Showing papers in "Tectonics in 1985"

Cooling history of the NW Himalaya, Pakistan

Abstract: Fission-track and 40Ar/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.

Alleghenian reconstruction and subsequent evolution of the Gulf of Mexico, Bahamas, and Proto-Caribbean

Abstract: A detailed model for the evolution of the Gulf of Mexico, the Bahamas and the Proto-Caribbean is built within the framework provided by a detailed initial Alleghenian (western Pangean) reconstruction and an accurate subsequent relative-motion history between North America and Gondwana (northern Africa and South America). The Alleghenian reconstruction closes all pre-Jurassic oceans; accounts for Jurassic attenuation of continental crust by restoring that attenuation to original prerift continental thicknesses; incorporates an improved Equatorial Atlantic fit between northern Brazil and the Guinea margin of Africa; quantitatively removes changes in shape of northern South America due to Late Cretaceous and Cenozoic accretion and internal deformation; includes pre-Mesozoic continental crust presently underlying the western Bahamas and southern Florida; and correlates Late Paleozoic geology of Yucatan with its neighboring continental masses. Extension occurred within the Gulf of Mexico from Late Triassic to earliest Cretaceous time, but seafloor spreading was delayed until the Late Callovian. This divided a single Gulf-wide salt basin into the Louann and Campeche salt provinces. The Yucatan block progressively rotated about 43 degrees counterclockwise away from the Texas-Louisiana margin around a pole in northern Florida. The Tamaulipas-Golden Lane-Chiapas fault zone of eastern Mexico is interpreted as the remains of an initially intracontinental transform system along which Yucatan migrated. Attenuated continental crust beneath southern Florida and the western Bahamas, termed here the Florida Straits block, migrated approximately 300 km out of the eastern Gulf, approximately along Central Atlantic flow lines. These rotations are consistent with recently suggested magnetic anomaly trends in the Gulf of Mexico (Shepherd et al., 1982; S. Hall, personal communication, 1984). The Proto-Caribbean formed synchronously by a fan-like rotation of Yucatan away from Venezuela.

Gravity anomalies, flexure of the Indian Plate, and the structure, support and evolution of the Himalaya and Ganga Basin

Abstract: Bouguer gravity anomalies along four profiles across the Western Himalaya and Ganga Basin show large deviations from local isostatic equilibrium. A deficit of mass characterizes the Ganga Basin, and an excess underlies the Lesser Himalaya. Both can be understood if the Indian plate is flexed down by the distributed load of part of the mountains. The cross sectional shape of the Ganga Basin seems to be controlled by the deflection of the Indian plate, which we compute assuming the Indian plate to overlie an inviscid fluid. From the shapes of both Bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate. If the Ganga Basin is a steady state feature, then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin. If the rate of convergence of India and the Himalaya were constant, that rate should equal the distance divided by the corresponding age. We find a rate of 10 to 15 mm/a for the last 15 to 20 Ma, which is consistent with a large part of the 50 mm/a rate of convergence between India and Eurasia being absorbed by the eastward extrusion of parts of Tibet. Profiles of Bouguer gravity anomalies show only a small peak or plateau over the southern edge of the Lesser Himalaya, implying that the boundary between the light sediments of the Ganga Basin and the heavier crustal rocks of the Lesser Himalaya is not sharp and that there exists some light material beneath the range. We infer that some sediment deposited in the Ganga Basin has been underthrust beneath the Lesser Himalaya, but the quantity is small; most of this sediment probably is scraped off the Indian plate to make the foothills of the range. We find that the load of the High Himalaya is too large to be supported solely by elastic stress in the Indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate. The observed gradient in Bouguer gravity anomalies increases from an average of about 1 mGal/km over the Ganga Basin and Lesser Himalaya to about 2 mGal/km over the High Himalaya. This increase in the gravity gradient implies that the Moho dips more steeply (10°–15°) beneath the High Himalaya than beneath the Lesser Himalaya (2°–3°). We interpret this steepening of the Moho to be due to a weakening of the plate, which allows it to bend more sharply beneath the High Himalaya than farther south. With the inclusion of a weak segment of Indian plate beneath the High Himalaya, calculated anomalies show a somewhat increased gradient beneath the High Himalaya, but when the weight of the entire Himalaya is used as a load on the plate, calculated anomalies are more negative than observed. Therefore an external force system is needed to support much of the weight of the High Himalaya, as well as to bend the plate sufficiently beneath the High Himalaya. The magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of India's mantle lithosphere, stripped of its crust and underthrust beneath southern Tibet.

The Najd Fault System, Saudi Arabia and Egypt: A Late Precambrian rift‐related transform system?

Abstract: The Najd Fault System is a complex set of left-lateral strike-slip faults and ductile shear zones that strike NW-SE across the Precambrian of Arabia and Egypt. This system was developed during the interval 540–620 Ma. It is up to 400 km wide with an exposed length of 1100 km; inferred buried extensions of the Najd give it a total length of 2000 km. It is the best exposed and may be the largest pre-Mesozoic zone of transcurrent faulting on earth. Previous models for the Najd Fault System suggest it formed as a result of a major Late Precambrian continent-continent collision. This model is not preferred here because (1) the lack of evidence for a pre-Late Precambrian continent to the east of the Najd Fault System; (2) the difference between the orientation of the Najd Fault System and that predicted by slip-line theory; (3) the younger age of Najd movements compared with that of collisional sutures in the Arabian Shield; and (4) lack of evidence for wide-spread crustal uplift that would be expected to accompany collision. A new model for the origin of the Najd Fault System accounts for each of these objections: The Najd Fault System formed in response to a broad zone of NW-SE directed crustal extension that accompanied juvenile continental crustal formation in northernmost Afro-Arabia. This model also accounts for the following observations: (1) Strands of the Najd parallel the direction of extension in the North Eastern Desert of Egypt and Sinai; (2) the timing of the principal rifting movements (ca. 575–600 Ma) overlap with those of the Najd (ca. 560–620 Ma); (3) in spite of observation (2), the Najd Fault System is not recognized in northernmost Afro-Arabia; instead the Najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached. The Najd Fault System thus represents a set of continental transforms developed in response to a major episode of Late Precambrian extensional continental crust formation in northernmost Afro-Arabia.

Geological manifestations of ridge collision: Evidence from the Golfo de Penas-Taitao Basin, southern Chile

Abstract: Recent geological and geophysical studies in the southern Andes adjacent to the intersection of the Chile Rise with the Peru-Chile Trench (ANT-NAZ-SAM triple junction) have revealed a number of features and a Neogene geologic history that are unique along the Pacific margin of South America. This history includes (1) development of a Tertiary-Quaternary marine basin with up to 3 km of sediment infill (Golfo de Penas-Taitao basin, GTB), (2) disruption of the region by a series of faults with both normal and strike slip movements, and (3) localization of silicic, near-trench volcanism and epizonal plutonism and related hydrothermal activity. The northern portion of the GTB began to subside in the Late Miocene (possibly earlier), and has subsequently been deformed, uplifted, and exposed. Gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today (Golfo de Penas). Subsidence and uplift have thus occurred diachronously in the region, although it is unclear when subsidence began in the Golfo de Penas. Tectonic disruption of the region is likely related to the Liquine-Ofqui fault (LOF), a major, NS-trending, crustal shear zone that curves westward and terminates in the Golfo de Penas. The LOF has both down-to-the-west and right lateral offset and separates the main Andean Cordillera on the east from a large crustal block (the Chiloe block) on the west. We hypothesize that the GTB has developed as a pull-apart basin in response to northward movement of the Chiloe block along the LOF. We propose a dynamic model whereby a stress gradient that decreases longitudinally away from the Chile Rise/Peru-Chile Trench intersection is set up because the youngest, most buoyant, oceanic lithosphere is being subducted at the triple junction. The Chile Rise is viewed as a type of indenter which is acting to drive the Chiloe block northward in front of the northward-migrating triple junction. This model explains the unique set of geologic features found in the region, and suggests that ridge-trench interactions may be an important factor in orogenesis at active continental margins.

Quaternary normal and reverse faulting and the state of stress in the central Andes of south Peru

Abstract: 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.

Origin, damping, and pattern of development of faults in granite

Abstract: Observations of brittle fractures in the granite of the Massif de la Borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults. It seems that all faults originate by shear along a preexisting plane which can be an important fracture (joint, former fault) or a microflaw (pore, grain boundary). The shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault. We also found an arrangement of “horsetail” fractures damping the movements on the faults. We then propose a pattern for the development of the faults. They originate on preexisting plane and propagate by initiating en echelon cracks. The movements are damped by horsetails which permit, by faults connection in relays, the creation of larger faults.

The North China Basin: An example of a Cenozoic rifted intraplate basin

Abstract: The North China basin was a stable continental region that has undergone several distinct phases of rifting and subsidence during the Mesozoic and Cenozoic eras. The last major phase, featuring block faulting, rapid subsidence and widespread calc-alkaline basaltic volcanism, began during early Tertiary time. This intraplate rifting appears to be the result of an approximately 30% extension and thinning of the lithosphere beneath the northeastern China. By the late Tertiary period, active rifting had slowed and postrift thermal subsidence had begun. This regional extension and subsidence resulted in the present day North China basin, a large saucer-shaped, oil producing basin with thinner crust and lower upper mantle P and S wave velocities than the surrounding regions. During the Quaternary period, however, the subsidence rate increased in the North China basin. The frequent occurrence of destructive earthquakes and observations of relatively high heat flow suggest that the Quaternary tectonic activity differs from the late Tertiary simple thermal subsidence pattern and may indicate a new phase of rifting.

Uplift of Tibetan Plateau

Abstract: A history of the elevation and crustal thickness of the Tibetan Plateau since the continental collision at 45 Ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume. In our reconstructions, the Indochina block was attached to the southern margin of Eurasia before being “extruded” to the southeast as evidenced by the opening of the South China Sea. This opening affected the mass input into Tibet so that Tibet did not uplift to an elevation higher than 1 km until early Miocene (when spreading of the South China Sea ceased). The model then gives a steady rise of Tibet from Middle Miocene to its present elevation.

Evolution of a turbiditic foreland basin and analogy with an accretionary prism: Example of the Eocene South‐Pyrenean Basin

Abstract: The South-Pyrenean Tertiary Basin was an east-west trending foreland trough which developed from Early Eocene to Miocene times in a north-south trending compressive regime. A turbiditic series of Lower and Middle Eocene age (Hecho Group) enables us to define the typical features of the tectono-sedimentary evolution of the basin: (1) slope and shelf derived carbonate turbidites and megaturbidites bear witness to the seismic activity of the basin borders, (2) the southern basin border, evidenced by the onlap of the turbidites onto slope and shelf deposits, migrated progressively southwards in time and (3) the deformation pattern is characterized in the Meso-Cenozoic sediments by a complex thrust-fold system of thin-skinned tectonics style; its specific geometry and kinematics were influenced by the pre-deformation geometry (onlap). This thrust-fold system is related to south-directed basement nappes, and its front advanced progressively southwards in time. From this data, we infer an evolutionary model for this highly mobile basin: it was a flexural basin linked to the northward bending of the lithosphere in the foreland of a south-directed crustal accretionary prism; as the nappes advanced, the basin migrated progressively southwards, in the foreland direction. As for the actual basin evolution, many analogies with accretionary prism-trench systems of active margins exist.

Petrologic constraints on imbrication, metamorphism, and uplift in the SW Tauern Window, eastern Alps

Abstract: The pressure-temperature-time-deformation (PTtd) history of two tectonic units in the southwest Tauern Window (Austria/Italy) is evaluated from P-T paths of metamorphism and uplift, geochronologic data, and observations on fabric development. Petrologic data indicate burial of the Lower Schieferhulle (LSH) to depths of ≥ 35 km and the adjacent Upper Schieferhulle (USH) to only 24–28 km; the units are now separated by only 2 km of section. Metamorphism was postkinematic in the LSH and synkinematic in the USH. Correlation of P-T path reversals calculated from zoned garnets in the two units, however, indicates that juxtaposition of the LSH and USH occurred prior to metamorphism. Temperature-time and depth-time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface. Both the petrologic data and the depth-time profiles for the LSH and USH point to a history of differential uplift in the early stages of decompression. Textural features indicate that the LSH behaved as an essentially rigid body during uplift, whereas the USH experienced extensive ductile shearing. The PTtd data for the LSH and USH are incorporated into a model for continental collision in the Eastern Alps. Initial tectonic uplift of the LSH was accompanied by ductile thinning of the USH and overlying Austroalpine units. Final uplift and erosion affected all of the units “en masse”.

Thermal evolution of convergent plate margins: Thermal modeling and reevaluation of isotopic AR‐ages for Blueschists in the Franciscan Complex of California

Abstract: Modeling of heat flow through a wedge-shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of Ar-isotopic ages and metamorphic progressions across blueschist belts. If convergence rates are maintained at rates of a few centimeters per year or more, material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where Ar-loss by diffusion and recrystallization effectively ceases. Maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider. Because of this difference in thermal history, K-Ar and 40Ar/39Ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth. Application of this thermal modeling to the Franciscan subduction complex of northern California indicates that the coherent blueschists in the Eastern Belt (typically with 115 to 125 m.y. Ar-isotopic ages) and the high-grade blueschist blocks in the mud matrix melanges of the western portion of the Central Belt (typically with 140 to 150 m.y. Ar-isotopic ages) could both have formed during a single, prolonged period of continuous convergence.

Tectonics of slow spreading mid-ocean ridges and consequences of a variable depth to the brittle/ductile transition

Abstract: Geophysical evidence, especially microearthquakes that extend into the upper mantle beneath the inner floor of the Mid-Atlantic and Gorda Ridges, indicates that the axial magma chamber episodically freezes beneath slow spreading ridges. Freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences. Most importantly, large near-axis faults will intersect as the brittle/ductile transition deepens, causing some of the faults to become locked. The topographic effect of fault locking will be a variation in the width of the inner floor from narrow ( 10 km) when a magma chamber is absent. The deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle. The nature of these faults at depth is unknown, but the following hypotheses are presented: (1) the faults may be planar and pass downward into aseismic ductile shear zones, and (2) the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow. Such low-angle normal faults are common in continental extensional domains. Large-scale tilting (50°–70°) is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks (similar to toppling dominoes). Calculations of temperatures at the brittle/ductile transition are made assuming olivine and diabase rheologies for the mantle and crust, respectively. The temperature is approximately 800°C when the transition is at 8 km depth, and approximately 600° when the transition is at 2 km depth. The strength of the lithosphere varies dramatically with the rise and fall of the brittle/ductile transition as magma chambers form and subsequently freeze. Strong lateral temperature gradients will be present when a magma chamber is formed, resulting in localization of faulting within thin weak crust above the chamber. When the magma chamber freezes, lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area; this results in active faulting occurring over a width of as much as 60 km. Some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges. The entire crustal section of the Josephine ophiolite was tilted by approximately 50° prior to deposition of overlying pelagic sediments. This tilting probably took place by faulting, suggesting that oceanic faults extended into the upper mantle. Possible oceanic faults have been identified in the Josephine ophiolite, but their oceanic origin cannot be established beyond doubt.

Cenozoic collision of the Lesser Antilles Arc and continental South America and the origin of the El Pilar Fault

Abstract: It is proposed that the Cenozoic tectonic record of the southern Lesser Antilles arc and northeastern continental South America can be explained by ongoing right-oblique collision between the arc and continent. The collision has proceeded by the transport of the leading edge of the arc across the slope and outer shelf of a former north facing passive margin of the South American continent. The overriding began in the study region near the Gulf of Cariaco in eastern Venezuela in late Eocene or Oligocene time and has migrated with a generally SE vector. Suturing has occurred between the arc and continent after the attainment of a critical distance of overlap; today's point of suturing lies in the Paria Peninsula. East of there, overriding continues. Major tectonic elements engaged in or created by the collision are the southern Lesser Antilles magmatic arc, forearc basin, the Araya-Tobago terrane, a South American foreland thrust and fold belt, and a foreland basin. The Araya-Tobago terrane is thought to consist of sediments of South American provenance that were accreted to the Lesser Antilles forearc during its transit of an ocean basin and the continental slope and outer shelf. The emplacement of the magmatic arc and the Araya-Tobago terrane caused tectonic imbrication of shelf strata to propagate ahead of the arc front as a foreland thrust and fold belt. Tectonic loading of the shelf also caused subsidence of a major foreland basin on the continentward side of the thrust belt. It is proposed the El Pilar fault exists between the Gulf of Cariaco and the Paria Peninsula as an active right slip fault but not east of Paria. It is not a throughgoing transform fault between the South American and Caribbean plates. The El Pilar fault exists where the overlapping arc and the continent are sutured and takes up a suture-parallel component of convergence between arc and continent. The eastern tip of the fault propagates east with the point of suturing. Reconstructions of the Cenozoic collision of the Lesser Antilles arc and the South American continent suggest that the arc lay somewhat north and west of its present position in the Eocene. This conclusion differs from that of plate reconstructions that assume that the arc was the leading edge of a Caribbean plate that has moved east from Pacific longitudes since the Eocene.

Structural discordance between neogene detachments and frontal sevier thrusts, central Mormon Mountains, southern Nevada

Abstract: Detailed geologic mapping in the Mormon Mountains of southern Nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens. A newly discovered, WSW-directed low-angle normal fault, the Mormon Peak detachment, juxtaposes the highest levels of the frontal most part of the east-vergent, Mesozoic Sevier thrust belt with autochthonous crystalline basement. Palinspastic analysis suggests that the detachment initially dipped 20–25° to the west and cut discordantly across thrust faults. Nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross-section through the thrust belt in the footwall, while highly attenuated remnants of the hanging wall (nowhere more than a few hundred meters thick) structurally veneer the range. The present arched configuration of the detachment resulted in part from progressive “domino-style” rotation of a few degrees while it was active, but is largely due to rotation on younger, structurally lower, basement-penetrating normal faults that initiated at high-angle. The geometry and kinematics of normal faulting in the Mormon Mountains suggest that pre-existing thrust planes are not required for the initiation of low-angle normal faults, and even where closely overlapped by extensional tectonism, need not function as a primary control of detachment geometry. Caution must thus be exercised in interpreting low-angle normal faults of uncertain tectonic heritage such as those seen in the COCORP west-central Utah and BIRP's MOIST deep-reflection profiles. Although thrust fault reactivation has reasonably been shown to be the origin of a very few low-angle normal faults, our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be. We conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis, it may never be assumed.

Geometry of Cenozoic extensional faulting: Dixie Valley, Nevada

Abstract: Precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of Basin and Range Cenozoic extensional faulting at depth. Previous studies have attempted to identify simple horsts and grabens, tilted crustal blocks with planar faulting, or tilted crustal blocks with listric faulting in the shallow crust. Normal faults defining these crustal blocks may root (1) individually in the ductile lower crust, (2) in regional or local low-angle detachment faults, or (3) in igneous intrusions or decoupling surfaces produced by the intrusions. The present study, in Dixie Valley, west-central Nevada, makes use of a seismic reflection survey, gravity models, seismograms from earthquakes occurring on December 16, 1954, and geometrical block models. These data show a structurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step-faulted floor to the southeast. The northwest bounding fault is moderately dipping (50°) and planar to a depth of 3 km. The southeast boundary is step-faulted, and altogether the faults indicate an extension of 20% across the valley at the rate of 0.38 mm/y for the last 8 my. Synthetic earthquake seismograms confirm a focal depth of 15 km and fault dip of 62° for the Fairview Peak earthquake and suggest that the focal depth of the Dixie Valley earthquake was also 15 km instead of the previously reported 40 km. Local microearthquakes cluster around 10–15 km. The geometrical block models indicate that crustal horst-graben faulting and planar, high-angle normal faults rooted in a low-angle detachment surface do not readily account for development of the subsidiary (step) faults found in Dixie Valley. Extension of the crust by intrusion may develop high-angle faults and, with further intrusion, may develop the subsidiary faults and produce a complex, sagged, asymmetric graben like Dixie Valley.

Structure of the Xigaze Ophiolite, Yarlung Zangbo Suture Zone, southern Tibet, China: Genetic implications

Abstract: 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.

Neogene block tectonics of eastern Turkey and northern South America: Continental applications of the finite difference method

Abstract: Continental plate boundary zones are, generally, wide diffuse zones of deformation within which, in the upper 5 to 15 km, may be recognized blocks, which are bounded by fault zones within which strain is highly concentrated and along which slip rates are fairly high (2–30 m Ma−1). These blocks may be irregular flakes, defined by older crustal inhomogeneities of an upper brittle crust below which strain occurs in a ductile and more homogeneous lower crust. With examples from the convergent plate boundary zone of eastern Turkey and the southern Caribbean plate boundary zone of northern South America, we use plate slip rates and trends and slip rates of faults to construct block vector diagrams to deduce the sums of relative motion among block mosaics, slip rates and trends of ill defined block boundaries, and the extent to which blocks are internally strained.

Intraplate deformation by block rotation and mesostructures along the Dead Sea Transform, northern Israel

Abstract: Combined analysis of mega- and meso-structures yield the strain, inferred stress and tectonic history of the margin of the Sinai sub-plate in northern Israel, along the Dead Sea transform. Three major stress regimes were developed and have affected the region since the middle Miocene: one is regional and controlled the left-lateral shear along the Dead Sea transform. This one has been active since post-early Miocene, is characterized by σ1 trending c. 345° and deformed the plate margin by horizontal shear and contemporaneous block rotation. This style of deformation is similar to simple shear. The second one was local and may be subdivided into distinct stress and strain domains. It was characterized by σ1 trending E-W and σ3 trending N-;S, which produced a conjugate set of strike-slip faults. The region underwent N-S elongation by horizontal shear and block rotation similar to the pure shear mechanism. This stress regime was active in late Miocene to early Pliocene time. The third and youngest stress regime is extentional with σ1 vertical and σ3 oriented N-S. This one has been active since post-middle Pliocene time producing E-W normal faults, and minor N-S elongation. The second and third stress regimes show local distribution and developed as a consequence of the bent geometry of the transform vis-a-vis the plate-slip vector. Finite element and photoelastic models predict the development of local stress fields and elongation of the plate margin parallel and subparallel to the transform plate boundary. The strain of the plate margin in northern Israel and southern Lebanon indicate remarkable elongation parallel to the Dead Sea transform by a factor of 1.5–2.1 since the Miocene. This process of intraplate deformation and the consequent strain is unique to the parts of the plate margin which are adjacent to the bent segment of the transform.

The Hidaka Shear Zone (Hokkaido, Japan): Genesis during a right‐lateral strike‐slip movement

Abstract: The Mesozoic structure of Hokkaido, which is characterized by a wide spread ophiolite sequence unconformably overlain by a Cretaceous flysch sequence, is cut by a north-south trending metamorphic belt, the Hidaka Belt. This contribution aims to demonstrate that the Hidaka Belt is a wide right-lateral strike-slip shear zone. The movement is contemporaneous with the metamorphism and the magmatism of Oligo-Miocene time.

Paleomagnetic results from Honduras and their bearing on Caribbean tectonics

Abstract: Paleomagnetic results have been obtained from 37 sites in central and western Honduras and one site in north-western Nicaragua. The rocks are limestones, sandstones, and granites and range in age from 140 to 60 m.y. Regional fold-tests demonstrate that the rocks carry a prefolding (pre-Laramide) magnetization. The data indicate a very large clockwise rotation of the sampling area during the Early Cretaceous followed by a counter-clockwise rotation in the Late Cretaceous. The changes in latitude are minor during this time period. It is highly unlikely that these large rotations apply to the entire Caribbean plate. Rather, they reflect the motion of a smaller area known as the Chortis Block. This interpretation is supported by paleomagnetic data from neighboring areas. The results also suggest that the Chortis Block sutured onto southern Mexico/Guatemala in Latest Cretaceous time in agreement with geochemical data and several plate tectonic models. These new data together with published paleomagnetic data from around the Caribbean imply that a rigid Caribbean plate did not exist until well into the Tertiary.

Strain patterns in models of spreading-gliding Nappes

Abstract: Three experiments have been carried out in order to study the progressive and finite strain in nappes where gravitational spreading and gliding occur together. These experimental models were made from analogue materials able to collapse under their own weight on an inclined plane. Throughout most of the models the stretch trajectories in a vertical plane parallel to the flow show a low-amplitude sigmoidal pattern consistent with a previous theoretical model. Other stretch trajectories exist at the front and back ends of each model. The kinematic significance of all these trajectories is discussed in detail. Highest strain intensities are always found next to the base of the model. Strain paths calculated at different stages of flow indicate clearly that nowhere in the model is the motion simple. There is a complex combination in time and space of simple and pure shear, except at the base of the model where the motion approximates a simple shearing. Near the two parallel lateral sides of the model a lateral boundary effect can be observed from the strong curvature of the transverse markers (surface grid and vertical layers). Strain paths have also been estimated in this complex zone where wrench shearing is superimposed upon vertical shortening and thrust shearing. The geological implications of these experimental models are discussed in relation to recent theoretical studies and field work.

The Cenozoic evolution of the North China Basin

Abstract: The Cenozoic evolution of the North China basin consists of early Tertiary rifting and differential subsidence, late Tertiary regional subsidence and Quaternary rifting and differential subsidence. Present Cenozoic sediment fill shows that Tertiary subsidence was greatest where the present thickness of pre-Cenozoic crust is least. In contrast, Quaternary subsidence has been greatest where the present thickness of pre-Cenozoic crust is greatest. The present crustal structure, geologic history and Cenozoic sediment fill of the basin suggest a nearly constant crustal thickness of ∼40 km at the onset of rifting (middle Eocene). If the present thickness of pre-Cenozoic crust is due solely to crustal thinning from early Tertiary mechanical extension then the central North China basin has been extended at least 30% of its initial width. This is greater than estimates of total Mesozoic plus Cenozoic extension derived from basement faulting, ∼20%. At the present time any possible discrepancies cannot be resolved due to uncertainties in the data. Tertiary subsidence of the basin, derived from backs tripping the present sedimentary sequence, is not satisfactorily explained by either a simple uniform or two-layer extensional model. No single extension parameter (uniform) or pair of extension parameters (two-layer) can account for the variable sediment thicknesses or the alternating horst and graben configuration of the basin. We can, however, use the parameters derived from simple one- or two-layer extensional modeling to infer lower bounds for both the average regional extension of the Bohai and the local extension of the depressions within the basin. For example, we find that the Neogene subsidence of a major part of the basin, the central Bozhong depression, has been nearly as rapid as that of young oceanic crust. Furthermore the present heat flow in this area is consistent with that for oceanic crust of 16–24 million years of age (post rift time). This indicates that the subsidence may be due to decay of a lithospheric thermal anomaly that is nearly as large as that in young oceanic crust. This is difficult to understand with a simple extensional framework because reported estimates of early Tertiary extension from basement faulting are only 10–30%. Possible sources of this excess heat are Mesozoic tectonics, effects of plate subduction at the Pacific margin, or energy input from regional tectonics during the early Tertiary. Our conclusions cast doubt upon the adequacy of simple extensional models to explain the Cenozoic evolution of the North China basin; however these conclusions can only be tentative due to inadequacies in the basic data set.

Paleomagnetic results for Eocene volcanic rocks from northeastern Washington and the tertiary tectonics of the Pacific Northwest

Abstract: 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.

Uplift, faulting, seismicity, thermal spring and possible incipient volcanic activity in the Lesotho-Natal Region, SE Africa: The Quathlamba Hotspot Hypothesis

Abstract: Various features of the Lesotho-Natal region of south-eastern Africa, namely, its anomalously high topographic elevation, the occurrence of numerous thermal springs, a few enigmatic CO2 gas exhalation sites, and a significant level of current seismicity in certain zones, suggest that there is some neotectonic activity here. In 1983, there were also media reports of a small volcanic eruption in Lesotho. It has been suggested that a process of continental margin-parallel warping was in operation during the Plio-Pleistocene period to produce the apparent uplift and seaward tilting of older geomorphological land-surfaces, but the fundamental geophysical cause of this process remained unclear. This “cymatogeny” or anorogenic plateau uplift has recently been related to absolute motion of the African continent over the former position of an oceanic spreading ridge but the hotspot reference frame was not used in the absolute motion modelling. As an alternative hypothesis for the Lesotho-Natal phenomena, the existence of a mantle hotspot near 30°S, 29°E is invoked, its Cenozoic track is modelled, and the ages of a chain of volcanic seamounts in the Mozambique Basin are predicted. The results appear to confirm the self-consistency of the hotspot frame as a preferred reference system for lithospheric absolute motion.

a Paleozoic suture in eastern Gondwanaland

Abstract: A new tectonic interpretation of southeastern Australia, Tasmania, and north Victoria Land, Antarctica, is given, in which a major N-S trending lineament, representing a suture zone, is recognized separating essentially shallow water shelf and continental deposits of latest Cambrian or Early Ordovician age from deeper water Ordovician flysch deposits. Lateral offset of this lineament, which is interpreted as a suture along which thrust telescoping and major strike-slip faulting occurred, could adequately explain the present distribution of major rock types in the three areas and also removes a number of problems previously encountered in correlation.

Transtensive tectonics in the Strait of Sicily: Structural and volcanological evidence from the Island of Pantelleria

Abstract: A detailed structural analysis carried out on the volcanic products of the island of Pantelleria has shown that the large-scale setting is dominated by N-S trending normal faults and NW-SE right-lateral strike-slip faults, while mesostructural data show a more complex pattern which is characterized by four systems of discontinuity. The volcanological features show a marked difference between a north western sector characterized by products of a basaltic nature and a south central one in which evolved products connected with the presence of a magmatic chamber are present. The present-day stress field, as derived from structural analysis, is characterized by a maximum compression oriented NNW-SSE which allowed the activation of right-lateral strike-slip faults bordering the Pantelleria trough giving rise to a thinning of the underlyng crust. Inside this main dextral shear zone it is possible that the development of pull-apart basins bounded by N-S trending normal faults could generate further thinning of the already thinned out crust zone and give rise to the magmatic phenomena known to occur mostly along the N-S trending extension structures. Such a model would therefore help to understand the characters of the volcanism associated with a zone of continental collision as well as its distribution, which is not rigorously in accordance with a classical rifting process.

Middle Cambrian paleomagnetism of the Avalon Terrane in Cape Breton Island, Nova Scotia

Abstract: Interbedded volcanics and sediments of the Bourinot Group (Mid-Cambrian) in central Cape Breton Island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the Avalon terrane with respect to the North American craton. Both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations. Dual-polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of D=293°, I=−66° (N=9, k=41.8, α95=8.1°). The sediments reveal, upon detailed thermal demagnetization, complex multivectorial remanences. However, a few samples show characteristic components and a large number of samples (38) have intersecting great-circle remagnetization trajectories; when combined, this yields a mean direction of D=267°, I=−65° (k=16, mean radius of error ellipse is 8.5°). The mean directions for the Bourinot rocks correspond to a paleolatitude of 49°S±11°, which is different from the range of possible values for the predicted Mid-Cambrian paleolatitudes for Nova Scotia (29°S to 18°N), inferred from the North American apparent polar wander path. This discrepancy between predicted paleolatitudes and the observed paleolatitude implies that Nova Scotia was not part of North America during the Mid-Cambrian, but rather, they were separated by an ocean with a minimum width of 1000 km (20°±10°). This interpretation of the relative positions of Nova Scotia and North America based on paleomagnetism is consistent with the geological record in the northern Appalachians and supports previous interpretations of the dissimilar Early Paleozoic faunas in Avalon and North America. It is paleomagnetically quite plausible that the Avalon terrane formed part of a larger continent, comprising Africa (Gondwana) and Armorica during the Cambrian.

east‐trending dextral faults in the western Great Basin: An explanation for anomalous trends of pre‐Cenozoic strata and Cenozoic faults

Abstract: Two postulated major east trending strike-slip fault zones, here named the Coaldale and Excelsior fault zones, are delineated mainly on the basis of faults in Cenozoic rocks in eastern California and western Nevada and coincide with apparent major right-lateral offsets in the distribution and facies of pre-Cenozoic rocks. Apparent right-lateral offset of pre-Cenozoic rocks on the Coaldale fault zone is 60 to 80 km and on the Excelsior fault zone is 45 to 55 km. This offset accounts for major disruption in trends of pre-Cenozoic rocks in eastern California and western Nevada, a disruption previously interpreted as the result of either an originally curving continental margin or tectonic distortion of originally linear trends by large-scale bending (oroflexural folding) or by crustal-scale folding related to north-east-southwest compression. Main offset on the Coaldale and Excelsior fault zones appears to be late Mesozoic in age. A pre-mid-Cretaceous age is indicated because mid-Cretaceous and younger plutonic rocks are not cut by major strike-slip faults on line with the fault zones. Major movement is probably younger than the Dunlap Formation, which is dated paleontologically as Early Jurassic but which may also contain rocks as young as Cretaceous. Local reactivation of the faults in the Cenozoic, perhaps under a different stress regime, accounts for offset of Cenozoic rocks. The Coaldale and Excelsior fault zones terminate major northwest trending late Cenozoic and possibly older faults. The Owens Valley-White Mountain fault system in eastern California and the Furnace Creek fault zone in southeastern California and westernmost Nevada terminate northward at the Coaldale fault zone. A system of south-east trending right-lateral and high-angle faults in western Nevada, including the Bettles Well fault, terminate southward at the Excelsior fault zone. Major movement on northwest trending faults such as the Furnace Creek and Bettles Well fault may be mostly late Cenozoic in age, but present information does not preclude the possibility that these fault zones initiated prior to, and are offset by, the Coaldale and Excelsior fault system. The unusual trends of pre-Tertiary rocks and of the Coaldale and Excelsior fault zones in eastern California and western Nevada appear to be restricted to the Walker Lane belt, a broad northwest trending structural zone characterized by right-lateral shear in western Nevada and eastern California. Possibly initial shear produced the northwest trending faults, and a later “kink” in this system produced oroflexural folding and the east-trending right-lateral Coaldale and Excelsior fault zones. This “kink” may have resulted from a change in the stress regime and structural complexities where the shear zone obliquely crossed the Paleozoic continental margin.

Rheological controls on the geometry of deep faults and the tectonic delamination of the continental crust

Abstract: Recent seismic reflection profiles indicate that the deep faults in the continental crust may decouple deep within the crust or at the Moho. In the contribution it is argued that the decoupling horizons were initially zones across which variations in flow strength occur due to mineralogical differences in the principal rock types. These can result in the tectonic delamination of the crust and at the Moho. Calculations reveal marked strength differences across the Moho. Once initial delamination occurs, weak planes of mylonite develop along such surfaces and are preferentially reactivated during any subsequent deformation. It is postulated that during tectonism, the continental crust (1) does not act, at depth, as a single rigid entity, (2) should not be considered as being firmly attached to the upper mantle and (3) is capable of moving independantly of the upper mantle.