Tectonic Wedge Research Articles

Décollement in the Alpine system: an overview

Decollement in the Alpine system is implied in the «nappe» concept. Jura decollement, originally proposed byBuxtorf (1907), is the latest addition north of the Alps. The rheology of evaporites shows easy feasibility of Jura decollement which is required by material balance. In eastern Switzerland the front of the Alpine foreland thrust belt in the Molasse is a typical triangle structure (tectonic wedge), an important decollement structure also in other places and scales. The Helvetic nappes are flat- and-ramp structures similar to the Jura but translated by greater amounts and deformed under larger overburden. Corresponding basement deformation was largely ductile (basement lobes) and lagged behind. The Penninic nappes are usually metamorphosed in various degrees, except some cover nappes transported far into the foreland. Coordination between basement and displaced cover is uncertain. The Austroalpine cover nappes of Switzerland and adjacent Austria as a rule are only weakly metamorphosed and resemble the southern Alps in both stratigraphy and its expression in tectonics. The basement part of both are essentially upright slivers not more than five kilometers thick with a frontal ramp fold. Splitting apart of the sedimentary sequence particularly in the Carnian with tectonic wedging is frequent in both domains. Extensional decollement, occasionally accompanied by tectonic denudation and high cooling rates, occurred during several episodes, most notably during early Jurassic rifting and between the compressional phases (Gosau, Oligocene — early Miocene episodes). The main strike-slip elements of the transpressive Neoalps are the Insubric fault system with its dextral Riedel shears and the dextrally arranged brachyanticlines of the late Alpine windows which again imply decollement.

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.

On the tectonics of the Ligurian Apennines (northern Italy)

Four nappes have been recognized in the Ligurian Apennines. In the Lavagna Nappe very low-grade metamorphism is combined with very large, originally W-facing isoclinal folds. In the other nappes, no evidence for metamorphism is found and all eutectonic folding was originally E- to NE-facing. Tectonic transport along the major nappe contacts was in an E- to NE-direction. A tectonic model is presented, which explains the generation of the large, originally W-facing folds as a result of originally E-inclined subduction within a young oceanic basin. Young oceanic lithosphere (maximum age approximately 25 Ma) subducted beneath oceanic lithosphere with a maximum age of approximately 40 Ma, under the influence of horizontally oriented compressional forces. Within the tectonic wedge, associated with the subduction, originally W-facing isoclinal folding and metamorphism occurred. Decrease and/or termination of compression resulted in the cessation of the subduction movements, followed by uplift of the region above the subducted plate by means of buoyancy. This uplift formed a slope from which sequences slid in an E- to NE-direction, causing E- to NE-facing folds. Ultimately, detachment and thrusting of gravitational nappes took place, by which process rock sequences of oceanic origin have been externally transported to attain ensialic (continental) domains. The Triassic-Early Oligocene tectonic events recognized in the Ligurian Apennines correlate quite well with the events that preceded the collision phase of the Alps.

Tectonic wedges and offset Laramide structures along the Polochic Fault of Guatemala and Chiapas, Mexico: Reaffirmation of large Neogene displacement

Fault wedges along the Polochic fault of northern Guatemala and Chiapas, Mexico record left‐lateral slip of from 20 to 65 km that accompanied documented Neogene slip of 130 km across this North American—Caribbean plate boundary fault. Wedges can be correlated to source areas consistent with left‐lateral displacement along the Polochic fault. They occur adjacent to restraining bends in three areas where changes in strike of the fault have been mapped. These wedges, which have been previously interpreted as allochthons emplaced along low‐angle reverse faults, are bounded by high‐angle faults. Folds that terminate against the Polochic fault are part of a regional fold belt that originated during the Campanian through Paleocene (Laramide) orogenic event. A previous contention that some of these major folds are secondary and related to fault movement is shown to be incorrect. The 130 km model of left‐lateral offset across the Polochic fault is reviewed along with evidence for and against a Neogene time of major displacement. Additional evidence for the model is presented: (1) Late Cretaceous granitoid rocks from near Motozintla, Chiapas, Mexico (northern fault block), are correlated with granitic rocks of the Santa Maria Batholith in the region of Huehuetenango, Guatemala (southern block), (2) mineralization at the contacts of the granitoid intrusives of the two areas is correlated, (3) points of deflection of Laramide structural axes are correlated across the fault, (4) a major thrust sheet similar to that of the Sierra de Santa Cruz in eastern Guatemala is reassembled by reversing 130 km of left‐lateral slip which has offset serpentinites north of the fault in western Guatemala and those south of the fault in central Guatemala.

Collision along an irregular margin: a regional plate tectonic interpretation of the Canadian Appalachians

An idealized plate tectonic model for the pre-Carboniferous development of the Canadian Appalachians explains the 400 km dextral offset of tectonostratigraphic zones from Quebec and northern New Brunswick to Newfoundland and the up to 600 km offset of oppositely verging belts of Acadian deformation from the Gaspé Peninsula to eastern Newfoundland. It is proposed that these offsets, which occur at the St. Lawrence promontory, result from the collision of an irregular North American passive continental margin with island arc and continental crust to the east, along an east-dipping subduction zone. The line of subduction is assumed to have been linear and the subducting slab to have maintained its mechanical integrity during collision. A "jigsaw fit" of the opposite sides of the Iapetus Ocean is made unnecessary by invoking lithospheric delamination and tectonic wedging during the Acadian orogeny in Newfoundland. The model is consistent with surface geology and recent deep seismic reflection observations from north of Newfoundland.

The role of fracture zones during early Red Sea rifting: structural analysis using Space Shuttle radar and LANDSAT imagery

The structural pattern of the Red Sea region, an area where continental lithosphere is cut by a young oceanic rift, was studied by means of satellite and space shuttle radar imagery. The Space Shuttle Mission 2 made two passes over the Red Sea bracketing a region transitional between the southern Red Sea, where true seafloor spreading started about 5 Ma BP, and the northern Red Sea where crust is probably continental, though thinned and stretched. Structures on these passes define bands of shearing in the African and Arabian margins parallel to the Dead Sea Fracture Zone (000–020°). Combinations of these data with LANDSAT information reveal four shear bands along the margins of the Red Sea and one oblique to the Gulf of Suez. We have named the northern three Red Sea shear zones detected on the SIR-A data, the Brothers–Dead Sea Fracture Zone, the Zabargad Fracture Zone and the Shagara Fracture Zone. Two sets of normal faults were also detected, trending 330–340° (parallel to the northern Red Sea coastline) and 310–320° (parallel to the axial deeps in the Red Sea). A map of the 310–320° faults bordering the Red Sea resembles a V-shaped wedge narrowing to the north near the Gulf of Suez. These faults may have formed as a result of the most recent push of seafloor spreading. If this is so, then there may be a propagating tectonic wedge moving from south to north which thins the continental crust up to distances of 300 km from the axis of spreading. Following continental deformation, oceanic crust may be emplaced in 'hot points' along the rift axis. These 'punctiform' oceanic spreading centres should propagate in the same direction as continental rifting. During this episode palaeo-shear zones probably act as barriers ('locked zones') to the axial propagation. One such case is at the Zabargad Fracture Zone where the oblique impact of the propagating rift against the palaeo-shear may have placed the Red Sea's western side under compression and uplift while the eastern side may have been subject to oblique extension with the formation of pull-apart basins. The Space Shuttle imagery allows integration of some of the northern Red Sea morphotectonic features with structures on the continent.

Seismic reflection geometry of the Columbia River Fault Zone and East Margin of the Shuswap Metamorphic Complex in the Canadian Cordillera

Vibroseis (CONOCO, Incorporated) seismic reflection data, recorded near the transition between the Cordilleran fold and thrust belt and the east margin of the Shuswap metamorphic complex of the southern Canadian Cordillera, outline an east tapering wedge geometry in the upper crust. The wedge comprises part of the Shuswap metamorphic complex and may be the result of either a foreland‐dipping duplex or tectonic wedging. The geometric wedge separates the overlying miogeoclinal rocks of the Selkirk fan structure from the crystalline basement (?) rocks below, and delineates the eastern limit of the Monashee complex. Brittle motion on the Columbia River fault zone produced an east dipping, listric normal fault which may have offset the upper portion of the Monashee complex. The Columbia River fault zone is imaged over the entire 15 km of the profile and can be traced to a depth of at least 10 km. Reflections from the lower crust are generally subhorizontal and discontinuous, and they may be a manifestation of crustal extension. The crust‐mantle transition is probably represented as reflections with travel times of 11.0–12.0 s.

The southeastern Canadian Cordillera: Thrust faulting, tectonic wedging, and delamination of the lithosphere

Displacement of tectonic wedges, bounded above and below by zones of thrust faulting with opposing vergence, has produced tectonic delamination at various stratigraphic levels and times in the Rocky Mountain fold and thrust belt of Canada. The ‘triangle zone’, along the eastern edge of the belt, comprises an easterly tapering wedge of imbricate thrust-slices and delaminated Upper Cretaceous molasse deposits. The Porcupine Creek anticlinorium is a fan fold resulting from the insertion of a wedge of Proterozoic rocks along a zone of delamination at the base of the Lower Paleozoic shale succession. In the Selkirk fan structure, which involves the metamorphic infrastructure of the Cordillera, delamination has occurred as the result of the insertion of an apparently allochthonous mass (a ‘suspect terrane’) between the detached parautochthonous supracrustal rocks and the autochthonous basement complex. The diagnostic feature of tectonic wedging and delamination is a reversal in vergence between the bottom and the top of the wedge. Conspicuous tectonic overprinting (‘polyphase folding’) characteristically occurs above the wedge, and is commonly described as ‘backfolding’ and ascribed to polyphase orogeny. Tectonic wedging and delamination are widepread in compressional fold belts of various ages. Examples occur in the Cordilleran tectonic collage of accreted terranes and in early Proterozoic (Wopmay Orogen), Paleozoic (Caledonide-Appalachian) and Cenozoic (Alps) mountain belts. Moreover, tectonic wedging and delamination occur over the whole spectrum of scales from the microscopic to the lithospheric. They are an inherent feature of ‘flake tectonics’ and of the development of large overthrusts such as crystalline basement sheets and obducted ophiolites, all of which comprise a strong upper layer that has been delaminated from the rest of one lithospheric plate, and has overlapped another lithospheric plate that has been wedged beneath it. Geometric and kinematic relationships discernible in well-documented natural prototypes of tectonic wedging and delamination on a small scale provide a basis for elucidating relationships that are more difficult to establish directly on a large scale.

Geo-tectonic framework of the Himalaya of N Pakistan

In the Karakorum Range there is a structurally complicated Cretaceous are comprising the Kohistan sequence. On its northern side the Northern Suture consists of a mega-mélange and is bounded to the S by tightly folded pillow-bearing volcanics and sediments. To the S the Kohistan Plutonic Belt consists of (southwards): (a) early foliated and late post-tectonic tonalites and diorites, (b) aplites and pegmatites (up to 30% of rock volume), (c) basic dykes up to 10 m thick, (d) the Chilas Complex, a stratiform cumulate body over 300 km long and 8 km thick (chromite-layered dunites, gabbros and norites) with a low pressure granulite-facies mineral fabric of tectonic origin, (e) an amphibolite belt with a complex mixture of other rocks, and (f) the Jijal Complex, a 200 km 2 tectonic wedge of high pressure granulites and chromite-layered dunites. Cumulate graded units in the Chilas Complex show that it is folded by an isoclinal anticline (F 1 ). The mid-upper crust of the are is folded by a 50 km half-wavelength F 2 , syncline. The whole Kohistan sequence with its two phases of isoclinal folds was tilted during Himalayan collision so that the structures are now subvertical. The Southern Suture (Main Mantle Thrust) has a wedge of glaucophane schists. The Indian plate contains a basement of psammites and schists intruded by Cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales.

The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan

The Jijal complex, covering more than 150 sq. km in the extreme north of Pakistan, is a tectonic wedge of garnet granulites intruded in the south by a 10 × 4 km slab of ultramafic rocks. The granulites are divisible into plagioclase-bearing (basic to intermediate) and plagioclase-free (ultrabasic to basic) types, the two types reflecting differences in bulk chemistry. Garnet + plagioclase + clinopyroxene + quartz + rutile ± hornblende ± epidote is the most common assemblage. The plagioclase-free rocks are composed mainly of two or three of the minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxene occurs in websteritic rocks devoid of epidote. Much of the amphibole and some epidote appear to be prograde products. Although variation diagrams do not reveal a genetic link between the two types of granulite, it is considered that they are comagmatic rather than the products of two or more unrelated magmas. The compositions of garnet (Py28–46 Alm 27–43Gro16–28), clinopyroxene (Mg44–34Fe5–17Ca51–49, Al2O3 3·0–9·9 per cent), orthopyroxene (with up to 5·5 per cent Al2O3), amphibole (with up to 16·3 per cent Al2O3 and high Alvi/Aliv), and the abundance of garnet suggest a high-pressure origin for the granulites. The rocks appear to have differentiated from a tholeiitic magma of oceanic affinity or they may be genetically related to the pyroxene granulites of Swat considered to have originally crystallized from a calc-alkaline magma of island arc or continental margin affinity. They probably crystallized in the ancient Tethyan crust/upper mantle (or less likely in a continental margin), later to be metamorphosed to granulites (670–790 °C, 12–14 kb) during the collision of the Indian-Asian landmasses, and carried upwards during later Himalayan orogenic episodes. The ultramafic rocks are alpine-type in nature and devoid of garnet. They are dominated by diopsidites; dunites, peridotites, and harzburgites together form <50 per cent of the area of outcrop. The chemistry of the rocks, and their olivines (Fo92–89) and clinopyroxenes (Mg49.5–48Fe2.8–5.2Ca47.4–46.8) are similar to those of alpine complexes of the harzburgite subtype. It is not clear whether they represent a faulted slab of suboceanic crust/upper mantle, mantle diapirs in deep orogenic roots, or dismembered ultramafic rocks differentiated from a basaltic magma. They seem to have a complex history; their present mineralogy is suggestive of high grade metamorphism (800–850 °C, 8–12 kb). They are magmatically unrelated to the garnet granulites and were probably intruded into the latter as plastic crystalline material after both had been independently metamorphosed, but before the entire complex was carried tectonically into its present surroundings. The abundances of the diopsidites is in marked contrast to other alpine-type complexes and the possibility of Ca and Si metasomatism during or before their metamorphism should not be totally ruled out.

Examining the Tectonic Wedging Hypothesis in the NW India Himalaya

DOI = 10.3126/hjs.v5i7.1343 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.168