West-dipping Reflections Research Articles

Fossil divergent double-subduction zone in the Great Xing’an Range, NE China: Evidence from a deep seismic reflection profile

Divergent double-subduction is an important component of plate tectonics and continental growth at modern active convergent margins. However, such a process is rarely identified by deep seismic reflection (DSR) profiles, which casts doubt on its past significance. We report here a 416 km DSR profile of large dynamite shots across the Great Xing’an Range (NE China), which shows multiple arc-shaped reflections in the upper-middle crust of the Xing’an terrane, and densely layered reflection structures suggesting tectonic nappes in the lower crust. This new evidence, together with recent geological studies, indicates that the Xing’an terrane may not be a micro-continental block as previously thought, but a Paleozoic accretionary complex. In addition, there is a strong west-dipping reflection at the boundary between the Xing’an and Erguna terranes in the western DSR profile extending from the lower crust to the lithospheric mantle, suggesting that this fossil subduction zone is most likely the Toudaoqiao-Xinlin suture, which separates the Xing’an and Erguna terranes. In addition, an east-dipping strong reflection was also found in the eastern profile between the Xing’an and Songliao terranes, corresponding to the Hegenshan-Nenjiang suture at the surface. Given the newly identified crust-mantle structure, the opposite subduction zone, and other geological evidence, we propose a new model that the Xing’an terrane was developed by divergent double-subduction of the Paleo-Xing’an Ocean (part of the Paleo-Asian Ocean in the area) from ca. 500 Ma to ca. 300 Ma. Our study provides the first DSR evidence for a divergent double-subduction in the Paleozoic, which emphasizes its importance in past accretionary orogens.

Crustal structure of the Middle Urals based on seismic reflection data

Abstract EUROPROBE-related seismic reflection surveys in the Middle Urals, Russia (latitude 56-62°) since 1993 have led to an increased understanding of the crustal structure and tectonic evolution of this region. A 400 km long profile now extends from the foreland basin in the west well into the West Siberian Basin in the east. Bivergent structures characterize the upper crust of the Uralide orogen, whereas the middle and lower crust generally contain gently west-dipping reflections. A crustal root is imaged down to almost 60 km beneath the exposed Urals. Below the foreland and the West Siberian Basin the lower crustal reflectivity is pronounced and the Moho lies at a depth of 40-45 km. Below the foreland on the recently acquired Serebrianka-Beriozovka profile, two sets of late arriving (20-25 s) reflections are present. One set reflects from a zone in the mantle at about 60-70 km depth that strikes ENE and dips about 45° to the SSE. The other set may represent imbricated lower crust. Major events during the Palaeozoic tectonic evolution of the Middle Urals were: continental and oceanic rifting (Late Cambrian to Early Ordovician); development of a passive continental margin (Mid-Ordovician to Mid-Carboniferous); intra-oceanic subduction below the Tagil arc (Silurian to Devonian); east-dipping subduction of the Baltica plate (Silurian to Early Devonian); possible subduction reversal with formation of the Alapaevsk island arc and the Krasnoturjinsk-Petrokamensk active continental margin (Devonian to Early Carboniferous); active building of a mountain belt and intrusion of collision-related granitic plutons (Carboniferous to Permian).

Products of 2.65–2.58 Ga orogenesis in the Slave Province correlated with Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) seismic reflection patterns

Reflection patterns along Lithoprobe Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) seismic reflection line 1 in the southwestern Slave Province are interpreted as products of tectonic wedging during late Archean lithospheric convergence. The interpretation is aided by application of new seismic imaging techniques, by correlation of upper crustal reflection patterns to known geology, and by a comparison of lithospheric reflection patterns to similar convergent zones elsewhere. In the Yellowknife area, reflection patterns consist of (1) east-dipping reflections at 12–14 s that project into the upper mantle, (2) a wedge-shaped body in the lower crust with an east-dipping reflection fabric that is truncated on the west by (3) a series of west-dipping reflections that outline thrust-and-fold structures in the upper crust. The similarity of these reflection patterns to those of the Proterozoic Fort Simpson – Hottah collision zone ~300 km to the west provides support for the interpretation that reflection patterns beneath the Slave Province are also products of collisional tectonics. Rocks within the Slave Province preserve evidence of a ~2.65–2.58 Ga pan-Slave orogenic event, in which the >2.9 Ga Central Slave Basement collided with the ~2.7 Ga juvenile eastern Slave Province. Their suture is interpreted to be a west-dipping surface at 4–5 s (12–15 km) beneath Yellowknife and to project to the surface east of the profile. In the lower crust and upper mantle, east-dipping reflections are interpreted to delineate a coeval subduction zone and accretionary wedge. The upper crustal thrust-and-fold structures are likely linked to gold-bearing shear zones at Yellowknife. These results provide tantalizing evidence that processes similar to those of modern convergent zones were operational at 2.65–2.58 Ga.

Crustal structure and evolution of the Trans‐Hudson orogen: Results from seismic reflection profiling

A 400‐km‐long deep seismic reflection transect across northeastern Montana and northern North Dakota reveals the crustal‐scale structural fabric of the Early Proterozoic Trans‐Hudson orogen beneath the Williston basin. Comparison with deep seismic reflection data across the Canadian portion of the same orogen ∼700 km to the north reveals first‐order similarities in crustal architecture but documents significant along‐strike variation in orogenic evolution. Both transects display a broad crustal‐scale antiform axial to the orogen. In the north, geologic data suggest that this antiform is cored by an Archean microcontinent. In the south, west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the Archean Wyoming craton. Within the Wyoming craton, the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the Early Proterozoic Trans‐Hudson orogen as interpreted from potential field and drill core data. On the basis of subsurface geochronologic data from the crystalline basement and by analogy with the Glennie domain within the exposed Trans‐Hudson orogen in Canada, we suggest that the southern antiform is cored by an Archean crustal fragment that was caught up in the terminal collision of the Wyoming and Superior cratons during Hudsonian orogeny. The eastern side of the Trans‐Hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections. We interpret the easterly dip of these reflections as evidence that the Superior province was thrust westward over the interludes of the orogen during terminal collision. Although juvenile Early Proterozoic terranes characterize the exposed segment of the Trans‐Hudson orogen in Canada, limited drill core information within the Dakota segment of the orogen shows a predominance of granulitic Archean age crust. This difference in basement lithologies along strike within the orogen may indicate the following: either juvenile crust comparable to that exposed in the northern Trans‐Hudson was never present in the south, or it was removed by progressive over thickening, erosion, and/or faulting. Postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north.

Seismic reflection profiles across the central Kapuskasing uplift

The central Superior Province is transected by the intracratonic Kapuskasing uplift, which contains rocks exhumed from 30 to 35 km paleodepth. As part of the Lithoprobe Kapuskasing transect, approximately 52 km of 16 s seismic reflection data were collected in the central segment of the uplift along three profiles that traverse the northern Groundhog River block, the bounding Saganash Lake fault, and the eastern Val Rita block. The seismic sections have the following characteristics in common: (i) a complexly reflective uppermost portion (< 1 s) limiting correlation of reflective zones and surface features; (ii) numerous subhorizontal, east- and west-dipping reflection zones; and (iii) a significant reduction in reflectivity beyond the refraction-defined Moho (~ 14 s). Beneath the Groundhog River block a series of straight, west-dipping (~ 20°) reflection zones between 2 and 10 s is underlain by subhorizontal reflections in the lower crust. Across the Saganash Lake fault, the Val Rita block is characterized by a maze of discontinuous, curvilinear reflections with general easterly dip down to 8 – 10 s, below which west-dipping events are prominent. A north–south cross profile reveals a highly reflective crust with dominantly horizontal reflection geometry below the Saganash Lake metavolcanic belt, and a steep truncation of reflection zones down to at least 7 s, which correlates with the surface trace of the Nansen Creek fault. This fault resembles well-known strike-slip faults in intraplate settings. The Saganash Lake fault, variably interpreted as a west-side-down normal fault with up to 15 km of throw or a major strike-slip zone, may be visible as a west-dipping, weakly reflective zone steeply truncating east-dipping reflections and becoming listric at depth. This interpretation accords with surface geological observations and gravity models for the structural geometry of the region in which the Groundhog River block is a thin thrust sheet of granulite perched on Abitibi belt rocks and truncated on the west by the crustal-scale Saganash Lake fault. Alternatively, the fault could be a seismically unresolved major transcurrent structure juxtaposing blocks with disparate reflection patterns in the upper 8 s. Limited amounts of late strike-slip motion have been inferred from various geophysical studies.

Is the Sevier Desert reflection of west-central Utah a normal fault?

A prominent west-dipping reflection that can be traced in seismic-reflection profiles over an area of 7000 km 2 beneath the Sevier Desert basin of west-central Utah is generally referred to as the Sevier Desert detacbment and is widely regarded as one of tbe best examples of an upper-crustal low-angle normal fault. The absence of evidence for fault-related deformation in drill cuttings and core from two industry boreholes that intersect this feature casts doubt on the fault interpretation. The existing interpretation is based mainly on the observation that high­ angle normal faulting is restricted largely to Tertiary sedimentary and volcanic rocks above the reflection. An alternative explanation is tbat the bigh-angle faults are related to the withdrawal of early deposited lacustrine salt, which even today is as much as 2 km thick. Reevaluation of the seismic data suggests that the Sevier Desert reflection consists of two spatially and genetically distinct segments: a shallow segment here interpreted as an unconformity between Paleozoic and Tertiary strata and a fortuitously aligned deeper segment that is traceable to mid- and lower­ crustal levels and that appears to represent a thrust fault related to the Cretaceous Sevier orogeny.

Crustal structure of the Valhalla complex, British Columbia, from Lithoprobe seismic-reflection and potential-field data

The Valhalla complex, situated in the Omineca crystalline belt in southeastern British Columbia, is a Cordilleran metamorphic core complex bordering the suture zone between Quesnellia and North American rocks. The region is tectonically interposed between a convergent plate margin along Canada's west coast and the stable North American craton, and is characterized by a crustal thickness of ~ 35 km, high surface heat flux, and elevated lower crustal electrical conductivity. In this study, Lithoprobe deep-crustal seismic-reflection data, potential-field data, and geological constraints have been used to gain a better understanding of crustal structure in the vicinity of the Valhalla complex. Analysis of Bouguer gravity and total-field aeromagnetic data indicates that mafic oceanic rocks and various syn- and post-accretionary granitoid plutonic rocks are not major constituents of the upper crust underlying the complex. The seismic data reveal a moderately reflective upper crust and image several fault zones, including a very high amplitude, west-dipping reflection that is interpreted as a significant Late Cretaceous or Paleocene thrust fault. The fault-zone reflectivity may be related to compositional heterogeneity and (or) seismic anisotropy associated with mylonites. The lower crust appears to be nonreflective, in contrast with other areas of high surface heat flux and elevated lower crustal conductivity. Taken together, the various data show that the Valhalla complex is likely cored by North American metasedimentary rocks and reveal features related to the Jurassic to Paleocene compressional fabric, which has been largely overprinted at the surface by subsequent Eocene extension.

Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex

Summary. LITHOPROBE has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern Canadian Cordillera. These reflection profiles, obtained during the Fall of 1985, extend from the Rocky Mountain thrust and fold belt, across the Rocky Mountain Trench, Purcell anticlinorium, Kootenay Arc, Nelson batholith and Valhalla gneiss complex. North American basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the Kootenay Arc. The Purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous North American basement. Proterozoic supracrustal rocks, thickened by folding and thrusting, occupy the core of the anticlinorium. Steeply dipping surface structures of the western Purcell anticlinorium and Kootenay Arc appear to be truncated at 3 - 4 s (9-12 km) by a gently east-dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below. Beneath the Kootenay Arc, at a travel time of 9-10 s (27-30 km), the North American basement seems to be truncated by the major east-dipping Slocan Lake fault zone, which can be traced from its surface exposure at the east edge of the Valhalla gneiss complex eastward to near the base of the crust. A high amplitude, west-dipping reflection underlies the Valhalla complex and may be related to a major compressional shear zone.

Crustal structure of the Basin and Range–Sierra Nevada Transition from COCORP deep seismic-reflection profiling

Deep seismic-reflection data collected by COCORP along a 145-km-long line in westernmost Nevada and easternmost California as part of the 40°N latitude transect of the western Cordillera reveal the geometry of three crustal-scale features associated with the Walker Lane and the transition from the Basin and Range extensional province to the Sierra Nevada crustal block. The Walker Lane, a zone of late Cenozoic strike-slip and normal faults crossed in the central third of the line, is underlain by a zone of moderately west-dipping reflections that terminate at a discontinuous mid-crustal zone of subhorizontal reflections extending across most of the line at depths ranging from 14 to 23 km. We interpret the mid-crustal reflections as an active detachment accommodating strike-slip and extensional displacements. It may coincide with the base of the Sierran batholith, a shear zone, and/or the transition from brittle to ductile crustal deformation. The dipping reflections are inferred to be from a moderately dipping (45°) fault that soles in this mid-crustal detachment and accommodates the strike-slip and normal displacement of the northern Walker Lane. Discontinuous reflections at 9.5–10.2 s two-way travel time (28–30 km) on the eastern half of the line are interpreted as the Moho, similar to the Moho imaged by COCORP lines farther east in Nevada. Moho reflections are imaged at 11.0–11.5 s (33–35 km) in the western third of the line, and project westward toward similar reflections as deep as 12–13 s (37–41 km) beneath the eastern Sierra Nevada, and so the crust appears to thin gradually from the Sierra Nevada into the Basin and Range. The lower crust east of the Walker Lane is relatively highly reflective, characteristic of COCORP data from other Nevada lines, but the lower crust west of the Walker Lane is less reflective, similar to COCORP data from the northern Sierra Nevada. The change in reflectivity of the lower crust also appears to occur gradually, with no sharp offsets or boundaries. On the basis of the COCORP data, therefore, the transition between the Basin and Range and Sierra Nevada appears to be a wide (∼100 km) zone of crustal thinning possibly accompanied by a change in deformation fabric that may correspond to changes in lower crustal rheology or degree of deformation. This transition zone is dominated by a moderately dipping fault underlying the Walker Lane and accommodating strike-slip and normal displacement down to a mid-crustal detachment level.

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

Cocorp deep seismic reflection traverse of the interior of the North American Cordillera, Washington and Idaho: Implications for orogenic evolution

Deep seismic reflection data collected by the Consortium for Continental Reflection Profiling along a transect of the interior of the northwestern U. S. Cordillera indicate the presence of crustpenetrating faults; these faults developed during an orogenic cycle consisting of Jurassic to Paleocene accretion‐related crustal thickening followed by Eocene crustal thinning. The reflection data also suggest that the Moho is a young, dynamic feature which developed a relatively smooth, flat geometry beneath a complexly deformed crust, probably during an Eocene episode of crustal extension. The profiles reveal prominent west dipping reflection sequences (and associated diffractions) interpreted as a Mesozoic thrust system of crustal dimensions that linked accretion‐related shortening in the western Cordillera with the thrust belt in the eastern Cordillera, east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated Eocene crustal extension, and essentially flat Moho reflections at 33–35 km beneath complexly deformed crust. These features are consistent with a sequence of (1) late Mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the North American passive continental margin along a Jurassic thrust (2) propagation of crust‐penetrating thrusts to shallower levels in the east, where they probably formed the sole of the Rocky Mountain Thrust Belt (3) Eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and (4) development of a flat Moho geometry during Cenozoic extension and magmatism. These data provide new information about the structure of the entire crust in the Cordilleran interior and offer a working model, both for evolution of Cordilleran core complexes and possibly for other collisional orogens.

Adirondack-Appalachian crustal structure: The COCORP Northeast Traverse

Reflection surveys from northeastern New York and central Vermont, the first elements of a COCORP seismic traverse of Grenville and Appalachian terranes in the northeastern United States, indicate that an unusual zone of layered reflections characterizes the lower crust beneath the eastern Adirondacks and that Grenville-age “basement” of the Green Mountains may actually be allochthonous, perhaps thrust over Paleozoic shelf rocks. The top of the Adirondack deep layered sequence is found at about 20 km beneath the Marcy meta-anorthosite massif. It appears to dip about 10° northwestward and wedge out against more horizontal reflections at about 28 km. This intracrustal sequence may correspond with a previously inferred increase in electrical conductivity. Possible interpretations range from its being a layered igneous complex to its representing a tectonically buried metasedimentary assemblage. Although graphitic schists and iron or sulfide segregations in such an assemblage might contribute to its electrical characteristics, the problematical issue of water in the lower crust—if and how it got there and remained—is again raised, if not resolved. Other intrabasement features include major east- and west-dipping reflections, possibly corresponding to faults; arched reflections that may represent folds; a well-defined lens-shaped zone virtually devoid of reflections; and possible Moho reflections. Unequivocal sub-Moho reflections are absent. Superimposed at shallower levels on the eastern part of the traverse are Appalachian structures. Reflections down to about 4.5 km beneath the Taconic Mountains are interpreted to represent Paleozoic shelf sediments of the Middlebury synclinorium underlying the Taconic allochthons. Possibly correlative, although less well defined, reflections appear to extend in disrupted fashion beneath the Precambrian rocks of the Green Mountains, suggesting a major basement overthrust. The inferred decollement for this overthrust steepens rapidly beneath the east flank of the Green Mountains. East-dipping reflections at the east side of the Green Mountains may represent another major thrust zone involving basement and overlying metasedimentary slices. The basement overthrusting suggested by this study is similar in certain respects to the style, if not the extent, of that demonstrated by previous COCORP surveys in the southern Appalachians.