Deep Crustal Reflectors Research Articles

Paleoproterozoic thick-skinned tectonics in the Central Indian Tectonic Zone: implications on the tectonic reconstructions of cratonic nuclei

ABSTRACT The Satpura Mountain Belt, or the Central Indian Tectonic Zone (CITZ), is a Paleoproterozoic mountain belt of Central India, with Mesoproterozoic and Neoproterozoic rejuvenation history. Structural studies carried out in the southern margin of this ancient orogenic belt have identified a two-phase evolution. The documentation of mapped fold interference of Type 2 ‘boomerang-shaped’ pattern and the spread of early lineations over the hinge area of second-generation folds provide an estimate of ~40-43° angle between the regional axial directions of the folds of the two orogenic episodes. The map patterns indicate the presence of basement-involved recumbent fold nappes placed above the sedimentary cover sequence. The identified décollement structure on the surface provides information regarding the deformation of the upper crust in ductile manner. The patterns of the deep crustal reflectors mapped through previous deep seismic reflection study interpreted during the present investigation brought out the thick-skinned deformation in the region as well as identification of crustal low-velocity zones of lower strength and a domal geometry of the lower crust. The regional thrusts are found to deform deeper crustal layers, one thrust affecting the crust-mantle boundary. The behaviours of the upper and lower crust in terms of rheological layering have been used to interpret the regional structural style of the lithosphere. The structural features of the region do not support the argument regarding the status of the Central Indian Shear as a suture, but indicate it to be the outermost emergent thrust towards the foreland side. The patterns of dip of the thrusts and thrust transport directions indicate a northward dip of the South Indian Block during the subduction stage and the position of the collisional suture of the Satpura orogen to the north of the Sausar belt. A tectonic model has been proposed here to explain the formation of the Satpura Mountain Belt or CITZ involving subduction-collision at ~2250 Ma, followed by erosion, stretching and basin development between 2100 and 1500 Ma. A collisional event in the eastern and western craton margins of India, at ~1500 Ma, caused the second uplift of the Satpura Mountain Belt and its northern margin, developing a temporal unconformity of ~300 Ma gap between the Semri and Kaimur Groups (Vindhyan Supergroup). A third event in the region at ~950 Ma was similar to the second event. No continental amalgamation took place along the CITZ during the tectonothermal events at ~1500 Ma and ~950 Ma; these involved reactivation of old thrusts and basal accretion which were the effect of far-field stresses emanating from the orogenic events at the eastern margin of the Bastar Craton and western margin of the Aravalli Craton.

Deep crustal reflectors under Reydarfjördur, eastern Iceland:crustal accretion above the Iceland mantle plume

Summary Deep seismic reflection profiling on the Faroe–Iceland Ridge Experiment (FIRE) imaged a sequence of dipping events in the upper to |-crust beneath the Tertiary lava pile in Reydarfjordur, eastern Iceland. These events dip towards the spreading centre at which the crust was formed, and continue down to ∼ 6 s two-way time, corresponding to depths down to 15–18 km. Similar crustal reflections have been observed previously within the crust in other areas of Iceland, although the reflective events located by previous surveys have not been interpreted rigorously. In this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava πle. The synthesized lava pile sections are based on field measurements made in the Reydarfjordur area: the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions, and some intercalated tuffaceous clastic horizons. We compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas. We conclude that the upper and mid-crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface, with the main reflections resulting from packets of flows of similar thicknesses. The reflectors that we have imaged beneath Iceland are similar to those defining the seaward-dipping reflectors on the volcanic continental margins, and we therefore suggest that the seaward-dipping reflections commonly result from packets of lavas with similar distributions of thicknesses. The occurrence of lavas down to 15–18 km beneath Reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust, although the volcanic processes forming the crust in Iceland generally lead to advection of extrusive material to mid-crustal levels. The production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity. The advection of frozen lava flows to mid-crustal levels contributes to the maintenance of the subsolidus crust that is observed on Iceland, and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques.

Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization

Segments of the root to the Caledonian collision zone are exposed along the west coast of Norway, allowing the study of processes and petrophysical conditions at the deepest crustal levels. P-T conditions prevailing in these crustal root zones correspond to eclogite facies. Eclogitization is associated with marked changes in petrophysical properties, notably a density (10–15%) and Vp increase which may give a mantle signature to an eclogitized crust. The observed ductility enhancement may be caused by transformation plasticity, reduction in grain size, formation of new mineral assemblages and presence of fluid. In the Western Gneiss Region and the Bergen Arcs, eclogites form along shear zones, interpreted as fluid pathways with sharp boundaries to their metastable protolith. This makes eclogite shear zones potential deep crustal reflectors. Dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available. Timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the P-T space, but rather on the introduction of fluids into the system. Eclogites are not only passive recorders of P-T-t paths illustrating very deep subduction of crust (> 100 km); rapid changes in petrophysical properties make fluid-induced eclogitization a dynamic process that influences geodynamics. Stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn-eclogitic pseudotachylytes in the metastable protolith. Deformation, including seismicity, enhances fluid infiltration and further eclogitization. Dense and rheologically weak eclogites may delaminate and sink into the mantle. In the absence of fluids, crustal doubling may occur without metamorphic re-equilibration. The evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to P and T.

Problems interpreting deep crustal reflectors beneath the Moine thrust between Shetland and the Scottish Highlands

AbstractOffshore seismic reflection profiles crossing the Caledonian orogenic front (locally, ?Moine thrust) between Shetland and the Scottish Highlands show a singular coherent east-dipping reflection underlain by a highly reflective dipping zone in the middle crust extending down to the Moho discontinuity. This reflector pattern varies spatially with respect to the eastern edge of the Precambrian Lewisian foreland as well as to previously mapped locations of the Moine thrust. Southwest of Shetland, the reflector pattern coincides with the offshore projection of the Moine thrust, but further south, toward the Highlands, the reflector diverges to the west of the thrust and actually underlies autochthonous Lewisian foreland basement. Where this reflector pattern diverges from the Moine thrust, two interpretations are possible: the prominent reflector is part of a basement imbricate thrust duplex within the footwall (or ‘lower plate’) of the Moine thrust that merges with the thrust zone further north and/or it was originally a Proterozoic normal fault which, further north around Shetland, was reactivated or over-printed by the Moine thrust. The latter interpretation supports the importance of pre-existing crustal structure in controlling Caledonian compressional deformation.

Eclogite-facies shear zones—deep crustal reflectors?

Strongly foliated eclogite-facies rocks in 30–150 m thick shear zones of Caledonian age occur within a Grenvillian garnet granulite-facies gabbro-anorthosite terrain in the Bergen Arcs of Norway. The predominant eclogite-facies mineral assemblages in the shear zones are omphacite + garnet + zoisite + kyanite in gabbroic anorthosite and omphacite + garnet in gabbro. Eclogite-facies rocks in shear zones are generally fine-grained; alternating omphacite/garnet- and kyanite/clinozoisite-rich layers define gneissic layering. A strong shape preferred orientation of omphacite, kyanite, and white mica (phengitic muscovite and/or paragonite) define the foliation. The anorthositic eclogites show omphacite b-axis maxima approximately normal to the foliation and c-axis girdles within the foliation plane. P-wave velocities ( V p ) determined at confining pressures to 600 MPa for samples from eclogite-facies shear zones range from 8.3 to 8.5 km s −1 and anisotropy ranges from 1 to 7%. The few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation. The fast propagation direction lies within the c-axis girdles (parallel to foliation) and the slow propagation direction is parallel to the b-axis concentration (normal to foliation) in samples for which omphacite crystallographic preferred orientation was determined. V p for the granulite-facies protoliths average about 7.5 km s −1. High calculated reflection coefficients for these shear zones, 0.04–0.14, indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high-pressure conditions but escaped thermal reactivation.

Acoustic attenuation in oceanic gabbro

SUMMARY Compressional wave velocity and attenuation were measured in oceanic Layer 3 from nearly 0.5 km of recovered core at Ocean Drilling Program Site 735 in the southern Indian Ocean. High-frequency (400 kHz) acoustic transmission experiments were conducted on 117 water-saturated minicore samples under room conditions of temperature and pressure on board the drillship JOlDES Resolution. Although experimental errors were large in some deformed samples, the average of our measurements of attenuation, Qp', on these samples is 0.049 f 0.032. Agreement between 'spectral ratio and inversion estimates of Qp' indicates that acoustic noise in the data does not significantly reduce the measurement accuracy. Ultrasonic tests on a 1 per cent porosity sample at elevated effective pressures from 10 to 500MPa showed an increase in V, from 6.8 to 7.0kms-' and a decrease in Qp' from 0.026 to 0.023. The small pressure dependence of Qp' in this low-porosity gabbro suggests that it is possible to use measurements at room conditions for relative interpretation as a function of depth. The measurements of Qp' show (1) an overall decrease as a function of depth, (2) a decrease in the average variance in the measurement with depth, and (3) an inverse relationship between average V, and average Qp', although Qp' shows greater variation in these rocks than V,. Our explanation for these results is that rock fabric has a significant effect on V, and Qp'. A simple deduction from these results is that seismic data will be strongly influenced by the degree of alteration and tectonism in similar crustal rocks. Particularly in fracture zones, seismic reflections from lithologic contacts may be attenuated, and deep crustal reflectors, identified as faults or as the Mohorovic discontinuity, may result from V, and Qp' contrasts caused by alteration and tectonism within Layer 3, not from the lithologic contact itself.

Shear wave constraints on a deep crustal reflective zone beneath Vancouver Island

Analysis of teleseismic receiver functions recorded in the northern Cascadia subduction zone show that large P‐to‐S converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors (the “E” zone) beneath central Vancouver Island. These data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey. Modeling of the amplitudes, polarities, and arrival times of the P‐to‐S converted phases demonstrates that the E zone is a prominent low‐velocity layer, with an S velocity 1.0±0.2 km/s lower than the regions above and below. The E zone dips 7°±5° in the direction N10°E±20° and extends from depths of 36.5 to 41 km at ALB‐B on central Vancouver Island. The estimated S velocity contrast, when considered with refraction P velocities, yields a Poisson's ratio of 0.34 for this zone. Given the uncertainty in ΔVp and ΔVs, this value may range from 0.27 to 0.38. The low P and S velocities and high Poisson's ratio combined with electrical resistivity measurements suggest a porosity of 0.1–1.9% and crack aspect ratios of 0.001–0.01, values consistent with a region containing thin, fluid‐saturated cracks and thus supportive of the interpretation of this feature as a major shear zone.

Physical properties of deep crustal reflectors in southern California from multioffset amplitude analysis

Analysis of reflection waveforms before stack can constrain the physical properties of reflectors in the deep crust. To simplify this analysis, recorded amplitudes are assumed to be reflections from weak elastic heterogeneities. With these assumptions, trends in reflection amplitudes with offset may indicate whether the signs of a reflector’s density and rigidity contrast agree with or oppose the sign of its Lamé’s parameter contrast. The slope of the trend indicates the degree of Poisson’s ratio contrast. No attempt is made to invert for the individual modulus or density contrasts. By examining only gross amplitude‐versus‐offset (AVO) trends, deep reflections constrain some crustal properties. Two seismic reflection surveys in the Mojave Desert recorded deep reflections that show amplitude changes with offset. Both the 1985 Calcrust Ward Valley survey in the eastern Mojave and the 87 km COCORP Mojave line 3 in the western Mojave incorporate long offsets of 10 km or more. Prestack traces are equalized using a quantile technique assuming a constant noise level at large time, then corrected for spherical divergence. Gross AVO trends that are summarized for each survey in amplitude trend stacks suggest that the strongest reflectors in the middle and deep crust represent Poisson’s ratio contrasts of at least 10 percent. In the eastern Mojave, a transition to a basal‐crustal zone, at ∼23 km depth, may include an increase in Poisson’s ratio with depth. Poisson’s ratio may also increase at the Moho.

Deep crustal reflections in Australia 1957-1973--II. Crustal models

In a companion paper, we described experiments conducted by the Australian Bureau of Mineral Resources (BMR) between 1957 and 1973 to record deep crustal seismic reflections. At 24 of the 46 sites investigated, events were recorded at times up to 10-13 s. At some sites, only one or two shots were recorded; where more comprehensive investigations were made, the events were consistent with deep reflections, though they lacked continuity in many cases. The results were sufficiently encouraging that we have now analysed the events on the hypothesis that they represented deep crustal reflectors, including the Moho. In 1976, BMR introduced digital recording, and successfully carried out several long deep crustal profiles since then. This paper analyses only the recordings made with analogue equipment prior to 1973. In a few places, the reflection structure could be compared with nearby refraction traverse results; some events correlated well with first or second-order discontinuities in the refraction velocity-depth curves, while for others there was no correspondence. It is difficult to generalize as to which types of refractors give rise to reflections. The refraction surveys show that the Moho is not always a boundary with a sharp change in velocity across it. In some areas in Queensland the deepest reflector correlates well with the refracting Moho. For reflection shots not near refraction surveys, we converted travel times to depths using simplified velocity models based on refraction surveys in similar geological environments, and on isostatic balance considerations using gravity and elevation data. In central Australia, if the deepest reflector is the Moho, a relatively thin and light crust is implied. However comparison with refraction models makes it seem probable that a heavier, thicker crust could exist, with the Moho being deeper than the deepest reflector. Also in the Tasman Geosyncline, it is probable that the Moho is deeper than the deepest reflector.

Off‐line acquisition of crustal reflection and refraction data

A piggy‐back seismic reflection survey was carried out in 1984 as part of the DEKORP 2‐S profile. The signal from explosive‐sources was digitally recorded on 120 channels. The objectives were: a) To record crustal reflections at a wide range of incidence angles (5‐85 km offset) b) To provide 3‐D control of deep crustal reflectors seen on the main profile.An important consideration throughout the fieldwork was to avoid interference with the main reflection survey. This experiment showed that explosive charges as small as 5‐90 kg may provide sufficient energy for observations from the steep‐angle to the wide‐angle range. The off‐line position of the receivers allowed the reconstruction of steep‐angle reflection profiles parallel and perpendicular to the main reflection line. A strong diffraction cluster on the main line could be correlated laterally, and indications about the diffractors' shape could be obtained.

On the use of seismic reflection surveys from oil exploration in deep crustal studies

A very large number of seismic reflection surveys is carried out by the oil industry throughout the world. Some of these surveys are designed to study deep oil traps and may use field parameters which, for the most part, are not significantly different from those used in deep crustal reflection studies. The one parameter which always varies is the record length (listening time). In the case of a vibratory source, the record length can be increased at the processing stage by the equivalent reduction of vibration time through partial correlation. We have used an oil exploration survey from a deep sedimentary basin in the coastal plain of Israel and extended its record length using this technique. We show that if a survey with appropriated field parameters, i.e., a survey for a deep target is chosen, deep crustal reflectors can be traced. Since a COCORP type deep crustal reflection study was also carried out in the same region, we can compare the two sets of results. We note that in this case the extended oil exploration record is at least equivalent to, and probably of better quality than, the COCORP type survey. This result is due mostly to a better S/N ratio in the oil exploration survey where input power was significantly larger than in special purpose study. This result indicates the as yet untapped potential of oil exploration data in deep continental crustal studies.

Twenty years of deep seismic reflection profiling in Germany—a contribution to our knowledge of the nature of the lower Variscan crust

Summary Near-vertical reflection studies in Germany have concentrated on the following objectives: (1) The nature of crustal reflectors (2) A detailed velocity structure (3) Zones of increased tectonic activity, e.g., faults. Approximately 20 years ago, the identity between reflectors in the near-vertical and the wide-angle range was established, wide-angle events being strongly influenced by velocity gradients. A pronounced, but limited low-velocity body below the Urach geothermal anomaly was detected by using an especially long spread length. An alternating polarity of deep crustal reflectors could be demonstrated. A deep fault zone, cutting the whole crust at steep angles, and shallow-angle overthrust structures were detected in the Variscan area. There is generally a strong reflectivity in the Variscan lower crust between the Conrad level at 6 s two-way travel time, i.e., 18 km, and the Moho at 9–10 s, i.e., 27–30 km depth. The lower crust seems to consist of thin lenses or lamellae with alternating velocity. They were probably generated by an extensive syn- and post-orogenic melting process leaving behind mafic and ultramafic ‘splinters’ in the lower crust and a compact ultramafic residue in the uppermost mantle, the Moho being formed as a new and uniform boundary. The poorly reflective upper crust is probably dominated by vertically oriented plutons in a rigid environment.

STATISTICAL EVALUATION OF DEEP CRUSTAL REFLECTIONS IN GERMANY

By a routine method of observation during exploration reflection measurements, sufficient data were compiled to allow significant statistical conclusions concerning deep crustal reflectors. The appearance of deep crustal reflections differs considerably from that of reflections from shallow sedimentary beds. They correlate only over short distances, seldom more than a few kilometers. The reflection times display quite a scatter, even over small areas. In many cases the reflected signal is typically broadened to continue over a few cycles. Following this reflection band along a profile, the main energy is frequently shifting from one phase to another until finally it disappears completely. A new method of wide angle reflection observations using a common reflection point technique was used to support and confirm results of the statistical analysis.