Showing papers in "Tectonics in 2003"

Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: Termination of the central Asian orogenic belt

Abstract: [1] The Solonker suture records the termination of the central Asian Orogenic Belt (CAOB). However, tectonic development of the Solonker suture is poorly understood. We report new field data for the Ondor Sum melange in the Ulan valley, and present a new evaluation of the orogenic belt extending from the southern Mongolia cratonic boundary to the north China craton within the context of a new geological framework and tectonic model, which incorporates relevant data from the literature. The southern accretionary zone between the north China craton and the Solonker suture is characterized by the Mid-Ordovician-Early Silurian Ulan island arc-Ondor Sum subduction-accretion complex and the Bainaimiao arc. This zone was consolidated by the Carboniferous-Permian when it evolved into an Andean-type magmatic margin above a south dipping subduction zone. The northern accretionary zone north of the Solonker suture extends southward from a Devonian to Carboniferous active continental margin, through the Hegenshan ophiolite-arc accretionary complex to the Late Carboniferous Baolidao arc associated with some accreted Precambrian blocks. This northern zone had consolidated by the Permian when it developed into an Andean-type magmatic margin above a north dipping subduction zone. Final subduction of the central Asian ocean caused the two opposing active continental margins to collide, leading to formation of the Solonker suture in the end-Permian. Predominant northward subduction during final formation of the suture gave rise in the upper northern plate to a large-scale, postcollisional, south directed thrust and fold belt in the Triassic-Jurassic. In summary, the CAOB underwent three final stages of tectonic development: early Japanese-type accretion, Andean-type magmatism, and Himalayan-type collision.

Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western Tibet

Abstract: [1] In the Shiquanhe area of far-western Tibet, mid-Cretaceous strata lie unconformable on ophiolitic melange and Jurassic flysch associated with the Bangong-Nujiang suture zone. On the basis of our mapping and geochronologic studies, we suggest that these Cretaceous strata were shortened by >57% over a north south distance of 50 km during Late Cretaceous-early Tertiary time. The Late Cretaceous Narangjiapo thrust placed Permian strata >20 km over ophiolitic melange and Cretaceous strata. North of the Narangjiapo thrust, >40 km of shortening was accommodated by the Late Cretaceous-early Tertiary south directed Jaggang thrust system that involves Jurassic flysch and Cretaceous strata, and roots into a decollement within ophiolitic melange. The most recent shortening was accommodated to the south of the Narangjiapo thrust, along the north dipping Shiquanhe thrust. The Shiquanhe thrust cuts flat-lying 22.6 ± 0.3 Ma volcanic rocks and underlying folded, Tertiary nonmarine strata in its footwall and was likely active during slip along the Oligocene Gangdese thrust system of southern Tibet. Ophiolitic melange and structurally overlying Jurassic flysch near Shiquanhe are interpreted to represent remnants of a subduction-accretion complex and forearc basin, respectively, that were obducted southward onto the margin of the Lhasa terrane during Late Jurassic-Early Cretaceous closure of the Bangong-Nujiang Ocean. Subsequent imbrication of the obducted sheet could have produced the two east-west trending belts of ophiolitic melanges, separated by ∼100 km, in western Tibet. Late Cretaceous-early Tertiary thin-skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of Lhasa terrane rocks beneath the obducted ophiolitic melange and the Qiangtang terrane to the north.

Tectonic evolution of the early Mesozoic blueschist-bearing Qiangtang metamorphic belt, central Tibet

Abstract: This is the published version. Copyright 2003 American Geophysical Union. All Rights Reserved.

Formation of the Maturín foreland basin, eastern Venezuela: Thrust sheet loading or subduction dynamic topography

Abstract: [1] The Maturin Basin in eastern Venezuela is considered a good example of a peripheral foreland basin. Earthquake and tomographic data indicate that eastern Venezuela is affected by the oblique subduction of the South American Plate underneath the Caribbean Plate. New forward flexural isostatic modeling of eastern Venezuela has been carried out in order to determine whether the Maturin Basin was generated purely by thrust sheet loading from the Serrania and Monagas Foreland Thrust Belts. A sequence of forward models from middle Miocene to Present was generated for 3 profiles across the Serrania del Interior Thrust Belt, the Monagas Foreland Thrust Belt, and the Maturin Foreland Basin. The predictions of these models are constrained using seismic reflection and well data. The flexural isostatic modeling shows that thrust sheet loading associated with the Serrania del Interior and Monagas Foreland thrust belts is insufficient to generate the observed subsidence within the Maturin Basin. Dynamic fluid flow modeling of subduction related dynamic topography of eastern Venezuela has been used to investigate the influence of South American Plate subduction on the generation of the accommodation space observed in the Maturin Basin. Fluid flow modeling of subduction related dynamic topography suggests that the subduction of the South American lithospheric mantle caused downward deflection of the South American crust affecting the Maturin Basin and the Serrania Thrust Belt. This modeling suggests that the Maturin Basin subsidence has two components: 55% related to thrust sheet loading and 45% driven by continental subduction.

Contemporary strain rates in the northern Basin and Range province from GPS data

Abstract: We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada–Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)–North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada–Great Valley (SNGV) microplate, and a narrow north-south elongate region in the central Great Basin (CGB) occupying the longitude band 114–117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr^(−1) oriented N47 ± 1°W and with respect to NA at a rate of ∼12.4 mm yr^(−1) also oriented N47°W, slower than most previous geodetic estimates of SNGV-NA relative motion, and nearly 7° counterclockwise from the direction of P-NA relative plate motion. We estimate CGB-CP relative motion of 2.8 ± 0.2 mm yr^(−1) oriented N84 ± 5°W, consistent with roughly east-west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr^(−1) concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV-CGB relative motion of 9.3 ± 0.2 mm yr^(−1) oriented N37 ± 2°W, essentially parallel to P-NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr^(−1) across a zone of width ∼125 km in the south (latitude ∼36°N), (2) 25 ± 5 nstr yr^(−1) in the central region (latitude ∼38°N), and (3) 36 ± 1 nstr yr^(−1) across a zone of width ∼300 km in the north (latitude ∼40°N). By construction there is no net extension or shortening perpendicular to SNGV-CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr^(−1) extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east-west extension rate of 10 ± 1 nstr yr^(−1) across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, range-bounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second-order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second-order pattern.

Conjugate strike-slip faulting along the Bangong-Nujiang suture zone accommodates coeval east-west extension and north-south shortening in the interior of the Tibetan Plateau

Abstract: This is the published version. Copyright 2002 American Geophysical Union. All Rights Reserved.

Late Paleozoic strike-slip shear zones in eastern central Asia (NW China): New structural and geochronological data

Abstract: New structural studies and 40Ar/39Ar dating in northwest China provide information about late Paleozoic strike-slip motions subsequent to accretional events, which built eastern central Asia during the Paleozoic. Two principal areas were affected by these large transcurrent motions. First, in the Tianshan range, main east-west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism. Associated biotites give ages ranging from 290 Ma to 245 Ma. The earlier N110 shearing occurred in western Tianshan, while the last one, dextral in whole Tianshan, occurred 250–245 Myr ago. Second, in the Chinese Altay region several NW-SE shear zones structured the area. The main motion is sinistral and occurred along the Erqishi zone at 280–290 Ma. It is followed by a complex succession of dextral and sinistral shearing episodes, leading to the northwestward structuring, dated at 245 Ma, of a metamorphic zone that was folded during a compressive event.

Tectonic shortening and topography in the central High Atlas (Morocco)

Abstract: [1] Three cross sections of the Moroccan High Atlas illustrate the structural geometry and relationship between tectonic shortening and topography in this Cenozoic intracontinental mountain range. The structure is dominated by thick-skinned thrusting and folding, essentially by inversion of Mesozoic extensional faults and by buckling of both pre-Mesozoic basement and its sedimentary cover. Detached, thin-skinned thrusting is limited and apparently related to basement underthrusting, which did not always create structural relief. Despite the high topography, tectonic shortening is moderate, with faults and folds being spaced and separated by broad tabular areas. Section restoration indicates that shortening decreases along strike from east to west in the High Atlas, while topographic elevation generally increases. This inverse correlation suggests that crustal thickening does not fully explain the observed topography and suggests a mantle contribution to uplift. This is supported by geophysical indications of a thin lithosphere and by alkaline volcanism in the vicinity. Mantle-related uplift, which occurs in a broad region, may also explain the scarce foreland basin record adjacent to the High Atlas. The relief of the Atlas Mountains is interpreted as a combination of isostatic and dynamic topography.

Crustal structure of the transpressional Variscan orogen of SW Iberia: SW Iberia deep seismic reflection profile (IBERSEIS)

Abstract: [1] IBERSEIS, a 303 km long (20 s) deep seismic reflection profile, was acquired across the Variscan belt in SW Iberian Peninsula. The acquisition parameters were designed to obtain a high-resolution crustal-scale image of this orogen. The seismic profile samples three major tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone, and the Central Iberian Zone, which were accreted in Late Paleozoic times. These terranes show a distinctive seismic signature, as do the sutures separating them. Late strike-slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures. The upper crust appears to be decoupled from the lower crust all along the seismic line, but some deformation has been accommodated at deeper levels. A sill-like structure is imaged in the middle crust as a 1–2 s thick and 175 km long high-amplitude conspicuous reflective band. It is interpreted as a great intrusion of mafic magma in a midcrustal decollement. Taking into account surface geological data and the revealed crustal architecture, a tectonic evolution is proposed for SW Iberia which includes transpressional collision interacting during Early Carboniferous with a mantle plume. The Moho can be identified along the entire transect as subhorizontal and located at 10 to 11 s, indicating a 30–35 km average crustal thickness. Its seismic signature changes laterally, being very reflective beneath the South Portuguese Zone and the Central Iberian Zone, but discontinuous and diffuse below the Ossa Morena Zone.

Production and loss of high‐density batholithic root, southern Sierra Nevada, California

Abstract: Received 20 February 2002; revised 21 March 2003; accepted 1 July 2003; published 18 November 2003. [1] Eclogites are commonly believed to be highly susceptible to delamination and sinking into the mantle from lower crustal metamorphic environments. We discuss the production of a specific class of eclogitic rocks that formed in conjunction with the production of the Sierra Nevada batholith. These high-density eclogitic rocks, however, formed by crystal-liquid equilibria and thus contrast sharply in their petrogenesis and environment of formation from eclogite facies metamorphic rocks. Experimental studies show that when hydrous mafic to intermediate composition assemblages are melted in excess of 1 GPa, the derivative liquids are typical of Cordilleran-type batholith granitoids, and garnet + clinopyroxene, which is an eclogitic mineralogy, dominate the residue assemblage. Upper mantlelower crustal xenolith suites that were entrained in mid-Miocene volcanic centers erupted through the central Sierra Nevada batholith are dominated by such garnet clinopyroxenites, which are shown further by geochemical data to be petrogenetically related to the overlying batholith as its residue assemblage. Petrogenetic data on garnet pyroxenite and associated peridotite and granulite xenoliths, in conjunction with a southward deepening oblique crustal section and seismic data, form the basis for the synthesis of a primary lithospheric column for the Sierra Nevada batholith. Critical aspects of this column are the dominance of felsic batholithic rocks to between 35 and 40 km depths, a thick (� 35 km) underlying garnet clinopyroxenite residue sequence, and interlayered spinel and underlying garnet peridotite extending to � 125 km depths. The peridotites appear to be the remnants of the mantle wedge from beneath the Sierran arc. The principal source for the batholith was a polygenetic hydrous mafic to intermediate composition lower crust dominated by mantle wedge-derived mafic intrusions. Genesis of the composite batholith over an � 50 m.y. time interval entailed the complete reconstitution of the Sierran lithosphere. Sierra Nevada batholith magmatism ended by � 80 Ma in conjunction with the onset of the Laramide orogeny, and subsequently, its underlying mantle lithosphere cooled conductively. In the southernmost Sierranorthern Mojave Desert region the subbatholith mantle lithosphere was mechanically delaminated by a shallow segment of the Laramide slab and was replaced by underthrust subduction accretion assemblages. Despite these Laramide events, the mantle lithosphere of the greater Sierra Nevada for the most part remained intact throughout much of Cenozoic time. A pronounced change in xenolith suites sampled by Pliocene-Quaternary lavas to

Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India‐Eurasia collision in Myanmar

Abstract: Situated south of the eastern Himalayan syntaxis at the western margin of the Shan-Thai terrane the highgrade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallized during ~43 Ma high-grade metamorphism A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 309 plus or minus 07 Ma magmatic zircon The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time Mid- Cretaceous to earliest Eocene (120 to 50 Ma) I-type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (~43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma) This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma Similar syntectonic syenites and leucogranites intruded the Ailao Shan-Red River shear belt in southern China and Vietnam during the Eocene-Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina

The ultimate arc: Differential displacement, oroclinal bending, and vertical axis rotation in the External Betic‐Rif arc

Abstract: [1] The External Betic-Rif arc, which lies between the converging African and Iberian plates, is one of the tightest orogenic arcs on Earth. It is a thin-skinned fold and thrust belt formed in Miocene time around the periphery of the Alboran Domain, an older contractional orogen that underwent extensional collapse coevally with the formation of the thrust belt. Restoration of four sections across the thrust belt, together with kinematic and paleomagnetic analysis, allows a reconstruction of the prethrusting geometry of the Alboran Domain, and the identification of the following processes that contributed to the formation of the arc: (1) The Alboran Domain moved some 250 km westward relative to Iberia and Africa during the Miocene. This initiated the two limbs of the arc on its NW and SW margins, closing to the WSW in the region of Cherafat in northern Morocco. The overall convergence direction on the Iberian side of the arc was between 310° and 295°, and on the African side it was between 235° and 215°. The difference in convergence direction between the two sectors was primarily a result of the relative motion between Africa and Iberia. (2) Extensional collapse of the Alboran Domain during the Miocene modified the geometry of the western end of the arc: the Internal Rif rotated anticlockwise to form the present north trending sector of the arc, and additional components of displacement produced by extension were transferred into the external thrust belt along a series of strike-slip faults and shear zones. These allowed the limbs of the arc to rotate and extend, tightening the arc, and creating variations in the amounts and directions of shortening around the arc. The Betic sector of the arc rotated clockwise by 25° during this process, and the southern Rif rotated anticlockwise by ∼55°. (3) Oblique convergence on the two limbs of the arc, dextral in the Betics and sinistral in the southern Rif, resulted in strongly noncoaxial deformation. This had three related effects: (1) large rotations of individual thrust sheets resulted from the oblique propagation of thrusts away from the thrust front, followed by pinning and rotation as the thrust sheets peeled off, (2) continued oblique convergence resulted in distributed shear, particularly in the rear of the thrust wedge, causing rotation of stacks of thrust sheets on the scale of a few tens of kilometers, and (3) distributed shear in the orogen resulted in the rotation of folds as they amplified, the hinges migrating through the rock body, and rotating at a slower rate than the rock. These rotations were substantially larger than the bulk rotations of the limbs of the arc, and they strongly modified the orientations of folds, thrust traces, and the structural indicators of fault slip directions.

High-pressure metamorphism in the Aegean, eastern Mediterranean: Underplating and exhumation from the Late Cretaceous until the Miocene to Recent above the retreating Hellenic subduction zone

Abstract: [1] We report 40Ar/39Ar ages from various tectonic units in the Aegean and westernmost Turkey. On the basis of published geochronologic data and our 40Ar/39Ar ages we propose that the Aegean is made up of several high-pressure units, which were successively underplated from the Late Cretaceous until the Miocene. Ages for high-pressure metamorphism range from 80–83 Ma in parts of the Vardar-Izmir-Ankara suture zone in the north to 21–24 Ma for the Basal unit in the Cyclades and the external high-pressure belt on Crete in the south. Published seismic data suggest that high-pressure metamorphism is currently occurring underneath Crete. Younging of high-pressure metamorphism in a southerly direction mimics the southward retreat of the Hellenic subduction zone. We propose that distinct stages of high-pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat. By far most of the exhumation of the high-pressure units occurred early during the orogenic history in a forearc position.

Neogene extension and volcanism in the Kunlun Fault Zone, northern Tibet: New constraints on the age of the Kunlun Fault

Abstract: [1] The geology of the Jingyu basin (north Tibet) provides constraints on the relationship between localized E-W extension in the Qiangtang block and strike-slip motion along the Kunlun fault. Eocene to Oligocene SW-NE compression formed topographic relief in the present western Kunlun Shan. Pull-apart basins started forming in the western Kunlun ranges around 15 Ma, possibly coincident with the initiation of subduction of the Tarim-Qaidam lithosphere under the Kunlun Shan. Extensional tectonism is still active, and the Jingyu basin is extending, as a normal fault connected with the Kunlun fault was recently active. Localized extension allows Tertiary and Quaternary shoshonitic magmas to reach the surface from crustal depths of 50 to 60 km. Miocene strike-slip faulting indicates that the initiation of the Kunlun strike-slip fault is early Neogene in age and that the total amount of horizontal movement that occurred along the fault has to be reconsidered.

Proterozoic history of the Borborema province (NE Brazil): Correlations with neighboring cratons and Pan-African belts and implications for the evolution of western Gondwana

Abstract: [1] Geological and geochronological correlations between Borborema province (NE Brazil) and neighboring cratons and Brasiliano/Pan-African belts indicate that the Amazonian, West African, and Sao Francisco/Congo cratons and the basement of the Araguaia, Borborema, Nigerian, and Cameroon provinces were part of the Atlantica supercontinent This continent was established at the end of the Transamazonian/Eburnean cycle (∼20 Ga) and, apart from ubiquitous taphrogenesis in the 18–17 Ga interval, remained largely unaffected for the following 1 Ga Around 1 Ga an important magmatic event in Borborema province correlates with rifting episodes and anorogenic magmatism in the Sao Francisco, Congo, and Amazonian cratons These events are interpreted as failed attempts to break up Atlantica, which at this time may have been part of the larger Rodinia supercontinent Renewed extensional conditions in Borborema province during the middle and late Neoproterozoic are attributed to far-field stresses transmitted to the interior of Atlantica by outwardly dipping subduction zones that encircled its northern (present day coordinates) portion The rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the Borborema province essentially includes reworked intracontinental domains Regional deformation and metamorphism, starting at 650–640 Ma, and shear zone development, beginning at 590–595 Ma, were continuously developed through time and were synchronous throughout most of the Borborema, Araguaia, Cameroon, and Nigerian provinces Postorogenic conditions were reached 540–530 Myr ago, while active deformation was still occurring in other belts that accreted around Atlantica to form western Gondwana

Geochronological and geochemical constraints on Mesozoic suturing in east central Tibet

Abstract: [1] This paper reports isotopic, major and minor element geochemistry of igneous and metamorphic rocks from the Kokoxili and Yushu regions of central and eastern Tibet. The first region lies along the Kunlun suture, which separates the Bayan Har-Songpan Ganze (Songpan) terrane from the Tarim and Qaidam blocks. Two Kokoxili granitoids yield U-Pb zircon dates of 217 ± 10 and 207 ± 3 Ma (Late Triassic), which represent the time of emplacement, and Rb-Sr isochron dates of 195 ± 3 and 190 ± 3 Ma (Early Jurassic), which are interpreted as cooling ages. The geochemical signatures of these granitoids suggest that they are related to subduction continuing into the Late Triassic. In the Yushu area, three samples help constrain the age of the Jinsha suture, which separates the Songpan terranes from the Qiangtang blocks. A leucocratic granite and an orthogneiss in the suture zone yield U-Pb zircon dates of 206 ± 7 and 204 ± 1 Ma, respectively, and a paragneiss south of it, a U-Pb monazite date of 244 ± 4 Ma. The existence of coeval magmatism in both the Jinsha and Kunlun sutures suggests that the two subduction zones were simultaneously active. Combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions, we propose that the Kunlun and Qinling block boundaries, which were distinct in the Permian, subsequently formed a continuous, Late Triassic, northward subducting plate margin. Our data suggest that the Jinsha suture correlates with the Benzilan and Nan-Uttaradit sutures, which together belong to a major Late Triassic subduction zone.

Geodynamic significance of the Aeolian volcanism (Southern Tyrrhenian Sea, Italy) in light of structural, seismological, and geochemical data

Abstract: [1] The Aeolian volcanoes are located between the Southern Tyrrhenian Sea back arc and the Calabrian Arc forearc region Structural, geochemical and seismological data indicate that the early phases of volcanic activity (13 Myr) developed in the western sector along WNW-ESE tear faults controlling the southeastward migration of the forearc–back arc system This magmatism ceased when delamination processes affected the Calabrian Arc At 08 Myr, the volcanism migrated southeastward and concentrated on the “new” formed NNW-SSE tear faults related to the postsubduction extensional strain The compressive strain deduced by focal mechanism of earthquakes in the western sector explains the volcanism ending The still active volcanism in the central and eastern sectors develops on a NNW-SSE striking lithospheric discontinuity that crosses the ring-shaped volcanic belt Moho upwelling occurs in the area of active volcanism Fault-slip data and focal mechanisms from M > 5 earthquakes indicate that the NNW-SSE discontinuity moves in response to an oblique (strike-slip/normal) stress related to a WNW-ESE extension This direction of extension is consistent with that of the forearc region, where thrust-type events are lacking and the last compressive phases occurred during Pliocene The later phases of the Aeolian volcanism are related to the melting of shallower source(s) consistent with a continental rift magmatism The Aeolian Islands represent a postcollisional, rift-type volcanism emplaced in an older collision zone

INDEPTH III seismic data: From surface observations to deep crustal processes in Tibet

Abstract: [1] During the summer of 1998, active-source seismic data were collected along a transect running 400 km NNW-SSE across the central Tibetan Plateau as the third phase of project INDEPTH (International Deep Profiling of Tibet and the Himalaya). The transect extends northward from the central Lhasa block, across the Jurassic Bangong-Nujiang Suture (BNS) at 89.5°E, to the central Qiangtang block. A seismic velocity model for the transect to ∼25 km depth produced by inversion of P wave first arrivals on ∼3000 traces shows (1) a ∼50-km-wide region of low velocity (at least 5% less than surrounding velocities) extending to the base of the model at the BNS; (2) sedimentary cover for the southern Qiangtang block that is ∼3.5 km thick; (3) a distinct interface between sedimentary cover and Qiangtang basement or underplated Jurassic melange in the central Qiangtang block; and (4) evidence that the Bangoin granite extends to a depth of at least 15 km. The BNS has little geophysical signature, and appears unrelated to the ∼5 km northward shallowing of the Moho which is associated with the BNS in central Tibet. Geophysical data along the main INDEPTH III transect show little evidence for widespread crustal fluids, in contrast to the seismic “bright spots” found in southern Tibet and to magnetotelluric evidence of fluid accumulations in eastern Tibet. A comparison between the global average and Tibetan velocity-depth functions offers constraints for models of plateau uplift and crustal thickening. Taken together with the weak geophysical signature of the BNS, these velocity-depth functions suggest that convergence has been accommodated largely through pure-shear thickening accompanied by removal of lower crustal material by lateral escape, likely via ductile flow. Although we cannot resolve the details, we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of Tibet as a series of accreted terranes.

Crustal architecture and tectonic evolution of the Gulf of Cadiz (SW Iberian margin) at the convergence of the Eurasian and African plates

Abstract: [1] The Gulf of Cadiz, located at the southwestern Iberian margin, is characterized by widespread seismicity, compressional and strike-slip fault plane solutions and by a large, elongated positive free-air gravity anomaly, the Gulf of Cadiz Gravity High (GCGH). Multichannel seismic profiles across and along GCGH, together with bathymetric and gravity data, allow us to study in detail the tectonic architecture and crustal structure of the Gulf of Cadiz. The upper shelf and slope of the Gulf of Cadiz includes the main structural domains of the Betic fold and thrust belt. In the middle part of the Gulf, the Paleozoic basement crops out on the shallow Guadalquivir Bank and is associated with the largest signature of the GCGH, whereas toward the outer part of the Gulf, the basement deepens progressively. A large NW-SE normal fault and conjugate NE-SW faults define a prominent basement high associated with the GCGH. Modeling of the GCGH suggests localized crustal thinning of 10 km along the central part of the Gulf of Cadiz, probably generated during the Mesozoic rifting episode between the Iberian and African plates. Concentric wedges of fold and thrust belts and large allochthonous masses were emplaced in the Gulf of Cadiz during the Neogene compressional phase. The final emplacement of these units becomes progressively young from the SE (pre-early Langhian) toward the foreland in the NW (late Tortonian). Seafloor surface ruptures, pockmarks, and submarine landslides provide evidence of active faulting in the Gulf of Cadiz. To accommodate the present-day convergence between the African and Eurasian plates, previously extensional faults have probably been reactivated and inverted at depth, as suggested by the intermediate depth seismicity.

Magallanes‐Fagnano continental transform fault (Tierra del Fuego, southernmost South America)

Abstract: [1] Multichannel seismic reflection profiles, gravity measurements, and bathymetric soundings, in conjunction with field geological reconnaissance and remote sensing images, reveal with unprecedented detail the morphostructure of a major segment of the South America–Scotia plate boundary in the Tierra del Fuego region. This segment, known as the Magallanes-Fagnano fault system, is a continental transform margin arranged in an en echelon geometry, along which prominent asymmetric basins were developed. Data acquired off the Atlantic coast of Isla Grande (the main island of Tierra del Fuego), in its central and eastern part, and in the central and western Magallanes Strait image the surface and subsurface structure of the transform fault and its associated basins. The Magallanes-Fagnano fault system is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near-vertical faults. In the Atlantic sector, the fault system trends broadly N70°E and seems to be composed by a single master fault, along which a highly asymmetric basin has formed. At around 63°W, the fault terminates by splaying into secondary normal faults that dissipate the horizontal displacement along the system. In the central eastern part of Isla Grande, the fault segments have been principally identified from analyses of remote sensing images on the basis of their morphological expression. These segments are located within river valleys and are generally associated with localized gravity minima. Lago Fagnano, a 105-km-long, E-W trending depression, is a large, mostly asymmetric pull-apart basin developed within the principal displacement zone of the Magallanes-Fagnano fault system. Restraining bends and overlapping step-over geometry characterize the central part of the Magallanes Strait. Along the western part of the fault system, in the vicinity of the Pacific entrance of the Magallanes Strait, asymmetric sedimentary basins have also developed. The sedimentary architecture of the basins formed within the principal displacement zone of the fault, in which the thick end of the depositional wedge abuts the transform segment, suggest simultaneous strike-slip motion and transform-normal extension, a common feature found in other continental transtensional environments. Strike-slip faulting in the Tierra del Fuego region is also documented along other prominent lineaments which parallel the Magallanes-Fagnano fault system. Along at least two of these lineaments, characterized by a remarkable morphological expression, widespread Quaternary activity occurs. The present-day motion between the South America and Scotia plates is slow (<5 mm/yr). Also the modern seismicity monitored in the Tierra del Fuego region is low (individual events <3.5 in magnitude). The low seismicity may be explained by the slow relative motion between plates and may be further affected by slip partitioning along the different segments which make up the Magallanes-Fagnano fault array, and along the subsidiary wrench lineaments that traverse the region.

Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision

Abstract: [1] Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift (700–850 m) likely due to preferential basal erosion under the center of Lima Basin. Long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km myr−1. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km myr−1) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4–11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long-term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust.

Exhumation tectonics of the ultrahigh-pressure metamorphic rocks in the Qinling orogen in east China: New petrological-structural-radiometric insights from the Shandong Peninsula

Abstract: [1] In eastern China, the Sulu area is recognized as the eastern extension of the Qinling-Dabie Belt, which is famous for its ultrahigh-pressure (UHP) metamorphism. Although numerous petrologic and geochemical works are available, structural data are still rare. This paper provides the first extensive study of bulk geometry and kinematic analysis of the Shandong Peninsula. The study area is divided into three tectonic areas by Cretaceous faults, namely, a southern UHP belt or Sulu area, a northern migmatite area, and an eastern eclogite and migmatite area or Weihai area. Conversely to the deeply entrenched idea that the later area belongs to the North China Belt, and the two others to the South China Block (SCB), we argue that all three areas are parts of the SCB. Structural, petrologic, 40Ar/39Ar, and U/Pb data comply with this new interpretation. In the North Shandong area, mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the Sulu and Weihai eclogites which also experienced a granulite facies overprint before migmatization. The circa 210–200 Ma age of the main ductile deformation is related to an extensional event during the Triassic (or Indosinian) orogeny. This date corresponds to the temperature climax, but the time of the pressure peak, i.e., the real age of the UHP metamorphism is discussed.

Paleozoic tectonic evolution of the northern Xinjiang, China: Geochemical and geochronological constraints from the ophiolites

Abstract: [1] The northern Xinjiang of China is a composite orogenic belt in the south central part of the Altaids formed by subduction-accretion mainly during the Paleozoic. Geochronologic and geochemical data from the Paleozoic ophiolites and associated assemblages are reported here to place constrains on the tectonic evolution of this region and to reconstruct its history, using a model of subduction-accretion plus large-scale map-view strike-slip faulting. Geologic and geochronologic review indicates that the Paleozoic ophiolites and associated assemblages of the western and eastern Junggar regions are tectonically related, occurring as a pattern with the early Paleozoic Tangbale-Mayile-Hongguleleng-Aermantai complexes in the center, the late Paleozoic Darbut-Karamaili and the northern Tianshan complexes in the south, and the late Paleozoic Kekesentao-Qiaoxiahala-Kuerti complexes in the north. Available geochemical data suggest that the Paleozoic ophiolites of the northern Xinjiang are mainly oceanic fragments of these three types: mid-ocean ridge basalt, oceanic island basalts, and island arc basalts. The Paleozoic subduction-accretion complexes grew mainly along different segments of the Kipchak arc, and in associated with migration of magmatic arc. The Paleozoic ophiolite-associated sedimentary sequences and subduction-related volcanic rocks exhibit an evolution history from intraoceanic (early Paleozoic) to marginal settings (late Paleozoic), implying coeval strike-slip stacking with subduction-accretion processes, and by which to transport the respective complexes together and further to amalgamate with neighboring units of the Altaids. Subduction-accretion processes terminated in the mid-Carboniferous in this region. In the late Permian-early Triassic, a counterclockwise rotation of fault blocks in a giant sinistral strike-slip and extension domain between Altay and Tianshan, lastly generated present configuration of the Paleozoic subduction-accretion complexes in the northern Xinjiang.

Cenozoic deformation and tectonic style of the Andes, between 33° and 34° south latitude

Abstract: [1] The Andes of Argentina and Chile between latitudes 33° and 34°S are composed from west to east of an Oligocene to Miocene volcanic arcs and the Neogene east-vergent Aconcagua fold and thrust belt of the Cordillera Principal, and the basement-block faulted Cordillera Frontal. A regional cross section suggests that shortening across the Andes was achieved by thrusting along detachments at several levels in the crust. While thin-skinned deformation along newly formed thrusts occurred in Mesozoic sequences of the eastern Cordillera Principal, reactivation of preexisting Jurassic and Oligocene normal faults has resulted in additional hybrid thick- and thin-skinned structures in the western Cordillera Principal. Five major thrusting events are recognized in this part of the Andes: (1) Early to Middle Miocene tectonic inversion of the extensional faults in the western Cordillera Principal, (2) Middle to Late Miocene development of the Aconcagua fold and thrust belt, (3) Late Miocene uplift of Cordillera Frontal, (4) Late Miocene–Early Pliocene out-of-sequence thrusting in the Cordillera Principal, and (5) Pliocene to present deformation of the foreland.

Backstop geometry and accretionary mechanics of the Sunda margin

Abstract: The convergent Sunda margin off Indonesia displays all geological features characteristic of an accretion-dominated subduction zone. A combined interpretation of prestack depth-migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high-resolution bathymetric data and for the first time allows the exact mapping of backstop regimes. Initially, the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin. Increasing material strength of the outer high due to lithification formed a dynamic backstop, which controls accretion today. An out-of-sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area. The existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc, which is observed in regimes of frontal as well as of oblique subduction. Mass balance calculations, which account for porosity changes and metamorphism, indicate a subduction history dominated by accretionary processes since the late Eocene. Accretion is associated with the low values of basal friction inferred for the Sunda margin. Structural investigations of conjugate fault planes indicate a very weak basal detachment. Effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments, whereas overpressuring within the frontal accretionary prism is negligible.

Architecture of the Focşani Depression: A 13 km deep basin in the Carpathians bend zone (Romania)

Abstract: [1] In front of the SE Carpathians Bend a very deep basin (Focsani Depression) developed in Miocene to Recent times. An important part of its subsidence occurred after the main stages of thrusting in the Carpathians. Apparently, the basin lies in the “wrong” place and evolved in the “wrong” time. In this study, we constrain its architecture and evolution by analyzing a large database consisting of more than 1000 km two-dimensional seismic lines and more than 60 wells. Around 13 km thick, Badenian-to-Quaternary (<16.5 Myr) sediments were deposited in the central part of the Focsani Depression. During the Badenian (16.5–13 Myr), the foreland (south of Trotus fault) underwent NE-SW directed extension and NW trending basins opened in the eastern Moesian platform. A NW-SE oriented area of subsidence stretched from the Transylvania basin through the Focsani Depression to the SE of the Moesian platform while thrusting was going on in the East European/Scythian platform, East Carpathians, and Getic Depression. Starting with the Sarmatian (13–10 Myr), the Focsani Depression depocenter moved out of the Carpathian belt coeval with the exhumation of the south and the East Carpathians north of the Trotus fault. The basin became wider and was tilted toward the belt. Tilting was accompanied by dextral shearing mainly along the Intramoesian and Peceneaga-Camena faults. After Sarmatian times, subsidence occurred practically only SSE of Trotus fault. During Meotian-Pontian (10–5 Myr), subsidence slowed down. Stronger, Pliocene-Quaternary subsidence is coeval with normal faulting and shearing in Moesian platform. The western margin of the Focsani Depression was then tilted eastward, coeval with the exhumation of the bend zone and opening of the intramontane basins in the inner part of the belt.

Two phases of Mesozoic north‐south extension in the eastern Altyn Tagh range, northern Tibetan Plateau

Abstract: [1] The >300-km long, east striking Lapeiquan fault lies in the eastern Altyn Tagh range along the northern margin of the Tibetan Plateau and was interpreted as a north dipping thrust in early studies. However, our mapping shows that the fault is a south dipping normal fault juxtaposing Archean-Proterozoic gneisses beneath an early Paleozoic volcanic and sedimentary sequence. Its dip angle varies from <30° to ∼60°. The central fault segment is expressed as a 30–50 m thick ductile shear zone with well-developed mylonitic fabrics and stretching mineral lineations, where the eastern and western segments are characterized by cataclastic deformation. Kinematic indicators such as asymmetric boudinage, asymmetric folds, and minor brittle and ductile faults within the fault zone consistently indicate a top-south normal-slip sense of shear. The age of the Lapeiquan fault is constrained by two types of information. First, a sequence of Early-Middle Jurassic sediments is locally present in the hanging wall of the Lapeiquan fault. The clasts of the Jurassic strata, particularly the stromatolite-bearing, cherty limestone and purple quartzite, can be correlated uniquely with those in the footwall of the fault. We interpret that the Early-Middle Jurassic strata were deposited in an extensional basin related to motion along the Lapeiquan fault. Second, 40Ar/39Ar thermochronologic analyses indicate two prominent cooling events in the Lapeiquan footwall. The older event occurred in the latest Triassic-earliest Jurassic between ∼220 and 187 Ma, while the younger event occurred in the latest Early Cretaceous at ∼100 Ma. Because the 220–187 Ma cooling ages are widespread in the Lapeiquan footwall, we suggest it to represent the main stage of faulting. We interpreted the younger phase of fault motion at ∼100 Ma to have been related to fault reactivation. The deformation was aided by motion along the south dipping Qiashikan normal fault that merges with the eastern Lapeiqaun fault. From the regional tectonic setting, it appears that Mesozoic extension in northern Tibet to have occurred in a back arc setting during northward subduction of the Tethyan oceanic plate. The findings of Mesozoic extensional structures in northern Tibet suggest that compressive stress induced by collision of the Qiangtang and Lhasa terranes with Asia was not transmitted beyond northern Tibet. This in turn implies that the popularly inferred contractional setting for Mesozoic evolution of the Tian Shan north of Tibet needs a reevaluation based on a combination of both structural and sedimentological observations.

Neogene to ongoing normal faulting in the inner western Alps: A major evolution of the late alpine tectonics

Abstract: [1] Widespread brittle extension occurs in the internal zones of the southwestern Alps, forming a dense fault network, which overprints the compressional structures. The extension has been followed by transcurrent motions. In terms of paleostress this extension is radial to the belt close to the Crustal Penninic Front and multidirectional in the eastern part of the internal zones. The paleostress field of the strike-slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults. Globally, we propose that the internal zones of the southwestern Alps underwent a single Neogene transtensive tectonic regime. To strengthen our structural results, the current tectonics has been analyzed using seismotectonic tools. This approach allowed the recognition of several active faults. Moreover, the ongoing tectonics is coherent with the observed brittle deformation. The fault network controls the present-day seismicity. The combined seismotectonic and structural approaches give coherency and continuity from the Neogene to ongoing extensional tectonics in the southwestern Alps. In the framework of the whole western Alps a synthesis of our results with neotectonic-related data allowed us to provide an accurate map of the Neogene to present kinematics of the belt. Extension, which appears as a major feature of the internal zones, has been coeval with the propagation of thrusts in the outer zones during Neogene times. The geodynamic processes, which rule this major tectonic evolution of the belt, remain a matter of debate.

Shortening of an overriding plate and its implications for slip on a subduction thrust, central Hikurangi Margin, New Zealand

Abstract: [1] At subduction plate boundaries, strains accumulate within the overriding plate in response to motion of the down-going plate. To a first approximation, margin-normal interplate slip and shortening in the overriding plate sum to the total plate convergence. Therefore margin-normal interplate slip can be estimated by measuring shortening in the overriding plate and total plate convergence. Here we quantify margin-normal interplate slip by measuring shortening of the upper plate along a profile which extends up to 480 km across, and approximately normal to, the central Hikurangi Margin, New Zealand. Long-term deformation is estimated for periods of millions of years using data from seismic reflection lines and cross sections, while contemporary strain rates are derived from Global Positioning System (GPS) geodetic velocities. These data permit comparison of plate convergence rates determined by seafloor spreading data with the rates of contraction over short (∼7 years) and long (∼2.5 and 5 Myr) periods of time. On this part of the plate boundary, total margin-normal shortening in the overriding plate over the last ∼5 Myr was ∼9–28 km and accounts for ∼6–19% of the total convergence (145–160 km) at rates of ∼2–6 mm/yr. The remaining ∼80% or more of plate convergence has been accommodated by slip on the subduction thrust. GPS contraction along approximately the same profile indicates ∼14–16 mm/yr of contemporary shortening across the terrestrial part of the upper plate, which is approximately modeled by ∼80% interplate coupling on the subduction interface down to depths of ∼30 km. The contrasting proportions of inferred interplate coupling for contemporary and geological data sets are consistent with stick-slip behavior on the plate interface and with slip being principally achieved during intermittent large earthquakes. The general shapes of short- and long-term shortening profiles are similar and suggest that the highest strains occur at, or immediately east of, the Mohaka Fault. These strain profiles are consistent with a model where high strains in the upper plate are concentrated near the down-dip end of the contemporary locked zone on the plate interface. This model requires that the down-dip end of the interseismic locked zone has, on average, been approximately stable over a period of at least 2.5–5 Myr.

Deformation history of the high‐pressure Lycian Nappes and implications for tectonic evolution of SW Turkey

Abstract: [1] In southwestern Turkey, the Lycian nappe complex which overlies the autochthonous Menderes Massif and Bey Dag platform, consists from base to top of metasediments, a melange unit and an ophiolitic sequence. Fresh Fe-Mg-carpholite occurrence in the metasediments attests to a high-pressure low-temperature metamorphic event. We report the distribution of Fe-Mg-carpholite and its breakdown products (e.g., pyrophyllite and chloritoid) on the Bodrum peninsula, south of the Menderes crystalline massif. The distribution of Fe-Mg-carpholite and its relics shows that the low-grade high-pressure metamorphism affected a widespread area in the lower units of the Lycian Nappes. Analysis of the ductile deformation in HP-LT metasediments indicates shear senses top-to-the-northeast to top-to-the-east. Most of this deformation is contemporaneous with the retrogression of high-pressure low-temperature parageneses and is therefore coeval with exhumation from a depth of about 30 km. At the top of the Menderes Massif “cover series,” close to the contact with the Lycian Nappes, similar eastward displacements are observed and trajectories of the stretching lineations are continuous from the Lycian Nappes to the Menderes Massif across the contact. These observed movements are incompatible with the southward transport of the Lycian Nappes over the Menderes Massif. We discuss the regional tectonic implications and conclude that the Lycian Nappes and the southernmost part of the Menderes Massif were exhumed in two stages: (1) Eocene (?) top-to-the-NE shear (syn-orogenic extension?), (2) Miocene deformation contemporaneous with the Aegean extension.