bluuebunny/crossref_metadata_2025 huggingface dataset:
Active deformation of the Pacific Northwest Continental Margin
Authors: Adams John
Published: February 2008

Tilted and uplifted marine terraces in southern Oregon show progressive landward tilting of the coastal ranges at about 5–16 × 10−8 rad. yr−1 for the last 0.25 m.y. Tide gauges in Washington and British Columbia, and ten resurveyed leveling lines running inland from the coast, indicate contemporary landward (down‐to‐the‐east) tilt rates of about 1–12 × 10−8 rad. yr−1 averaged over periods of from 10 to 50 years. The leveling lines traverse, and the terraces cut across, dipping Cenozoic strata: Pleistocene (dips to 3°), Mio‐Pliocene (dips to 30°) and Eocene (dips to 60°). Southern Oregon from Cape Blanco to the Siletz River shows geodetic or terrace tilting in the same direction as the underlying stratal dips. Hence present‐day deformation continues past deformation of the coastal ranges and is most likely related to active subduction of the Juan de Fuca plate. The steep stratal dips, lack of major active faults and historic earthquakes, and presence of very young bedding‐plane faults suggest that much of the onshore deformation and shortening within the overlying North American plate is taken up by folding rather than thrust faulting. Present shortening rates across north‐trending folds near the coast are between 0.02 and 1.9 × 10−7 yr−1. The rate of shortening decreases rapidly eastwards from the Juan de Fuca ‐ North American plate boundary. A total of about 25 mm yr−1 of permanent shortening could be occurring within the North American plate; most of it in the westernmost 40 km. The landward tilt and shortening rates are similar to those above many other subduction zones that have experienced great thrust earthquakes. While a high strain rate measured near Seattle, Washington, has been interpreted as elastic strain accumulation before a thrust earthquake, the low level of historic seismicity and the similarity of short‐ and long‐term deformation rates suggest alternatively that the subduction beneath Washington is aseismic. The issue has considerable implications for seismic hazard evaluation in the Pacific Northwest and could be resolved by a search for the effects (or lack of effects) of prehistoric great earthquakes.
Cooling history of the NW Himalaya, Pakistan
Authors: Zeitler Peter K.
Published: February 2008

Fission‐track and40Ar/39Ar cooling ages indicate that the late‐Tertiary cooling history of the Himalayan ranges of northern Pakistan is largely a function of uplift and erosion. Interpretation of cooling ages which range from under 0.5 Ma to over 80 Ma suggests that during the late Tertiary, long‐term uplift rates at least doubled, from under 0.2 mm/yr to in some cases well over 0.5 mm/yr. Uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions. The association of very rapid uplift and erosion with the Nanga Parbat‐Haramosh Massif can be explained by a locally vigorous collision of India with Eurasia near a promontory of Indian crust. The resultant rapid uplift of the Nanga Parbat‐Haramosh Massif reactivated the Main Mantle Thrust melange zone with a reversed sense of motion. Discontinuities in the coolingage distribution along the Main Mantle Thrust in the southern Swat‐Hazara region may be the result of the thermal effects of overthrusting.
Structure of the Xigaze Ophiolite, Yarlung Zangbo Suture Zone, southern Tibet, China: Genetic implications
Authors: Girardeau J. , Mercier J. C. C. , Yougong Zao
Published: February 2008

Three massifs in the Xigaze area (Southern Tibet, China), have been preserved from the intense deformation observed everywhere else in the Yarlung Zangbo Suture Zone. These massifs have similar lithologies and display a remarkable homogeneity in the attitude of most magmatic and tectonic structures such as lava flow planes, dolerite dikes and sills, and foliation planes in peridotites. This indicates that these ophiolites are only slightly dismembered, inasmuch as they locally display complete sequences from pillow lavas at the top to fresh Cr‐diopside‐bearing harzburgites at the base. Gabbroic rocks are rare and are present as screens between dolerite intrusives or in small‐sized pockets at the limit between mafic and ultramafic rocks. One of the most important feature of this ophiolite is the occurrence of dolerite sills and dikes throughout the whole sequence indicating that this ophiolite results from to two successive magmatic events with (1) creation of the crustal part of the oceanic lithosphere with deposition of minor cumulates in small magma chambers over mantle residual rocks and (2) intrusion of dolerite sills and dikes throughout the whole sequence.
Paleomagnetic results for Eocene volcanic rocks from northeastern Washington and the tertiary tectonics of the Pacific Northwest
Authors: Fox Kenneth F. , Beck Myrl E.
Published: February 2008

The direction of remanent magnetization for 102 sites in Eocene volcanic and volcaniclastic rocks of the O'Brien Creek Formation, Sanpoil Volcanics, and Klondike Mountain Formation suggests approximately 25° of clockwise rotation of a 100 by 200 km area in northeastern Washington. The volcanic rocks consist chiefly of rhyodacite and quartz latite flows, with intercalated ash flow tuff and volcaniclastic layers. These rocks have been sampled at 102 sites distributed among five volcanotectonic depressions: the Toroda Creek, Republic, Keller, and First Thought grabens and the Spokane‐Enterprise lineament. The volcanic rocks probably range in age from 55 m.y. to about 48 m.y., and the 50‐ to 48‐m.y.‐old volcanic rocks within this suite appear to be rotated as much as the older rocks. Previous investigators have shown that 40‐m.y.‐old and younger plutonic rocks of northwestern Washington are not rotated; hence we infer that the north‐central Washington rocks were rotated to their present declination between 48 and 40 m.y. B.P. (during the middle and/or late Eocene). During early Eocene time this region was extended in a westward direction through crustal necking, gneiss‐doming, diking, and graben formation. Internal deformation of the region related to this crustal extension was extreme, but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation; hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area. Faults active during Paleogene time appear to define boundaries of a triangular crustal block (the Sanpoil block), encompassing much of northeastern Washington, northern Idaho, northwestern Montana, and adjacent parts of British Columbia. The faults include the Laramide thrusts of the Rocky Mountain thrust belt, the strike‐slip faults of the Lewis and Clark line, and strike‐slip faults of the Straight Creek‐Fraser zone. We suggest that during early Eocene time the Sanpoil block was extended westward through crustal necking and dilation and then during the middle Eocene was rotated clockwise and thrust over the craton in a final stage of Laramide thrusting. The “motor” driving these deformations presumably was interaction of North America with oceanic lithosphere off its western margin; such interaction probably involved right‐oblique underthrusting and dextral shear.
Mode of crustal shortening adjacent to the Alpine Fault, New Zealand
Authors: Allis R. G.
Published: February 2008

The late Cenozoic episode of crustal shortening in the South Island, New Zealand, which has caused present day uplift rates of greater than 10 mm/yr and has formed the Southern Alps, may have also caused clockwise rotation of the Alpine Fault by up to 20°. In contrast to the northern end of the Alpine Fault, which appears to have remained fixed relative to the Australian plate, increasing westward movement of the Fault trace has occurred southward along the fault. This westward movement may exceed 100 km where the Fault crosses the coastline in northern Fiordland. Most shortening adjacent to the southern end of the Alpine Fault has occurred within Australian plate continental crust which has been thrust beneath Pacific plate continental crust. This zone of continental underthrusting merges with the Fiordland subduction zone further south, where Australian plate oceanic crust is being subducted. Gravity modelling of the continental collision zone beneath the Southern Alps indicates that the leading edge of underthrust Australian plate crust may be close to a relatively sharp 10 km change in Moho depth. Inward dipping thrust faults on both plates mark the outer limits of the collision zone, which is over 300 km wide near the southern end of the Alpine Fault. The probable increase in crustal thickness due to the late Cenozoic shortening suggests that the amount of crustal thickening is similar in size to the amount of eroded crust.
Ductile thrusting in the Himalayas: Shear sense criteria and stretching lineations
Authors: Brunel Maurice
Published: February 2008

The most useful criteria used for the determination of the sense of shear can be found in the huge Himalayan ductile shear zone known as M.C.T. They allow the determination of the sense of shear in relation with an unequivocal southward‐directed intracontinental thrust. The various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution. However, most of the observed shear‐criteria are due to late nappe emplacement postdating the climax of the metamorphism. The most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt. It parallels everywhere the shear direction and thus indicates the transport direction of the nappes. The observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence.
block rotations in the Rio Grande Rift, New Mexico
Authors: Brown Laurie L. , Golombek Matthew P.
Published: February 2008

Additional paleornagnetic data from the north‐central Rio Grande rift support previous suggestions for counterclockwise rotation of a large crustal block. Thirteen sites from intrusive and extrusive rocks associated with the Oligocene Espinaso Formation in the area around the Ortiz Mountains, central New Mexico, yield magnetic directions of I = 42.1° and D = 337.8° (alpha‐95 = 10.3°). Comparison with the expected direction for the Oligocene indicates 17.8° ± 11.4° of counterclockwise rotation. This agrees with previous studies of Miocene to Pliocene rocks north of the study area that also show about 15° of counterclockwise rotation and corroborates the timing of this motion as post 5 Ma. The extent of the rotated block now includes areas in and adjacent to the Espanola basin east of the Pajarito fault zone, from the San Luis basin in the north to the Albuquerque basin in the south. Extensive published geologic and structural information indicates that counterclockwise block rotation in the Rio Grande rift probably results from left slip along the rift. The rotated block is diamond shaped and bounded by major fault zones (Embudo fault to the north, Pajarito fault zone to the west, Tijeras‐Canoncito fault zone to the southeast, and Picuris‐Pecos fault to the east). Structural consequences of the counterclockwise block rotation are uplift at the acute ends (Sandia and Picuris uplifts) and subsidence at the obtuse ends (Velarde graben and Santa Fe embayment).
Deep crustal structure and tectonic history of the Northern Kapuskasing Uplift of Ontario: An integrated petrological‐geophysical study
Authors: Percival J. A. , McGrath P. H.
Published: February 2008

The northeast trending Kapuskasing uplift transects the east‐west belts of the central Superior Province over a distance of some 500 km. Granulite to upper amphibolite facies rocks of the uplift form three distinct geological‐geophysical entities: from south to north, the Chapleau, Groundhog River, and Fraserdale‐Moosonee blocks. Uplift of the granulites along a moderately northwest dipping crustal‐scale thrust fault is attributed to an early Proterozoic compressional event. Major northeast‐striking faults that bound the Kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet‐orthopyroxene‐plagioclase‐quartz assemblages to determine vertical displacement. Granulites in the Kapuskasing zone have 7‐ to 9‐kbar signatures whereas those in the Quetico belt to the west indicate metamorphic pressure of 4–6 kbar. Individual calibrations of the barometer yield consistent pressure differences of 2–3 kbar, suggesting 7–10 km of west‐side‐down movement on the faults. Modelling of gravity and aeromagnetic gradients indicates westerly dips of 60°–65°, with west‐side‐down offset of up to 14 km. These major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage. Differences in the configuration of individual blocks of the Kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults. Thus the Groundhog River and southern Fraserdale‐Moosonee blocks are perched thrust tips analogous to the Sangre de Cristo Range of the Laramide uplift province, whereas the southern Chapleau block is a tilted slab with similar configuration to the Laramide Wind River Range. Pop‐up geometry deduced for the northern Fraserdale‐Moosonee block resembles the structure of the Laramide Uinta Mountains. A normal fault crosses the surface trace of the basal thrust fault between the Groundhog River and Fraserdale‐Moosonee blocks and causes a 65‐km “gap” without granulites. This article contains supplementary material.
A paleomagnetic study of tectonically deformed red beds of the Lower Glarus Nappe Complex, eastern Switzerland
Authors: Hirt A. M. , Lowrie W. , Pfiffner O. A.
Published: February 2008

A paleomagnetic study at 15 sites (315 samples) in Permo‐Triassic red beds of the Helvetic nappes of eastern Switzerland isolated stable characteristic and secondary remanent directions. Thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction. Secondary directions are scattered but may have been acquired during the Alpine deformation in the Oligocene and Miocene. The characteristic directions also show large between‐site scatter. Anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage, and that there is a north‐south lineation. This anisotropy pattern implies that the magnetic grains were realigned by the Alpine deformation. It is not possible to explain the characteristic remanent magnetization deviations on a site‐by‐site basis, but the site mean directions are streaked along a great circle, the down dip direction of which is approximately parallel to the regional fold axes (60°/20°). This spread could be explained by a penetrative simple shear deformation associated with the nappe‐internal deformation.
Thermochronologic evidence of major tectonic denudation associated with detachment faulting, Northern Ruby Mountains ‐ East Humboldt Range, Nevada
Authors: Dokka Roy K. , Mahaffie Michael J. , Snoke Arthur W.
Published: February 2008

Fission track (FT) thermochronology studies on igneous and metamorphic rocks from the Ruby Mountains‐East Humboldt Range metamorphic core complex provide important constraints on the timing and nature of major mid‐Tertiary extension in northeast Nevada. Rocks from within the Ruby‐East Humboldt detachment (brittle‐ductile normal‐sense shear) zone were analyzed; the dominant lithology studied was variably mylonitic mafic orthogneiss. Nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the Oligocene (circa 36 Ma) Harrison Pass pluton were also studied. FT ages on apatite, zircon, and sphene (except one 18.4‐Ma apatite from the basal portion of the detachment zone) are concordant and range in age from 26.5 Ma to 23.6 Ma; all ages overlap at 1 σ between 25.4 Ma and 23.4 Ma. These data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above ∼285°C (sphene closure temperature) to below ∼70°C (lower temperature limit of track stability in apatite) near the beginning of the Miocene (minimum cooling rate of 40°C/Ma). The length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly (Fish Canyon ash flow tuff). The lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval, but apparently only cooled to a temperature within the zone of partial annealing of apatite (∼70°–130°C). Residence of these rocks in this zone led to a partial retention of fission tracks, thus resulting in a reduced age. Confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history (remained hotter longer) than structurally higher levels of the zone. FT data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23.4 Ma. Rapid cooling of the region is considered to reflect large‐scale tectonic denudation (intracrustal thinning), the vertical complement to crustal extension. Rocks originating in the middle crust (10–15 km) were quickly brought near the surface along the Ruby‐East Humboldt detachment fault (brittle‐ductile simple shear zone) and juxtaposed against brittlely extended rocks deformed under upper crustal conditions.
Paleomagnetism and crustal rotations along a shear zone, Las Vegas Range, southern Nevada
Authors: Nelson Michael R. , Jones Craig H.
Published: February 2008

In southern Nevada, Paleozoic isopachs, Mesozoic structures, and mountain ranges trend north–south, but as they approach the right‐lateral, west‐northwest‐trending Las Vegas Valley Shear Zone (LVVSZ), they are bent clockwise up to 90° in oroflexes 30 km long. In the Las Vegas Range the declination of the characteristic magnetization of the Cambrian Bonanza King Formation has likewise been rotated clockwise more than 50°. For samples from eight sites (13–23 samples per site) in the Las Vegas Range and in its extension south of the LVVSZ, stepwise thermal demagnetization to 600°C removed recent and Tertiary overprints, revealing a nearly horizontal, late Paleozoic viscous partial thermoremanent magnetization (VPTRM) which predates both the Tertiary oroflexural bending and the Sevier thrusting in the area. The declination of the characteristic magnetization (after correcting for the dip of bedding) varies from 133° ± 2° at a site 24 km north of the shear zone to 196° ± 4° 13 km north of it and 204° ± 4° 11 km south of the shear zone. This parallels a similar change in the structural trend. The paleomagnetic data indicate that the Las Vegas Range is broken into rather small blocks (<5 km) which rotate in response to more continuous deformation at depth. This deformation of the upper crust can be modelled using a thin non‐Newtonian viscous plate model, which when applied to the Las Vegas Valley Shear Zone predicts rotations similar to those seen in the Las Vegas Range.
The northeastern Brazil and Gabon Basins: A double rifting system associated with multiple crustal detachment surfaces
Authors: Castro Augusto Canellas M.
Published: February 2008

Analysis of structural, stratigraphic, and gravimetric data from the Reconcavo, Tucano, and Sergipe‐Alagoas basins in northeastern Brazil and the Gabon basin in Africa shows that these basins originated in a double rifting system associated with multiple crustal detachment surfaces and that the direction of dip of the detachment surfaces was reversed at the Vaza‐Barris fault system. This geometry is in agreement with models and data from modern rifts, but it requires the existence of a predominantly extensional stress regime in the northern part of the South Atlantic during the early stages of oceanic evolution. Evidence for the existence of such a stress regime is given by application of kinematic constraints to Early Cretaceous reconstructions of the South Atlantic Ocean. Interpretation of the regional geologic setting suggests that the double rifting system and the point of final continental rupture were controlled by the preexisting structural orientation of the tectonic provinces in the Precambrian basement.
Lithoprobe seismic reflection structure of the Southeastern Canadian Cordillera: Initial results
Authors: Cook F. A. , Green A. G. , Simony P. S. , Price R. A. , Parrish R. R. , Mikereit B. , Gordy P. L. , Brown R. L. , Coflin K. C. , Patenaude C.
Published: February 2008

Nearly 270 km of crustal seismic reflection data obtained by Lithoprobe in the southern Canadian Cordillera provide a geometric link between the Rocky Mountain foreland thrust and fold belt, the Purcell anticlinorium, and the extensional regime superimposed on the crystalline core zone. Autochthonous North American basement and its overlying deformed and transported cover can be traced from the thrust and fold belt, beneath the Rocky Mountain trench, to 20 km depth beneath the central part of the Purcell anticlinorium. The Purcell anticlinorium is cored by foreshortened Proterozoic supracrustal rocks that were carried northeastward on a series of west dipping imbricate thrust faults. These faults crop out within and east of the anticlinorium and converge downward with subhorizontal detachments above the autochthonous North American basement. Beneath the western Purcell anticlinorium and Kootenay Arc, reflections associated with the Purcell stratigraphy and its underlying crystalline basement terminate at about 20 km depth and may be truncated against the east dipping Eocene Slocan Lake fault zone. The Slocan Lake fault zone is clearly imaged from the surface to about 12 km depth and can probably be followed discontinuously to about 25 km depth. A west‐dipping, high amplitude reflection from beneath the Valhalla gneiss complex outlines the domal geometry of the complex and is probably related to an east verging compressional shear zone.
Early Miocene change in direction of least principal stress, southwestern United States: Conflicting inferences from dikes and metamorphic core‐detachment fault terranes
Authors: Best Myron G.
Published: February 2008

Previous investigations have suggested that the direction of least principal stress in the western United States has shifted from northerly to easterly between 50 to 30 Ma. An effort is made to more tightly constrain the time of this shift using igneous dike‐host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 Ma. In such systems, the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock. These systems indicate that a northerly least principal stress orientation persisted more recently than previously believed, until probably after 30 Ma, possibly about 26 Ma, and conceivably as recently as 18 Ma in some areas. The data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift. However, the change could be diachronous—earlier in the southeast and later in the northwest. Major easterly trending magmatic and tectonic elements of Tertiary age in the Great Basin of Utah and Nevada are compatible with the dike data. However, numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes. This inconsistency is not clearly resolvable, but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by Mesozoic compression.
Paleomagnetism of the Hervey Group, Central New South Wales and its tectonic implications
Authors: Li Z. X. , Schmidt P. W. , Embleton B. J. J.
Published: February 2008

Paleomagnetic work on the Hervey Group in southeast Australia reveals a predeformational magnetization as suggested by the retention of a predeformational compaction produced magnetic fabric and the effect of deformation on both the magnetic fabric and the direction of the magnetic remanence in cleaved rocks. The formation mean direction D=29.2°, I=−19.3° with α95=15.5° gives a latest Devonian‐Early Carboniferous palaeomagnetic pole at 54.4°S, 24.1°E with DP=8.4°, DM=16.2°. Comparing this pole with existing poles from Australia and North America, it is suggested that: (1) The Lachlan Fold Belt of southeast Australia has been part of cratonic Australia since at least the late Devonian, and consequently, the Late Palaeozoic palaeomagnetic data from the LFB may be applied to the whole of Gondwanaland. (2) Cratonic Australia and the New England Fold Belt were all in an equatorial position during the Early Carboniferous. (3) There was probably a rapid anticlockwise rotation of Gondwanaland during the Late Devonian around an axis close to Australia. (4) A “V”‐shaped ocean existed between Laurussia and the northern margin of western Gondwanaland during the late Devonian‐Early Carboniferous. The Appalachian‐Hercynian Orogenic Belt is probably the result of the closure of this ocean.
A thin‐skinned tectonics model for an arcuate fold and thrust belt: The Cantabrian Zone (Variscan Ibero‐Armorican Arc)
Authors: Pérez‐Estaún A. , Bastida F. , Alonso J. L. , Marquínez J. , Aller J. , Alvarez‐Marrón J. , Marcos A. , Pulgar J. A.
Published: February 2008

The Cantabrian Zone constitutes the external zone of the Variscan orogenic belt in the northwest of the Iberian Peninsula. Within it a large number of thrusts and folds can be observed which define the form of the Asturian Arc. Tectonostratigraphically, two units can be distinguished: a pretectonic one and a syntectonic one. The time of the transition between the two units lies close to the Devonian‐Carboniferous boundary. The major allochthonous units of the Cantabrian Zone display varied geometries (duplexes, imbricated thrusts, out‐of‐sequence thrusts, etc.). The allochthonous units were emplaced in a foreland propagating sequence with movement age ranging from Westphalian B to Stephanian. Movement directions changed from one major unit. As a whole, the movement directions converge toward the core of the Asturian Arc, and are best interpreted in terms of a progressive series of rotational displacements, leading to a final disposition of major units similar to that of the leaves of a photographic iris. The folds were initiated at the same time as the motion of each nappe unit, and each type of structure represents the hanging wall adaptation to the development of a lateral and longitudinal staircase thrust surface topography. These folds suffered tightening after the emplacement of the thrust unit in which they occur, as a result of the different direction of movement of the next thrust unit propagating from beneath.
flexural rotation of normal faults
Authors: Buck W. Roger
Published: February 2008

A conceptual model is proposed for the generation of low‐angle normal faults in Metamorphic Core Complexes. The model is based on three assumptions: (1) the isostatic response to normal fault motion is of regional extent; (2) when a fault segment is significantly rotated from the optimum angle of slip, relative to the crustal stress field, it is replaced by a new planar fault oriented in the optimum direction; and (3) the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust. The stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations. Active low‐angle normal faults are difficult to reconcile with rock mechanics theories, earthquake focal mechanism studies, and geochronologic results indicating rapid cooling of core complex rocks. The model does not require active fault slip on low‐angle faults. The flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation. The anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust. This can explain the observed short wavelength of topographic response to normal fault loads. The model results in: (1) a nearly flat‐lying abandoned normal fault (or “detachment”) below slices of upper plate rocks and sedimentary infill; (2) a strong contrast in metamorphic grade across the abandoned “detachment”; (3) rapid movement of lower plate rocks from midcrustal depths to shallower depths. These results are qualitatively in agreement with geologic observations for core complexes.
Early Mesozoic southward migration of Cordilleran Transpressional Terranes
Authors: Avé Lallemant Hans G. , Oldow John S.
Published: February 2008

Oblique plate convergence can cause forearc, arc, and back arc regions of active margin orogenic belts to behave as semi‐independent terranes relative to bordering continental and oceanic lithospheric plates. These terranes, called here “transpressional terranes,” are bound by the subduction zone on one side, by arc‐parallel strike‐slip faults in the forearc, in the volcanic arc, or in the back arc region on the other and are underlain by throughgoing basal decollements. These terranes undergo shortening in a direction perpendicular to the subduction zone and migrate as coherent structural bodies parallel to the subduction zone along bounding strike‐slip faults. Many terranes of the North American Cordillera may have behaved in this manner and perhaps did not travel across the Pacific basin but rather were always peripheral to North America. We propose that during the Triassic to mid‐Cretaceous, portions of the Cordillera borderland generally were displaced southward with respect to the North American craton due to left‐oblique convergence. Since the mid‐Cretaceous, they moved northward due to right‐oblique subduction. The proposed displacements are consistent with structural, paleomagnetic, and paleontologic data and with the relative and absolute motions of North America and the Pacific basin plates based on hotspot reference frame models.
The Goodman Swell: A lithospheric flexure caused by crustal loading along the midcontinent rift system
Authors: Peterman Zell E. , Sims P. K.
Published: February 2008

Rb‐Sr biotite ages of Archean and Early to Middle Proterozoic crystalline rocks in northern Wisconsin and adjacent Upper Peninsula of Michigan describe a regionally systematic pattern related to differential uplift. An “age low” occurs in northern Wisconsin where values range from 1070–1172 Ma for rocks with crystallization ages of 1760 to 1865 Ma. These values overlap with the main episode of mafic igneous activity (1090 to 1120 Ma) along the Midcontinent rift system (MRS). We interpret these low biotite ages as registering closure due to cooling below the 300°C isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the Goodman swell. The position of the Goodman swell is independent of the Archean and Early Proterozoic regional geologic framework but closely linked with the curvilinear (concave to the south) MRS. We interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the MRS. The southward concavity of load axes resulted in flexural interaction that localized the swell. Because of rapid uplift the swell may have been a source area for clastic sediments in the rift sequence.
Pre‐Pliocene Extension around the Gulf of California and the transfer of Baja California to the Pacific Plate
Authors: Stock J. M. , Hodges K. V.
Published: February 2008

Late Miocene (12–5 Ma) extension around the edges of the Gulf of California has been alternatively attributed to “Basin and Range” extension, back arc extension, or development of the Pacific‐North America plate boundary. This extension was ENE directed and similar in structural style to extension in the Basin and Range province. Timing constraints permit nearly synchronous onset of this deformation in a belt extending SSE from northernmost Baja California to the mouth of the gulf. Where this extensional faulting continued through Pliocene time to the present, synchronous with motion on the modern transform plate boundary in the Gulf of California, no change in direction of extension can be resolved. Revised constraints on Pacific‐North America plate motion support the development of this late Miocene extension as a component of Pacific‐North America displacement that could not be accommodated by strike‐slip displacement along the existing plate boundary west of the Baja California peninsula. This scenario implies that transfer of Baja California from the North America plate to the Pacific plate was a gradual process, beginning about 12–10 Ma, when motion of the Pacific plate relative to North America was partitioned into separate regimes of strike‐slip and dip‐slip displacement on opposite sides of Baja California.
Extensional faulting in southern Klamath Mountains, California
Authors: Schweickert Richard A. , Irwin William P.
Published: February 2008

Large northeast striking normal faults in the southern Klamath Mountains may indicate that substantial crustal extension occurred during Tertiary time. Some of these faults form grabens in the Jurassic and older bedrock of the province. The grabens contain continental Oligocene or Miocene deposits (Weaverville Formation), and in two of them the Oligocene or Miocene is underlain by Lower Cretaceous marine formations (Great Valley sequence). At the La Grange gold placer mine the Oligocene or Miocene strata dip northwest into the gently southeast dipping mylonitic footwall surface of the La Grange fault. The large normal displacement required by the relations at the La Grange mine is also suggested by omission of several kilometers of structural thickness of bedrock units across the northeast continuation of the La Grange fault, as well as by significant changes in bedrock across some northeast striking faults elsewhere in the Central Metamorphic and Eastern Klamath belts. The Trinity ultramafic sheet crops out in the Eastern Klamath terrane as part of a broad northeast trending arch that may be structurally analogous to the domed lower plate of metamorphic core complexes found in eastern parts of the Cordillera. The northeast continuation of the La Grange fault bounds the southeastern side of the Trinity arch in the Eastern Klamath terrane and locally cuts out substantial lower parts of adjacent Paleozoic strata of the Redding section. Faults bounding the northwestern side of the Trinity arch generally trend northeast and juxtapose stacked thrust sheets of lower Paleozoic strata of the Yreka terrane against the Trinity ultramafic sheet. Geometric relations suggest that the Tertiary extension of the southern Klamath Mountains was in NW‐SE directions and that the Redding section and the southern part of the Central Metamorphic terrane may be a large Tertiary allochthon detached from the Trinity ultramafic sheet. Paleomagnetic data indicate a lack of rotation about a vertical axis during the extension. We propose that the Trinity ultramafic sheet is structurally analogous to a metamorphic core complex; if so, it is the first core complex to be described that involves ultramafic rocks. We infer that Mesozoic terrane accretion produced a large gravitational instability in the crust that spread laterally during Tertiary extension.
Geophysical constraints on the deep structure along the Ecors Pyrenees Line
Authors: Daignieres M. , de Cabissole B. , Gallart J. , Hirn A. , Surinach E. , Torne M.
Published: February 2008

The structure of the central part of the Pyrenean range, traversed by the ECORS vertical reflection seismic line, is further constrained by a joint interpretation of complementary and reprocessed data. Wide‐angle, low‐frequency profiling of Moho topography indicates an abrupt termination of the northern, European Moho at the North Pyrenean fault and a range of possible geometries of a deeper, Iberian Moho which does not seem to extend significantly to the north of this fault. When reprocessing vertical reflection data to 25 s two‐way travel time, strong clear straight reflections are discovered in the part of the section under the northern foreland, long after the Moho reflection. Their strong northward dip indicates that they originate in fact at reflectors at depth beneath the North Pyrenean fault, which may correspond to the connection between the two levels of Moho, the reflections then being from the northward inclined limit between the wedge of Iberian crust underlying the southern edge of the European mantle. A range of possible shapes is allowed by the conjunction of these seismic data, which is compatible with the results of a detailed gravity profile. This would indicate that the imbrication of European into Iberian lithosphere has been of limited extent with respect to the surface trace of the North Pyrenean fault, provided no strong density contrast in the mantle part of the section contributes significantly to the gravity anomaly.
Forward modeling of the Southern Apennines
Authors: Endignoux L. , Moretti I. , Roure F.
Published: February 2008

Forward kinematic modeling has been developed to reconstruct the successive geometries of geological sections. The method is tested here on a geological section across the southern Apeninnes, where syntectonic Neogene deposits are of help in determining the timing of the deformation that progressively affected more eastern parts of the Apulian margin. Well‐known Triassic and Paleogene decollement levels induced thin‐skinned tectonics in the Mesozoic and Cenozoic sedimentary cover, but the basement geometry is only poorly revealed by subsurface data. Two distinct hypotheses have been tested: (1) thin‐skinned tectonics and (2) basement‐involved tectonics. The shortening of the prism is estimated to be between 170 and 210 km, whether or not the basement is involved in the imbricated stack of thrust sheets. The results are discussed in the light of regional constraints (i.e., regional flexure, uplifts, etc.), which favor basement‐involved structures at depth. In addition to the effects of compression, other phenomena affected the geometry of the prism and have to be taken into account. Because of the recent uplift of the belt, the inner parts of the Apenninic wedge have emerged and undergone erosion. Part of this eroded material has been simultaneously trapped in piggyback basins, which record both the geometric and temporal evolution of the prism. Normal faulting also affected the western part of the southern Apennines, reactivating older thrust as detachment surfaces along which block rotation occurred.
A geophysical profile of the southern margin of the midcontinent rift system in western Lake Superior
Authors: Hinze William J. , Braile Lawrence W. , Chandler Val W.
Published: February 2008

A 70‐km‐long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5‐s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ∼2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault‐bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic‐filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate‐ to high‐angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region.
Relaxation of continental lithosphere: An explanation for Late Cretaceous reactivation of the Sabine Uplift of Louisiana‐Texas
Authors: Nunn Jeffrey A.
Published: February 2008

With few unconstrained assumptions, a simple quantitative model for flexural isostasy between two crustal blocks of different thickness that subsequently undergoes partial relaxation of accumulated stress can explain reactivation of the Sabine Uplift in the mid‐Cretaceous. Thin salt over the Sabine Uplift indicates that it was a positive area in the Middle Jurassic but began to subside during the Late Jurassic. During the Late Jurassic and Early Cretaceous, the Sabine Uplift had no topographic expression and was the center of a large, flat bottomed basin covering most of east Texas and north Louisiana. Reactivation of the Sabine Uplift in the mid‐Cretaceous caused a minimum of 150 m of uplift and extensive erosion of Lower Cretaceous rocks. During the Late Cretaceous and Tertiary, the Sabine Uplift resumed subsiding but at a slower rate than the adjacent basins. Since the mid‐Cretaceous, the Sabine Uplift has risen more than 1 km relative to the East Texas Basin. We suggest that this structural history is consistent with the following simple quantitative model for flexural isostasy. During formation of the Gulf of Mexico in the Late Triassic‐Early Jurassic by rifting and extension of the lithosphere, brittle deformation of the crust created small‐scale wavelength of tens of kilometers, lateral variations in crustal thickness. Initially, the newly formed margin was in point‐wise Airy isostatic equilibrium. Thus synrift and/or early postrift subsidence was characterized by a series of rift valleys or half grabens separated by uplifts. However, as the margin cooled and contracted, the lithosphere became stronger, and subsequent loading was regionally compensated. Thus postrift subsidence was characterized by a broad regional downwarp. If, at some later point in time, the rigid portion of the lithosphere was weakened or relaxed, then lateral density variations would have been recompensated at shorter wavelengths. Thus areas of little or no crustal extension would have been uplifted and areas of high extension would have experienced additional subsidence. Gravity and total tectonic subsidence interpretations agree that the Sabine Uplift region is underlain by thicker crust than the adjacent East Texas and North Louisiana basins. Possible mechanisms for relaxation of continental lithosphere are thermal rejuvenation, horizontal in‐plane stress, viscous relaxation, and extensional faulting.
Origin of the Chichibu Sea, Japan: Middle Paleozoic to Early Mesozoic Plate construction in the northern margin of the Gondwana continent
Authors: Otoh Shigeru , Yamakita Satoshi , Yanai Shuichi
Published: February 2008

Pre‐Jurassic plate tectonic development of southwest Japan is discussed in the light of rifting tectonics using the symmetrical rock arrangement. In particular, a geodynamic model is presented of Middle Paleozoic to Early Mesozoic plate construction of the Chichibu Sea. The model involves five steps: (1) the deposition of Siluro‐Devonian rocks on the Gondwana continental margin; (2) incipient rifting into Hida + Hida Marginal continental block on the north and Kurosegawa continental block on the south, forming Chichibu Sea plate in Carboniferous time; (3) parts of the Chichibu Sea plate were subducted beneath the Kurosegawa continental block and newly formed Maizuru Island Arc in Permian time; (4) the rifting and subduction ceased in latest Permian time and the Chichibu Sea remained stable with the width of over 1000 km in Triassic time; and (5) the Kurosegawa continental block collided with the Hida + Hida Marginal continental block in Jurassic to earliest Cretaceous times. The geodynamic model is further discussed in the context of breakup and accretion tectonics.
Tectonic history of the Lunksoos Composite Terrane in the Maine Appalachians
Authors: Wellensiek M. Reid , van der Pluijm Ben A. , Van der Voo Rob , Johnson Rex J. E.
Published: February 2008

Several Ordovician volcanic terranes are preserved between the Paleozoic North American margin and the Avalon Terrane in the northern Appalachians of Maine. They have been interpreted as remnants of an island arc on the basis of their current position in the Appalachians or as ocean islands near the margin of Avalon on the basis of paleontologic data. A study of one of these terranes, the Lunksoos terrane in northern Maine, was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution. Paleomagnetic study of the pre‐Caradoeian (pre‐Late Ordovician) Stacyville volcanics revealed three components of magnetization. The final magnetization removed (D=90°, I=36°, α95=11.8°) is interpreted to be primary and yields a paleolatitude of approximately 20°S; however, a Late Ordovician remagnetization cannot be excluded. The Stacyville volcanics now overlie a Cambrian and Lower Ordovician sequence (the Grand Pitch and Shin Brook formations) that is believed to have been deposited at the southern side of Iapetus, possibly near the margin of Avalon. During the Middle and Late Ordovieian, Avalon was at paleolatitude s higher than 40°S, while the volcanics were either formed or remagnetized at a paleolatitude of 20°S. The pre‐Caradocian Stacyville volcanics therefore could not have been deposited directly on the Grand Pitch‐Shin Brook sequence but must have been tectonically emplaced. The Rockabema Diorite which intrudes the entire Cambro‐Ordovician sequence constrains the timing of this emplacement to be pre‐late Early Silurian. These relationships indicate that the Lunksoos area is a composite terrane within the Maine Appalachians. The tectonic emplacement scenario is unnecessary if the Grand Pitch and Shin Brook formations were not located on the south side of Iapetus during the Ordovician. This alternative, however, poses conflicts with the interpretation of faunal affinities.