Extension Of Upper Crust Research Articles

Lower crustal thinning in the Rhinegraben: Implications for recent rifting

Seismic near‐vertical and undershooting experiments carried out in the southern upper Rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties. Significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings. A 12‐ to 14‐km‐thick reflective lower crust beneath a 15‐km‐thick transparent upper crust of the eastern graben shoulder (Black Forest) probably originated during Permo‐Carboniferous reequilibration of thickened Variscan orogenic crust. Thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder (the Dinkelberg block) is interpreted to be related to Cenozoic extensional faulting. A discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension. Congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust. In the transitional Dinkelberg block, such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity; it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area.

Seismic stratigraphy and tectonic development of Virgin River depression and associated basins, southeastern Nevada and northwestern Arizona

Virgin River depression is a Neogene basin, with a surface area that exceeds 1,500 km 2 in the Basin and Range structural province of south-eastern Nevada and northwestern Arizona. The depression formed within the foreland of the Sevier orogenic zone, a region that was characterized in Paleogene time by a flat-lying section of Cambrian to Cretaceous platform strata about 5 km thick. Well data from Mobil Virgin 1A on Mormon Mesa reveal 2,000+ m of Neogene basin fill that consists mostly of the Muddy Creek Formation (4-10 Ma), the red sandstone unit (10-12 Ma) of Bohannon (1984), and the Lovell Wash Member of the Horse Spring Formation (12-13 Ma). Seismic reflection data from six primacord and two vibroseis lines show that the Muddy Creek Formation uniformly fills Virgin River depression to a depth of 1-2 km. Two older and less-extensive basins, the Mormon and Mesquite, lie beneath the Muddy Creek and are separated from one another by a complex buried ridge. The basins are mostly filled with rocks of the red sandstone unit and Lovell Wash Member to depths locally exceeding 6 km. Two older members of the Horse Spring Formation, the Rainbow Gardens and Thumb, also occur in Mormon basin (the western one), where they rest disconformably on the pre-Tertiary strata. The basins are east-tilted half grabens that are bounded on the east and southeast by large listric normal fault systems. The faults that bound Mormon basin are buried by the Muddy Creek Formation, but the Piedmont fault, on the east side of Mesquite basin, cuts Quaternary alluvium. The Virgin River depression formed in three stages. The period from 24-13 Ma is characterized by slow subsidence in Mormon basin and little noticeable deformation of the basin substrate. The Mormon and Mesquite basins became fully differentiated during the period from 13-10 Ma. This stage is associated with large displacements on the normal faults bounding both basins and the buried ridge. Proterozoic crystalline rocks were exposed locally, providing a source for part of the red sandstone unit deposited in the basins. Tectonic denudation during the 13-10 Ma stage locally removed large amounts of the pre-basin section. By 10 Ma, most of the fault activity had ceased, the ridge between the basins was overlapped, and Virgin River depression began to subside uniformly over a wide area. This stage lasted until the commencement of the modern period of dissection associated with the Colorado River. Our structural analysis suggests that upper crustal extension within the basin, mostly during the 13-10 Ma stage, might have exceeded 60%. The basin subsidence was partly due to extension in the upper crust and partly due to viscous flow in the deeper crust beneath the basin. It is not clear to us what caused the uplifts that flank the depression, but isostatic rebound due to tectonic denudation remains aviable possibility.

Space-time patterns and tectonic controls of Tertiary extension and magmatism in the Great Basin of the western United States

Structural and stratigraphic relations in the Great Basin indicate widespread pre-middle Miocene crustal extension that appears to define two north-trending belts. Most extension in these belts was Oligocene age, but locally it began earlier or lasted into early Miocene time. The eastern belt straddles the Nevada-Utah border and includes the Snake Range, Nevada, area, with its southern end near 37.5°N and its western edge at the Seaman-Butte Mountains breakaway. The southern boundary of the eastern belt is occupied by the 26-15 Ma Caliente caldera complex and approximately coincides with the present east-west-trending margin of the Great Basin north of Saint George, Utah. Crust north of this boundary extended approximately east-west before volcanism began at 30-32 Ma, but to the south, extension began after about 15 Ma. This boundary may have been a rooted zone of left-slip faults that allowed the footwall of the Stampede detachment to move west relative to unextended terrain to the south. The eastern margin of the eastern belt is probably located near the present eastern edge of the Great Basin, but its northern end is poorly defined. The western belt runs from the Funeral and Grapevine Mountains, California, to the Ruby Mountains, Nevada, and north-northeast to the Albion Range, Idaho. Tens of kilometers of crustal extension occurred at least locally in both belts, but magnitude of extension is poorly known for large areas of each. Tertiary volcanism in the Great Basin began in the north in Eocene time with predominantly effusive volcanism and swept southward, ending with voluminous Oligocene-Miocene ignimbrite eruptions from calderas in an irregular, discontinuous belt between Marysvale, Utah, and Reno, Nevada. A result of the south-ward migration of volcanism is that the onset of extension in both belts was syn- or post-volcanic in the north but was pre-volcanic in the south. Late Paleogene extension and crustal magmatism coincided in both time and space only locally, where south-migrating magmatism overlapped active north-south-trending extensional belts. Most calderas in the southern Great Basin formed in previously extended belts or on their margins. Southward migration of ignimbrite sources was apparently blocked by unextended crust to the south. In contrast, volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to, or during, crustal extension. This is consistent with observations in the southern Basin and Range, where volcanism and crustal extension were generally synchronous, and volcanism was dominantly effusive. Thus, caldera formation may be controlled by the distribution of upper-crustal extension, although the physical mechanism for this control remains speculative. The space-time patterns of late Paleogene extension in the Great Basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin, but being driven by gravitational collapse of thick crust. Space-time patterns of Tertiary volcanism in the Basin and Range also appear to conform to patterns of thermal weakening or destruction of the subducted slab. Both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts, because both predict close spatial and temporal association of extension and magmatism, which is not generally observed.

Geodynamics of rifting and implications for hydrocarbon habitat

Abstract Rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub-lithospheric upper mantle, combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system, and by far-filed stresses controlling the interaction of lithospheric plates moving relative to each other. Mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting. Therefore, it is questioned whether a distinction between “active” and “passive” rifts is still justified. Lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere. Magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust. This could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data. The duration of the rifting stage of palaeorifts and passive margins is highly variable. Depending on plate interaction, crustal separation can be achieved after as little as 7 Ma and as much as 280 Ma of rifting activity. The availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved. The structural style of rifts is controlled by the physical state of the lithosphere, the mode and amount of its extension and the lithological composition of pre- and syn-rift sediments. Subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere. Post-rift basin subsidence is mainly governed by the decay of rift-induced thermal anomalies; densification of the lower crust and intra-plate stresses provide overprinting effects. In view of this it is unlikely that quantitative analyses of post-rift basin subsidence give a direct measure of lithospheric extension. Tectonically active rifts, palaeorifts and passive margin basins contain major hydrocarbon provinces. Their hydrocarbon charge can rely exclusively on pre-rift, syn-rift or post-rift sedimentary sequences or a combination thereof. Maturation of source-rocks can be achieved during the syn- and/or post-rift stage of basin evolution. During rifting, conductive and convective heat transfer accounts for elevated geothermal gradients; these play an important role in the maturation of pre and syn-rift source-rocks; thermal gradients decrease asymptotically during the post-rift stage.

A stretching model for the Oslo Rift

Data on the tectonic and magmatic activity, crustal thickness, seismic velocity and gravity have been integrated to constrain a stretching model for the Late Carboniferous/Early Permian Oslo Rift. The crustal structure and numerical modelling of temperature and melt generation during rifting suggest that the Oslo Rift in Skagerrak developed in response to crustal stretching with a β-factor between 1.4 and 1.6. However, upper crustal extension by faulting accounts for a stretching factor of only 1.1–1.2. Thermal expansion of the lithosphere due to an increased asthenospheric potential temperature, the intrusion of melted mantle material into the lower crust and probably a lowering of the erosional base by a fall in sea level all contributed to the erosion of 2–4 km of Lower Palaeozoic pre-rift sediments over large areas. The lateral extent of the thermal anomaly reached far beyond the Oslo and Skagerrak grabens.

Baikal rift zone: Structure and geodynamics

The Cenozoic Baikal rift zone is superimposed on the Caledonian Baikal fold belt, representing the suture between the Precambrian Siberian craton and several microcontinents. The Baikal rift zone consists of a system of disconnected fault-bounded basins and extensional and wrench faults that straddles a major arch, having a topographic relief of 2–3 km. Rifting activity commenced during the Oligocène and is still active as evident from high seismicity of the Baikal rift zone. Across the Baikal rift zone, upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8–11 km across the Baikal basins as compared to the adjacent unextended areas. The discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir. Indeed, geophysical data indicate that the Sayan-Baikal dome coincides with such a diapir, the top of which is located at the crust/mantle boundary. This intrusion is held responsible for Oligocene and Quaternary doming of the rift zone. Development of the intra-continental Baikal rift system is thought to be related to asthenospheric diapirism (active rifting); however, intra-plate stresses in conjunction with the Himalayan collision may have played some role.

The Valencia trough (western Mediterranean): An overview

The Valencia trough, located between the Spanish mainland and the Balearic islands, corresponds to a late Oligocene to recent sedimentary basin that is characterized by a highly attenuated continental crust. Its northwestern part evolved in response to a late Oligocene to early Miocene rifting phase that was followed by a period of post-rift subsidence and a late Miocene-Pleistocene phase of renewed crustal extension and magmatism. Its southeastern part is characterized by late Oligocene to mid-Miocene compressional deformation and subsequent extension. In the axial parts of the basin the crust has been thinned by a factor of 3 whereby the lower crust is either absent or very thin. Upper crustal extension appears to be considerably smaller than indicated by the crustal configuration of the basin. A major problem in understanding the evolution of the Valencia trough is the lack of knowledge about the physical state and structure of the lithosphere before late Oligocene extension. Evolution of the northwestern part of the basin appears to be linked with the development of the oceanic Provençal Basin. Middle Miocene compressional deformation of the Balearic fold belt, forming the southeastern margin of the Valencia trough, appears to have interfered with the tensional evolution of the latter, which only resumed after crustal shortening in the Balearic domain had given way to extensional tectonics during the late Miocene.

Deep crustal structure of the Rhine Graben from dekorp-ecors seismic reflection data: A summary

Results of two deep reflection seismic lines crossing the northern and southern part of the Rhine Graben, which were recorded in 1988 by the French ecors and the German dekorp teams, indicate that the asymmetry of this rift persists to the base of the crust. The brittle/ductile transition zone approximately coincides with the top of the lower crust. Ductile shear zones appear to affect the crust/mantle boundary. Upper crustal extension by faulting accounts for a divergence of the graben flanks by 5–7 km, whereas the crustal configuration of the Rhine Graben suggest an extension value of some 17 km.

Seismic reflection evidence for seismogenic low-angle faulting in southeastern Arizona

Focal mechanisms for large (M > 6) earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from {approximately}30{degree} to {approximately}70{degree}. Geologic relations suggest that low-angle faults have accommodated large-scale upper-crustal extension. These disparate observations are often reconciled by arguments that low-angle faults move aseismically or rotate to low angles from initially high angles. Seismic reflection data from the Tucson basin in southeast Arizona image a low-angle normal fault (the Santa Rita fault) that crops out along the trend of late Quaternary fault scarps caused by large-magnitude (M {approximately} 6.7-7.6) earthquakes. Velocity-independent dip analysis from shot records of the Santa Rita fault indicates that it has a true dip of {approximately}20{degree} to a depth of at least 6km. This observation suggests that low-angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper-crustal extension depend on local conditions of stress and preexisting geologic structure.

Magmatic-tectonic interaction during early Rio Grande rift extension at Questa, New Mexico

Pre-caldera and caldera-related igneous rocks near Questa, New Mexico, provide a well-constrained example of upper-crustal extension over a magma chamber. Extensional strain exceeding 200% occurred over many square kilometers in a region now underlain by a batholith. The most intense deformation followed emplacement of intrusions related to formation of Questa caldera at 25.7 Ma and preceded the cooling of 24.6 Ma plutons. Detailed mapping of the southern part of Questa caldera indicates the sequence (1) extension concurrent with plutonism, resulting in an overlying carapace of highly tilted blocks bounded by low-angle faults; (2) renewed intrusion into overlying units, with the orientation of pluton tops, dikes, and molybdenum-bearing alteration zones controlled by previously formed low-angle faults; (3) reactivation or continued motion of low-angle normal faults, possibly aided by hydrothermal fluids; and (4) high-angle normal faults that formed after the magmatic system waned. Early high-angle faults remained active, again possibly aided by fluids, during their rotation to low angles. At upper-crustal levels, the magma chamber is believed to have provided a zone of ductility into which overlying brittle normal structures were rooted. At mid-levels in the crust, regional magmatism thermally softened the crust and allowed it to deform in a ductile manner. Regional extension appears to have been focused in a thermally and mechanically weakened area during the period of maximum plutonic activity. The features at Questa provide insights into processes that may have occurred in similar systems in the Basin and Range province and elsewhere.

A flexural-cantilever simple-shear/pure-shear model of continental lithosphere extension: applications to the Jeanne d’Arc Basin, Grand Banks and Viking Graben, North Sea

Abstract Mathematical models have been constructed of the geometric, thermal and flexural-isostatic response of the lithosphere to extension by faulting (simple-shear) in the upper crust and plastic, distributed deformation (pure-shear) in the lower crust and mantle. Models involving upper-crustal extension by both listric and planar faults have been developed. These coupled simple-shear/pure-shear models have been used to calculate extensional sedimentary basin geometry, subsidence history and crustal structure. Basin geometry and subsidence history are controlled by fault geometry (planar or listric), the amount of fault extension, fault dip, the depth of the transition from simple-shear to pure-shear, and the flexural rigidity of the lithosphere during both syn-rift and post-rift stages of basin formation. For the planar fault model, footwall and hangingwall blocks are considered to behave as two interacting flexural cantilevers; the response of these cantilevers to the isostatic forces produced by extension generating footwall uplift and hangingwall collapse. For a set of adjacent planar faults the lateral superposition of flexural footwall uplift and hangingwall collapse generates the familiar ‘domino’-style block-rotation of such multiple fault systems. The listric fault model assumes that the hangingwall collapses onto a rigid footwall by vertical shear, and that the tectonic denudation of the upper crust by faulting generates isostatic uplift producing limited footwall uplift. Deep seismic reflection data and earth-quake seismology suggest that the fundamental basement faults controlling lithosphere extension are planar. It is argued that the vertical shear construction of hangingwall collapse onto a rigid footwall is inappropriate for basement response. The coupled simple-shear/pure-shear models of extensional basin formation, using both listric and planar fault geometries, have been applied to the formation of the Jeanne d’Arc basin, Grand Banks, and the Viking Graben of the northern North Sea. The numerical modelling shows that the crustal thinning, thermal and sediment-fill loads generated during and after lithosphere extension need to be distributed flexurally in order to generate the observed basin depth and geometry. The flexural-cantilever, planar-fault model provides closer agreement to observed basin depth and subsidence than the listric-fault model. The planar fault model also produces more footwall uplift than a listric fault model. Erosion of this footwall uplift generates substantial isostatic uplift through unloading, leading to a large underestimate of the horizontal displacement on basin-bounding faults. The low values of effective elastic thickness obtained by extensional basin modelling are substantially less than the thickness of the cool, brittle upper crust. It is suggested that flexural bending stresses associated with lithosphere extension on planar faults are sufficiently large to generate brittle failure within the upper crust, so producing the low values of effective elastic thickness. This is consistent with the predictions of the flexural-cantilever model.

High heat flow in the Jornada del Muerto: a region of crustal thinning in the Rio Grande rift without upper crustal extension

The Jornada del Muerto is a large basin located in the Rio Grande rift of southern New Mexico. Although syn-rift crustal thinning is likely in the region, geologic data suggest little if any syn-rift tectonism has occurred in the upper crust. This implies crustal thinning occurred by processes affecting the mid to lower crust. There is little syn-rift subsidence or sedimentation and little Cenozoic volcanism in the immediate study area; instead, the Jomada del Muerto seems to be a stable crustal block exhibiting remnant Laramide compression. Interestingly, high heat flows are estimated regionally in the area (~95 mW m −2), with very high heat flows in the southern Jornada del Muerto (~ 125 mW m −2). The N-S heat flow profile across the study area has a small half width (~ 5–11 km) which implies relatively shallow thermal sources. From analysis of possible surface anomalies generated by subsurface isothermal steps, one may suggest that the thermal sources generating the very high heat flows must be within ~ 10–15 km of the surface. A high crustal thorium concentration, measured in Precambrian rocks at one of the heat flow sites, may indicate that the surface heat flow anomaly in the southern Jomada del Muerto is caused by radiogenic anomaly. A variety of other heat sources also seem possible, such as: convective overturn in the ductile mid to lower crust, mid to lower crustal intrusion, and volatile streaming. It is suggested that these crustal heat and mass-transfer mechanisms are in turn generated by deeper thermal sources. Possibilities might include a mantle plume, or local thickening of a massive lower crustal tabular intrusion thought to underlie large regions of the southern Rio Grande rift.

Late Cretaceous extension in the hinterland of the Sevier thrust belt, northwestern Utah and southern Idaho

Cover rocks of the Raft River metamorphic core complex, located in the Sevier belt hinterland, preserve a structural and metamorphic history that predates the middle Tertiary extension of the region. In the eastern Raft River Mountains, Cambrian( )-Permian rocks form two allochthons that occur in the hanging wall of the mid-Miocene Raft River detachment fault. Dramatically attenuated, metamorphosed Cambrian( )-Pennsylvanian strata of the lower allochthon yield Late Cretaceous muscovite {sup 40}Ar/{sup 39}Ar plateau ages (ca. 82-90 Ma) that are interpreted to date cooling during final stages of layer parallel extension. Devonian-Permian rocks exposed in the Black Pine Mountains underwent east-west extension ({approximately} 160%) associated with syntectonic growth of white mica along cleavage. {sup 40}Ar/{sup 39}Ar whole-rock spectra of associated slates also indicate a Late Cretaceous metamorphism. Together, field and {sup 40}Ar/{sup 39}Ar results suggest that Late Cretaceous extension occurred in the Sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland. Sufficient topography must have been present to drive upper-crustal extension in the eastern hinterland.

Geothermal measurements in the northern Red Sea: Implications for lithospheric thermal structure and mode of extension during continental rifting

The northern Red Sea is a continental rift in the process of transition from continental to oceanic rifting. We present 191 new heat flow measurements from the northern Red Sea forming three traverses across the water covered portion of the rift. The heat flow across the rift systematically increases from values of about 125 mW/m2 seaward of the coasts to average values greater than 250 mW/m2 in the axial depression. The heat flow measurements are evaluated for environmental disturbances. These are found to be generally small. The largest estimated disturbance results from the relief of the seafloor and of the top of a subbottom evaporite layer. The relief on these surfaces can account for the 20% point to point scatter typically observed in the heat flow measurements. Limits are placed on systematic disturbances to the heat flow pattern across the rift. The estimated largest systematic disturbance results from sediment blanketing which may cause a reduction in the heat flow on the order of 10%. The heat flow variation across the rift is then utilized to examine the lithospheric thermal structure and the geometry and mechanism of extension. We employ a two‐dimensional time‐dependent numerical technique to follow the advection and diffusion of heat for a simple shear model and various pure shear models of lithospheric extension. The model of simple shear lithospheric extension along a planar shallow dip detachment produces significantly lower than observed heat flow, results in relatively small amounts of thermal thinning of the lithosphere compared to pure shear extension, and does not result in conditions likely to generate melt. The pure shear models with widening or constant width zones of extension also do not match the observed heat flow nor generate partial melt. The modeling study indicates that the actively extending region must become narrower through time in order to match the heat flow high in the center of the rift, generate some degree of partial melt and produce crustal subsidence within the observed width of the rift. A narrowing of the zone of active extension is also consistent with the pattern of tectonic activity inferred from sedimentary structures observed on seismic reflection profiles which indicate an abatement of activity in the marginal areas and intense present‐day activity in the axial depression. In all of the models, an additional component of lithospheric heating other than that produced by extension appears necessary to match heat flow values in the marginal and near coastal areas, as well as explain the observed magnitude of rift flank uplift. Several scenarios in which the zone of extension eventually becomes narrow are consistent with the heat flow data alone, including those resulting in seafloor spreading for the past several million years. However, additional information provided by gravity, magnetic, and seismic reflection and refraction data appear to be most consistent with a history in which a period of relatively uniform extension across the rift is followed by a concentration of extension toward the center of the rift. Such a model retains a small thickness of continental crust in the center of the rift and does not necessitate active seafloor spreading in the northern Red Sea. The three heat flow traverses indicate that maximum lithospheric thinning is occurring within the axial depression in the northern Red Sea which is very close to initiating seafloor spreading. There is evidence that upper crustal extension is locally concentrated in specific areas along the strike of the axial depression forming “deeps” associated with large recent intrusions. These deeps may constitute a precursory stage to seafloor spreading and appear to be nucleating at discrete and regularly spaced sites along the axis of the rift. The deeps are located midway between regularly spaced cross‐trending fault zones or accommodation zones which, in the marginal areas, separate sets of fault blocks along strike. Crustal extension appears to be taken up more diffusely in the accommodation zones than in the intervening fault block domains which are characterized by fewer, larger faults. As a result, initial crustal rupturing and emplacement of large intrusive bodies occurs halfway between the accommodation zones where more focused crustal extension forms the deeps. Comparison with other rifts suggests that the pattern of fault block domains separated by cross‐trending fault zones is a common feature, that it originates early in the development of a rift, and that it may continue to influence the development of the rift up to the establishment of a continuous seafloor spreading axis.

Thrust tectonics in the Scandes - upper crustal extension during Scandian compression

Thrust tectonics in the Scandes - upper crustal extension during Scandian compression

Midplate Seismicity Exterior to Former Rift-Basins

Abstract Midplate seismicity associated with some former rift-zones is distributed diffusely near, but exterior to, the rift basins. This “basin-exterior” seismicity cannot be attributed to reactivation of major basin-border faults on which upper-crustal extension was concentrated at the time of rifting, because the border faults dip beneath the basins. The seismicity may nonetheless represent reactivation of minor faults that were active at the time of rifting but that were located outside of the principal zones of upper-crustal extension; the occurrence of basin-exterior seismicity in some present-day rift-zones supports the existence of such minor basin-exterior faults. Other hypotheses for seismicity exterior to former rift-basins are that the seismicity reflects lobes of high stress due to lithospheric-bending that is centered on the axis of the rift, that the seismicity is localized on the exteriors of rift-basins by basin-interiors that are less deformable in the current epoch than the basin exteriors, and that seismicity is localized on the basin-exteriors by the concentration of tectonic stress in the highly elastic basin-exterior upper-crust. All of the hypotheses considered here appeal to the presence of a rift zone to explain the characteristics of the basin-exterior seismicity, but the hypotheses differ in their implications for the seismic risk of former rift-zones and in their implications for the causes of midplate earthquakes in general.

Crustal extension and magmatic processes: COCORP profiles from Death Valley and the Rio Grande rift

New crustal-scale information on the interaction between normal faulting and magmatic activity is provided by recent COCORP deep seismic reflection profiles in Death Valley, California, and by reprocessing of the COCORP data from the Socorro area of the Rio Grande rift, New Mexico. The most striking feature on the seismic sections from these areas is a prominent, subhorizontal series of reflectors at mid-crustal depth. Previous studies have suggested that thin tabular magma bodies lie within the mid-crustal reflective zones. In addition, because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the COCORP data, these mid-crustal horizons are here inferred to be detachments or zones of tectonic decoupling. Upper-crustal Cenozoic faults do not appear to penetrate deeper than 15 km in Death Valley and 13 km in the Rio Grande rift. In Death Valley, these faults are relatively planar, with moderate dips (20° to 35°), and appear to bound large basement rocks. One such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central Death Valley to the surface location of a 690,000-yr-old basaltic cinder cone. Listric and low- to moderate-angle normal faults are evident on the reprocessed New Mexico data and constitute the structural component of upper-crustal extension. In particular, a listric master fault traceable to depths as great as 13 km is inferred to underlie the Albuquerque basin. Unlike the Death Valley data, no faults are observed to merge with the Socorro magma body per se. Rather, subhorizontal to moderately west-dipping packages of reflections are imaged between the base of the faulted upper crust (13 km depth) and the mid-crustal magma body (about 20 km depth). The middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion. Events seen in the middle and lower crust are generally subhorizontal, and prominent layering is observed. A band of reflections attributed to the crust-mantle boundary is evident on most seismic sections. The upper mantle appears seismically transparent. On some of the profiles, the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections, at least one of which is a layer of magma. This association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes.

Subsidence History of Australia's Southern Margin: Constraints on Basin Models

The subsidence history of Australia's southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation. The first phase is characterized by rapid tectonic subsidence (about 50 m/m.y.) which lasted for about 25-30 m.y. during the Early Cretaceous. The period of rapid subsidence represents the rifting phase of margin development. This early stage is correlated with associated rifting phenomena: faulting and extension of the brittle upper crust, formation of grabens and half-grabens, and the rapid accumulation of volcanogenic detritus (Otway Group). The amount of extension (20-50%) is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin The model includes uniform extension and thinning of the continental lithosphere, a prolonged extensional event followed by simple cooling, horizontal and vertical conduction of heat, and radioactive heat production by the crust. The rift phase may have begun earlier, but the most rapid and widespread subsidence occurred during the Early Cretaceous. The end of rapid and widespread subsidence occurred during the Early Cretaceous. The end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly (about 90 Ma) adjacent to the continent-ocean boundary (COB). The second stage of margin development is the post-rift (or drift) phase, a period of much slower rates of subsidence and relative tectonic quiescence. Subsidence during the post-rift stage is modeled as cooling and contraction of extended continental lithosphere. Estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the COB. Estimates of extension from the geometry of basement blocks near the COB do not agree with the analysis from subsidence modeling. Upper crustal extension seems to record only a small proportion of total lithospheric extension. Alternative or supplementary mechanisms of extension are explored, with emphasis on implications from a model that includes localized shear along a lithospheric detachment system.

The structure and evolution of the Wessex Basin, southern England: an example of inversion tectonics

The Wessex Basin of southern England consists of four main sub-basins. Each sub-basin is thought to be fundamentally controlled by the normal reactivation of Hercynian basement thrust and wrench (or transfer) faults. Post-Hercynian polyphase reactivation of both east-west thrust and northwest-southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete (Permian-Cretaceous) depocentres. While rifting was confined to the western portions of the area during the Permo-Triassic, easterly migration in the Mesozoic allowed the initiation of the Weald and Channel Basins. The sub-basins represent therefore, simple sinistral pull-apart basins opening along northwest-southeast faults. This same fault direction is postulated to have also played a major role in controlling and/or modifying the development of European basins in general.Major pre-Aptian uplift and erosion is largely attributed to the thermal effects (and hence the isostatic uplift of the basin) associated with continental rifting of the Bay of Biscay and the North Atlantic margins. Major inversion (and hence basin destruction) began in the Late Cretaceous-Early Tertiary with the uplift of the Channel, Weald and Vale of Pewsey Basins and culminated in the Oligocene-Miocene, synchronous with the Helvetic phase of Alpine deformation. Wessex Basin inversion was facilitated by the thrust reactivation of Variscan faults above an intracrustal detachment surface, the same surface which allowed upper-crustal extension and hence basin initiation in the Permo-Triassic.The structural development of the Wessex Basin, both during extension (basin formation phase) and compression (inversion phase), was ultimately controlled by the plate motion of Africa relative to Europe. Such plate motions successfully predict the observed NW-SE sinistral motion from the Stephanian to the Aptian, E-W sinistral motion during the Aptian-Cenomanian, and largely NW-SE dextral motion from the Cenomanian to the present day.

The thermal and mechanical development of the Wessex Basin, southern England

Summary The Wessex Basin of southern England is an E-W-trending Late Palaeozoic to Tertiary extensional basin formed on Early Palaeozoic Variscan crust. Extension occurred over a protracted period from Late Carboniferous to Cretaceous times resulting in the evolution of four main depocentres or sub-basins; the Channel basin, Winterborne-Kingston Trough, Vale of Pewsey, and Weald basins. Each sub-basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults. The basement faults represent primary structures associated with the fold-and-thrust terrain of the Devonian-Carboniferous Variscan Orogeny. Subsequent, but synchronous, reactivation of both thrust and transfer faults tended to compartmentalize the basement, leading to discrete depocentres and the production of general, rhomboidal-shaped, sub-basins (characteristic of pull-apart basins in strike-slip terrains). To investigate the thermo-mechanical properties of the lithosphere during basin formation, 50 boreholes within the Wessex Basin were back-stripped to isolate the driving tectonic subsidence responsible for basin formation. The resultant driving subsidence is characterized by a gentle, sometimes negative, exponential form, with superimposed rapid, discrete, subsidence events. As the geological development of the Wessex Basin was characterized by polyphase extension and the utilization of crustal detachments, the lithospheric model used to determine δ (crustal extension above the detachment) and β (stretching below the detachment) included the effects of depth-dependent stretching and finite re-rifting. From the back-stripping analysis, the basin-wide-averaged upper-crustal extension and lower-crustal/sub-crustal lithospheric stretching were 11 and 5% respectively. The distribution of δ is strongly influenced by the location of growth faults, and hence basement thrust-fault reactivation, suggesting that δ is reflecting the brittle failure and collapse of hanging wall blocks. Repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence. In contrast, β is uniformly distributed across the basin, suggestive of a ductile failure of the lower crust and responsible for the low-magnitude thermal subsidence. Strain compatibility between the upper-crust and lower-crust/sub-crustal lithosphere is maintained partly within the basin (5%), and partly external to the basin. As the Variscan basal thrust was the fundamental relay (in the sense of Wernicke) responsible for the Wessex Basin formation, we postulate that strain balancing induced a cumulative (essentially thermal) isostatic uplift (of ≈ 60 m) under the northern margin of the Armorican Massif.