Eastward Translation Research Articles

New insights on the tectonics along the New Hebrides subduction zone based on GPS results

At the New Hebrides (NH) subduction zone, ridges born by the subducting Australia plate enter the trench and collide with the overriding margin. Results from GPS surveys conducted on both sides of the trench and new bathymetry maps of the NH archipelago bring new light on the complex tectonics of this area. Convergence vectors present large variations that are not explained by Australia/Pacific (A/P) poles and that define four segments. Vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors. Slow convergence (i.e., 30–40 mm/yr) is found at the central segment facing the D'Entrecasteaux Ridge. The southern segment moves faster than A/P motion predicts (89 to 124 mm/yr). Relatively to a western North Fiji basin (WNFB) reference, the northern and southern segments rotate in opposite directions, consistently with the extension observed in the troughs east of both segments. Both rotations combine in Central Vanuatu into an eastward translation that “bulldozes” the central segment into the WNFB at ∼55 mm/yr. That model suggests that the motion of the central segment, forced by the subduction/collision of the D'Entrecasteaux ridge, influences the motion of the adjoining segments. The New Caledonia archipelago is motionless with respect to the rest of the Australia plate despite the incipient interaction between the Loyalty ridge and the NH margin. Southeast of the interaction area, convergence is partitioned into a ∼50 mm/yr trench‐normal component accommodated at the trench and a ∼90 mm/yr trench‐parallel component, close to the A/P convergence, and presumably accommodated by a transform boundary at the rear of the NH arc.

Palaeozoic palaeogeography: A North Atlantic viewpoint

Palaeozoic palaeogeography, highlighting the North Atlantic Caledonian evolution and the destruction of the Iapetus Ocean and the Tornquist Sea, is recapitulated with reconstruction maps from Early Ordovician to Mid-Devonian times. In the Early Ordovician (Trem-adoc-Arenig), Laurentia, Siberia, and the North China Block were positioned in equatorial latitudes, Baltica was located at intermediate southerly latitudes, whilst Avalonia and the European Massifs were located together with the North African part of Gondwana in high southerly latitudes. During the Ordovician, Baltica drifted northwards and approached Siberia while undergoing counter-clockwise rotations. Aval-onia rifted away from Gondwana during Arenig-Llanvirn time, and the Tornquist Sea, separating Avalonia and Baltica, narrowed gradually during the Ordovician followed by Late Ordovician ‘soft docking’ of Eastern Avalonia and Baltica prior to their joint collision with Lau-rentia. The main collisional event between Baltica and Laurentia occurred at c. 425 Ma and was marked by deep subduction of Baltican crust beneath Laurentia with concomitant eastward translation of nappes over the Baltican margin. Deep subduction was a function both of rapid motion of Baltica (8-10 cm/year) toward a stationary Laurentia and precedence of prolonged subduction of large volumes of cold litho-sphere. Shortly after collision, in Emsian times, these rocks were exhumed by extensional collapse.

Paleomagnetism of Eocene Basalts of Mayreau, West Indies: Implications for Contrasts in Tectonic Rotation in the Southeastern Caribbean

Paleomagnetic directions were determined from 29 sites on the Grenadine island of Mayreau. Most data come from the middle Eocene Mayreau Basalt, which has been interpreted to be oceanic crust of the Grenada Basin uplifted in the half-horst of the southern Lesser Antilles arc platform. The objective was to assess the existence in the Mayreau Basalt of anomalies in paleomagnetic declination and inclination relative to Eocene South America and to other terranes in the Caribbean-South American plate-boundary zone. Primary and early deuteric magnetic directions were extracted that yielded site-means with α95 ≤ 15° from 15 sites in the Mayreau Basalt. Demagnetization removed a pervasive north-down component of low unblocking temperature. Tests demonstrated that the Eocene rocks were not remagnetized during Neogene island-arc magmatism and that the magnetization occurred before folding. The mean of site-means for the Mayreau Basalt is not different at the 95% confidence level from expected Eocene directions at Mayreau, from either North or South America. Therefore, there has been no net vertical-axis tectonic rotation or latitudinal transport of the Mayreau Basalt relative to the continents. Moreover, the eastward translation of the Grenada Basin as part of the Caribbean plate relative to the American plates must have been about a Euler pole that is geographically nearly 90° from the Grenada Basin. This result contrasts markedly with the large declination anomalies relative to South America in Tobago, Aruba, and other localities in the plate-boundary zone bordering northern South America. If the declination anomaly in Tobago is Eocene or younger and results from vertical-axis rotation in the plate-boundary zone, the difference in motions between Tobago and Mayreau must have been taken up at the southern Grenada Basin deformation front, with right-oblique subduction of Grenada Basin lithosphere and large rotation of the Tobago terrane.

GCM Simulations of Eastern Australian Cutoff Lows

The ability of the CSIRO-9 General Circulation Model (GCM) to capture surface cutoff lows over eastern Australia is investigated by comparing composites of ten GCM cases with ten observed lows. The lows are also studied individually to compare their development and movement, as well as synoptic features, which may have been smoothed out in the compositing process. Finally, the incidence of all such lows in the 1 × CO2 and 2 × CO2 simulations are examined to determine the possible effects a doubling of CO2 will have on their occurrence. The GCM surface lows were found to develop from an upper-level cutoff low in a manner similar to the observed lows. In both sets, this development took place between the upper-level subtropical and polar jets in all seasons except summer, where only one jet was evident. Latent heat release appeared to play an important role in the intensification of the surface lows. The main difference between the two sets of cutoff lows was that the GCM surface lows tended to develop farther to the east of the upper-level cutoff, the upper-level features were less intense and occlusion did not take place. As a result, the GCM lows had a greater eastward translation compared to the observed lows, which often meander along the east coast for several days while they intensify. These features appear to be related to the low resolution of the GCM. The frequency of east Australian cutoff lows was underpredicted in the WM by about 45% in the 1 × CO2 simulation, with the greatest underprediction occurring in autumn and winter. Analysis of upper-level jet structure indicated that the GCM produced a poor simulation of the dual jet structure aloft, which may account for this problem. The 2 × CO2 simulation produced even fewer cutoff lows over eastern Australia. This was probably caused by the reduced baroclinicity due to increased warming of polar regions, which resulted in an even weaker dual jet structure. The cast Australian cutoff lows were found to be more intense in the 2 × CO2 Simulation, suggesting the greater role played by latent heat effects once development has been initiated.

Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan connection

The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental mountain building at different stages of development. The region is structurally dominated by several major E—W left-lateral strike-slip fault systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be traced along the surface for over 350 km. The eastern two-thirds of the fault system ruptured during a major earthquake (M = 8.3) in 1957, whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the length of the fault system. Detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross-sectional geometries, with thrust faults rooting into oblique-slip and strike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, will lead to growth and coalescence of the restraining bends into a continuous sublinear range, possibly obscuring the original strike-slip fault system; this may be a common mountain building process.The largely unknown Gobi-Tien Shan fault system is a major left-lateral strike-slip fault system (1200 km + long) that links the southern ranges of the Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Tien Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Quaternary activity. The total displacement is unknown, but small parallel splays have apparent offsets of 20 + km, suggesting that the main fault zone has experienced significantly more displacement. Field investigations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fault system. The spatial relationship between topography and active faults in the Barkol Tagh and Bogda Shan strongly suggests that these ranges are large, coalescing, restraining bends that have accommodated the fault's left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien Shan formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system is one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low, E–W trending corridor between the northern edge of the Tibetan Plateau and the Hangay Dome, presumably in response to the Indo-Eurasian collision 2500 km to the south.

The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico)

Terrane analysis has established differences in lithology, style of deformation, metamorphism, and radiometric ages among several basement complexes in southern Mexico. In this paper, we identify similarities in the geologic history of the adjacent Acatlán, Oaxaca, and Xolapa complexes through a U/Pb zircon study on crustal sources and timing of magmatic, metamorphic, and tectonic activity in the Xolapa complex. Our regional tectonic scenario is based on both new chronologic and previously established structural data. Inherited zircon in the Xolapa complex indicates the presence of a Proterozoic (1.0–1.3 Ga) crustal component with an age range that overlaps Grenville crystallization dates (1.0–1.2 Ga) from the Oaxaca basement, and an inherited Grenville crustal component (1.0–1.1 Ga) identified in zircons from the Acatlán complex. The Proterozoic component in the Xolapa complex indicates either that it received sediments from a continental region of Grenvillean age, for example the Oaxaca basement, which is the closest exposed Grenville crust in southern Mexico, or that the Xolapa complex has a Grenville basement. In the latter case, the Xolapa complex has been modified by widespread high‐grade metamorphism and large‐scale migmatization. Metamorphism and migmatization occurred from 66 to 46 Ma and locally continued into Oligocene time. Magmatism in the Xolapa complex terminated with crustal growth by plutonism, which is characterized by a systematic pattern of eastward‐younging crystallization ages, from 35 Ma in the west (west of Acapulco) to 27 Ma in the east (east of Puerto Angel). Metamorphism and migmatization in the Xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the Farallon and North American plates (e.g., rate and direction of subduction) and the early evolution of the Caribbean. This plate reorganization triggered sinistral transtension distributed across the southern Mexican continental margin and the eastward translation of the Chortis block. The eastward younging of the Xolapa plutons also is consistent with the motion of the Chortis block during eastward displacement of the Caribbean plate. This induced a shift of the magmatic arc from its Cretaceous to early Tertiary position along the Xolapa and Chortis blocks to its present mid‐Mexican position. We conclude that a single tectonic framework accounts for the Mesozoic and Cenozoic geologic history of the southern Mexican terranes.

Amount and style of Late Cenozoic Deformation in the Liupan Shan Area, Ningxia Autonomous Region, China

The structures of the Liupan Shan area are characterized by numerous active thrust and strike‐slip faults that suggest thin‐skinned deformation. The structural history of this area can be divided into three phases that probably overlap one another in time and are parts of a single protracted deformation. The oldest Cenozoic deformational phase occurred probably between late Pliocene and early Quaternary time and produced some of the folds and thrust faults in the Liupan Shan and Yueliang Shan. During this phase, deformation was the result of approximately N50°E shortening, and the amount of shortening seems to have been about 1–2 km. The second phase of deformation was dominated by left‐lateral strike‐slip faulting (left slip) on the N60°W striking Haiyuan fault zone and shortening on north‐south trending structures; shortening was associated with a transfer of the left‐slip displacement on the Haiyuan fault zone to shortening in areas farther east. Shortening occurred by thrust faulting in the Liupan Shan and Xiaoquan Shan and by folding in the Madong Shan. During this phase the orientation of shortening changed to N60°W. The average amount of shortening on the north‐south trending folds in the Madong Shan is about 6.3–7.8 km. Most of the shortening on the Liupan Shan and Xiaoguan Shan thrust faults also occurred during this phase and amounted to a minimum of 4.8–6.3 km and 6.6–7.6 km, respectively, also with an orientation of N60°W. During the third phase of deformation, about 1–1.5 km of late Pleistocene to Recent left slip occurred on the Xiaokou fault, which was transferred into oblique left‐slip thrusting in the Liupan Shan. At this time, deformation in the Madong Shan and Xiaoguan Shan ceased or was reduced to a very slow rate. The present, active left‐slip on the Haiyuan fault zone is accommodated by shortening in the Liupan Shan area. The total displacement along the Haiyuan fault is essentially the same as the total amount of shortening in the Liupan Shan area. The sequence and interaction of strike‐slip and thrust faults in the Liupan Shan area seem to apply to the folds and thrust faults farther north in the southern Ningxia region. Thus the northeastern margin of the Tibetan Plateau appears to grow by shortening oriented northeast and by left slip faulting that transfers material from farther to the west. The total left‐slip in the entire southern Ningxia region probably is less than 20–25 km and may be absorbed by shortening within this region. Thus the eastward translation of crustal fragments in the northern part of the Tibetan Plateau may not extend east of southern Ningxia, and if large‐scale eastward displacement has occurred, it must lie farther south.

Recent tectonics around the island of Timor, eastern Indonesia

Eastern Indonesia is the site of active collision between the Banda Arc and the Australian continent. The zone of collision is a poorly understood area and, in particular, controversy exists regarding the sites and styles of Holocene deformation in the vicinity of the island of Timor. This paper describes evidence for late Holocene to historically active sea-floor and shallow subsurface tectonic activity in the area, mapped using the GLORIA sidescan sonar system and single-channel seismic reflection data. Our main conclusion is that the zone of plate contact and major compressional deformation, which lies along the Java Trench west of 119° 45′E, continues directly eastward into the Timor Trough. There is little evidence for recent upper plate shortening north of this major plate boundary in the vicinity of the islands of Savu and Timor. Changes in deformational style, from small-scale uniform deformation at the Java Trench to mud diapirism and broad thrust-bounded folds further east, partly disguise the continuity of the deformation zone. Lateral changes in the style of deformation appear to be controlled only by the thickness and facies of the subducting sedimentary section. Where thin, such as at the Java Trench, uniform small-scale folding results. Further east, in the Timor Trough, a thicker sedimentary sequence deforms into buckled thrust sheets. The location of a large mud diapir field in the Timor Trough south of Savu appears to correlate with a thick sequence of Early Cretaceous marine shale that underlies the Australian shelf. The same shale may be the source of mud diapirs on Timor, demonstrating that, within the convergence zone, sediment facies rather than precise tectonic setting is the principal control on the distribution of the diapirs. A zone of strike-slip, thrust, and possibly normal faults observed to the north of Timor, between the islands of Alor and Wetar, appears to have a complex origin. Some evidence suggests that it is part of a left-lateral offset which cuts across the entire arc, through Timor. However, the apparent south-easterly thrusting of the Banda Sea beneath Wetar, and the deep pull-apart basin developed south-east of Alor, seems also to indicate eastward translation of Wetar relative to the arc further west. This may be driven by deformation of the Banda Sea under north-south compression. We see no evidence for major thrusting north of Wetar, indicating that, at this longitude, little cross-arc compression is accommodated by shortening on its northern side.

Formula omitted] mineral age record of a polyorogenic evolution within the Seve and Köli nappes, Trøndelag, Norway

In the central Scandinavian Caledonides, eugeoclinal terranes represented in the Köli Nappes were thrust > 500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe Complex are suspected to have been derived from the outer margin of continent Baltica. Hornblende within amphibolite at one locality within westernmost exposures of Seve Nappe units in Trandelag, Norway, displays an internally discordant 40Ar 39Ar age spectrum which suggests a polyphase thermal evolution including: 1. (1) initial post-metamorphic cooling through argon retention temperatures at c. 475–480 Ma; 2. (2) partial rejuvenation during a Silurian (c. 400–425 Ma) thermal overprint. Muscovite within the exposure was more extensively rejuvenated during the Silurian overprint. At twelve other Trandelag sample locations, hornblende, muscovite and biotite within both the Seve and Köli Nappe Complexes record post-metamorphic cooling ages between 420 and 435 Ma and display no direct evidence of a pre-Silurian thermal record. However, the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre-Silurian tectonothermal history. Comparison with 40Ar 39Ar mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: 1. (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the Late Cambrian-Early Ordovician over a west-dipping subduction zone; related arc volcanic activity occurred in more outboard, western sectors; 2. (2) ductile imbrication of high- and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); 3. (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltica continental margin; 4. (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian. Crystalline Baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high-temperature eclogues. Previously reported crystallization ages of c. 425 Ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures. Following eastward nappe transport, the basement experienced rapid uplift and resultant adiabatic decompression. This was followed by rapid cooling through argon retention temperatures at 390–400 Ma.

Shipboard and altimetric studies of rapid Gulf Stream variability between Cape Cod and Bermuda

The temporal variability of the Gulf Stream between Cape Cod and Bermuda was examined using shipboard ADCP measurements, satellite infra-red (AVHRR) and altimeter (GEOSAT) data for a period of about 2 weeks in April 1989. Dominant changes were a southward shift and rotation of the Gulf Stream due to a meander, a westward translation of a warm-core ring and an eastward translation of a cold-core ring. Terms in the along-track (nearly cross-stream) and cross-track momentum balances from the ADCP data showed that horizontal advection of horizontal momentum was the principal ageostrophic term, exceeding both measured temporal changes and estimated vertical velocity terms by about a factor of five at a depth of 100 m. Cross-track pressure gradients (not measured) needed to be about 20% higher than expected geostrophically and, if not balanced by the horizontal advection terms, could force particles across and out of the Gulf Stream to the south at speeds of 0.1 m −1. Cross-track ADCP velocities agreed closely with those estimated from GEOSAT: both gave peak velocities in the Gulf Stream of about 2.0 ± 0.1 m s −1. Errors were of the same order as the ageostrophic terms in the along-track momentum balance. Except near the Bermuda Rise, GEOSAT-derived absolute velocities could be quantitatively related to major oceanographic signals, such as the Gulf Stream and rings.

Cenozoic Plate tectonic history of the northern Venezuela‐Trinidad Area

Geological and geophysical data, coupled with recent plate tectonic reconstructions, suggest that the Cenozoic geologic history of the northern Venezuela‐Trinidad area has been dominated by strike‐slip displacement of discrete crustal blocks. Allochthonous terranes within the area include metavolcanic rocks of the Cretaceous Villa de Cura Group and metamorphic rocks of the Precambrian to Cretaceous Cordillera de la Costa. A relatively competent crustal block (Margarita Block) is defined by an outline around the metamorphic basement of Margarita Island, the Araya/Paria peninsula, the Northern Range of Trinidad, and Tobago Island. Reconstruction of the Margarita Block to its original position requires at least partial closure of the Falcon Basin, closure of the Bonaire and Cariaco basins, and restoration of about 50 km of motion on both the Oca and Bocono faults. Post middle Eocene eastward translation of the Caribbean plate caused eastward motion of the Margarita Block. A minor change in relative plate motion during the late Oligocene or early Miocene produced a right step in the Moron fault, forming the Cariaco pull‐apart basin and El Pilar fault zone. Maximum offset on El Pilar fault is estimated to be no more than 125 km, though displacement along the entire fault zone may have been greater. Transpressional stresses between the Caribbean plate and northern South America caused folding of the Serrania del Interior of Venezuela and the Central Range of Trinidad. Eastward migration of transpressional stresses at the southeastern corner of the Caribbean‐South American plate boundary is being accommodated by formation of oblique thrusts, transpressive anticlines, and downwarping of the crust. Bouguer gravity data suggest that Jurassic‐aged Atlantic oceanic crust is being depressed as the Caribbean plate expands into the Demerara Plateau area. This study suggests that the faults and transtensional/transpressional/compressional structures identified in this study are the result of stresses produced during the large eastward translation of the Caribbean plate since the Paleocene, and are not the product of a shear couple.

Contrasting accreted terranes in the southern Appalachian Orogen, basement beneath the Atlantic and Gulf Coastal Plains, and West African orogens

A variety of lithostratigraphic units may be outlined within the pre-Mesozoic crystalline basement of the Atlantic and Gulf Coastal Plains. These include: (1) a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern Alabama and southeastern Mississippi (Wiggins Uplift). Hornblende and biotite within higher-grade units record 40Ar/ 39Ar post-metamorphic cooling ages of ∼310−300 Ma; (2) a suite of contrasting igneous rocks (granite, basalt, and agglomerate) and serpentinite which occurs along the Brunswick-Altamaha Magnetic Anomaly in southwestern Alabama; and (3) an extensive, apparently coherent tectonic element (the Suwannee Terrane) comprised of undeformed granite (in which biotite records 40Ar/ 39Ar post-magmatic cooling ages of ∼530−525 Ma), low-grade felsic metavolcanic rocks, a suite of high-grade metamorphic lithologies (St. Lucie Metamorphic Complex in which hornblende yields 40Ar/ 39Ar post-metamorphic cooling ages of ∼515−510 Ma), and a succession of Lower Ordovician-Middle Devonian sedimentary rocks (characterized by Gondwanan fauna). The West African Orogens (Mauritanides, Bassarides, and Rokelides) record a locally complex, polyphase tectonothermal evolution. The earliest event corresponded to westward rifting of a continental fragment from the West African Craton at ∼700 Ma. This led to development of a rift-facies lithotectonic succession which included sedimentary units and intracontinental igneous sequences. Rifting was limited, and a western ensialic arc began to develop by ∼680 Ma. The associated convergence culminated in an episode of folding and metamorphism at ∼650 Ma (Pan-African I orogenesis). Following widespread deposition of late Paleozoic glacial and flyschoid sediments, a second tectonothermal event occurred between ∼550 and 500 Ma (Pan-African II orogenesis). The West African Orogens were marked by general tectonic quiescence throughout most of the early and middle Paleozoic. Late Paleozoic (∼300 Ma) collision of Gondwana and Laurentia resulted in eastward translation of previously tectonized Mauritanide units over their foreland, and emplacement at highest structural levels of previously imbricated nappes which include sequences with uncertain palinspastic origins. The late Paleozoic (Hercynian) transport was largely intracontinental, and only westernmost portions of the exposed Mauritanides record an associated penetrative tectonothermal overprint. Characteristics of pre-Mesozoic basement units which comprise the Suwannee Terrane in the Florida subsurface suggest they represent extensions of the Bassaride-Rokelide Orogen. There is no apparent record of Paleozoic tectonothermal activity in the Suwannee Terrane. This contrasts markedly with penetrative late Paleozoic reworking of basement units in the Wiggins Uplift. This suggests that a major dextral transcurrent fault was active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various basement units of the Coastal Plain are not correlative with any of the non-Laurentian terranes exposed in the southern Appalachian Orogen (e.g., Carolina terrane of the eastern Piedmont) which accreted in the Ordovician-Devonian to exterior positions along the eastern margin of the North American craton. These exotic Appalachian terranes were transported into their present structural positions during late Paleozoic collision of Gondwana and Laurentia. They are separated from Gondwanan elements of the Suwannee Terrane by a suture approximately marked by the Brunswick-Altamaha Magnetic Anomaly.

Palaeomagnetic constraints on Himalayan-Tibetan tectonic evolution

Palaeomagnetic data from the Lhasa, Qiangtang and Kunlun Terranes of the Tibetan Plateau are used with data from stable Eurasia, eastern China and Indochina, to test different models of crustal thickening in the Tibetan Plateau, to attempt a Carboniferous palaeogeographic reconstruction, and to calculate the relative motion between the South China Block and the Indochina Block. The data suggest that since the onset of the India—Eurasia collision, the Lhasa Terrane has moved 2000 + 800 km north with respect to stable Eurasia. This indicates that strong internal defomation must have taken place in southern Eurasia since the collision, and thus challenges the model of large-scale underthrusting of the Indian subcontinent beneath the Tibetan Plateau as the mechanism for crustal thickening in Tibet. Palaeomagnetic results from the Kunlun Terrane show that it was at 22° south latitude during the Carboniferous. A Carboniferous reconstruction is presented in which the Kunlun and Qiangtang Terranes, several Indochina terranes, and the North and South China Blocks are grouped together. These units of continental crust all share the specific tropical and subtropical Cathaysian flora, and the group is therefore called the Cathaysian composite continent. To test the model of propagating extrusion tectonics, we have used newly available palaeomagnetic results from South China and Indochina to calculate probable displacements. This exercise suggests a rotation of about 8° of Indochina with respect to the South China Block that is smaller than the predicted rotation of 40°, A large eastward translation of the South China Block relative to the Indochina Block of about 1500 km is consistent with the palaeomagnetic data.

Polyphase tectonothermal evolution of the Scandinavian Caledonides

Summary A polyphase tectonothermal evolution characterizes many of the allochthonous sequences which constitute the Scandinavian Caledonides. Initial Late Cambrian consumption of Iapetus oceanic crust occurred outboard of the Baltoscandian margin with development of an island-arc above a west-dipping subduction complex. Imbrication, polydeformation and variably high-grade metamorphism of Baltoscandian miogeoclinal rocks and underlying Precambrian crystalline basement occurred within an accretionary prism which developed in the subduction complex. This resulted in eastward younging, diachronous metamorphism throughout the Early and Middle Ordovician. Cooling during subsequent uplift was regionally variable, and is reflected by the wide range of radiometric dates reported for the Baltoscandian allochthons (455–535 Ma). This early Caledonian activity has been termed the Finnmarkian orogeny; however, it is best viewed as a complex, diachronous series of tectonothermal events which variably affected the Baltoscandian margin from the Late Cambrian through the Middle Ordovician. Almost complete closure of the Iapetus Ocean appears to have been accomplished by the Late Ordovician when eugeoclinal terranes which had developed along the Laurentian margin were tectonically juxtaposed with eroded remnants of the Late Cambrian-Middle Ordovician arc. Initial imbrication of the Laurentian eugeoclinal sequences with the arc occurred during the Early Silurian (Late Llandovery). These were subsequently imbricated and infolded with predeformed Baltoscandian allochthons. The resultant composite allochthon was subsequently thrust eastward onto Silurian sedimentary successions deposited over previously metamorphosed Caledonian basement. This sequence of Early Silurian to Early Devonian tectonic events has been collectively termed the Scandian orogeny; however, they are regionally diachronous and should be bracketed with the local controls available. Scandian dates of 425–430 Ma are locally recorded in allochthons along eastern portions of the orogen and they probably date initial nappe translation to higher crustal levels along the Baltoscandian margin. In lower structural units Scandian dates range from 410 Ma in central to 385 Ma in western portions of the orogen. These are interpreted to date rapid cooling during the uplift associated with a crustal rebound which followed eastward translation of overriding allochthons.

Geodynamic evolution of the mauritanide, bassaride, and rokelide orogens (West Africa)

Abstract The Mauritanide, Bassaride and Rokelide orogens occur along the western edge of the West African Craton. These record a polyphase tectonothermal evolution, including Pan African I (c. 650 Ma) and Pan African II (c. 550 Ma) events together with local Hercynian (late Paleozoic) overprinting. Pan African I activity is most penetratively recorded in the Bassarides, and resulted from late Proterozoic collision of a western continental structural block. Pan African II orogenesis increases in intensity from the southern Mauritanides through the Bassarides and dominates the Rokelides. This tectonothermal activity appears to reflect collision of the West African and Guyanean Cratons during assembly of Gondwana. Hercynian activity is concentrated along the margin of a western continental block which underwent relative eastward translation during collision of Gondwana and Laurentia. This resulted in extensive thrusting of intracontinental foreland sequences (external nappes) and more ductile imbrications of pre-deformed and metamorphosed late Proterozoic rift sequences and western calc-alkaline igneous successions (internal nappes).

On the equatorial Pacific response to the 1982/1983 EI Nino—Southern Oscillation event

The ocean's response to the 1982/83 El Nino-Southern Oscillation event was the largest ever documented. In this paper the authors explore the effects of certain details of the observed zonal wind stress anomaly field upon the ocean's response using a linear, adiabatic, reduced gravity, analytical model. The increase in magnitude of the downwelling response over the composite of previous El Nino events is attributed to the eastward translation of the observed westerly wind anomaly and the double peaked downwelling at the South American coast is attributed to the amplitude modulation of this anomaly. Effects of an easterly anomaly which appeared to the west of the dateline midway through the event are also considered.

Initiation of subduction by post-collision foreland thrusting and back-thrusting

While postulated causes of initial subduction and trench formation include underthrusting, controls on its location and age have not been determined. Consideration of the age of subduction zones bordering five collisional orogens suggests that subduction may have been initiated by foreland thrusts and back-thrusts. Foreland thrusts develop within a continental foreland on the subducting plate mostly within 50 my of collision with an arc system; where the foreland is narrow the thrusts may intersect the continent-ocean crust boundary. Back-thrusts develop in the fore-arc or back-arc area on the overriding plate within 10 to 20 my of collision, and can result in tectonic burial of the magmatic arc; where the arc system is oceanic the back-thrusts may intersect the arc-ocean crust boundary. Possible examples of subduction initiated by foreland thrusts are the start of subduction in the late Jurassic beneath the northern Sunda Arc, and at the end-Miocene in the Negros Trench. Examples of back-thrusts which have initiated or may initiate subduction are the late Cenozoic eastward translation of Taiwan over the Philippine Sea plate, the incipient southward subduction of the Banda Sea beneath Timor, and the W-dipping back-thrust comprising the Highland Boundary Fault zone and postulated early Ordovician thrusts to the SE in Scotland. The suggested relationship of subduction to collision helps to explain the persistence of Wilson cycles in the still-active late Mesozoic to Cenozoic orogenic belts and implies that orogeny will cease only with collision between major continents.

Tectonic Significance of Microstructures in Idaho-Wyoming Thrust Belt and Hinterland: ABSTRACT

Mechanics of foreland thrust belt development can be explained using paleostress orientations from dynamic analyses of microstructures in quartz, calcite, and dolomite. Deformation in the hinterland must also be considered since the two basic models--gravity spreading and lateral tectonic compression--predict substantially different stress fields in this region. Petrofabric studies in the Meade plate show that compression was dominantly layer-parallel, trending approximately east-west. On overturned fold limbs, compression at 50-80° to bedding suggests a locking angle which agrees well with existing theoretical and experimental analyses of kink-folding. Observed kink-fold geometries may be a necessary result of ramp configurations in the decollement thrust surface. These data are in accord with either of the two principal models. Dynamic analyses at scattered localities in the southern Idaho hinterland show primarily layer parallel or subparallel, east-west compression in all demonstrably allochthonous rocks at all structural levels. Fold vergence and local overturning indicate eastward translation along the undated, but probably Mesozoic, younger-over-older thrusts characteristic of this region. Near metamorphic core complexes, Tertiary thermal events may have affected preservation of older microstructures. Parautochthonous Precambrian metasediments between foreland and hinterland record compression at high angles to bedding. The age and origin of these microstructures are unknown at present. These studies indicate that maximum compression was nearly horizontal and oriented approximately east-west throughout southeastern Idaho during thrust belt activity. Therefore, the lateral tectonic compression model is favored. End_of_Article - Last_Page 669------------

Status of Antler Orogeny in Central Idaho--Clarifications and Constraints from Pioneer Mountains: ABSTRACT

Structural telescoping of argillaceous lower Paleozoic sequences with coeval calcareous and quartzitic rocks in central Idaho has commonly been presumed to be analogous to the Roberts Mountains thrust in Nevada. However, all datable thrusts in the Pioneer Mountains are post-middle Permian to pre-Eocene in age; if Antler age thrust existed, they were reactivated or obscured by major post-Antler movement. Furthermore, differences in fabric or structural style between thrusted argillaceous pre-Mississippian sequences and calcareous-arenaceous Pennsylvanian-Permian sequences, which have been cited in support of Antler orogenesis, are equivocal. These differences more likely resulted from disharmonic response to entirely later thrusting in rocks of different competencies or at different structural levels. The Copper Basin Formation, a dominantly clastic deposit requiring an argillite-chert-quartzite western highland source in Mississippian time, remains the only evidence in central Idaho for an Antler highland. The Copper Basin now occurs in two superimposed allochthons which, when palinspastically restored, require that their original depositional basin extended at least 50 to 75 km west from the present Pioneer Mountains. Thus, in Mississippian time, the Antler highland reached no farther east than westernmost Idaho. Emplacement of the Pioneer Mountains allochthons during mainly Mesozoic time involved (1) tectonic slices of high-grade metamorphic rocks and Precambrian crystalline basement, (2) eastward movement and imbrication of Antler detritus, and (3) thrusting of argillaceous facies lower Paleozoic rocks from the Antler highland over the tectonic remnants of its own debris. At least 100 km of post-Antler eastward translation is estimated; a comparable amount of earlier facies telescoping could be accommodated within the Antler highland based on reasonable facies reconstructions, but thrusting has not yet been demonstrated to have accompanied Antler highland development in Idaho. End_of_Article - Last_Page 700------------

Phanerozoic thrusting in Proterozoic belt rocks, northwestern United States

Newly identified listric thrust faults show eastward translation north of the Lewis and Clark line across all 800 km of Belt terrane from Spokane, Washington, on the west to Glacier National Park on the east. Right-lateral slippage on the line was accompanied by clockwise rotation on the thrusts. These movements were probably in response to the complex plate interactions that began about 200 m.y. ago along the western continental margin. Gravity and magnetic data suggest that basement rock is involved in the thrusting. Reconcentration of strata-bound copper sulfides along bedding-plane shears may help form ore near one thrust. West of the Rocky Mountain Trench, Phanerozoic strata that elsewhere have oil and gas potential may have been stepped up by the thrusting, and eroded, rather than extending beneath most of the thrust belt.