bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismotectonics of New Guinea: A model for arc reversal following arc‐continent collision
Authors: Cooper Patricia , Taylor Brian
Published: February 2008

The structure and evolution of the northern New Guinea collision zone is deduced from International Seismological Center (ISC) seismicity (1964–1985), new and previously published focal mechanisms and a reexamination of pertinent geological data. A tectonic model for the New Guinea margin is derived which illustrates the sequential stages in the collision and suturing of the Bewani‐Toricelli‐Adelbert‐Finisterre‐Huon‐New Britain arc to central New Guinea followed by subduction polarity reversal in the west. East of 149°E, the Solomon plate is being subducted both to the north and south; bringing the New Britain and Trobriand forearcs toward collision. West of 149°E the forearcs have collided, and together they override a fold in the doubly subducted Solomon plate lithosphere, which has an axis that is parallel to the strike of the Ramu‐Markham suture and that plunges westward at an angle of 5° beneath the coast ranges of northern New Guinea. Active volcanism off the north coast of New Guinea is related to subduction of the Solomon plate beneath the Bismarck plate. Active volcanism of the Papuan peninsula and Quaternary volcanism of the New Guinea highlands are related to slow subduction of the Solomon plate beneath the Indo‐Australian plate along the Trobriand Trough and the trough’s former extension to the west, respectively. From 144°–148°E, seismicity and focal mechanisms reveal that convergence between the sutured Bismarck and Indo‐Australian plates is accommodated by thrusting within the Finisterre and Adelbert ranges and compression of the New Guinea orogenic belt, together with basement‐involved foreland folding and thrusting to the south. The Finisterre block overthrusts the New Guinea orogenic belt, whereas the Adelbert block is sutured to New Guinea and overthrusts the oceanic lithosphere of the Bismarck Sea. Along the New Guinea Trench, west of 144°E, seismicity defines a southward dipping Wadati‐Benioif zone, and focal mechanisms indicate oblique subduction. Only this oldest, westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity.
The tectono‐magnetic signature of the old red sandstone and pre‐Devonian strata in the HÅSteinen Area, western Norway, and implications for the later stages of the Caledonian Orogeny
Authors: Torsvik T. H. , Sturt B. A. , Ramsay D. M. , Vetti V.
Published: February 2008

The structural fabrics recorded from Devonian rocks and substrate in the Håsteinen area, Western Norway, show near E‐W trends. In the Devonian rocks these fabrics are a consequence of post‐depositional regional folding at a considerable depth, presumably attended by the aquisition of an thermoviscous (TVRM) or thermochemical (TCRM) remanence (group A). Post or possibly late syntectonic group A remanences (declination=210, inclination = +18, α95=9) constitute the principal remanence in both Devonian rocks and their substrate, whereas a younger group B magnetization (declination=047, inclination=+59, α95=7) relates to early Mesozoic shearing. The palaeomagnetic (group A) and structural data from the Håsteinen and Kvamshesten Old Red Sandstone Massifs favor a regional compressive N‐S shortening event (Svalbardian‐Solundian), which probably occurred during latest Devonian times. Some recent extensional‐basin models, appealing to listric‐normal faulting to account for both the sedimentation and the subsequent deformation of the Western Norwegian Devonian rocks, are rejected on the basis of inadequate structural information.
Is the Brunswick magnetic anomaly really the Alleghanian suture?
Authors: Tauvers Peter R. , Muehlberger William R.
Published: February 2008

The southeastern margin of the United States presents a complex picture of accreted Paleozoic terranes and a rift‐related Mesozoic overlap assemblage buried by a thick veneer of Coastal Plain sediments. Basement terranes have been defined on the basis of drilling and geophysical data. Their relationship to each other and to plate tectonics is in debate. The Brunswick magnetic anomaly in Georgia has been cited as the location of the late Paleozoic suture between North America and a continental fragment with African affinities known from the northern Florida subcrop. This paper poses a new plate model for this region based upon the following assumptions: (1) that deep well tests furnish a reasonable data set for the discrimination of various basement rock types across the region and (2) that the Brunswick anomaly is the outboard limit of the North American Piedmont/Avalon terrane. We propose that the area between the northwest limit of “African” rocks and the trace of the Brunswick anomaly is a region of crustal overlap, as African‐related rocks were overthrust to the northwest. The original irregular outline of the North American continental margin and the landward extensions of oceanic fracture zones are intimately related to the kinematics of suspect terranes in the Appalachian orogen. The implications of the model suggest that differential shortening during collision and differential extension during subsequent rifting accommodate movements on repeatedly activated transforms.
Crustal deformation and regional metamorphism across a terrane boundary, Coast Plutonic Complex, British Columbia
Authors: Crawford M. L. , Hollister L. S. , Woodsworth G. J.
Published: February 2008

The Coast Plutonic Complex between Prince Rupert and Terrace, British Columbia, developed in two stages between mid‐Cretaceous and mid‐Eocene time. The early stage (>100–70 Ma) involved crustal thickening as the amalgamated Alexander and Wrangellia terranes were emplaced against the Stikine terrane to the east. This thickening resulted from tectonic stacking of crustal slabs, lubricated by intrusion of melt generated at the base of the thickened crust. Emplacement of westward directed thrust slabs along the western margin of the orogen was accompanied by intrusion of a high pressure epidote‐bearing tonalite pluton and associated sills. Thrusting resulted in inversion of the metamorphic sequence as supracrustal rocks buried early in the collision were tectonically exhumed. Pluton emplacement also accompanied eastward directed thrusting of high temperature gneisses over low‐grade rocks on the eastern margin of the orogen as the tectonic welt was backthrust over Stikinia. The earliest recognized events in the core of the orogen involve pervasive ductile deformation accompanied by emplacement of tonalite sills between about 85 and 50 Ma. Uplift of the orogen core between 60 and 48 Ma coincided with and was facilitated by emplacement of large volumes of tonalitic magma, accompanied by anatexis and the development of ductile shear zones. One of these shear zones, the Work Channel lineament, presently separates the core of the orogen from the schists to the west. The main orogenic cycle ended by 48 Ma when the rocks cooled rapidly through the biotite and hornblende Ar blocking temperatures. The time and space associations of deformation, metamorphism, and plutonic intrusion imply that substantial differential movement occurred across zones which contained fluid during deformation of the Coast Plutonic Complex. In the region of westward directed thrusting and inverted metamorphism, melts generated in the tectonically thickened lower crust and fluids released during metamorphic recrystallization of supracrustal rocks, which were rapidly buried during tectonic thickening, acted to reduce the strength of the crust. In the core zone of the complex, injections of tonalite magma into the lower crust induced pervasive anatexis. The weakened core zone yielded along both low angle and steep shear zones that were further lubricated by melt as the relatively buoyant hot crust rose rapidly in response to continued compression and underthrusting.
South Island, New Zealand, and transverse ranges, California: A seismotectonic comparison
Authors: Yeats Robert S. , Berryman Kelvin R.
Published: February 2008

Both the South Island of New Zealand and the Transverse Ranges of California have (1) a major right‐slip fault marking the plate boundary, (2) a left‐stepping bend in the fault resulting in ranges and basins bounded by active reverse faults that strike 45° counterclockwise to the regional strike of the main fault, (3) a set of right‐slip faults on one side of the constraining bend, with most of the total displacement concentrated On the fault stepped farthest left from the continuation of the fault on the other side of the bend, and (4) intersections between the set of faults and the main fault within the bend not present or poorly defined. The faults with most of the displacement (Alpine‐Wairau, San Andreas) may pre‐date the constraining bend, The reverse fault province southeast of the Alpine fault is separated from that fault by the Southern Alps, which may be underlain by crust that is too hot and too weak to sustain major reverse faults. Average reverse fault recurrence intervals in New Zealand and California are 10³ to 2 × 104 years as compared to less than 10³ years for the right‐slip master fault, Seismicity is high but patchy in the Nelson‐Buller region of the northern South Island, low but patchy in the Transverse Ranges, and very low in Central Otago in the southern South Island, all reverse fault provinces. Seismicity patterns in both regions, poorly related to overall late Quaternary fault distribution, correspond largely to individual rupture events on portions of Quaternary faults, and the seismic inactivity of many of these faults may simply represent quiescence between seismic faulting with long recurrence intervals. The geology and Seismicity of the three reverse‐fault provinces support the suggestion of C. R. Allen [1975] that late Quaternary fault history is a better long‐term guide for assessment of large‐magnitude earthquake hazard than instrumental and historical Seismicity for faults with long recurrence intervals.
Northward translation of “BAJA British Columbia” along the Late Cretaceous to Paleocene margin of western North America
Authors: Umhoefer Paul J.
Published: February 2008

Paleomagnetic data suggest that a large part of British Columbia and southern Alaska was at the latitude of Baja California, Mexico, in early Late Cretaceous time. This allochthonous block (Baja BC) includes the Insular (Talkeetna in Alaska) and Intermontane composite terranes, the miniterranes between them, and possibly part of the Omineca metamorphic belt. Assuming that Baja BC was adjacent to North America by middle Cretaceous, an analysis using plate kinematics of the Pacific basin produces a model of northward movement of Baja BC which is compatible with the known geology of the reconstructed margin of western North America. Baja BC moved north 2400 km as part of the KuIa plate along a transform margin with North America from 85 to 66 Ma. This time interval is marked by the cessation of igneous activity in the Sierra Nevada, development of wrench faults and basins along coastal California, and initiation of Laramide‐style tectonics to the east. Within Baja BC, diminished igneous activity to the west was coeval with possible wrench‐tectonic basins along its eastern margin. By the end of the Cretaceous, Baja BC was positioned from Oregon to northern British Columbia. Between 66 and 56 Ma, Baja BC became attached to North America in a dextral‐transpressive stage, during which the Kula‐‐North America plate boundary expanded to encompass the whole block with a series of northwest trending strike‐slip faults, a fold and thrust belt in the east, and large‐scale uplifts and the formation of an incipient volcanic arc on the west. Southern Alaska was the site of convergence during the whole of the period of northward translation of Baja BC. A Late Cretaceous subduction zone in Alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of Baja BC collided with interior Alaska. The collision culminated in the Paleocene with formation of the McKinley granites and coeval dextral slip on the Denali fault system.
Seismic rupture associated with subduction of the Cocos Ridge
Authors: Adamek Scott , Tajima Fumiko , Wiens Douglas A.
Published: February 2008

The April 3,1983, Costa Rica earthquake (Mw = 7.4, Ms = 7.3) occurred where the aseismic Cocos Ridge has been partially subducting beneath the Caribbean plate. The epicenter is at the eastern edge of the 70 × 110 km² initial aftershock area, which later shows considerable expansion. P wave directivity analysis indicates that the rupture front propagated westward 63 km from the epicenter to where the major moment release was truncated. The source time history of the 1941 event (Ms = 7.5) which occurred in the same epicentral area appears to be similar to the 1983 event but with more complexity and a slower initiation of moment release. The low stress drop of the 1983 event, 2–3 bar, and low ratio of seismic slip to tectonic slip, 0.15 for the 1983 event, suggest that the coupling between the descending slab and the overlying plate is weak. Yet, a recurrence time for large events (mag > 7.0) in this zone of about 40 years is implied by the 1904, 1941, and 1983 events, suggesting that the Cocos Ridge causes increased partial coupling compared with that in the adjacent subduction zone. The slow source rupture process may reflect the nature of the contact area between the Costa Rica arc and the subducting plate in the vicinity of the bouyant Cocos Ridge. The normal faulting event of July 1, 1979, (mb = 5.3) is consistent with present‐day uplift of the Burica Peninsula, which previous studies suggest is related to the subduction of the Cocos Ridge. The Burica and Osa peninsulas may behave as detached blocks which influence the distribution of partial coupling reflected in the aftershock expansion pattern.
Correlation of early Cretaceous Blueschists in Washington, Oregon and northern California
Authors: Brown E. H. , Blake M. C.
Published: February 2008

The protolith and metamorphic histories of Early Cretaceous blueschists that occur in Washington, Oregon, and California are remarkably similar. These blueschists are the Shuksan metamorphic suite of northwestern Washington, the Condrey Mountain schist of northern California and southern Oregon, and the Pickett Peak terrane of northern California and southwestern Oregon. We suggest that these bodies were coextensive during their formation. Dispersion of the blueschists occurred in Late Cretaceous time by lateral translation of up to 900 km along the continental margin in response to oblique plate convergence.
Determination of the tectonic stress tensor from slip along faults that obey the Coulomb yield condition
Authors: Reches Ze'ev
Published: February 2008

A new method to calculate the stress tensor associated with slip along a population of faults is derived here. The method incorporates two constraints: First, the stresses in the slip direction satisfy the Coulomb yield criterion, and second, the slip occurs in the direction of maximum shear stress along the fault. The computations provide the complete stress tensor (normalized by the vertical stress), and evaluation of the mean coefficient of friction and the mean cohesion of the faults during the time of faulting. The present method is applied to three field cases: Dixie Valley, Nevada, Wadi Neqarot, Israel, and Yuli microearthquakes, Taiwan. It is shown that the coefficieet of friction of the three cases varies between μ < 0.1 to μ = 0.8. Further, it is demonstrated that previous stress inversion methods implicitly assume zero friction along the faults.
Kinematics of orocline and arc formation in thin‐skinned orogens
Authors: Marshak Stephen
Published: February 2008

The regional traces of folds, faults, and foliations found in many fold‐thrust belts are bent in plan view. It proves valuable to distinguish between two types of bent orogens based on their kinematic evolution: oroclines (or rotational arcs) are bent orogens in which segments of the orogen change strike during the evolution of the bend, and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend. The kinematic evolution of a bend that forms in a thin‐skinned orogen can be described in terms of three parameters: the displacement path trajectories followed by points along the strike of the orogen, the magnitude and distribution of tangential extension along the strike of the orogen, and the change in position of the endpoints of the orogen with respect to a reference line. Only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline; thus the distinction between these two types of bends is not always of major tectonic significance. Compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs (as can be simulated with a sand wedge model). Interaction between a fold‐thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation. Orocline formation is also associated with noncoaxial reactivation of thrust faults. Tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines. Two examples, one from the Umbrian Apennines and one from the Makran Range, demonstrate how movement on cross‐strike fault arrays can accommodate tangential extension in thin‐skinned bends.
Emplacement of foreland thrust systems
Authors: Cello Giuseppe , Nur Amos
Published: February 2008

Emplacement‐driving mechanisms for thrust systems have been briefly reviewed and compared with distinctive structural styles in thrust belts. A B‐type mode, which implies a push from the rear and an elastic‐brittle behavior of the rock mass, appears to be suitable as a general model for the emplacement of thrust sheets in thin‐skinned fold and thrust belts where stratal shortening of sedimentary covers and imbrication of upper level crustal rocks occur. In such an environment that the mechanical behavior of the crustal section involved the mode, and timing of deformation, and the geometry of thrust systems are all strongly dependent upon the coupling between induced tectonic strain and transient, close to lithostatic pore fluid pressure buildup over a critical areal extent. The model presented here does not require a weak basal horizon for the detachment of thrust sheets, and pore pressure Pp does not need to be high at once under the entire length of the thrust wedge. Rather, the transient buildup of Pp and hence the spatial and temporal progressive weakening of the rock mass are directly coupled with deformation.
Out‐of‐Sequence Thrusts
Authors: Morley C. K.
Published: February 2008

Out‐of‐sequence thrusts (OOST) are those thrusts which do not obey the foreland propagating or in‐sequence deformation style. They include both isolated thrusts which develop hindward of the thrust front and sequences of break‐back thrusts which propagate from the foreland to the hinterland. Two end‐members of a series of OOST types are recognized: (1) older in‐sequence thrusts which are reactivated along their entire length and (2) completely new thrusts which propagate through already deformed thrust sheets. Between the two end‐members are thrusts composed partially of reactivated in‐sequence thrust sequences and partially of new, entirely out‐of‐sequence segments. OOSTs can be initiated for a variety of reasons including: (1) keeping the orogenic wedge at critical taper, (2) break‐back sequences from the suture zone in the overriding plate, (3) ramping to overcome a sticking point which inhibits in‐sequence thrust propagation, and (4) during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust. Many different types of thrust behavior including gravity sliding, plucking, and derivation of isolated horses from ramps may mimic some of the characteristics of OOSTs. Consequently, it may be difficult to conclusively prove an OOST origin for a complex thrust geometry.
Permian and Triassic rocks of the Mojave Desert and their implications for timing and mechanisms of continental truncation
Authors: Walker J. Douglas
Published: February 2008

Upper Paleozoic and lower Mesozoic rocks in the Mojave Desert and adjacent areas record the change from a passive continental margin to a subduction margin in the southwestern United States. This change was apparently accompanied by truncation and reorientation of the continental margin in the Mojave Desert and western Basin and Range Province. From Late Proterozoic to Mississippian time the western margin of North America had a northeast‐southwest trend through Nevada and eastern California. Truncation of the margin began in Pennsylvanian time when a northwest trending, left‐lateral strike‐slip fault zone formed across the Antler orogenic belt, the Cordilleran miogeocline, and the more cratonal character Mojave Desert. Rocks of the Antler belt were apparently displaced southward from east central California to their present position in the northwestern Mojave Desert. Sedimentary rocks deposited during this interval record an active continental borderland, characterized by alternately subsided and uplifted marine basins. This transcurrent faulting continued to Late Permian time. Magmatic activity and deformation started in the Mojave Desert in Late Permian time and affected both miogeoclinal‐cratonal and displaced Antler rocks. Magmatic activity ceased and marine sedimentation resumed in Early Triassic time. Lower Triassic rocks constitute an overlap sequence on both deformed and undeformed miogeoclinal‐cratonal rocks and possibly on displaced Antler rocks. Facies trends in these rocks are northwest‐southeast, indicating the new margin trend. Truncation and reorientation of the margin were completed therefore by Early Triassic time, or possibly as early as Late Permian time. Upper Permian magmatism and deformation in the Mojave Desert are older than, and distinct from, the early Triassic Sonoma Orogeny of Nevada. Lower Triassic rocks in the Mojave Desert are post tectonic with respect to deformation in that region, whereas correlative Lower Triassic rocks in Nevada are pretectonic to syntectonic with respect to the Sonoma Orogeny. This difference reflects the variation in deformation and tectonic setting along a newly developed Cordilleran margin. The southern part of the Cordillera was apparently a subduction margin as early as Late Permian or Middle Triassic time, while the central (Nevada segment) was the site of collision tectonics.
Internal deformation in clay models of extension by block faulting
Authors: Kautz Steven A. , Sclater John G.
Published: February 2008

Clay and sand have provided useful analogues for the failure by block faulting of continental crust under extension. We performed experiments on clay models of such faulting and found that the total horizontal displacement on the major faults provided only 40–50% of the known extension. Similar experiments on sand showed that the displacement provided 70–80% of the known extension. Examination of the differential tilt in both models also underestimated the extension. The hidden extension is due to internal deformation within the blocks. Assuming that the sand and clay deform exclusively by slip and rotation on four families of faults, and that these faults have many scales, we propose that the hidden extension occurs on faults below the visible scale. We observed much of the deformation in sand because the grain size, the lower limit for faulting, was close to the minimum observable scale for the model. On the other hand, in clay, two or three orders of faults lay below this scale, giving rise to a hidden extension of 50% or more. We cannot apply our results directly to the block faulting behavior of continental crust. However, our method of analysis suggests an approach that may have value. Deformation within the continental crust occurs at many scales. Some of these scales remain hidden when determining the amount of extension. In such cases, our postulated behavior for clay may present a useful analogue as it suggests that deformation within the blocks may play an important role. For example, estimates of extension found by summing the horizontal displacement on normal faults observed on multichannel seismic lines are lower than those given from analyses of the subsidence of many sedimentary basins and continental margins. Hidden deformation at a scale below that observable on the seismic line could account for this difference.
Geologic structure of the northern New Caledonia Ridge, as inferred from magnetic and gravity anomalies
Authors: Collot J. Y. , Rigolot P. , Missègue F.
Published: February 2008

Bathymetric, gravity, and magnetic data collected in the southwest Pacific Ocean over the northern New Caledonia ridge show that the main geological units known from the island of New Caledonia extend northward from this island, beneath the Grand Lagon Nord, the Grand Passage, and the d'Entrecasteaux reefs. These data support the model of tectonic evolution of the New Caledonia region proposed by Kroenke [1984]. We interpret a linear axial gravity low that extends from southern New Caledonia to the Grand Passage as evidence for the continuity of the thick pre‐Permian to Jurassic core of the island. The Belep magnetic pattern, which covers the southwestern half of the Grand Lagon Nord, shows linear, high relief (1000–2000 nT) anomalies that are locally associated with a 120‐mGal gravity high. These magnetic and gravity anomalies resemble anomalies measured over the west coast of New Caledonia, suggesting that the Cretaceous to Eocene basaltic complex of the coastal area is overlain by ophiolite remnants as far north as the western d'Entrecasteaux reefs. The similarity between the Belep magnetic pattern and a highly magnetic province evident 200 km southwest across the New Caledonia basin, along the Fairway ridge, indicates that volcanic rocks lie symmetrically on both sides of the New Caledonia basin. We suggest that part of these volcanic rocks were emplaced during the middle Cretaceous when the New Caledonia ridge rifted from the Australian margin. Simple gravity models of an elongated gravity high, having peak values in the range of +130 to +150 mGal, suggest that the ophiolite, which was thrust over New Caledonia during the Upper Eocene, extends along the east side of the Grand Lagon Nord and as far north as the d'Entrecasteaux reefs. Gravity and magnetic models suggest that in the area of the d'Entrecasteaux reefs, the ophiolite belt includes two subunits. The first subunit underlies the Huon‐La Surprise platform and may include highly serpentinized ultramafic rocks without basaltic oceanic crust. The second subunit, which lies beneath the Guilbert ridge, may include ultramafic rocks as well as thin imbricate slices of oceanic crust. At the northern termination of the New Caledonia ridge, seismic and geopotential data evidence a major east‐west trending tectonic zone that separates the basement of the New Caledonia ridge from the d'Entrecasteaux zone, an arcuate oceanic feature extending northward from the ridge. Differences in structure, geophysical signatures and morphology evident between areas north and those south of the Grand Passage, together with the nearness of the Le Noroit massif west of the Grand Passage, suggest that contemporaneously with Eocene to early Oligocene subduction along the western New Caledonia margin, an arc‐ridge collision may have occurred near the northern termination of this subduction zone.
Uncertainties and implications of the Late Cretaceous and Tertiary position of North America relative to the Farallon, Kula, and Pacific Plates
Authors: Stock Joann , Molnar Peter
Published: February 2008

We present updated global plate reconstructions and calculated uncertainties of the Pacific, Kula, and Farallon/Vancouver plates relative to North America for selected times since 68 Ma. Improved magnetic data from the Indian Ocean decrease the uncertainties in. the global plate circuit approach; these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots. Major differences between these results and those of others are due to our use of more detailed Africa‐North America reconstructions, separate Vancouver and Farallon plate reconstructions, and the assumption of a rigid Antarctica plate during Cenozoic time. The uncertainties in the relative positions of the Pacific and North America plates since the time of anomaly 7 (26 Ma) range up to ±100 km in position, or from 1 to 3 m.y. in time. If the Mendocino triple junction initiated at about 28.5 Ma, its position would have been at 31.3°N ± 130 km relative to fixed North America. Unacceptable overlap of oceanic crust of the Pacific plate with continental crust of western North America in the anomaly 10 (30 Ma) reconstruction is a minimum of 340±200 km along an azimuth of N60°E and may be accounted for by Basin and Range extension. Pacific‐North America displacement in the past 20 Ma is found to be considerably less than that calculated by fixed hotspot reconstructions. Farallon (Vancouver)‐North America convergence velocity decreased greatly between the times of anomalies 24 and 21 (56 to 50 Ma), prior to the 43 Ma age of the Hawaiian‐Emperor bend and the often quoted 40 Ma “end” of the Laramide orogeny. A change in direction of Farallon‐North America convergence occurred sometime between 50 and 42 Ma and also may not correlate with the time of the Hawaiian‐Emperor bend. The lack of data from subducted parts of the Farallon and Kula plates permits many possibilities regarding the position of the Kula‐Farallon ridge, the age of subducted crust, or the position of oceanic plateaus during the Laramide orogeny, leaving open the question of the relationship between plate tectonic scenarios and tectonic style during Laramide time. Displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed North America during the interval between anomalies 30/31 and 18 (68 to 42 Ma) are found to be: Pacific plate, 1700±200 km northward; Farallon plate, 3200±400 km northeastward; Vancouver plate, 3000±400 km northeastward; Kula plate, if attached to the Pacific plate after A24 time, 2500±400 km northward.
Crustal‐scale thrusting and extension in the Hercynian Schwarzwald and Vosges, central Europe
Authors: Eisbacher Gerhard H. , Lüschen Ewald , Wickert Frank
Published: February 2008

The Hercynian outcrop areas in the Schwarzwald of SW Germany and Vosges of easternmost France consist from north to south of three major lithotectonic complexes: very low‐ to medium grade metamorphics of the Saxothuringian Belt, the polymetamorphic Central Gneiss Complex, and the tectonic melange of the Southern Complex. Juxtaposition of these three complexes occurred by oblique convergent motions in early Carboniferous time. The Saxothuringian Belt is part of a major SE dipping accretionary wedge that consists of lower Paleozoic meta‐sedimentary and metaigneous rocks and superimposed upper Devonian to lower Carboniferous synorogenic volcanics and elastics. The Central Gneiss Complex consists of polymetamorphic paragneisses, orthogneisses, metabasites, and meta‐ultramafics dipping to the NW. The Southern Complex consists of an imbricated melange of north to NW dipping synorogenic elastics, volcanics, and blocks of ultramafics and gneissic basement. The Central Gneiss Complex (CGC) experienced pervasive Hercynian low‐pressure/high‐temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins. This convergence (340–330 Ma) produced mylonitic‐cataclastic shear zones in the CGC. The most significant shear zones are the SE verging Todtnau fault zone between the CGC and the Southern Complex and the NW verging Lubine fault zone between the Central Gneiss Complex and the rocks of the Saxothuringian Belt. These two fault zones enclose a bivergent “pop‐up” of the polymetamorphic basement. The Todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies. Detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km. Crustal thickening during early Carboniferous convergence led to an extensional phase of crustal reordering (between about 330 and 280 Ma) that was accompanied by the massive rise of “posttectonic” granitic plutons from middle to high crustal levels. Simultaneous creation of a structural “lamination” in the refractory lower crust neglected the main convergent tectonic boundaries; it produced a basin‐and‐range type lower crust and mantle‐crust transition in early Permian time and involved a major advective input of heat.
Fission track analysis reveals character of collisional tectonics in New Zealand
Authors: Kamp Peter J. J. , Green Paul F. , White Stan H.
Published: February 2008

During the late Cenozoic, the Pacific plate has been obliquely converging with the Australia plate in South Island, New Zealand. This has resulted in westward obduction of continental crust of the Pacific plate, and consequently, a major mountain range, the Southern Alps, has formed east of the Alpine fault and within the leading edge of the Pacific plate. We have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the Southern Alps, starting at the Alpine fault, to better understand the uplift age and structure of the continental collision. The results show the following: (1) Uplift was underway at 7 Ma in the south at Haast Pass and by 5 Ma in the north at Arthur's Pass. (2) Profiles of the total amount of late Cenozoic uplift show an exponential increase toward the Alpine fault. Adjacent to the Alpine fault there has been 14.5 to 21.5 km of uplift in the vicinity of Haast Pass, 14.5 to 21 km near Mt Cook, and 12.5 to 15.5 km in the Arthur's Pass transect. Adjacent to the Alpine fault in the Mount Cook region the average uplift rate from ∼5.5 Ma to ∼1 Ma was 2.3 to 2.5 mm/yr; between ∼1 Ma and the present it has averaged 6.2 mm/yr. The strongly asymmetric uplift pattern is responsible for the exposure of a 13–25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the Alpine fault known as the Alpine schists. (3) Late Cenozoic uplift of Torlesse greywackes and Alpine schists was preceeded during the Jurassic‐early Cretaceous by 5–8 km of uplift. The Otago schist belt to the south was uplifted much more during this interval, and in addition, western parts of the Otago schists were uplifted 5–6 km during the late Cretaceous. (4) The fission track data do not support the notion of significant late Cenozoic shear heating about the Alpine fault, as previously suggested from spatial variations in K‐Ar ages of the Otago and Alpine schists.
Low‐angle extensional faulting and the emplacement of the Connemara Dalradian, Ireland
Authors: Williams D. Michael , Rice A. H. N.
Published: February 2008

A breccia mapped at the contact of the Connemara Dalradian and the unconformably overlying Silurian succession of north Galway shows evidence of having formed from strains developed at the brittle‐ductile deformation transition. This breccia was surficially exposed just prior to the onset of an Upper Llandovery marine transgression. The geometry of the breccia outcrop is consistent with its having developed along a regional low‐angle extensional fault. Two possible regimes of formation are proposed. In the first the breccia represents the remnant of a significant low‐angle strike‐slip extensional fault along which the Connemara Dalradian was laterally emplaced. In the second the extensional faulting may be a slightly later event, related to the rapid uplift of the Connemara Massif. Both models involve rapid footwall uplift, an event already inferred from metamorphic criteria. The fault may be compared to major extensional detachments such as those in the Basin and Range Province of the United States. However, this is the first description of an early Caledonian fault of this nature and may have important consequences for future tectonic studies in this (and other) orogen(s).
The evolution of forearc structures along an oblique convergent margin, central Aleutian Arc
Authors: Ryan Holly F. , Scholl David W.
Published: February 2008

Multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central Aleutian Ridge. Since at least late Miocene time this sector of the ridge has been obliquely underthrust 30° west of orthogonal convergence by the northwestward converging Pacific plate at a rate of 80–90 km/m.y. Our data indicate that prior to late Eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits; the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits. Beginning in latest Miocene or earliest Pliocene time, a zone of outer‐arc structural highs and a forearc basin began to form. Formation of these companion intraarc structures may be linked to the late Neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the Aleutian Trench. Initial structures of the zone of outer‐arc highs formed as the thickening wedge underran, compressively deformed, and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust. Forearc basin strata ponded arcward of the elevating zone of outer‐arc highs. However, most younger structures of the zone of outer‐arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge. Oblique convergence created a major right‐lateral shear zone (the Hawley Ridge shear zone) that longitudinally disrupted the zone of outer‐arc highs, truncating the seaward flank of the forearc basin and shearing the southern limb of Hawley Ridge, an exceptionally large antiformal outer‐arc high structure. Slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes. Uplift of Hawley Ridge may be related to the thickening of the arc massif by westward directed basement duplexes. In addition, the forearc is disrupted by structures transverse to the margin that occur where unusually high‐stress accumulations have resulted in the rupture of repeated great earthquakes. It is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central Aleutian Ridge. Great structural complexity, including the close juxtaposition of coeval structures recording compression, extension, differential vertical movements, and strike‐slip displacement, should be expected, even within areas of generally kindred tectonostratigraphic terranes.
Evolution and assembly of Archean Gneiss Terranes in the Godthåbsfjord Region, southern west Greenland: Structural, metamorphic, and isotopic evidence
Authors: Nutman Allen P. , Friend Clark R. L. , Baadsgaard Halfdan , McGregor Victor R.
Published: February 2008

The Godthåbsfjord region of southern West Greenland comprises several terranes that were assembled between 2750 and 2550 Ma and folded during amphibolite facies metamorphism. The terranes, which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories, are (1) the Færingehavn terrane containing the 3820–3600 Ma Amîtsoq gneisses, with granulite facies metamorphism at circa 3600 Ma, (2) the Akia terrane containing the 3070–2940 Ma Nûk gneisses, with granulite‐amphibolite facies metamorphism at circa 2980 Ma, (3) the Tasiusarsuaq terrane containing circa 2900 Ma gneisses, with granulite facies metamorphism at circa 2800 Ma, and (4) the Tre Brødre terrane containing the 2800–2750 Ma Ikkattoq gneisses, with amphibolite facies metamorphism at 2800–2750 Ma. Metamorphic assemblages and structures formed prior to terrane assembly, were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly. Recognition of the region as consisting of several terranes indicates that the anatomy of some Archean high‐grade gneiss complexes may resemble that of orogens of Proterozoic and Phanerozoic age that formed as a consequence of plate tectonic processes.
Blueschists in major suture zones of China
Authors: Liou J. G. , Wang Xiaomin , Coleman R. G. , Zhang Zh. M. , Maruyama S.
Published: February 2008

Blueschist facies rocks and C‐type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of China and along the Circum‐Pacific and Tethys‐Himalayan belts. At least forty blueschist localities have been described. Blueschists around the Pacific margins and the Himalaya‐Alpine belts are mostly Mesozoic in age; only those in Taiwan and New Caledonia are Cenozoic. Intracratonal blueschists of China are older than Late Permian, when the collision‐amalgamation presumably occurred. Several alleged Precambrian blueschist localities [Ye, 1987] are shown in the recently published Metamorphic Map of China. A Precambrian(?) blueschist belt in Anhui‐Hubei (central China) extends for about 2300 km. However, except for the pre‐Sinian blueschist terrane in Aksu, the age determination for Precambrian blueschist metamorphism elsewhere has not been confirmed. Characteristic features of Chinese blueschists include the following: (1) blueschist facies metamorphism predates the continental collision‐amalgamation, (2) most blueschists have undergone multistage metamorphism, commonly with increasing temperature and/or decreasing pressure for later events, (3) assemblages include sodic amphibole + epidote + albite + quartz + phengite + chlorite + sphene; lawsonite, jadeitic pyroxene, and aragonite are not common, and (4) protoliths are mainly mafic, pelagic, and clastic rocks. Blueschist facies metamorphism in China may be divided into two types on the basis of the imposed geothermal gradient, hence the progressive change in mineral assemblage. The rare lawsonite‐bearing blueschists produced at lower temperatures and higher pressures are confined to the Inner Mongolia and Yarlung Zangbo suture zones where aragonite and possibly jadeitic pyroxene + quartz occur, and the blueschists are associated with subgreenschist facies rocks. Most other blueschists of China formed at higher temperatures along the transitional blueschist‐greenschist facies boundary where epidote, sodic, and calcic amphiboles are ubiquitous, and the blueschists are interlayered with greenschist facies rocks. The common occurrence of intracratonal blueschists in major suture zones of China and elsewhere in Eurasia indicates pre‐Mesozoic subduction‐accretion of oceanic crust and flysch sediments. These zones of accretion represent growth around the periphery of major Precambrian cratons. This fact, together with the distribution of ophiolite belts and available paleomagnetic data, provides evidence for an extremely mobile history of plate movement in Eurasia. However, the Paleozoic intracontinental blueschists and ophiolites of China have formed before, rather than during, final closure between cratons. They do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction, underplating, and imbrication of oceanic materials similar to the accretion history of westernmost North America.
Devonian paleomagnetism of Yunnan Province across the Shan Thai‐South China Suture
Authors: Fang Wu , Van der Voo Rob , Liang Qizhong
Published: February 2008

A paleomagnetic study of Devonian sedimentary sequences from western and eastern Yunnan, China, yields contrasting paleolatitudes for these two areas. They indicate an equatorial position for eastern Yunnan, as part of the Yangtze Paraplatform or South China Block, and a paleolatitude of about 40° for western Yunnan as part of the Shan State (or Shan‐Thai) Block, which probably was part of the Gondwana supercontinent. The majority of samples is strongly overprinted by a present‐day field magnetization, but thermal demagnetization has been successful in isolating ancient, dual‐polarity characteristic directions of magnetization in rocks from eastern and western Yunnan, with results from the latter passing the fold test. The paleopole from eastern Yunnan is located at 8.9°N, 10.4°E (declination/inclination = 279.2°/+2.7°, k = 13, alpha 95 = 6.9°, N = 33 samples from 7 sites). The paleopole from western Yunnan is located at 66.5°N, 133.8°E (declination/inclination = 18.0°/+62.0°, k = 21, alpha 95 = 5.8°, N = 31 samples from 7 sites), yielding a paleolatitude of 42°. The two blocks studied are separated by the Red (Yuan) River and Lancang (Mekong) River fault zones, one of which is inferred to be an ancient suture.
Topographic and volcanic asymmetry around the Red Sea: Constraints on rift models
Authors: Dixon Timothy H. , Ivins Erik R. , Franklin Brenda J.
Published: February 2008

The Red Sea rift is asymmetric, with the locus of uplift and Tertiary volcanism in Saudi Arabia, 200–400 km east of the present rift axis. One model for this asymmetry involves simple shear extension on an east dipping, low‐angle master fault that penetrates the lithosphere. Uplift and volcanism would mark the location where the fault enters the asthenosphere. However, observed seismicity, regional heat flow, and sedimentation are not in accord with this model's predictions. An alternate model involves interaction of upwelling asthenosphere in the early rifting stage with a nearby crustal weak zone such as a suture that controls rift location. In general, the location of the weak zone and subsequent rift will not coincide with the locus of mantle upwelling, leading to several asymmetric rift features. In this model, Tertiary volcanism in Saudi Arabia marks the location of initial upwelling, and uplift is due to crustal thickening associated with magmatic underplating and crustal intrusion. The incipient crustal rift and the locus of mantle upwelling will tend to align as rifting continues and stable seafloor spreading develops, implying relative migration of the lithosphere and asthenosphere. Absolute plate motion models for Africa and strongly asymmetric spreading in the Red Sea are consistent with northeast migration of the incipient rift and adjacent lithosphere with respect to a zone of asthenospheric upwelling.