bluuebunny/crossref_metadata_2025 huggingface dataset:
Quaternary normal and reverse faulting and the state of stress in the central Andes of south Peru
Authors: Sébrier Michel , Mercier Jacques Louis , Mégard François , Laubacher Gérard , Carey‐Gailhardis Evelyne
Published: February 2008

Field studies in the Andes of southern Peru show that in the High Andes and Pacific Lowlands, Quaternary and Recent faults are normal. This extensional tectonics postdates compressional deformations of Pliocene‐early Quaternary age. In the sub‐Andes the observed deformations are compressional; they affect early Quaternary deposits. Some of the faults separate Quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects. Striations on the fault planes indicate N–S trending extension in the High Andes and Pacific Lowlands. The total amount of crustal stretching is small, probably of the order of 1% during the last 1–2 m.y. In the sub‐Andes, folds and faults affecting Neogene and early Quaternary deposits result from N–S shortening. Nevertheless, it is supposed that this N‐S shortening is of early quaternary age. The present‐day compression probably strikes E‐W, judging from focal mechanisms in the sub‐Andes of central Peru, southern Bolivia, and northwest Argentina. Data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the Andes of southern Peru. The High Andes and Pacific Lowlands, subjected to N‐S trending extension, are bounded by two zones of E‐W trending compression: the sub‐Andes to the east, and the contact between the convergent Nazca and South America plates to the west. In our model the maximum horizontal compressive stress trajectory σ Hmax is roughly parallel with the E‐W convergence between the two plates; σ Hmax corresponds to σ 1, in the sub‐Andes and to σ 2 in the High Andes. The latter situation is caused by the elevated mass of the High Andes, where σ zz (the vertical stress) is inferred to be σ 1. Thus the third principal stress axis, being orthogonal to the other two axes, it is oriented N‐S, allowing extension to occur in that direction. On the other hand, in the sub‐Andes σ zz is σ 3, and horizontal E‐W shortening occurs. The state of stress in the Andean continental crust above the 30° dipping slab appears to be different from that in the Andes of Central Peru situated above the flat subducting segment. In this region, compressional deformantion affect a wider part of the Cordillera.
The Cretaceous‐tertiary deformation of the Lhasa Block and its implications for crustal thickening in Tibet
Authors: England Philip , Searle Michael
Published: February 2008

The strong Tertiary deformation of the northern margin of the Indian plate and of the Tsangpo suture zone is not matched by comparable deformation within the Lhasa block, yet the southern margin of the block was the site of large‐scale intrusive activity (the Gangdese, or Transhimalayan, batholith) throughout the late Cretaceous and was, therefore, presumably weaker than its surroundings. This poses a problem for any approach that interprets the deformation of Tibet in terms of diffuse crustal thickening. However, there is evidence that the Lhasa block was, in elevation as well as in magma type, an Andean margin by the end of the Cretaceous, and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern Lhasa block. This suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast. When subjected to boundary conditions representing the collision between India and Asia, the sheet accommodates convergence by diffuse deformation over a region comparable in size with the Tibetan plateau; in the absence of an elevation contrast this deformation consists of approximately 100% thickening strain over the region. A precollision elevation contrast of 3000 m in the Lhasa block would have resulted in its experiencing roughly 40%, rather than 100%, postcollisional thickening; smaller elevation contrasts would have resulted in greater thickening, and vice versa. These calculations have implications for the mode of crustal thickening in northern Tibet: As the available evidence suggests that the topographic expression of the earlier Mesozoic tectonic activity in southern Asia was erased before the India‐Asia collision, we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the Lhasa block during the Tertiary.
From faulted blocks to megamullions and megaboudins: Tethyan heritage in the structure of the western Alps
Authors: Tricart Pierre , Lemoine Marcel
Published: February 2008

Along the Grenoble‐Pelvoux‐Viso transect in the Western Alps, we schematize the geometrical relationships between (1) ductile, marly‐calcareous, sedimentary covers and (2) their more rigid substratum. This substratum may be the pre‐Alpine granitic‐gneissic basement, the Triassic dolomites, or even the Jurassic ophiolites. Basement mullions, a few kilometer size, are characteristic of the external part of the Western Alps arc (Dauphiné zone). They strongly contrast with the dolomitic or ophiolitic boudins, also a few kilometer size, in the inner part of the arc (Piedmont zone). The Dauphiné megamullions formed and then became accentuated, during the Oligocene and then the Neogene tectonic phases, which correspond to the last major Alpine contractions, at relatively shallow structural levels. In the sedimentary cover, upright or inclined folds were associated with slaty or strain‐slip cleavage development and low grade metamorphism. The corresponding stretching directions, locally subvertical, trend normal to the regional fold and mega‐mullions axes, and to the mountain belt itself. The Piedmont megaboudinage took place during the earlier, major Eocene, Alpine phase (which, but for some exeptions, did not affect the Dauphiné zone). In the very ductile “Schistes lustrés” that surround the dolomitic and ophiolitic megaboudins, generalized intrafolial recumbent folding was associated with blueschist metamorphism. The deformation, characteristic of a deep structural level, was a strong flattening within a subhorizontal plane and a preferential elongation parallel to the fold axes. Owing to regional horizontal festooning, those axes trend normal, oblique, or even parallel to the Piedmont zone, i.e., to the incipient mountain belt (restored initial trends). Coeval megaboudinage was probably omnidirectional (chocolate tablet pattern). The Eocene Piedmont structure has not been really obliterated during the later Alpine deformations leading to megamullions development in the Dauphiné zone. We summarize the very numerous stratigraphical, sedimentological, and tectonic field observations which lead us to regard each Dauphiné megamullion as directly inherited from a previous pre‐Alpine faulted block. Each Piedmont megaboudin has a similar origin. The corresponding paleofaults are in fact normal and/or strike‐slip faults of truly Tethyan age: Their synsedimentary offset occurred either during the Liassic‐Middle Jurassic passive margin rifting evolution of the southern edge of the European continent or during the Late Jurassic opening up of the Ligurian oceanic Tethys. We analyze how those Tethyan faults partly induced the style of the Alpine collisional structures, according to the regional characters of the successive synmetamorphic deformations, i.e., to their localization in the Western Alps mountain belt.
Deep structure of the foreland to the Caledonian Orogen, NW Scotland: Results of the Birps Winch Profile
Authors: Brewer Jonathan A. , Smythe David K.
Published: February 2008

The WINCH marine deep seismic reflection profile crosses the Hebridean shelf, the Proterozoic foreland to the Caledonian orogen, west of Scotland. The data quality is very good. The upper crust is largely devoid of coherent seismic reflections, although this may in part be due to acquisition techniques being inappropriate for this problem. In contrast, the middle and lower crust (10–25 km depth) exhibits good reflections; the mid crust contains reflectors which may be relics of early Palaeozoic, Caledonian (or earlier Grenvillian) eastward‐dipping thrust zones, which pass into an acoustically strongly layered lower crust. The Outer Isles Thrust is mapped from the surface to the mid crust, and tied into its land outcrop on north Lewis. Reactivation of this thrust offshore, in a normal sense, in Mesozoic times caused the formation of the Minch and North Lewis Basins. The Mono is defined by a strong band of reflections at about 27 km depth, which correlates well with results from the HMSE explosion seismic survey. Moho depth is apparently rather uniform in the area of the foreland crossed by WINCH. WINCH and HMSE results together suggest a mean crustal velocity for the Hebridean shelf of 6.4+0.1 km s−1. The eastward‐dipping Flannan Thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth, well into the upper mantle. Neither the Flannan Thrust nor the Outer Isles Thrust appear to pass straight through the reflective lower crust, suggesting that the lower crust is a region of high strain. The Outer Hebrides is a positive block probably formed as an isostatic response to Mesozoic normal faulting which reactivated the Outer Isles Thrust.
Effects of lithospheric in‐plane stress on sedimentary basin stratigraphy
Authors: Karner Garry D.
Published: February 2008

In‐plane lithospheric stress, though known to exist, has generally been ignored in the quantitative modeling of basin stratigraphy. However, low levels of intraplate lateral stress can induce observable plate deformations (10–100 m of vertical motion) if the lithosphere contains a preexisting deformation (such as a sedimentary basin). The importance of in‐plane stress in modifying the stratigraphy of a sedimentary basin has been assessed using an elastic plate model for the lithosphere. A compressive in‐plane stress generally induces basin subsidence with peripheral uplift (shoreline regression), while a tensile in‐plane stress induces basin uplift and peripheral subsidence (shoreline transgression). These simple results are complicated by variations in crustal thickness, as now two interfaces are involved, that is, the sediment/basement interface of the sedimentary basin and the Moho topography; the isostatic state of a sedimentary basin therefore ultimately controls the resultant deformation induced by in‐plane stress. The implications of in‐plane stress modification of basin stratigraphy are profound: regionally correlatable transgressions and regressions of basin interiors (e.g. cyclothems) and passive continental margins (the third‐order variations of the Vail et al. [1977a] coastal onlap curve), may be tectonically produced by the interaction of stress‐induced base level changes and a basin tectonic driving subsidence. In‐plane stress, its generation, magnitude, and variation, is considered a consequence of plate boundary reconfigurations during continental collisions, rifting, and subduction/obduction.
Paleogeographic and tectonic implications of quartzose sandstones of Barbados
Authors: Kasper D. C. , Larue D. K.
Published: February 2008

The provenance of Paleogene sandstones on the island of Barbados is relevant to various models of the tectonic evolution of the southern Caribbean area. Modal point counts of 26 sandstones from Barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin. Abundant detrital polycrystalline quartz grains have affinities to a spectrum of low‐ to high‐rank metamorphic/plutonic crystalline source rocks. Microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances. The complex heavy‐mineral suite of the terrigenous sandstones, including glaucophane, chloritoid, titanite, zircon, and aluminosilicates, supports derivation from orogenic and cratonal terranes. Due to the complex history of the South American‐Caribbean plate boundary and poor age constraints on timing of deposition, widely contrasting models have been proposed concerning the provenance of the sandstones. Convergence between the Aruba‐Orchila arc and continental South America during the Paleogene resulted in the formation of an E‐W trending foreland fold‐thrust belt and foredeep. Dominant sediment dispersal was to the east–northeast along tectonic strike. The present study shows that the terrigenous sandstones on Barbados probably were deposited in deep‐sea fans to the north of present‐day Araya Peninsula during and shortly after the late middle Eocene. Probable source terranes include the Guayana Shield, the Caribbean Mountain System/interior foreland fold‐thrust belt, and possibly (to a lesser extent) the Lesser Antilles magmatic arc or predecessor arc.
Paleozoic terranes of the central Argentine‐Chilean Andes
Authors: Ramos V. A. , Jordan T. E. , Allmendinger R. W. , Mpodozis C. , Kay S. M. , Cortés J. M. , Palma M.
Published: February 2008

The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower‐middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian‐Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian‐Devonian shelf‐slope‐oceanic basin assemblage bounded by mélanges on both sides and bearing many stratigraphic similarities to the lower‐middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carboniferous continental, deltaic, and marine strata demonstrates that these three terranes were sutured together and part of South America by the end of the Devonian. Subsequent Permo‐Carboniferous plate interactions more closely resembled the modern Andean margin, with eastward subduction, accretionary prism formation, and minor terrane emplacement exposed along the present coast of Chile and eastward migrating arc magmatism from the present coast of Chile to western Argentina.
The metamorphic internal zone of Wopmay Orogen (Early Proterozoic), Canada: 30 km of structural relief in a composite section based on plunge projection
Authors: King J. E.
Published: February 2008

In Wopmay Orogen, an early Proterozoic volcano‐sedimentary marginal prism and its underlying Archean structural basement, deformed together during the 1885 Ma Calderian orogeny, are exposed in oblique view as a result of regional cross folding. A composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic‐internal zone of the orogen. A sole thrust separates Archean basement and a thin mantle of autochthonous, early Proterozoic strata from the eastward transported Calderian allochthon. The allochthon is characterized by a transition from external thrust‐fold belt geometries in the east to polyphase, isoclinal recumbent folds and thrusts in the west. The center of the zone contains the Hepburn Batholith which has been been transported, along with the deforming, hot allochthon, over the exposed basement. Both basement and allochthon are deformed by late, large‐scale, upright folds. The metamorphic‐internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened, thickened (to over 30 km), metamorphosed and thrust eastward while still hot over the relatively cool Archean craton. The basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large‐scale folding due to continued compression during uplift of the internal zone.
Origin, accretion, and postaccretionary evolution of the Bridge River Terrane, southwest British Columbia
Authors: Potter Christopher J.
Published: February 2008

The Bridge River terrane of southwestern British Columbia, Canada, is one of a number of crustal fragments in the western part of the North American Cordillera which originated elsewhere in continental margin or oceanic settings and have subsequently been incorporated into North America. Detailed geologic analysis of individual accreted terranes such as Bridge River is essential in order to elucidate the processes of continental accretion and the postaccretionary evolution of accreted crust. The Bridge River terrane was probably derived from a mid‐Mesozoic ocean‐margin setting and consists of three principal elements: (1) the Carpenter Lake assemblage (CLA), (2) Bridge River schists (BRS), and (3) the Shulaps ultramafic body. The CLA is mid‐Triassic to mid‐Jurassic in age and consists of ribbon cherts, both pillowed and tuffaceous greenstones, limestone olistoliths, and coarse volcaniclastic rocks which were metamorphosed in prehnite‐pumpellyite facies and are nonpenetratively deformed on a regional scale. Parts of the CLA are intensely faulted; the faulting in part records Mesozoic accretion‐related shortening but also includes structures which offset dikes of probable Tertiary age. The BRS, separated from the CLA by a late high‐angle fault, was metamorphosed in prehnite‐pumpellyite through lower amphibolite facies, is locally twice‐deformed, and contains large‐scale recumbent folds. A Mesozoic phase of deformation and metamorphism was related to thrust emplacement of the Shulaps ultramafic complex above the BRS. The thrust zone between the Shulaps and the BRS, characterized by a thick serpentinite melange, cuts lower plate metamorphic rocks, which preserve an inverted metamorphic gradient. Tertiary deformation resulted in recumbent folding of the earlier, thrust‐related fabric in the BRS, probably during a postaccretion strike‐slip deformational regime. Early phases of the deformation of all three elements of the Bridge River terrane probably record accretion. The current position and age of the Bridge River terrane imply accretion later than the Intermontane terrane and earlier than the Insular terrane. However, its incorporation into North America may have been closely linked to one or both of these other crustal blocks. Early structures in the Bridge River area have been strongly overprinted during Late Cretaceous and early Tertiary strike‐slip faulting and northward transport.
Character of transverse faults in the Elat Pull‐Apart Basin
Authors: Ben‐Avraham Zvi , Garfunkel Zvi
Published: February 2008

The Elat Deep is a large pull‐apart basin, about 50 km long, within the Gulf of Elat (Aqaba). Its southern end displays many details of the way in which a pull‐apart basin is terminated at one of its ends. A marine geophysical study has revealed that the southern edge of the basin consists of a complicated deformed zone which contains several distinct active faults, and faults which were recently active. They strike at a large oblique angle to the long dimension of the Elat Deep. Most of the faults display dipslip offsets, although a lateral slip component may also be present on some faults.
An open‐system, two‐layer crustal stretching model for the Eastern Great Basin
Authors: Gans Phillip B.
Published: February 2008

New geological and geophysical information from the eastern Great Basin sheds light on how Cenozoic extensional strain is partitioned throughout the crustal column and provides broad constraints on how much new material was added to the crust as a consequence of synextensional magmatism. The total amount of extension across the eastern half of the northern Basin and Range province is estimated to be 141 km or 77%, but varies dramatically on more local scales. Individual domains, several tens of kilometers across, have experienced supracrustal extensional strains that vary from 15% to 300%. Volcanism in this region initiated in the early Oligocene and has continued sporadically to the present. The major eruptions appear to be both temporally and spatially associated with large‐magnitude extension. A COCORP deep seismic reflection profile across the eastern Great Basin indicates a uniform present crustal thickness of 30 to 35 km. The preextensional crustal thickness is inferred to have tapered westward from the 40‐ to 45‐km thickness of the Colorado Plateau to a maximum of 50 km in the thickened root zone of the Sevier fold and thrust belt. Simple mass‐balance calculations employing the present and inferred preextensional crustal thicknesses together with the amount of extension suggest that approximately 5 km of the present crustal thickness was added to the crust during Cenozoic extension and magmatism. The lack of relief on the reflection Moho suggests that the crust as a whole may have stretched quite uniformly. A model for crustal stretching that incorporates a heterogeneously deforming (brittle) upper crust decoupled from a more uniformly deforming (ductile) middle and lower crust, together with a significant flux of mantle‐derived magmas into the crust, best explains the geological and geophysical observations from the eastern Great Basin. This open‐system, two layer stretching model has important thermal and mechanical implications and helps account for many of the enigmatic aspects of Cenozoic extension and magmatism in the western United States.
Stress orientation determined from fault slip data in Hampel Wash Area, Nevada, and its relation to contemporary regional stress field
Authors: Frizzell Virgil A. , Zoback Mary Lou
Published: February 2008

Fault‐slip data were collected from an area of relatively young faulting in a seismically active part of the Nevada Test Site 12 km NW of Mercury, Nevada. The data come primarily from intensely faulted Miocene tuffaceous sedimentary rocks in Hampel Wash, which is bounded on the north by the Quaternary ENE trending Rock Valley fault and on the south by a parallel unnamed fault. Data from faults with known sense of displacement exhibit a bimodal distribution of slip angles (rakes). Faults exhibiting steep rakes (typically 75° to 90°) cluster about a N30°–35°E strike; most dip 65° to 80°. Faults having shallow rakes (generally less than 20°) exhibit a wide range of strikes (from N6°W to N80°E) and mostly dip between 80° and 90°. The predominant N30°–35°E strike of the steep‐rake faults and the quasi‐conjugate nature of a consistent subset of the shallowrake faults suggest a maximum horizontal stress orientation of about N30°–35°E and a least horizontal principal stress direction of N55°–60°W. Analysis of the data using a least squares iterative inversion to determine a mean deviatoric principal stress tensor indicates a normal‐faulting stress regime (S1 vertical) with principal stress axes in approximately horizontal and vertical directions (S1, trend = N 19°E and plunge = 82°N; S2, N30°E and 8°S; and S3, N60°W and 2°E). The maximum horizontal stress, S2, was found to be nearly intermediate in magnitude between S1 and S3. The N60°W least horizontal principal stress orientation obtained from the fault‐slip inversion agrees with our geometric analysis of the data and is consistent with a modern least horizontal principal stress orientation of N50°–70°W inferred from earthquake focal mechanisms, well bore breakouts, and hydraulic fracturing measurements in the vicinity of the Nevada Test Site. This solution fits all the data well, except for a subset of strike‐slip faults that strike N30°–45°E, subparallel to the normal faults of the data set. Nearly pure dip‐slip and pure strike‐slip movement on similarly oriented faults, however, cannot be accommodated in a single stress regime. Superposed sets of striae observed on some faults suggest temporal rotations of the regional stress field or local rotations within the region of the fault zone.
Change from Late Tertiary compression to Quaternary extension in southern Tibet during the India‐Asia Collision
Authors: Mercier Jacques‐Louis , Armijo Rolando , Tapponnier Paul , Carey‐Gailhardis Evelyne , Lin Han Tong
Published: February 2008

Field studies in Southern Tibet show that subsequent to folding resulting from the India‐Asia collision (50–40 Ma), reverse faults and backthrusts formed as a result of NNE‐SSW and NNW‐SSE trending compressions. They affect conglomerates of Oligocene‐Miocene age and are probably contemporaneous with shearing on the Himalayan Main Central Thrust and folding of Paleogene basins along the northern border of Southern Tibet (25–16 Ma). Rift systems postdate compressional structures. They were active during the Pleistocene and probably during the Pliocene (5–2 Ma). Change from a compressional to an extensional regime in Tibet probably occurred during the Late Miocene (10–5 Ma). This change is tentatively correlated with uplift inferred by Chinese workers from Paleogene to present‐day variations of the Tibetan vegetation. Recent and active faulting in the High Plateau results from a WNW‐ESE striking extension; this is in agreement with Tibetan focal mechanisms. The Himalayan Lowlands are subjected to a NNE‐SSW trending compression as demonstrated by superficial deformations and seismicity. Therefore the horizontal maximum compressive stress trajectory is roughly parallel to India‐Asia convergence, σ Hmax being σ 1 in the Lowlands and σ 2 in the High Plateau. This change results from the increase of the vertical principal stress value due to the weight of the elevated mass, σ zz is σ 3 in the Lowlands and σ 1 in the High Plateau. Thus the third principal stress axis is σ 2 in the Lowlands and σ 3 in the High Plateau, allowing extension to occur in the WNW‐ESE direction. Extensional tectonics are significantly different in Southern and Northern Tibet. Southern Tibet being constrained between the Himalayan syntaxes, the E‐W extensional principal deviatoric stress value σ 3 is of rather small magnitude, and the N‐S trending σ 2 value is extensional, as demonstrated by analysis of Recent faults. Normal components prevail along all the azimuths; in the field this favors formation of rift systems of large axial extension. On the contrary, north of the Himalayan syntaxes, Northern Tibet “flows” easily eastward. The E–W extensional principal deviatoric stress value σ 3 is of higher magnitude, and the N‐S stress σ 2, is compressional, as shown by inversion of focal mechanism data. This favors strike‐slip components on faults which deviate from the N‐S direction, as expressed in the field by shorter N‐S trending graben linked with normal strike‐slip faults. These faults permit some N‐S shortening in Northern Tibet, without thickening, in response to India‐Asia collision. In high plateaus which are subjected to lateral compression such as the Andean Altiplano and Tibet, the extensional direction is basically controlled by the convergence direction, being roughly orthogonal to the latter.
Felsic igneous rocks within the 3.3‐ to 3.5‐Ga Barberton Greenstone Belt: High crustal level equivalents of the surrounding Tonalite‐Trondhjemite Terrain, emplaced during thrusting
Authors: de Wit Maarten J. , Armstrong Richard , Hart Rodger J. , Wilson Allan H.
Published: February 2008

Felsic igneous rocks within the circa 3.5‐Ga ophiolitic Onverwacht Group of the Barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession. These felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession. Our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic‐ultramafic rocks of the Onverwacht Group. Rather, the data indicate that these felsic units are high‐level equivalents of the widespread gneissic to isotropic trondhjemite‐tonalite plutons which surround and intrude the greenstone belt. The felsic igneous rocks are predominantly shallow‐level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the Onverwacht Group by 40–50 Ma. The felsic intrusions were preferentially emplaced along thrust zones, cutting the ophiolite basement and its overlying sedimentary cover. Structural and chemical data imply that the hydrated ophiolitic rocks of the Barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting. Such structural imbrication of simatic crust can be best incorporated in a model advocating Archean intraoceanic obduction associated with shallow‐angle subduction, similar to that documented for the neotectonics of the Western Aleutians.
Dating transcurrent terrane accretion: An example from the Meguma and Avalon Composite Terranes in the northern Appalachians
Authors: Keppie J. D. , Dallmeyer R. D.
Published: February 2008

The polygenetic Minas Geofracture separates the Meguma and Avalon Composite Terranes of the Maritime Appalachians. Dynamically recrystallized muscovite and biotite occur within D3 shear zones which developed during dextral movement along the Geofracture. These record 40Ar/39Ar plateau ages between circa 360 and 370 Ma. Comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low‐pressure, contact metamorphic aureoles. These are similar to U‐Pb ages previously reported for igneous monazite, therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels. The D3 fabrics were superimposed on earlier (S1) structures which formed during initial amalgamation of the Meguma and Avalon Composite Terranes. Whole‐rock slate/phyllite samples collected outside high‐temperature portions of the plutonic contact aureoles yield 40Ar/39Ar plateau ages which correspond to groupings of circa 400–395 Ma, 390–385 Ma, and 380–375 Ma. These are interpreted to date diachronous formation of the S1 fabrics and concomitant low‐grade regional metamorphism. In the circa 25 Ma between initial amalgamation and granitic plutonism, it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening. This led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to D3 dextral shearing.
Strain and magnetic fabric in the Santa Catalina and Pinaleno Mountains Metamorphic Core Complex Mylonite Zones, Arizona
Authors: Ruf Amy S. , Naruk Stephen J. , Butler Robert F. , Calderone Gary J.
Published: February 2008

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference
and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains.
The Early Proterozoic structural and tectonic history of the south central Lake Superior Region
Authors: Ueng Wen‐long C. , Larue Dave K.
Published: February 2008

The early Proterozoic tectonic evolution of the south central Lake Superior region is complex, owing to the presence of four tectonostratigraphic terranes, which were affected by six phases of deformation. The four terranes are the passive margin of the Superior craton, two paraautochthonous passive margin terranes (Crystal Falls and Florence‐Niagara terranes), and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes. Four of the six deformational episodes accompanied subhorizontal shortening, while two were caused by subvertical shortening. The first and the most penetrative phase of deformation is marked by subhorizontal shortening in a NNE‐SSW direction. The second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area. The third, fifth, and sixth deformations mostly caused open folding, and shortening directions were NW, NE, and W, respectively. Because all the terranes in the south central Lake Superior region share parallel deformational histories, it is suggested that the accretion of these terranes occurred during the first deformational episode. After removal of younger deformational effects, including open folding of the suture zone, the tectonostratigraphic assemblages in this region show the following sequence from NNE to SSW: a platformal assemblage overlying sialic basement, a basinal assemblage of tholeiitic volcanic rocks overlain by deep‐water turbidites, an assemblage of basin floor deposits (Crystal Falls terrane) with apparently no demonstratably underlying crystalline basement, a fault‐bounded terrane with highly strained passive margin strata (Florence‐Niagara terrane), and a calc‐alkaline magmatic arc assemblage. Such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through Phanerozoic accretionary continental margins and therefore supports an arc‐continent collision model.
Contrasting modes of rifting: The Benue Trough and Cameroon Volcanic Line, West Africa
Authors: Okereke C. S.
Published: February 2008

The Benue trough of west Africa is commonly believed to be a rift feature that originated in the Cretaceous at about the time that Africa and South America began to separate. Bouguer gravity and available geological data in the trough indicate that its formation was probably the result of regional horizontal stresses in the lithosphere, causing crustal extension and surface subsidence. By contrast, the data for the adjoining Cameroon volcanic line suggests that the associated tensional stresses relate to mantle upwarp causing thinning of the lithosphere and regional crustal uplift similar to that associated with the Kenya rift. Thus the association of passive and active rifts seen in the Afro‐Arabia rift system is also a feature of the Cretaceous rift system in west Africa.
Variable extension in Lake Tanganyika
Authors: Morley C. K.
Published: February 2008

Lake Tanganyika is part of the east African rift system. The amount of extension along 22 dip section seismic lines in the lake has been measured. The maximum extension observed is 4.5 km (approximately 10%) at the central part of the rift and decreases to about 0.5 to 1‐km extension at the north and south ends of the rift. Deformation style changes with the amount of calculated extension. More complex rift geometries (paired half grabens and full grabens) are found in the central part where extension is greatest while single half grabens are present toward the northern and southern ends of the lake where extension is less. Local decreases in the amount of extension are superimposed on this regional trend. These drops tend to occur in regions defined as transfer zones and suggest a change from simple extension to more complex three‐dimensional strains (extension in more than the dip direction). Implied in the relationship between rift morphology and observed extension is the conclusion that Lake Tanganyika is a single rift segment that did not propagate with time but grew as a fixed length segment from day 1. Lake Tanganyika is a subsidence‐dominated rift, with up to 7‐km vertical displacement along major boundary faults for only 4.5‐km extension. The fairly steep, weakly listric boundary faults indicate that they sole out at deep (20–30 km) crustal levels.
Geochemical constraints on the origin of Croisilles and Patuki Ophiolites: Implications for Late Paleozoic‐Mesozoic tectonics in New Zealand
Authors: Sivell Warwick
Published: February 2008

Three stages of predominantly mafic (sub‐) seafloor magmatism accompanied the late Paleozoic evolution of both the Croisilles and Patuki ophiolites, Nelson Province, New Zealand. Stage 1 Croisilles and Patuki basaltic pillow lavas show strong light rare earth element (LREE) enrichment and low Zr/Nb ratios resembling alkali basalts from fracture zones and within‐plate (e.g., ocean island) settings. Analogous eruptives are not recognized in the Dun Mountain ophiolite or typical subduction‐related settings. Evolved Stage 2 basic lava flows and volcanic breccias show pronounced LREE‐depletion and high Zr/Nb ratios similar to N‐type mid‐ocean ridge basalts. Cross‐cutting dikes and irregular intrusions (Stage 3 magmas) that range from near‐primary basalts (Mg″ = 74; Al2O3/TiO2 = 20.9) to silicic plagiogranites, show decoupled rare earth element and high field strength element levels. They display evidence of extensive, polybaric, closed‐system fractionation adhering closely to geochemical trends of subduction‐related magmas. The composition of the most primitive dikes, and the presence of highly refractory magnesian Cr‐diopside and Cr‐spinel, indicate that Stage 3 magmas were derived by second‐stage melting of depleted upper mantle in a suprasubduction zone environment. Differences in mantle melting regimes (i.e., source composition plus/minus multiple melting) for the three magmatic episodes imply initial generation of the Croisilles and Patuki ophiolitic protoliths (Stage 1 and 2 magmas) in an ocean basin (most likely in a “leaky” transform‐related setting). Subsequent evolution (Stage 3 intrusives) took place in a forearc location after a time interval >50 Ma. The transition from oceanic to forearc regime could have occurred due to stepping‐out of a west dipping Benioff zone related to the Brook Street island arc, trapping a segment of oceanic lithosphere, and creating a tensional situation conducive to second‐stage mantle melting. The distinctive style of tectonic intrusion of Patuki ocean crust by subparallel bands of partially serpentinized ultramafic rocks (residual mantle after extraction of Stage 3 magmas) and formation of their metamorphic border zones (K‐Ar age 202–216 Ma) also occurred in response to this event.
Extensional collapse of orogens
Authors: Dewey J. F.
Published: February 2008

Lithospheric extension is sited, preferentially, along orogenic belts because they have a thicker continental crust, contain structural inhomogeneities, and suffer extensional orogenic collapse caused by body forces resulting from isostatically compensated elevation and sharp elevation gradients. Collapse occurs especially where rapid advective thinning of the shortened thermal boundary conduction layer occurs beneath an orogen and causes rapid uplift. Where boundary forces are compressional, extension is balanced by radial thrusting to form oroclinal loops around collapsed extensional basins. Where, as in the disruption of Pangea, boundary forces change rapidly from compressional to tensional, body force collapse is continued by general extension which may lead to continental splitting. Even where overall convergence is continuing, orogenic collapse may be enhanced by subduction rollback into small remnant oceans. The extensional collapse of orogens offers a partial explanation for why oceans cyclically close and reopen in roughly the same places, preservation of very high pressure metamorphic rocks, for the return of orogenic large crustal thicknesses to normal without very much erosional denudation with the widespread preservation of supracrustal sequences, high temperature metamorphic assemblages and the minimum‐melting granite suite.
A model for terrane accretion: Yukon‐Tanana and Slide Mountain Terranes, northwest North America
Authors: Hansen Vicki L.
Published: February 2008

To date, many North American Cordilleran geologists employing the concept of suspect terranes have emphasized differences in lithology, metamorphism, styles of deformation, and timing of these “distinct” tectonic units, arguing that each evolved separately from its neighbors. This practice of focusing on dissimilarity, and nomenclature used in terrane analysis, may be detrimental to our understanding of the evolutionary relationships between terranes. I present a model in which the Yukon‐Tanana (YT) and Slide Mountain (SM) terranes, which do exhibit certain lithotectonic disparities, are correlated within a unifying tectonic framework in which each terrane represents a different crustal level of the same late Paleozoic‐early Mesozoic magmatic arc and west dipping subduction complex formed during plate convergence outboard of western North America. The SM represents the upper crustal level of the arc and accretionary complex, and the heterogeneously metamorphosed and ductilely deformed YT represents the deeper crustal level. The Teslin suture zone (TSZ), the high‐P/T portion of the YT, formed the décollement between the YT‐SM composite terrane hanging wall and the descending North American plate; TSZ structural relations thus provide insight into the kinematic interaction of this composite terrane with North America prior to its late‐stage emplacement onto North America. The present‐day “collage” of suspect terranes clearly comprises a multitude of structurally dismembered and dissimilar lithotectonic assemblages. Nonetheless, it is possible to see order in the chaos, to reassemble the pieces in a tectonically meaningful fashion. The YT‐TSZ‐SM association in Yukon may serve as a model for tectonically coherent continental margin expansion.
Early Cretaceous near‐trench plutonism in southern Alaska: Atonalite‐Trondhjemite Intrusive Complex injected during ductile thrusting along the Border Ranges Fault System
Authors: Pavlis Terry L. , Monteverde Donald H. , Bowman John R. , Rubenstone James L. , Reason Michael D.
Published: February 2008

A group of leucotonalite to trondhjemite plutons in the western Chugach Mountains, Alaska, record a period of Early Cretaceous near‐trench plutonism along the paleo‐Alaskan/Aleutian subduction zone. The plutons were emplaced along the tectonic join between an older (Jurassic) arc massif and a younger (Cretaceous) melange terrane that was accreted by subduction beneath the crystalline massif. The plutonic rocks were injected during a major thrusting event that placed upper mantle(?) ultramafic masses atop the Cretaceous subduction assemblages. Deformation during this thrusting event apparently progressed from ductile to brittle conditions, and plutonic rocks were injected during both phases. Evidence for plutonism synchronous with ductile deformation includes boudinage or folding of leucotonalite dikes as well as a characteristic weak fabric within larger plutons. The latter is commonly associated with secondary mineral growth as well as deformation‐induced microstructures that suggest deformation under temperatures that decreased from lower amphibolite to subgreenschist facies conditions, conditions which are equivalent to the structural/metamorphic progression in adjacent schists. Paradoxically, however, the plutons locally crosscut low‐grade melange fabrics in the footwall as well as brittle fault networks within structurally higher parts of the crystalline hanging wall. The paradoxical field relationships are partially due to shuffling by early Tertiary(?) dextral strike‐slip faulting but are also indicative of plutonic emplacement over a period of time. The absolute age span of plutonism, however, need not have been long considering the syntectonic emplacement history. In particular, the deformation locally may have been melt‐dominated such that after initial injection of melts, the deformation was concentrated in the igneous masses. Lithologically, the plutons follow a low‐K series from diorite to muscovite‐garnet trondhjemite, a trend that is unusual in Phanerozoic terranes and is typically associated with Archean terranes. Moreover, the compositional trends are unlike the calc‐alkaline trend of near‐trench plutons studied by Hill et al. (1981). The tectonic mechanism for the near‐trench plutonic event is elusive, but shallow melting of amphibolite or metagreywacke along a young subduction zone provides the most reasonable explanations for the observations. However, an alternative model relating magmatic activity to a ridge‐trench encounter is also allowable. Nonetheless, in either model the genesis of the igneous rocks within a high‐temperature ductile thrust zone is atypical of modern subduction zones. Thus structural setting and lithologic association have many similarities to Archean systems, and the rocks may represent a young analog that ultimately may provide key petrogenetic constraints for the Archean system.