurian) and an upper detachment at the base of Los Monos Formation (Upper Devonian). This latter formation in the Ramos and Aguarague Ranges generates a clear disharmony between the lower and upper units. The shortening is resolved by a strong thickening of its original thickness, on the other hand, in the Madrejones and Campo Duran Ranges, part of the shortening is transferred to the foreland. The Carbonic and Tertiary units, are passively folded above the Los Monos Fm. These units show, in some cases, imbricated thrusts which displate the superficial anticlinal axis. Important hydrocarbon accumulations were more » discovered in each one of these structural units. Understanding their deformation mechanism would reduce risk in future exploratory prospects. « less

Thrust, extensional and strike-slip tectonics in an evolving Palaeozoic orogen—a structural synthesis of the Lachlan Orogen of southeastern Australia

The Lachlan Orogen preserves the best and most accessible evidence of the Palaeozoic development of the eastern margin of Gondwanaland. Its history is marked by a raising of freeboard from generally deep marine (Cambrian to Early Silurian), through mixed deep and shallow marine, volcanics and granite-emplacement (mid-Silurian to mid-Devonian), to fluviatile in the mid-Devonian to Early Carboniferous. A long deformation history from latest Ordovician to mid-Carboniferous is marked by formation of: (1) (near-)meridional structures—folds, cleavage and faults—in response to latitudinal shortening, and (2) (near-)latitudinal structures—folds, cleavage and faults—together with strike-slip movement on (near-)meridional faults in response to N-S and NE-SW shortening. Formation of these two sets of structures reflects a long history of oblique compression within the developing orogen. Partitioning of these deformation effects across the Lachlan Orogen leads to recognition of four meridional structural belts, each characterised by different structural histories. The common feature linking all four belts is the thin-skinned nature of deformation: steep surface contractional faults flatten with depth, to become parts of linked thrust systems soling into detachments lying at different levels in the upper and middle crust. The Lachlan Orogen is thus characterised by thin-skinned tectonics. The Southwestern Belt is allochthonous and preserves an Early to Middle Devonian east-vergent fold/thrust system which soles into the base of the Ordovician and also into the base of underlying Cambrian greenstones. Together with later strike-slip movement on major faults, this deformation records transpressional insertion of a terrane from the southeast. The Central Belt is characterised by N-S shortening, latitudinal folds and thrusts together with strike-slip movement on NNW-trending faults, and en echelon granites. Southward translation as a separate terrane occurred in the Early Silurian and mid-Devonian. Deep thrusting in the southern part of the belt has exhumed a metamorphic layer from depths of ⩾ 10 km. The Eastern Belt is a thin-skinned (near-)meridional thrust belt which has also experienced some strike-slip movement and N-S shortening. Thrusts detach in several major horizons: in the Late Ordovician/earliest Silurian (at a depth of ca. 4 km), within or at the base of the Ordovician, which is internally thickened with inferred duplexing and imbrication, and within a mid-crustal metamorphic layer at ca. ⩾ 10 km. Vergence reversals both parallel and perpendicular to the orogen indicate the presence of tear faults and tectonic wedges. Extensional faults and orogen-parallel extension are also features of the Lachlan Orogen, with the former able to account for the some of the “compressional” Silurian “orogenies” in the Eastern Belt. At deeper crustal levels, upper and mid-crustal shortening was balanced by over/underthrusting within and along boundaries of Precambrian continental basement terranes.

Blind duplex structure of the north Urals thrust belt front

New multichannel seismic-reflection profiles shot across the north Ural thrust front indicate the presence of previously unknown wedge-shaped foreland-verging blind duplex. The structure of this zone shows westward thrusting of the tectonic wedge of the Silurian-Artinskian assemblage into the foreland sedimentary cover. This process caused structural delamination of the basin succession, tilting of frontal molasse monocline above the leading edge of the low-taper allochthonous unit, thrust duplication and tectonic thickening of the wedge. Two distinct structural styles are recognized in the blind thrust. Its frontal zone formed by Artinskian shale has thin-skinned imbrication structure. The eastern internal zone consists of Silurian-Sakmarian carbonate-dominated sequences that have much more coherent thrust and fold deformations. This part of allochthonous unit forms a rigid “core” of the blind duplex. The structural geometry of the study area suggests its tectonic shortening of at least 60 km.

Evolution of the Qinling Orogenic Belt

Detailed field mapping, geophysics, and geochemistry show that the Qinling orogenic belt is a southward vergent tectonic wedge consisting of six regional thrust sheets (the Lushan, Luanchun, Qinling, Douling, Wudang, and Zhuqi thrust sheets). These regional thrusts underwent displacement at the end of the mid‐Triassic and the end of the late Jurassic due to the collision between the North and South China blocks and subsequent detachment. The Qinling orogenic belt is thus an Indosinian‐Yanshanian collision‐detachment zone. The so‐called “Caledonian orogeny” in the Qinling Orogenic Belt records a succession of extensional and partial melting events in the continental crust. The basic lavas in the lower Paleozoic were erupted into rifting zones (both in the northern Qinling subzone and southern Qinling subzone). At the same time, local melting of the transitional continental crust occurred and resulted in formation of S‐type granites.

Oblique collision and tectonic wedging of the South American continent and Caribbean terranes

Our studies of the surface structures, seismicity, and gravity anomalies of the Caribbean-South American plate-boundary zone of northeast Venezuela and Trinidad show that the crustal and upper lithospheric structure and kinematics of this zone are complex and differ markedly along strike. We propose a model of the zone9s lateral transition from near Trinidad west to Caracas, emphasizing three concepts: (1) the steepening, detachment, and then sinking away of Atlantic oceanic lithosphere that is (was) attached to the northern edge of continental South America; (2) the wedging seaward and imbricate thickening of the northern margin of the continent in the region of the detaching Atlantic slab; the continental wedge overrides the slab and is overridden by terranes attached to the Caribbean plate; and (3) that the structural and kinematic transitions are a consequence of the progressive oblique collision between continental South America and the overriding Caribbean terranes.

Interpretive Structural Cross Section from the Central Diablo Range to the San Joaquin Valley, California

Abstract An interpretation of the structure of the Central Diablo Range and its relation to the northern San Joaquin Valley is presented. The interpretation recognizes the need to separate in time and space at least three tectonic events: (1) formation of the basin of deposition of the Great Valley Sequence (GVS), (2) Paleogene convergence to produce the Diablo Range antiform, and (3) Late Cenozoic convergence to produce the present topography of the Range. The basin of deposition for the GVS was formed on two separate portions of the Foothills Terrane Arc. The Jurassic and Lower Cretaceous portion of the GVS, which has only been recognized in the Range, was deposited on oceanic crust on the west side of the Foothills Terrane Arc in a distal setting. The proximal Upper Cretaceous portion of the GVS was deposited on Foothills Terrane Arc after ductile deformation of the Arc substrate and tectonic stripping of the Arc superstructure. The ductile deformation within the Arc occurred synchronously with deposition of the distal portion of the GVS. It is suggested that the Arc superstructure detached in the Lower Cretaceous, slid slowly southwestward during the late Lower Cretaceous and Upper Cretaceous, and now forms the tectonic wedge beneath the Diablo Range, which has been interpreted by Wentworth and others (1984) from reflection seismic data. The Central Diablo Range is an anticline expressed by GVS and younger sedimentary rocks dipping "radially" away from a core of Franciscan Formation. Everywhere the GVS is separated from the Franciscan core by a complex fault zone (Jayko and others, 1987) called the "Coast Range Thrust." This "thrust" was initiated by gravity driven slumping of the GVS into the Franciscan trench, perhaps during the Late Cretaceous as a part of the detachment of the arc superstructure. The anticline was created in essentially its present form by Paleogene thrusting of the distal GVS and Franciscan package northeastward over the proximal GVS in the northern San Joaquin Valley on a gentle southwestward dipping thrust plane. Thrusting continued, perhaps intermittently, on the same thrust structures through the middle Miocene. The present elevation of the range is due to a reactivation of these same structures after deposition of the Plio-Pleistocene Tulare Formation. More than 2,600 feet of structural relief developed on the base of the Tulare Formation between the San Joaquin Valley and the Diablo Range during the latest Cenozoic at the latitude of the Panoche Hills. The interpretation is presented on a conceptually balanced cross section constructed along Garzas Creek. Potential hydrocarbon traps are indicated.

Geophysical evidence for the role of fluids in accretionary wedge tectonics

The seismic-reflection technique, by imaging lithological and structural boundaries, can largely define the framework upon which models of the fluid-flow régimes of accretionary wedges are hung. The distribution of fluid loss from the sediments that form the accretionary wedge and that lie beneath it, can be estimated from variation in seismic velocity as a measure of change in porosity, in conjunction with the interpretation of the structural evolution derived from seismic-reflection sections. Seismic techniques have detected regions of pronounced undercompaction believed to be associated with overpressured pore fluid, and detailed modelling has defined zones of localized fluid overpressuring, such as the decollements beneath wedges. The measurement of heat flow, directly, or indirectly from the methane-hydrate seismic reflector, can be used to detect the outflow of fluids, and map its variation in relation to structure and lithology. Geophysical techniques will achieve their full potential in constraining models of the behaviour of accretionary wedges, when calibrated from borehole measurements.

Lateral extrusion in the eastern Alps, PArt 2: Structural analysis

The late Oligocene‐Miocene tectonic style of the Alps is variable along strike of the orogen. In the Western and Central Alps, foreland imbrication, backthrusting, and backfolding dominate. In the Eastern Alps, strike‐slip and normal faults prevail. These differences are due to lateral extrusion in the Eastern Alps. Lateral extrusion encompasses tectonic escape (plane strain horizontal motion of tectonic wedges driven by forces applied to their boundaries) and extensional collapse (gravitational spreading away from a topographic high in an orogenic belt). The following factors contributed to the establishment of lateral extrusion in the Eastern Alps: (1) a rigid foreland, (2) a thick crust created by indentation and earlier collision, (3) a decrease in strength in the crust due to thermal relaxation, (4) a crustal thickness gradient from the Eastern Alps to the Carpathians, and, possibly, (5) a disturbance of the lithospheric root. Northward indentation by the Southern Alps causes thickening in and in front of the indenter and tectonic escape. Gravitational spreading attenuates crustal thickness differences. Indentation structures occur in the western Eastern Alps and comprise folds, thrusts, and strike‐slip faults. These structures pass laterally into spreading structures, which encompass transtensional and normal faults in the eastern Eastern Alps. The overall structural pattern is dominated by escape structures, namely, sets of strike‐slip faults that bound serially extruding wedges. Structural complexity arises from (1) interference of major fault sets, (2) accommodation of displacement differences between the Eastern Alps and their fore‐ and hinterland, (3) displacement transfer from the Eastern Alps toward the Carpathians which act as a lateral unconstrained margin, and (4) crustal decoupling, which partitions extrusion into brittle upper plate and ductile lower plate deformation. The kinematics of lateral extrusion is approximated by an extrusion‐spreading model proposed for nappe tectonics.

Evolution of deep structure along the Trans‐Alaska Crustal Transect, Chugach Mountains and Copper River Basin, southern Alaska

One of the most important results of the Trans‐Alaska Crustal Transect investigations is the discovery that more than one third of the North American plate in southern Alaska (Chugach Mountains and Copper River basin) consists of tectonically underplated oceanic lithosphere. In southern Alaska, exposed accreted tectonostratigraphic terranes, include, from south to north along the transect, the Prince William, Chugach, Peninsular, and Wrangellia terranes. Chief results from seismic refraction data include the following: (1) A sequence of layers of low and high velocity (5.7–7.8 km/s), more than 10 km thick, dips gently northward, with its top at the surface in the southernmost Chugach terrane and at more than 20‐km depth beneath the southern Peninsular terrane. A high‐velocity layer at the top of this sequence corresponds to metabasalt in the southernmost Chugach terrane. This sequence is interpreted to be tectonically underplated fragments of the Kula plate and its sedimentary overburden. (2) An intermediate‐velocity layer (6.35–6.5 km/s) at 9‐km depth beneath both the northern Chugach and southern Peninsular terranes appears to extend without offset across the deep projection of the suture between these two terranes, the Border Ranges fault system. (3) A crustal “root” between depths of 19 and 57 km beneath the northern Peninsular and Wrangellia terranes appears to extend without offset across the deep projection of the suture between these two terranes, the West Fork fault system. This crustal root also appears to abut the tectonically underplated sequence to the south. Unfortunately, in results 2 and 3 the apparent crosscutting and abutting relationships can not be unambiguously resolved. Three scenarios, A, B, and C, have been offered to explain the evolution of this structure. Scenarios A and B differ in interpretation of the depth extent of the northern Chugach and southern Peninsular terranes. In scenario A these terranes extend to only 9‐km depth, where they rest on an unknown middle crust, interpreted as a deeper part of the Peninsular terrane. In this scenario, the northern Chugach and southern Peninsular terrenes moved landward as a tectonic wedge, during tectonic underplating in the latest Cretaceous or early Tertiary, and uplifted the upper crust of the Peninsular terrane to the north. In scenario B, the northern Chugach and southern Peninsular terranes extend to 20‐km depth. In this scenario, they were compressed but not detached and displaced during accretion. Scenarios A, B, and C differ in interpretation of the deep crustal root that appears to abut the tectonically underplated sequence. Since the Peninsular and Wrangellia terranes were at or below sea level prior to the mid‐Cretaceous, the root was probably emplaced in mid‐Cretaceous or younger times. In scenario A the root is lower crust of North America that moved southward during tectonic underplating of the Kula plate. In scenario B the root is, in part, tectonically underplated rocks similar to the Kula plate sequence to the south. In scenario C the root is, in part, magmatically underplated rocks.

Precambrian structure and Late Pleistocene strike-slip tectonics around mega (southern Ethiopia)

Regional structure of Precambrian rocks suggests a tectonic style of large scale upright open synforms and antiforms, with axial-trace trending and fold axes plunging to the northwest. We propose that Precambrian structural highs, emerging through the present erosional surface, are northwest-southeast trending tectonic wedges formed by strike-slip faults. During the late Jurassic uplift of the Somali plateau, these faults initiated and were reactivated in the Late Pleistocene period. A right-lateral component is deduced from regional geometry of fault-pattern, spatter cones and maars alignment. In spite of the spectracular morphology shown by this fault system, geological considerations suggest that faults have reactivated ancient structures.

Formation and thrusting in some great valley rocks near the Franciscan Complex, California, and implications for the tectonic wedging hypothesis

On the western side of Sacramento Valley in northern California, the Upper Jurassic and Cretaceous Great Valley sequence dips east at 40° to 70° in what has been regarded as a tilted but otherwise undeformed east dipping homocline containing strata collectively up to 15 km in thickness. This paper reports the results of structural mapping in the Paskenta area which suggests that, in contrast, steep dips are caused by westward directed thrusting and folding which led to tectonic thickening of the sedimentary pile. In a mainly east dipping sequence, thrusts are recognized by changes in the structural facing and vergence of both folds and cleavage. Although mainly affecting undated rocks of the Great Valley sequence, one major thrust rides on a serpentine‐matrix melange which forms the substrate of the Great Valley sequence. Differential shortening occurring in different blocks during thrusting was accommodated by left‐lateral movement on the Paskenta and Elder Creek fault zones which acted as tear faults. Ductile and brittle faulting have also occurred throughout the sequence, in the melange, and on the tear faults. From work described in this paper and from other reports of thrusts and deformation in the Great Valley sequence in the foothills west of Sacramento Valley, the hypothesis is advanced that these strata have all been tectonically thickened by westward directed thrusting of possible early Tertiary age. While the regional applicability of this hypothesis awaits future detailed structural work, it is no longer sufficient to assume that east dipping strata have been merely tilted. The proposed thrusting model is compatible with the recent tectonic wedge model proposed for the Coast Ranges‐Sacramento Valley by Wentworth et al. (1984). With respect to such a crustal model in which a tectonic wedge of Franciscan Complex was thrust east above older continental basement but below rocks of the Coast Range ophiolite and Great Valley sequence, thrusts of this paper are back thrusts developed above a gently west dipping floor thrust. Back thrusting, shortening, and structural elevation of the Great Valley sequence are direct consequences of this wedging. The uplifted strata lie east of a poorly defined triangle zone and define the western limb of a regional synclinorium that has some analogies with the Alberta syncline of the Canadian foothills.

The stratigraphy of the 3.5-3.2 Ga Barberton Greenstone Belt revisited: A single zircon ion microprobe study

Recent field and geochemical studies indicate a need to test the stratigraphy of the ca. 3.5 Ga Barberton Greenstone Belt as it is presently adopted [1,2]. This work uses the ion microprobe SHRIMP, to attempt such a test. Results show that: (1) Volcaniclastic sediments of the Theespruit Formation (< 3453 ± 6Ma) could be younger than the (structurally) overlying mafic and ultramafic volcanics of the Komati Formation (3482 ± 5Ma). A major structural discontinuity may therefore exist between the two formations. (2) An age of 3538 ± 6Ma established for a tectonic wedge of tonalitic gneiss within the Theespruit Formation confirms the presence of a sialic basement and deformed unconformity below that unit. The tonalitic gneiss is the oldest unit yet recorded within the greenstone belt, equal in age to the older components of the adjacent Ancient Gneiss Complex. (3) The interpreted ages of felsic volcanic rocks from both the Hooggenoeg (3445 ± 8Ma) and Theespruit Formations and the nearby Theespruit Pluton (3437 ± 6Ma) are essentially the same, and corroborate field and geochemical evidence that the felsic units were probably cogenetic and emplaced simultaneously as high-level equivalents of trondjhemite-tonalite plutons that intrude the greenstone belt at its southwestern extremity. (4) Felsic-intermediate volcanic-volcaniclastic rocks locally separating the two major groups (the Fig Tree and Moodies Groups) which overlie the Onverwacht Group record a second major peak of tonalitic magmatism in the Barberton terrain at about 3250 Ma. This is close to the age of the Kaap Valley tonalite pluton which intrudes the Barberton Greenstone Belt at ca. 3226 Ma along its northwestern margin. The present results indicate the Barberton Greenstone Belt is part of an allochthonous sequence containing major tectonic and stratigraphic breaks, with a protracted history; of which the last 200 million years, at least, evolved within a tectonically active convergent environment.

Tectonic wedging in the forearc basin: Accretionary prism transition, Lesser Antilles forearc

The southern Lesser Antilles forearc contains an extensive 50‐ to 70‐km‐wide belt of active deformation at an arcward (inner) locus, between the structural high of the Barbados accretionary prism and undeformed forearc basin (FAB) of the Tobago Trough. This inner forearc deformation belt (IFDB) extends at least 400 km on strike between about 12°N and an undefined northern limit. The IFDB developed above crystalline basement of the FAB close to and perhaps above its probable subduction trace with Atlantic lithosphere. It is a thin‐skinned thrust and fold belt developed in thick FAB strata by arcward (westward) wedging of the accretionary prism, which may be translating obliquely with respect to it. Its western boundary is the inner deformation front (IDF), a bathymetric scarp underlain by fault propagation or detachment type frontal folds. The IFDB is mostly erosionally truncated south of 12°N and north of 13.5°N by an unconformity of probable late Miocene age. Seismic reflection studies permit subdivision of the IFDB into western and eastern divisions, each containing an upper and lower zone. Barbados Island exposes the eastern IFDB and provides calibration of nearby seismics and dating of strata and structural events. The eastern zones contain the inner flank of the accretionary prism and its allochthonous stratal cover, whereas the western zones are composed entirely of FAB strata that deformed ahead of the translating prism tip. Early displacements in the IFDB were taken up on a long (40–90 km) deep detachment into the FAB that issued rapidly from the prism floor and on a conjugate backthrust up the prism's inner (western) slope. The forward detachment's tip locked at the IDF, and progressive early contraction occurred by folding in the hanging wall of the conjugate fault system and by thrusting of FAB strata eastward across the prism's crest. The generation of early structures was followed by late deformation in which the major thickening of the IFDB has occurred. North of Barbados, such late deformation was modest. From near Barbados south, however, late deformation was accompanied by upramping of part of the accretionary prism to a high wedging level in the FAB strata. This mainly out‐of‐sequence wedging caused FAB beds in the ramped prism's footwall to imbricate as a duplex between the prism wedge and the early detachment and to propagate westward toward the IDF. Finally, after development of the FAB duplex, out‐of‐sequence thrusting of the high level prism wedge occurred at the back of the duplex. Balanced section models suggest arcward prism translation between 20 and 60 km and horizontal contraction of FAB strata of the western zone between 4% and 45%, both increasing southward. Development of the early long forward detachment and conjugate back thrust probably began in the early Miocene from 12.5° to 14°N but earlier (probably Oligocene) south of 12.5°N. The out‐of‐sequence upramping of the prism and duplexing of FAB beds probably occurred in late Miocene or Pliocene time. Wide areas of the IFDB became emergent in the Miocene. Tectonic loading of the lithosphere beneath the IFDB caused large Pliocene and Quaternary subsidence of the belt and eastward shift of the FAB depoaxis to the toe of the IDF. The Miocene and younger features of the IFDB and the Tobago Trough probably reflect a superposition of tectonism associated with the development of the Neogene Lesser Antilles arc on an older arc system. The IFDB and Neogene forearc basin were superposed on extant forearc elements and represent a major change in subduction or accretion phenomena. The IFDB is not the product of steady convergence and accretion.

Structural geometry and kinematic evolution of the eastern Cascades foldbelt, Washington and British Columbia

The eastern Cascades foldbelt is one of three structural domains lying within the complex collision zone between the Insular and Intermontane composite terranes in northern Washington and southern British Columbia. The foldbelt resides between the high-grade metamorphic backbone of the Cascade orogen on the west and rocks of the composite Intermontane terrane to the east. It encompasses the stratigraphically coherent, basalt-floored Jura–Cretaceous Methow basin as well as more chaotically disposed Permian–Jurassic oceanic rocks of the Hozameen terrane. Methow basin rocks are thought to have been sutured above the oceanic rocks prior to the middle Cretaceous contractional episode described in this report.Based on the analysis presented herein, between ca. 100 and 88 Ma the rocks in the foldbelt underwent shortening in an east-northeast – west-southwest direction by 50 km or more, largely by displacement on the east-vergent Jack Mountain – Chuwanten thrust system. The early stages of contraction occurred by the process of tectonic wedging, whereby rocks of the Hozameen terrane and western Methow basin translated eastward by delaminating eastern Methow basin strata along west-vergent thrusts. In later stages of shortening, the tectonic wedge became inactive and was carried piggyback atop the east-vergent Cascade Crest and Chuwanten faults.Presently available geochronologic data indicate overlap in the time periods during which eastern and western Cascades foldbelts were deforming and the Cascade metamorphic core was undergoing amphibolite-facies regional metamorphism. Therefore, contraction of rocks in the eastern foldbelt was an important product of the middle Cretaceous orogeny in the Cascades and must be considered in any regional tectonic model for orogenesis. The eastern foldbelt clearly accommodated substantially less shortening than the western foldbelt and is herein proposed to be a backthrust system in the rear of the predominantly west-vergent Cascade orogen.

Examples of strike-slip tectonics on platform-basin margins

Examples from the Southern Alps (North Italy) serve to illustrate the use of carbonate platform-basin relations to recognize strike-slip tectonics and to estimate amounts of displacements. It is noted that two-dimensional cross-sections across strike-slip faults cannot be balanced and three-dimensional reconstructions would be necessary to restore original paleogeographic shapes of platform-basin margins. The main characteristics of strike-slip tectonics are: 1. (a) tectonic omission of fragments of the original carbonate platform-basin system in two-dimensional section; 2. (b) subvertical dip of the fault; 3. (c) juxtaposition of differing thicknesses and facies of equivalent formations across faults, when these differences cannot be explained by normal growth faults; 4. (d) anomalous tectonic wedging of subvertical basinal ductile sediments in the core of the carbonate platform. Normal faults cutting inclined slope margins may simulate apparent horizontal displacements. Platform-basin margins influence thin-skinned thrust tectonics in that overthrust ramps may be guided by clinoforms, and oblique or lateral ramps may develop along margins that are at an angle of 0°–70° to the main compressional axis.

The structure of the Berzeliustinden area: Evidence for thrust wedge tectonics in the Tertiary fold-and-thrust belt of Spitsbergen

The Berzeliustinden area forms part of the Tertiary fold-and-thrust belt of western Spitsbergen. The relations of a basement-involved thrust fault, decollement structures, and a fault repeating part of the stratigraphy are investigated. The deforming mechanism is thought to be‘wedging’of a basement-involved thrust block into bituminous shaly beds of the overlying strata. The thrust fault thus does not continuously cut through to the surface, but lifts the overlying strata, forming a backward directed bedding-parallel thrust fault on top of the wedge. The presence of two bituminous shale formations, both potential splitting mediums for the wedge, complicates the structures. Many structural observations from adjacent areas of the fold-and-thrust belt also fit with this model. It is suggested that thrust wedges are common tectonic elements in the belt and might also be present further east beneath the relatively undeformed Tertiary strata of central Spitsbergen.