Tectonic Escape Research Articles

Timing and tectonic significance of Late Archaean, sinistral strike-slip deformation in the Central Pilbara Structural Corridor, Pilbara Craton, Western Australia

A north-south striking zone of sinistral strike-slip shear (20 × 100 km) and associated folding and thrusting within the central part of the Pilbara Craton is described. This zone, referred to herein as the Central Pilbara Structural Corridor (CPSC), was formed during regional D3 deformation and overprints two earlier sets of structures associated with granitoid doming. It is bound to the west by a continuous, north-south striking, curviplanar fault at greenschist-facies. In the east it is bound by a series of NE-SW striking fault segments located along sheared fold limbs, and by a kilometre-wide zone of amphibolite-facies mylonite within the western margin of the Shaw Batholith. Faults vary from narrow, high-level, brittle structures and retrograde breccia zones in the north, to broader, more ductile structures with S C mylonitic foliations in the south, suggesting a southerly increase in exhumation. A set of kilometre-scale, en-échelon folds with NE-SW striking, upright axial planes and NE-plunging axes characterize the interior of the CPSC and are wholly contained within the bounding faults, thus indicating coeval development with faulting during sinistral wrenching ( σ 1 = NW-SE). Strike-slip deformation in the CPSC is interpreted to have resulted from northwesterly indentation of Shaw Batholith (and contiguous rocks to the east). Westerly-directed tectonic escape of the Strelley Granite laccolith and associated supracrustal rocks from in between Shaw Batholith and North Pole Dome, was followed by ramping up the concave eastern margin of the Yule Batholith and eastward tilting during the transition from westerly to northerly displacement. Continued northward translation of the Strelley Granite at the leading edge of the system was terminated by constriction between the Carlindi Batholith and the North Pole Dome. Trailing greenstone successions were crumpled into a series of NE-trending, en-échelon folds and related faults. Kinematic evidence and structural geometry indicate that sediments of the Lalla Rookh Basin (De Grey Group) were deposited in advance of the north-moving Strelley Granite within a basin bounded by faults of opposite displacement, rather than in a pull-apart basin as previously suggested. The age of the D3 deformation is indicated by resetting of the ArAr and RbSr isotopic systems to ca. 2950 Ma in the Western Shaw region, and by UPb ages of syn-kinematic granitoid rocks.

U–Pb geochronologic constraints on Paleoproterozoic orogenesis in the northwestern Makkovik Province, Labrador, Canada

A 45 km wide, shear-zone-bounded segment of the northwestern Makkovik Province, Labrador, is underlain by Archean gneisses derived from the adjacent Nain craton. This lithotectonic block (Kaipokok domain) was reworked at high metamorphic grade, overthrust by supracrustal sequences (Lower Aillik and Moran Lake groups), and intruded by granitoid plutons during the Paleoproterozoic. Initial amphibolite-facies reworking of the Kaipokok domain at 1896 ± 6 Ma is indicated by U–Pb ages of metamorphic zircon from a foliated Kikkertavak metadiabase dyke. This is one of the oldest Paleoproterozoic tectonic events dated thus far in northeast Laurentia and may be linked with ca. 1890 Ma plutonism documented elsewhere in the Kaipokok domain. Intrusion of granitoid plutons at [Formula: see text], 1877 ± 5, and [Formula: see text] in the Kaipokok Bay area postdates early thick- and thin-skinned thrusting (possibly east to northeast directed) that involved Lower Aillik Group strata. U–Pb titanite ages of 1866–1847 Ma in part record a metamorphic event that followed this plutonic–tectonic activity. These early events are temporally and kinematically difficult to reconcile with accretion of juvenile Makkovikian terranes in the southeast and may instead be related to early stages of the ca. 1.91–1.72 Ga Torngat orogeny along the western margin of the Nain craton. In contrast, high-grade metamorphism, dextral shearing, and northwestward thrusting between 1841 and 1784 Ma, including crystallization of an Iggiuk granitic vein at 1811 ± 8 Ma, are in accord with accretion of Makkovikian terranes in a dextral transpressional regime (Makkovikian orogeny sensu stricto). Coeval sinistral transpression in the Torngat orogen suggests that both otogenic belts accommodated relative northward tectonic escape of the Nain craton during this interval.

The Carboniferous of Western Europe; the development of a petroleum system

A consistent stratigraphic study of the western European Carboniferous Basin has allowed the generation of eight palaeogeographic timeslices. These illustrate a model of large scale eastern (dextral) tectonic escape on the northern side of a convergent margin and integrate the diverse tectonic and sedimentary settings observed within the basin as a whole. This model can be compared to the present day eastern Mediterranean, where a similar range of diverse tectonic settings are observed. Reservoir and source distribution is discussed for each time-slice along with the present distribution of Carboniferous oil and gas fields. Due to the diversity observed no unique play type can be distilled from the data to explain the distribution of hydrocarbons within the studied area.

The deep crust of the Southern Rhine Graben: reflectivity and seismicity as images of dynamic processes

The Rhine Graben, part of the European Cenozoic rift system, deserves special attention because of its location in the foreland of the Alpine orogen. The Phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the Variscan orogeny, late-orogenic crustal re-equilibration to the interference of rifting and Alpine orogeny in Tertiary times.The Rhine Graben is one of the most detailedly studied continental grabens. Prospecting for hydrocarbons in its sedimentary fill and intensive pre-site studies for the Continental Deep Drilling Program (KTB) on its eastern crystalline flank (Black Forest) provided a comprehensive set of geoscience data. Seismic investigations ranging from a deep seismic network of near-vertical reflection to concurrent wide-angle refraction experiments were accompanied by seismological and geological surveys. Therefore, a direct observational comparison between the two depth definitions of crustal subdivisions, elastic vs. strength/rheological, was possible. In large parts of Western Europe deep seismic sounding (DSS) investigation distinguishes between a crystalline upper crust of comparatively low P-wave velocities (5.9–6.0 km/s) with little and discrete reflectivity and a strongly reflective lower crust with higher P-wave velocities (about 6.4–6.8 km/s). Both are separated by the Conrad discontinuity. In contrast, strength and rheological considerations define a brittle upper crust and a ductile lower crust. The boundary between these two realms is usually referred to as the brittle/ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres. The present brittle/ductile transition in the Rhine Graben region, as defined by the deepest hypocentres, does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies. This suggests that the laminated lower crust developed during the crustal re-equilibration phase at the end of the Variscan orogeny.The second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the Variscan lower crust. A remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern Rhine Graben and is possibly induced by the young rifting process.We suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the Alpine orogen. This allows us to propose the distinction between the influence of compression, extension, continental subduction and intra-crustal detachment during graben formation. The dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small-scale collisional escape tectonics associated with an intra-continental transform fault at the southern end of the Rhine Graben, the Rhine-Saoˆne Transform zone.

Northwestern Overthrusting and Related Lateral Escape During the Brasiliano Orogeny North of the Patos Lineament, Borborema Province, Northeast Brazil

The Borborema province (BP) of northeastern Brazil, located between the Sao Luis and Sao Francisco cratons, represents a branching system of Precambrian orogens of the South American platform. It is composed of segments of Archean and Proterozoic crust that were deformed by the convergence of the West African and São Francisco-Congo cratons during assembly of the Brasiliano collage (650 to 500 Ma), a period of intense orogenic activity considered to be the strongest and most pervasive tectonic event that affected the Precambrian of the South American platform. The tectonic and kinematic history of the Brasiliano/Panafrican orogeny is fundamental for reconstructing South American and African Precambrian geology. The correlation between Neoproterozoic tectonic processes occurring in both continents should use structural elements, of regional or local character, with identical kinematic and metamorphic conditions manifested in both basement and supracrustal units. North of the Patos shear zone, subhorizontal Brasiliano thrusts (0.65 to 0.58 Ga) affected the basement and the supracrustal Serido belt with such related regional D1/D2 structures as foliation, lineation, isoclinal folds, and related metamorphism. Overprinting the previous structures, regional folding with a vertical S3 foliation and an associated strike-slip shear zone were developed (0.58 to 0.52 Ga). The metamorphism is similar for all deformation phases, ranging from upper-greenschist to amphibolite facies with mineral assemblages including biotite and garnet throughout the Serido fold belt. We propose, on the basis of deformationai and kinematic reconstructions, that the structural evolution of the Serido fold belt was characterized by transition from a syn-collisional to a strike-slip regime. The transition between regimes occurred, progressively or instantaneously, by the switching of the maximum and intermediary strain axes of the strain ellipsoid. The entire tectonic history can be related to a frontal or oblique collision and lateral escape tectonics, with local, syn-collisional transpression and transtension. The Patos shear zone represents a final vertical shearing, juxtaposing different terranes of the northern and southern Borborema province.

Mode of extensional tectonics in the southeastern Betics (SE Spain): Implications for the tectonic evolution of the peri‐Alborán orogenic system

The Gibraltar arc, which closes the westernmost part of the Mediterranean basin, is a Miocene A‐type subduction arc formed by the continental collision of various pre‐Miocene terranes in the major zone of collision between the Iberian and African cratons. The hanging‐wall block, known as the Alborán domain, has undergone more than 300 km migration from a more easterly position, where it was the continuation of the Alpine Cretaceous‐Paleogene orogen. Contemporaneous with thin‐skinned thrusting in the footwall, the Alborán domain underwent two episodes of nearly orthogonal extension in which extensional systems developed with directions of extension varying from a NNW‐SSE system, orthogonal to the belt axis, in the late Burdigalian‐Langhian to a WSW directed orogen‐parallel one in the Serravallian. The superposition of these two systems resulted in a chocolate tablet megastructure. This extensional pattern is not satisfactorily explained in previously proposed models for the evolution of the arc. Orthogonal extension is plausible in a process of the gravitational collapse of an overthickened crust; nevertheless, orogen‐parallel extension is more difficult to explain in this context. We advocate that the WSW directed low‐angle normal faults formed during large‐scale extension in connection with important westward arc migration. The driving force of extension in a general context of convergence is controversial and varies between a convective removal model and a delamination model. Constraints on both the timing and the kinematics of extension, as presented in this paper, seem to support the contribution of both mechanisms. Convective removal may have started the process, but continued N‐S convergence could have resulted in westward tectonic escape and asymmetric lateral inflow of asthenospheric material accompanying lithospheric delamination.

Model for the formation of large, transtensional basins in zones of tectonic escape

Research Article| March 01, 1997 Model for the formation of large, transtensional basins in zones of tectonic escape Paul Mann Paul Mann 1Institute for Geophysics, University of Texas at Austin, 8701 North Mopac Expressway, Austin, Texas 78759-8397 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (3): 211–214. https://doi.org/10.1130/0091-7613(1997)025<0211:MFTFOL>2.3.CO;2 Article history first online: 02 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 Paul Mann; Model for the formation of large, transtensional basins in zones of tectonic escape. Geology 1997;; 25 (3): 211–214. doi: https://doi.org/10.1130/0091-7613(1997)025<0211:MFTFOL>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 SocietyGeology Search Advanced Search Abstract “Tectonic escape” is strike-slip-dominated motion of colliding continental or arc crust in the direction of subductable oceanic crust. I describe four examples of large transtensional basins in Cenozoic zones of tectonic escape and propose that these basins formed along newly formed, transtensional faults formed between the collided and escaping parts of the plate. The reoriented direction of the escaping plate, split along the new transtensional fault zone, is determined by the presence of a “free face,” or subductable oceanic crust. Continued subduction toward the free face is driven by the force of gravity acting on dense oceanic crust (“slab pull”). The four examples presented here show that a variety of transtensional deformational styles can be generated within this specific type of divergent tectonic setting. These transtensional styles include pull-apart basins at fault stepovers, more asymmetrical, rift-like basins thought to be formed under conditions of fault-normal extensional stresses, and single or multiple short spreading ridges. Transtensional styles may be controlled more by the type and age of crust present than the angle of reorientation of the escaping plate. 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.

The cause of N-S extensional tectonics in western Turkey: Tectonic escape vs back-arc spreading vs orogenic collapse

It is proposed that Late Cenozoic N-S extensional tectonics in western Turkey began in the latest Oligocene-Early Miocene (~24-20 Ma) and has been related to the spreading and thinning of an over-thickened crust (i.e. orogenic collapse) created by an earlier Palaeogene (~65-22 Ma) compressional regime. The occurrence of N-S extension immediately after the cessation of the compressional regime is attributed to hot lithosphere in western Turkey as indicated by pre- and syn-extensional magmatic activity. This early start for N-S extensional tectonics in western Turkey in the latest Oligocene-Early Miocene is inconsistent with the tectonic escape and back-arc spreading models. Those models cannot explain this early beginning of extension because their triggering events, that is the collision of Eurasia/Arabia in Middle Miocene (~14 Ma) and inception of the roll-back process in the Aegean in Middle-Late Miocene (~12 Ma), are too young.

Plastic velocity vector diagrams applied to indentation and transpression in the Alps

Alpine collision tectonics has affected the European Alps for more than 110 Ma and is probably still active today. The cumulative large-scale convergence in the Alps adds up to 1100 km. About 500 km of this convergence can be budgeted by escape tectonics, 300 km by continental and oceanic subduction, and an equal amount by crustal thickening. Considerable uncertainty still exists as to the kinematics of the collision; a transpressive indentation of a more or less rigid “Italy/Adria” block into a deformable European plate appears likely. In recent surveys of horizontal mass transfer in the Alps, as inferred from structural analyses, a dominant westwards directed flow regime has been recognized. Theoretical indentation models have been applied to explain the palaeo-flow field. The flow can be explained by: Case I normal indentation without transpression but a space restriction to the east. A palinspastic space restriction did not exist, on the contrary, remnant oceanic terrain survived longest in the east; Case II dextral transpression with no significant indentation. This would require an age progression of westward flow from the east (oldest) to the west (youngest) which is not observed; and Case III transpressive indentation with a sinistral shear component. This seems to be the most satisfactory solution as it explains large-scale sinistral lineaments, and leaves palinspastic space for the rapid rotation of Corsica and Sardinia.

The Irish Variscides: problems, perspectives and some solutions

ABSTRACTA review is presented of interpretations made to date of the Irish Variscides, and the various models of Late Palaeozoic crustal evolution. An examination is then made of the factors controlling Variscan strain in Ireland, including lithological competence, basement fabric and basement‐cover thickness. From this, the difficulties of recognizing Variscan ‘fronts’ and ‘fold belts’ are emphasized.In order to determine whether all putative Variscan strain is indeed Variscan. in age, new time‐temperature data (vitrinite reflectance; apatite fission track populations) are discussed, along with offshore data. A case is then presented for recognizing quite distinct episodes of both Variscan and post‐Variscan heating and deformation. Some of the implications for the Variscides to the east are briefly examined, and the suitability of a regional ‘escape tectonics’ model is tested.

Active tectonism in the intracontinental Middle Atlas Mountains of Morocco: synchronous crustal shortening and extension

Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE–SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE–SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa.

The Guitiriz granite, Variscan belt of northern Spain: extension-controlled emplacement of magma during tectonic escape

The Ollo de Sapo domain of the northern part of the Variscan belt of Spain, contains Precambrian and Ordovician metamorphic rocks intruded by the Guitiriz granite. The domain is bounded by two NS transcurrent shear zones. Detailed structural and anisotropy of magnetic susceptibility studies of the Guitiriz pluton reveal a syntectonic emplacement, relative to movement on the two NS trending bounding shear zones, characterized by: (1) development of NS trending structural and magnetic fabrics: (2) concordant structures in granites and country rocks; and (3) development of shear zones along the east and west granite margins. Gravity data show two roots trending 110°E extending down to 4.5 km from an otherwise thin (< 3 km), slab-like body. The Guitiriz pluton was emplaced in a magma chamber developed by local NS extension induced by an overall E-W shortening. The proposed emplacement model involves the northwards tectonic escape of a crustal wedge — the Ollo de Sapo Domain — bounded by two shear zones acting as conjugate strike-slip zones.

Docking and post-docking escape tectonics in the southern Philippines

Abstract The structure of the Philippine archipelago results from the juxtaposition, between the Late Miocene and the present, of a volcanic belt against fragments of the Eurasian margin and associated marginal basins. The southern Philippines offers the opportunity of studying the mechanics of the deformation from active contraction to a more complex post-docking setting. The docking period is characterized by a compression which began during the early Late Miocene in the central Philippines and has been recently studied in the island of Mindanao. There, deformation initiated in the Late Miocene-Early Pliocene on a NW-trending wrench zone, and continued until the Late Pliocene with thrusting on west-verging flats and ramps within the arc and east-verging thrusts within the Sangihe forearc. This deformation is still active to the south in the Molucca Sea. The post-docking period began in the Early Pleistocene in northern Mindanao and is represented by a new geodynamic framework with a paired subduction zone and strike-slip fault. However, convergence is still active in the Manila and the Negros trenches, although the Philippine fault is not offset. Large wrench faults, which reactivate the ramp faults of the collision stage, transfer strain from the Philippine fault to the Manila and Negros trenches. These observations imply active intra-arc extension and fragmentation within the Philippine mobile belt.

The sedimentary and tectonic setting of the Transvaal Supergroup floor rocks to the Bushveld complex

The Palaeoproterozoic Transvaal Supergroup floor to the Bushveld complex comprises protobasinal successions overlain by the Black Reef Formation, Chuniespoort Group and the uppermost Pretoria Group. The protobasinal successions comprise predominantly mafic lavas and pyroclastic rocks, immature alluvial-fluvial braidplain deposits and finer-grained basinal rocks. These thick, laterally restricted protobasinal sequences reflect either strike-slip or small extensional basins formed during the impactogenal rifting and southeasterly-directed tectonic escape, which accompanied collision of the Zimbabwe and Kaapvaal cratons during Ventersdorp times. The erosively-based sheet sandstones of the succeeding Black Reef Formation reflect northwand-directed compression in the south of the basin. Thermal subsidence along the Ventersdorp Supergroup and Transvaal protobasinal fault systems led to shallow epeiric marine deposition of the sheet-like Chuniespoort Group carbonate-BIF platform succession. After an estimated 80 Ma hiatus, characterized by uplift and karstic weathering of the Chuniespoort dolomites, slower thermal subsidence is thought to have formed the Pretoria Group basin. Widespread, closed basin alluvial fan, fluvial braidplain and lacustrine sedimentation, as well as laterally extensive, subaerial andesitic volcanism (Rooihoogte to Strubenkop Formations), gave way to a marine transgression, which laid down the tuffaceous mudrocks, relatively mature sandstones and subordinate subaqueous volcanic rocks of the succeeding Daspoort, Silverton and Magaliesberg Formations. Poorly preserved post-Magaliesberg formations in the Upper Pretoria Group point to possible compressive deformation and concomitant rapid deposition of largely feldspathic detritus within smaller closed basins.

A study of metabasite and metagranite chemistry in the Adola region (south Ethiopia): implications for the evolution of the East African orogen

The Adola belt forms the transition zone between the Neoproterozoic Arabian-Nubian shield and the Mozambique belt. In an attempt to assess its plate tectonic evolution, representative suites of magmatic rocks were analyzed for major and trace elements (including rare earth elements — REE). Based largely on REE and other trace element data, four tectonic environments are distinguished. These are: 1. i) continental basement (Awata gneiss); 2. ii) island arc (Daba and Bursano meta-igneous complexes); 3. iii) back-arc (Megado and Kenticha meta-igneous-sedimentary rocks); and 4. iv) MORB (Reji amphibolite). Boninitic meta-andesites mark the onset of back-arc tholeiitic volcanism in the Megado rocks. The recognition of MORB-type chemical characteristics from Reji makes the Adola region an important site in the East African orogen, in which a component of Neoproterozoic oceanic crust is preserved. The rock units that characterize the different tectonic settings from narrow belts up to 15 km wide signifying shortening during collision between east and west Gondwanaland. The extensive shortening and late-stage strike-slip components recognized in the Adola belt are consistent with the Adola region being geographically close to the Mozambique orogenic front during the Neoproterozoic. The results support the application of collision and tectonic escape models to the East African orogen.

From transpression to transtension: Oligocene-Miocene structural evolution of the Vienna basin and the East Alpine-Western Carpathian junction

Palaeostress analysis in the easternmost Alps and westernmost Carpathians revealed the existence of four different stress fields during Oligocene-Miocene times. The generalized Oligocene-late Middle Miocene maximal horizontal stress axis changes gradually from WNW-ESE to ENE-WSW and the Late Miocene axis trends to NNE-SSW. Except for the late Middle Miocene (late Sarmatian, 11 Ma) weak, transient stress field, the others reflect important deformations and are connected to the formation of the Vienna basin. The integration of microtectonic and stress-field data with other structural observations and sedimentological data resulted in a complete structural analysis. It suggests a four-stage evolution of the Vienna basin and its surroundings. The Late Oligocene and Early Miocene deformations were connected to progressively developed, NE-SW-trending sinistral strike-slip zones. During the earliest Miocene (Eggenburgian, 21-18.5 Ma), the deformations were characterized by transpression. At the end of the Early Miocene (17-16 Ma), the Vienna basin became a pull-apart structure for a short period. Middle and Late Miocene evolution (16-5.4 Ma) was characterized by a combination of extensional and strike-slip faulting (transtension) rather than a pure strike-slip or pure tensional regime.A combination of palaeomagnetic and tectonic data demonstrates that the stress field did not change during the Oligocene and Early Miocene. The compressional axis was oriented originally N-S, and than turned to the actual WNW-ESE or NNW-SSE orientation. The gradual counterclockwise rotation of the structures took place simultaneously with the development of the Late Oligocene-Early Miocene sinistral shear zones. These rotations affected mainly small crustal blocks between strike-slip faults. This mechanism permitted the transfer of sinistral slip toward the northeast through the entire junction area. All these deformations explain the mechanism of the tectonic escape of the whole East Alpine-North Pannonian block.

Basin superposition on the northern margin of the South China Sea

Basin superposition on the northern margin of the South China Sea is a product of pulsed basin development with geometries which changed through space and time, of changing strain fields, and of tectonically inherited, pre-existing zones of crustal weakness. This region of the South China Sea was welded together or assembled prior to the Cretaceous. Three distinct episodes of continental disassembly and accompanying basin evolution ensued, each episode commencing with rifting and the formation of grabens or half-grabens and each terminating with a gentle subsidence phase. The major trends of the faults and basins varied from predominantly NE for the first episode, through NE-EW for the second, to EW-NW for the third. The first episode occurred during the Late Cretaceous to Paleocene as a back-arc extensional domain induced by the dip steepening of a subducted oceanic slab. The second basin episode of the late Eocene to early Oligocene was initiated by the collision of the Indian plate with the Eurasian plate. The continued penetration of the former into the latter led to the tectonic escape and subsequent clockwise rotation of the Indochina block, creating regional transtension and left-lateral motion along the Red River Fault. The third episode of basin development occurred during the middle Miocene as a reactivated product of this collision. Owing to the clockwise rotation of Indochina, the approximately E-W trend of the simple shear principal displacement zone changed to northwest. By the Pliocene, the eastward extrusion of South China had exceeded that of Indochina, reversing the sense of Red River Fault displacement to right-lateral. A clockwise rotation of the Red River geosuture, together with considerations of the influence of zones of inherited crustal weakness, provides a model for the observed episodic changes in regional strain. Such a model best explains the post-Cretaceous evolution of superimposed basins on the northern margin of the South China Sea.

The Role of Extension in Mountain-Belt Life Cycles

Orogenic collapse anchor spreading occurs when the upper portion of a crustal welt (mountain belt) extends in response to gravitational forces and changing conditions when the positive topographic feature can no longer support itself and spreads laterally. This is perhaps similar to the dynamics of an ice cap. A literature review establishes orogenic collaps/spreading as a common tectonic phenomenon. It is a useful addition to the plate-tectonic paradigm, along with escape tectonics and suspect and exotic terrane analysis, when teaching mountain-belt processes and histories to undergraduates and laypeople. Orogenic collapse or spreading can help explain 1) why local extension occurs coevally within zones of plate convergence, 2) how high-pressure rocks can find their way to the surface quickly given that the interplay between erosion and isostasy is an inadequate mechanism by itself, 3) why mountain belts attain the heights they do, and 4) why ocean basins tend to open along old suture zones (as in the Wi...

Caledonian and precambrian history in Svalbard: a review, and an implication of escape tectonics

This article contains two parts, a review of geochronological data of the Svalbard basement, and a hypothetical application of the concept of escape tectonics on the Caledonian-Franklinian mobile belts of the Arctic. All Caledonian and older isotopic ages obtained from Svalbard are compiled and synthesized into a sequence of events from Proterozoic to middle Paleozoic times. The areal distribution of the rocks of various ages are considered in conjunction with the present understanding of the lithostratigraphy of the archipelago. Caledonian tectonothermal events in the Middle Silurian and Middle Ordovician are well defined by isotopic age determinations and stratigraphie evidence. An oceanic suture zone represents the early Caledonian subduction event. The Late Proterozoic-Early Ordovician low-grade metasediments occupy ca. 60% of the pre-Devonian basement of Svalbard. Early Paleozoic and latest Proterozoic isotopic age groups have no clear geological correlatives, however, they may reflect regional geological events recognized in other parts of Arctic margins. The Grenvillian and Svecofennian events have been well documented by the isotope systems and these data have made it possible to distinguish Late, Middle and Early Proterozoic rock units in Svalbard. The Middle and Early Proterozoic rocks occupy ca. 40% of the whole basement, and most of the crystalline basement has been formed essentially during these two tectonothermal events. These basement rocks are separated into three orogen-parallel structural units by steeply dipping faults in Svalbard. In the second part of this article, these faults and other orogen-parallel faults in the western part of the Arctic Caledonian-Franklinian mobile belts are summarized and considered in the context of “escape tectonics”.

Paleoproterozoic tectonic assembly of Northeast Laurentia through multiple indentations

Recent detailed geological mapping and UPb geochronological studies in the Torngat, Ungava and Baffin orogens of Northeast Laurentia have provided new information regarding the tectonic assembly of this region between ∼1.92 and 1.74 Ga. In contrast with an earlier hypothesis whereby the geometry of Archean continental fragments was thought to have resulted entirely from northerly indentation of the Superior Province and consequent extrusion tectonics, interpretation of recently acquired data shows that Superior Province indentation was the last of multiple collisional events during the formation of Northeast Laurentia. In the present model, we propose that Southeast Rae and North Rae provinces were separate prior to ∼1.92 Ga, when regional tectonism changed from extension to contraction. Westerly subduction and westward translation of the Archean Disko terrane and Nain Province between the North Rae and Southeast Rae provinces resulted in the development of a continental magmatic arc in the Cumberland batholithic complex and in the Torngat orogen, and deposition of the upper sequences of the Penrhyn-Piling-Karrat sequences in the Foxe and Rinkian belts of the Baffin orogen (1880–1895 Ma). Continental collision between the Disko terrane and North Rae Province produced shortening across the Rinkian and Foxe belts at, or before, 1876 Ma. Meanwhile, early arc magmatism in the Ungava orogen (1861–1898 Ma) accompanied foredeep or transtensional basin formation in the New Quebec orogen (∼1870–1883 Ma) and the onset of deformation in the southeastern Nagssugtoqidian orogen (∼1883 Ma). Nain-Southeast Rae provinces collision at ∼1860 Ma across the Torngat orogen caused renewed granitic magmatism and metamorphism in the Cumberland batholithic complex between 1857 and 1853 Ma. Initiation of felsic volcanism in the Makkovik-Ketilidian orogen at this time indicates a shift in the locus of plate subduction from the Southeast Rae Province to the southern margin of the Nain Province. In the Ungava orogen, northerly subduction changed to southerly subduction resulting in the generation of a second arc suite at between 1844 and 1826 Ma. Formation of the de Pas batholith in the hinterland of the New Quebec orogen at ∼1840 Ma accompanied arc-continent and Superior-Southeast Rae collision from 1845 to 1829 Ma. At this time, accretion of arc material in the Makkovik-Ketilidian orogen was contemporaneous with high-grade sinistral shearing along the reactivated Torngat orogen and the northwestern Nagssugtoqidian orogen. Southeast-directed thrusting along the southeastern margin of the Cumberland batholithic complex, and reactivation of the Rinkian belt is predicted to have also occurred at this time. At ⩽1826 Ma, the onset of arc-continent collision across the Ungava orogen initiated closure of the North Rae-Southeast Rae provinces juncture and generation of the Ford Lake plutonic suite. Subsequent Superior-Southeast Rae continental collision (∼1810 Ma) resulted in dextral “escape tectonics” in the Foxe and Rinkian belts, and thick-skinned folding of basement and cover sequences in the Ungava orogen. High-grade metamorphism and granitoid magmatism in the Makkovik-Ketilidian orogen also occurred. Reactivation of the Nain-Southeast Rae (east-directed thrusting) and Superior-Southeast Rae (west-directed thrusting) margins at ∼1790 Ma resulted in “pop-up” of the Southeast Rae Province between its more rigid neighbors. Final cooling and the emplacement of post-tectonic granitoid rocks took place at ∼ 1740–1790 Ma.