Reflective Lower Crust Research Articles

3D Crustal Architecture and Along‐Strike Variation in the Mid‐Northern South China Sea Rifted Margin

AbstractRifted margins often show apparent along‐strike variations at the continent‐ocean transition (COT) region, typically occurring gradually over hundreds‐to‐thousands of km between the “magma‐rich” and “magma‐poor” endmembers. In contrast, comparatively smaller‐scale lateral variabilities of rifting structure and faulting style in hyper‐thinned continental crust are not yet well described. The South China Sea (SCS) was recently defined as the archetype of a new “intermediate‐type” margin with neither excess magmatism nor mantle exhumation. However, along‐strike crustal variations along this new type of margin are still little explored. Based on previously unpublished 2D grid of seismic reflection lines and overlapping 3D seismic volumes, we have investigated the crustal structure, faulting style, and the lower crustal reflectivity along ∼400 km of the mid‐northern SCS margin. Our new maps of crystalline‐crust thickness and top basement show that the SCS extension developed in two segments with contrasting tectonic styles separated by a narrow boundary. The eastern domain, resembling a wide‐rift model, displays low‐angle detachment faults and high‐velocity lower crust mainly associated with syn‐rift magmatism indicating weak crustal rheology. The neighboring western domain, resembling a narrow rift model, displays steep crustal‐scale faults indicating comparatively strong rheology. The change from wide to narrow rift—or weak to strong rheology—occurs abruptly, which appears at odds with the current conventional wisdom of mostly gradual transitions over a long distance between rift models. We speculate that the inherited heterogeneity in mantle composition and 3D melt focusing effect may drive this abrupt along‐strike variability in the mid‐northern SCS.

Seismically constrained gravity inversions reveal magmatic and metamorphic processes at a major lithospheric boundary in northwestern Australia

Magmatic and metamorphic processes modifying the lower crust and uppermost mantle are key to understanding continental growth and evolution. However, our ability to identify and quantify such processes is challenged by inaccessibility, data sparsity, and uncertainties in defining properties at depth. As a result, knowledge of composition, metamorphic grade, and the nature of the continental crust-mantle transition is often limited. Here, we model the structure and properties of the crust and uppermost mantle between the West and North Australian Cratons to infer tectonic processes at this major lithospheric boundary. Seismically constrained 3D gravity inversions were used to estimate crustal and mantle densities and Moho depths. The application of isostatic principles and the joint interpretation of gravity, active and passive seismic models, well data, and outcrop data are used to identify and characterize anomalous masses that are otherwise unresolvable using supracrustal datasets. The results suggest a thick, dense crust with a mafic bulk composition, a high-density, reflective lower crust, and a low-density uppermost mantle beneath the southern Canning Basin. We interpret these crustal and mantle features as reflecting mafic mantle-derived magmatism and crustal anatexis related to Proterozoic-Cambrian Large Igneous Provinces. In contrast, the Proterozoic Paterson Orogen exhibits a thick, dense, high-grade metamorphic bulk crust with a felsic to intermediate composition, a high-density, low-reflectivity lower crust, and low-density uppermost mantle, inferred to result from convergent tectonics. The northern Canning Basin displays a thinner, less dense crust with an intermediate to mafic composition, and mafic, high-density, reflective lower crust, associated with mafic magma emplacement during rifting and passive margin formation. These variations in crustal structure reveal a complex interplay of Proterozoic terrane amalgamation to the cratonic margin, repeated intraplate high-volume mafic magmatic events, and rifting of the offshore basin, which have collectively shaped the deep crust and uppermost mantle in this region.

The transition from the Thomson Orogen to the North Australian Craton from seismic data

The transition between the North Australian Craton and the Thomson Orogen in the area south of the Mount Isa terrane lies under cover and is a critical element in interpretation of the nature of the Tasmanides. The location of the boundary between these domains is controlled by potential field results; the gravity and magnetic signatures are most sensitive to shallow structure, and so there is little information on structure at depth. Full-crustal reflection profiling crosses the boundary in a few locations but does not provide areal coverage. A deployment of 79 passive seismic stations spanning from southern Queensland into the Mount Isa terrane (AQT experiment) is exploited to examine the variations in crustal thickness and the nature of crustal structure across an area with no prior sampling. The analysis exploits the autocorrelation of the seismic signals that extracts the reflection response from the transmitted signals recorded at the surface, which can be migrated to provide an image of structure at depth. The combination of the active and passive seismic results and other geophysical data indicates that the structure of the northern and central Thomson Orogen is relatively homogeneous with a highly reflective, and magnetic, lower crust suggesting a common substrate across the region. The crust thickens and changes character in the North Australian Craton with variations under cover that can be linked to the areas of exposure. There is a zone approximately 100 km wide that separates the characteristic seismic signatures of the two domains indicating reworking at the craton margin. KEY POINTS A combination of passive and active seismic imaging sheds light on the transition from the Thomson Orogen to the North Australian Craton. The north central Thomson Orogen shows a consistent reflection character with a strong band of lower crustal reflectivity. The North Australian Craton shows seismic domains with varying character of reflectivity and crustal thickness. A transition zone about 100 km wide separates the distinctive styles of crustal architecture.

Seismic reflection imaging of the Juan de Fuca plate from ridge to trench: New constraints on the distribution of faulting and evolution of the crust prior to subduction

AbstractWe present prestack time‐migrated multichannel seismic images along two cross‐plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper‐middle crust. The regional difference in subduction bend‐faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate‐depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°–40°) lower crustal reflections are imaged in ~6–8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.

Deep reflection seismic imaging in SE Poland using extended correlation method applied to PolandSPAN™ data

Here I apply the extended correlation technique to a 156-km long regional seismic profile, PL1-5100, from the PolandSPAN™ survey located in SE Poland. This method allows the extension of the nominal record length of the survey (12s in this case) to much longer times (22s tested here), given that in the field the raw uncorrelated data were stored and a broadband sweep was used. Processing of the ground force correlated data compared to pilot sweep correlated one produced qualitatively similar results. Based on the consistency of the ground force measured along the profile, data were correlated with the mean of the ground force estimates. Results of the processing were validated against the coincident wide-angle reflection/refraction (WARR) profile (CELEBRATION 2000 CEL05 profile) and a deep reflection profile located further to the SE (POLCRUST-01 profile). I found a good correspondence between the wide-angle Moho signature and the near-vertical incidence (NVI) Moho reflectivity, both in terms of the depth and the reflection strength, with the exception of the area of the crustal keel (Moho >50km) interpreted from the WARR data, where the NVI reflectivity is observed at depth ca. 47km. Similarly to POLCRUST-01, line PL1-5100 is portraying the attenuated crust of the East European Platform margin with the dipping crystalline basement down to ca. 20km depth and the reflective lower crust. Two important fault zones: Izbica-Zamość Fault and Wilczopole Fault Zone are interpreted as the deeply-rooted crustal features, but there is no evidence for the Teisseyre–Tornquist Zone itself.

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

Multi-channel seismic (MCS) reflection profiles across the Pacific Plate south of the Alaska Peninsula reveal the internal structure of mature oceanic crust (48–56 Ma) formed at fast to intermediate spreading rates during and after a major plate re-organization. Oceanic crust formed at fast spreading rates (half spreading rate ∼74 mm/yr) has smoother basement topography, thinner sediment cover with less faulting, and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates (half spreading rate ∼28–34 mm/yr). MCS data across fast-spreading oceanic crust formed during plate re-organization contain abundant bright reflections, mostly confined to the lower crust above a highly reflective Moho transition zone, which has a reflection coefficient (RC) of ∼0.1. The lower crustal events dip predominantly toward the paleo-ridge axis at ∼10–30°. Reflections are also imaged in the uppermost mantle, which primarily dip away from the ridge at ∼10–25°, the opposite direction to those observed in the lower crust. Dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re-organization, where a Moho reflection is weak or absent. Our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt. The abundance and reflection amplitude strength of these events (RC∼0.15) can be explained by a combination of solidified melt that was segregated within the shear structures, mylonitization of the shear zones, and crystal alignment, all of which can result in anisotropy and constructive signal interference. Formation of shear zones with this geometry requires differential motion between the crust and upper mantle, where the upper mantle moves away from the ridge faster than the crust. Active asthenospheric upwelling is one possible explanation for these conditions. The other possible interpretation is that lower crustal reflections are caused by magmatic (mafic/ultramafic) layering associated with accretion from a central mid-crustal magma chamber. Considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re-organizations associated with ridge jumps or rift propagation, we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle, and therefore to shearing in the ductile lower crust or, alternatively, that plate reorganization could produce magmatic pulses which may lead to mafic/ultramafic banding.

Crustal structure in and around the Onikobe geothermal area, northeastern Honshu, Japan, inferred from the spatial variation of coda decay

The Onikobe area is an active geothermal area situated in the Ou backbone range of northeastern Honshu, Japan. It is home to calderas from the Tertiary to Quaternary eras and active volcanoes. A systematic spatial variation of Qc has been found in this area: Qc values are lower at stations in and around calderas than at other stations. The amplitude of coda waves with high Qcs decreases more slowly after a lapse time of around 7–10s than that with low Qcs. In the present study, to determine causes for these coda decay variations, coda envelopes were synthesized in a structure model in which high attenuation zones existed beneath the Onikobe and Sanzugawa calderas and where scattering coefficients were higher in the lower crust than in the upper crust. Using hypocenters shallower than 10km, envelopes were calculated for 256 station-hypocenter pairs with epicentral distances of less than 10km. It was assumed that the coda waves were composed of S–S scattered waves, and that the scattering was single and isotropic. The observed features of the Qc distribution were reproduced in the synthesis, and synthesized envelopes were found to mostly coincide with observed decay curves. The top of high attenuation zones was thus estimated as being deeper than 7.5km. The structure assumed for the synthesis was consistent with that of previous studies. We consider that the structure model used was appropriate, and that high attenuation zones beneath calderas and the reflective lower crust caused the spatial variation of the Qc and decay curves in the Onikobe area. We also consider that studies using coda decay would be beneficial in detecting high attenuation zones and the reflective lower crust.

Crustal structure and tectonic evolution of Enderby Land, East Antarctica, as revealed by deep seismic surveys

Subsurface crustal structures of the metamorphic terrains in Enderby Land, East Antarctica, were obtained on the basis of data from deep seismic surveys along with connecting reliable tectonic models. The lithospheric structure of the early-Paleozoic Lützow-Holm Complex (LHC) at the western side of Enderby Land was delineated by inland dipping velocity models and lower crustal reflectivity by the Japanese Antarctic Research Expedition. The seismic profiles across the Prince Charles Mountains (PCM) and Princess Elizabeth Land (PEL) by the Soviet Antarctic Expedition demonstrated the graben structure beneath the Amery Ice Shelf. The Lambert graben and the underlying rift structure in the central part of the profile appeared to be associated with the Cretaceous breakup. Tectonic evolution of the wider area in Enderby Land is discussed by combining the coastal structure with that toward the inland plateau of the Gambursev Subglacial Mountains (GSM), with geological aspects concerning the amalgamation and separation of the Gondwana supercontinent.

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south-central British Columbia, including across much of the Nechako–Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11–12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P-wave velocities of 6.45–6.98 km/s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle-derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east–west, consistent with north-northwest-trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks.

Seismic images of the continental Moho of the Indian shield

Abstract Deep seismic reflection profiling has imaged different patterns of the Moho across the Indian shield with a variety of tectonic environments from Archean to Recent. The character of the Moho varies from a discrete strong event, the base of strong coherent lower crustal sub-horizontal reflections, the base of dipping lower crustal reflections into mantle, to no clear reflection boundary. The seismic reflection data suggest a laminated lower crust in several places and offsets in the Moho at others. Kinematic and dynamic modeling of wide-angle reflection data across the Mesoproterozoic South Delhi Fold Belt and the Central Indian Tectonic zone suggests a laminated lower crust for these regions. In general, the lower crust of the Indian shield is heterogeneous. A clear Moho is identified in some of the Precambrian orogenic belts and sedimentary basins along with reflective lower-crust, whereas the cratonic areas exhibit a diffused Moho. Post-collisional extensional process, such as orogenic collapse, delamination, magmatic intrusions, low-viscosity ordering and underplating might have played a role in the generation of lower crustal laminated zone and formation of a younger Moho. The termination of lower-crustal reflectivity at the Moho with a transparent upper mantle need not necessarily indicate homogeneous upper mantle.

Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

Moho, crustal architecture and deep deformation under the North Marmara Trough, from the SEISMARMARA Leg 1 offshore–onshore reflection–refraction survey

Understanding further the nature and evolution at lithospheric scale of the Sea of Marmara on the North Anatolian Fault needs constraints on the deep crustal and Moho spatial variation. This has been probed here with offshore–onshore and OBS, Ocean Bottom Seismometer refraction seismics, in addition to coincident MCS, marine multichannel reflection seismic profiles over the whole North Marmara Trough. The diverse strikes of MCS profiles in a dense grid allow to avoid misinterpretation of late echoes in the deep basin as Moho reflections and attribute them to sidesweeps. Moho is instead positively identified from reversed observations of first-arrival head and refracted waves at the top of the mantle obtained at large offset by land stations. A significant and sharp reduction in its depth, on the order of 5 km occurs beneath both the eastern and western rims of the North Marmara Trough, with a more progressive crustal thinning from the south. The wide-angle reflections on OBS and land stations document in addition to Moho the top of a lower crustal reflective layer, which is also sampled by MCS profiles, and appears to follow Moho topography. The dense grid of MCS profiles along the southwestern margin of the North Marmara Trough reveals a dipping reflector through the upper crust with tilted basement blocks on top. This low-angle fault is suggested as a normal sense detachment extending in depth towards the reflective lower crust. The upwarp of the Moho and lower crustal layer towards the North Marmara Trough suggests that crustal thinning occurs mostly in the upper crustal part, with lateral transport of the material towards WSW in the footwall of the detachment, and possibly other features to the south, in the motion of Anatolia with respect to stable Eurasia oblique to the North Marmara Trough. Thinning can be accommodated in an asymmetric partitioning of the displacement on several branching faults at lithospheric scale.

Deep seismic reflection profiling across active reverse faults in the Kinki Triangle, central Japan

The Kinki triangle, central Japan, is marked by a dense distribution of active faults. For regional characterization of the crust in the Kinki metropolitan area to be used for seismic hazard mitigation, seismic reflection profiling was carried out using vibroseis trucks and explosive sources along a 135 km-long seismic line from Metropolitan Osaka to the Ise basin across several active reverse faults. The seismic reflection section portrays the west-dipping upper surface of the down-going Philippine Sea plate at 35 to 40 km depth, lower crustal reflectors at 27 km and mid-crustal reflectors at 15 to 16 km depth. Dipping reflections are dominant in the seismogenic upper crust and some of them can be traced as deeper extensions of active faults. A 60-km-wide uplifted zone between Osaka and Ise basins were formed as a pop-up structure bounded by double-vergent reverse fault systems. The mid-crustal reflectors beneath the uplifted zone form a slight convex shape and the lower limit of the hypocentral distribution of crustal earthquakes lies a few km above the reflectors. The dipping reflectors in the upper crust merged to the sub-horizontal, mid-crustal reflectors and are interpreted as a possible mid-crustal detachment. The hanging wall of active faults in the Osaka basin show back-tilting associated with reverse faults, suggesting a curved fault geometry and shallower detachment in the upper crust.

The crustal structure of the southern Nain and Makkovik provinces of Labrador derived from refraction seismic data

Data from a refraction seismic profile parallel to the coast of Labrador (Canada) were used to determine the crustal structure across the boundary of the Nain and Makkovik provinces, and to look for evidence for an offshore continuation of the Mesoproterozoic Nain Plutonic Suite (NPS). Seven seismometers recorded airgun shots along the 283 km long line. P- and S-wave velocity models were developed from forward and inverse modeling of travel times. The velocity model distinguishes three distinct zones. In the Saglek block of the Nain Province, the crust is divided into three layers with P-wave velocities between 5.8 and 6.9 km/s. Farther to the south, upper crustal velocities increase to 6.3–6.5 km/s and the Poisson’s ratio increases from 0.24 to 0.27. This zone correlates with a gravity low that is interpreted to outline the offshore continuation of the NPS. The upper crustal velocities are intermediate between anorthositic and granitoid rock samples collected from the NPS. A lower crustal reflector is limited to the area underneath the NPS and may be related to dioritic magmas. Mid-crustal and lower crustal velocities do not vary along the line and no underplating was detected. Within the Makkovik Province, upper crustal velocities of 5.9–6.2 km/s may indicate a dioritic composition similar to the Island Harbour Bay plutonic suite. Moho depth varies between 28 and 36 km with the maximum beneath the NPS. The variations could not be linked to effects of the Makkovikian orogeny but are thought to relate to Mesozoic rifting in Labrador Sea.

A first deep seismic survey in the Sea of Marmara: Deep basins and whole crust architecture and evolution

Increased source strength, streamer length and dense spatial coverage of seismic reflection profiles of the SEISMARMARA Leg 1 allow to image the deep structure of the marine North Marmara Trough (NMT) on the strike-slip North Anatolian Fault (NAF) west of the destructive Izmit 1999 earthquake. A reflective lower crust and the Moho boundary are detected. They appear upwarped on an E-W profile from the southern Central Basin eastwards, towards more internal parts of the deformed region. Thinning of the upper crust could use a detachment suggested from an imaged dipping intracrustal reflector that would allow upper crustal material to be dragged from beneath it and above the lower crust, accounting for the extensional component but also southwest motion of the southern margin of the NMT. Sections across the eastern half of the NMT, crossing the Cinarcik and Imrali basins, reveal several faults that are active reaching into the basement and have varying strike and proportions of normal and strike-slip displacement. They might be viewed as petals of a large scale negative flower-structure that spreads over a width of 30 km at surface and is rooted deeper in the lithosphere. Under the Central Basin a very thick sediment infill is revealed and its extensional bounding faults are active and imaged as much as 8 km apart down to 6 km depth. We interpret them as two deep-rooted faults encompassing a foundering basement block, rather than being merely pulled-apart from a jog in a strike-slip above a décollement. The deep-basin lengthening would account for only a modest part of the proposed 60 km finite motion since 4 Myr along the same direction oblique to the NMT that sidesteps the shear motion from its two ends. Thus differential motion occurred much beyond the deep basins, like subsidence involving the NMT bounding faults and the intracrustal detachments. The complex partitioned motion localized on active faults with diverse natures and orientations is suggested to represent the overburden deformation induced from horizontal plane simple shear occurring in depth at lithospheric scale, and in front of the North Anatolian Fault when it propagated through the region.

The structure of the lower crust at the Argentine continental margin, South Atlantic at 44°S

It is well established that the Argentine passive margin is of the rifted volcanic margin type. This classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope, manifested by seismic data as a seaward dipping reflector sequence (SDRS). Here, we investigate the deep structure of the Argentine volcanic margin at 44°S over 200 km from the shelf to the deep oceanic Argentine Basin. We use wide-angle reflection/refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times. The resulting P-wave velocity-depth model confirms the typical volcanic margin structure. An underplated body is resolved as distinctive high seismic velocity ( v p up to 7.5 km/s) feature in the lower crust in the prolongation of a seaward dipping reflector sequence. A remarkable result is that a second, isolated body of high seismic velocity ( v p up to 7.3 km/s) exists landward of the first high-velocity feature. The centres of both bodies are 60 km apart. The high-velocity lower-crustal bodies likely were emplaced during transient magmatic–volcanic events accompanying the late rifting and initial drifting stages. The lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the South Atlantic rift evolved by instantaneous breakup of longer continental margin segments. These segments are confined by transfer zones that acted as rift propagation barriers. A lower-crustal reflector was detected at 3 to 5 km above the modern Moho and probably represents the lower boundary of stretched continental crust. With this finding we suggest that the continent–ocean boundary is situated 70 km more seaward than in previous interpretations.

Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr‐Donets rift zone, Ukraine

Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat‐Dniepr‐Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide‐angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project together with gravity and magnetic data are analyzed for the structure and evolution of the Donbas Fold Belt, which is the uplifted and deformed part of the Dniepr‐Donets Basin. The seismic data are used for identification of large‐scale crustal structures and modeling of the seismic velocities of the crust and uppermost mantle. A ray‐trace‐based velocity and density model is derived by joint inversion of gravity and traveltime data. The inversion result reveals a zone of high density and velocity beneath the basin at middle to lower crustal levels, slightly offset to the NE of the rift axis. Full waveform synthetic seismograms, matching the observed data, show high‐amplitude and low‐frequency arrivals from this high‐density body as well as from the Moho. We interpret the high‐amplitude, low‐frequency signals as reflections from layered magmatic rocks, which intruded into the ductile lower crust during the main rift phase and subsequently were sorted by fractional crystallization. The intrusive material thickened the lower crust by approximately 50%. This may explain the enigmatic flat Moho topography across the rift zone which has been significantly stretched (β = 1.3). The rifting initiated in the late Devonian (Frasnien) as a consequence of back‐arc extension in relation to subduction of the Paleo‐Tethys Ocean. The subducting oceanic slab may have enriched the mantle with volatiles and created convection, leading to strong partial melting, upwelling, and continued rifting in the Famennien. We interpret the asymmetrical rift geometry as a combination of pure and simple shear tectonic processes.

Tectonic framework of a Paleoproterozoic arc-continent to continent-continent collisional zone, Trans-Hudson Orogen, from geological and seismic reflection studies

Vertical incidence seismic data were collected along a 300 km-long profile across the northwestern flank of the Trans-Hudson Orogen in Saskatchewan (line S2b). The present study integrates the seismic data with previously published geological maps and recent results from the La Ronge – Lynn Lake Bridge Project that provide new constraints on the lithological, structural, and tectonic framework of this collisional zone. An interpretative section, based on the seismic reflectivity and surface geological constraints, is presented. The integrated data suggest the following key elements. (1) Although an early foreland-vergent accretion history is suggested by surficial geological constraints, the predominant fabric is reflected by penetrative hinterlandward vergence. (2) A zone of more reflective lower crust with hinterland-verging fabric may represent "fossil" reflectors from a pre-collisional accretionary phase. (3) The Archean to Paleoproterozoic Sask Craton may extend in the lower middle crust to at least beneath Rottenstone Domain, consistent with what is observed along strike on other Lithoprobe seismic profiles. (4) The Needle Falls and Parker Lake shear zones do not appear to be associated with obvious reflectors, consistent with observations from Reindeer Lake, suggesting that they merely rework the Wathaman Batholith intrusive margin and are not fundamental sutures. (5) The La Ronge and Rottenstone domains show nearly identical seismic properties, consistent with recent mapping results in the Reindeer Lake area that suggest that they are temporally and, at least in part, lithologically related in the area covered by, and to the east of, seismic line S2b.

Must magmatic intrusion in the lower crust produce reflectivity?

The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin.

Contrasts in Tectonic style of the Central and Southern Appalachians, United States: Insights from seismic reflection data

Two reflection seismic transects, one across the central Appalachians in Virginia and the other across the southern Appalachians in Georgia, reveal a significant contrast in mid- and lower crustal reflectivity from east to west. Data from east of the Blue Ridge geologic province in Virginia and to the east of the Inner Piedmont in Georgia show a highly reflective crust extending from the near-surface to the Moho, including zones of east-dipping reflections, a sub-horizontal reflection signature at ∼7 seconds, and a west-dipping Moho. Reflection seismic data from west of the Blue Ridge in Virginia and Inner Piedmont farther south are characterized by reflector geometries related to deformation above a master decollement, leading to classic ‘thin-skinned’ tectonic structures in the overlying allochthon, and few if any apparent structures in the underlying basement. The location of the Iapetan rifted margin, the preexistence of favorably oriented structures to the east of this point, and sub-horizontal weak zones within the lower Paleozoic shelf strata have played critical roles in the distribution of seismic reflector geometry. Seismic reflection signatures seen in the southeastern United States are a result of multiple episodes of deformation from the early Paleozoic through the middle Mesozoic. Oblique stresses during late Paleozoic time produced transpression that manifested itself as predominantly strike-slip faulting to the east of the Blue Ridge/Inner Piedmont. Onlapping lower Paleozoic shelf strata responded to tectonic stresses through thin-skinned deformation above a master decollement during the late Paleozoic Alleghanian orogeny, aided in part by sub-horizontal zones of weakness in the strata. This partitioning of strain was supported via tectonic buttressing provided by Precambrian continental crust that was little deformed in the Taconic orogeny. During the Alleghanian orogeny, the variations in Valley and Ridge deformational style between the central and southern Appalachians were controlled by the original shape of the continental edge. Further deformation during Mesozoic extension occurred to the east of the Precambrian rift margin in the region where favorably oriented faults were reactivated, leading to the rotation of the fault zones from more steeply dipping initial orientations, the merging of the mid-crustal reflection zone with the Moho, and the formation of Mesozoic basins and antiformal reflections in the seismic sections.