Andrew J. Calvert

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E-W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N-S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end-member metamorphic core complex-type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Inverse Q filtering by Fourier transform

Although some attention has been paid to the idea that seismic migration is equivalent to a type of deconvolution (of the spatial wavelet), less thought has been given to the opposite perspective: that deconvolution (of the earth Q filter) might itself be equivalent to a form of migration. The key point raised in this paper is that a dispersive 1-D backward propagation can form the basis of a number of different algorithms for inverse Q filtering, each of which is akin to a particular migration algorithm. An especially efficient algorithm can be derived by means of a coordinate transformation equivalent to that in the Stolt frequency‐wavenumber migration. This fast algorithm, valid for Q constant with depth, can be extended to accommodate depth‐variable Q by cascading a series of constant Q compensations, as in cascaded migration. By combining a cascaded phase compensation with a windowed approach to amplitude compensation, we obtain an algorithm that is sufficiently efficient to be used routinely for pre...

ARCHEAN CONTINENTAL ASSEMBLY IN THE SOUTHEASTERN SUPERIOR PROVINCE OF CANADA

Between 1988 and 1993, seismic reflection and refraction surveys were acquired across the medium- to high-grade Opatica plutonic gneiss belt, the low-grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 Ma of various exotic terranes against a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65-km depth in the upper mantle below the Opatica belt. In contrast to the mainly orthogneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt comprises metasedimentary and igneous rocks, plus imbricated units of unknown affinity. Relict midcrustal accretionary complexes of substantial size, whieh are indicative of primary suture zones, are interpreted near the northern and southern limits of the Abitibi belt. An interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous. Evidence of large-scale extension appears to be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extension. Unusually high seismic P wave velocities, 7.5–8.2 kms−1, are present in the lower 8 km of the Abitibi crust, and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this process may be linked to early Proterozoic magmatism.

Archaean crustal growth and tectonic processes: a comparison of the Superior Province, Canada and the Dharwar Craton, India

Abstract We present a comparison of the processes involved in the tectonic evolution of two Archaean cratons, the Superior Province of Canada and the Dharwar Craton of India. These two cratons exhibit distinct map patterns, the Superior Province being dominated by elongate belts, while the Dharwar Craton is characterized by dome and basin features. We suggest that certain tectonic processes operating in the Phanerozoic, such as enhanced mantle plume activity, subduction of young (warm) oceanic crust, and faster than usual accretion of crust, may have been the norm during the Archaean. In the Superior Province rapid crustal growth occurred, largely due to horizontal tectonic forces. Models analagous to modern plate tectonics are applicable, but the rates of convergence and accretion exceeded those normal for the present day. Accreted crust was warm and subject to more ductile deformation than in modern accretionary zones. These accreted arc, ocean-floor and ocean-plateau fragments would have been underlain by a thick refractory, buoyant, warm lithospheric root that was rapidly underplated (or imbricated) below the recently accreted terranes. In the Dharwar craton a major thermal event appears to characterize its evolution at 2.5 Ga. Reheating of the lower and middle crust in response to magmatism and metamorphism, resulted in diapirism and growth of crust in a vertical sense. The southern Superior Province’s evolution reflects accretion at the margins of a protocraton, while the Dharwar craton’s tectonic environment may reflect plume impact and incipient rifting in the centre of an Archaean craton.

Crustal‐scale shortening and extension across the Grenville Province of western Québec

A deep seismic reflection survey shot in 1993 crosses almost the full width of the Grenville Province in western Quebec. The seismic transect provides a very clear image of the crust-mantle boundary and the most precise definition to date of the various Grenvillian terranes. The crust is around 44 km thick beneath the Grenville Front but thins rapidly to 36 km some 60 km to the southeast; it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect, far from the inferred location of the main Grenvillian collision. The Grenville Front zone, in which NW directed thrusting at about 1 Ga was followed by SE directed extension, is defined by discontinuous, SE dipping reflections, which extend down to the Moho. To the southeast, the overlying migmatitic Archean parautochthon, almost half of the transect, is characterised instead by NW dipping reflectors extending into the lower crust. These reflectors are in turn truncated to the southeast by a 12-km-thick zone of intense SE dipping reflections, the Baskatong crustal ramp. The base of the allochthonous terranes (Allochthon Boundary Thrust) is likely located at this ramp, which flattens out at around 30-km depth into the base of the relatively transparent intermediate crust. A highly reflective upper crustal deck corresponding to rocks of the Mont-Laurier terrane was thrust over the Baskatong crustal ramp and is represented further to the northwest by the klippelike Cabonga allochthon. The synformal, transparent Morin anorthosite-charnockite complex belongs to the same upper crustal level. Ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the NW directed propagation of tectonic transport during the Grenvillian orogeny, involving deformation and displacement along the Baskatong ramp with a relay into the Grenville Front zone. Postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the Grenville Front and affecting the crust up to 350 km away from the front.

Three-dimensional crustal structure of the Mariana island arc from seismic tomography

[1] A three-dimensional (3-D) seismic refraction survey was acquired over the Mariana volcanic arc at 14.5–18.5N and 145–147E. First-arrival traveltimes from this survey and from a separate 2-D survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3-D P wave velocity model using seismic tomography subject to smoothness constraints. The active arc, which initiated only 3–4 Ma ago, has an average crustal thickness of 18 km. Approximately 40 km to the east the inactive remnant of the rifted Eocene arc has an average crustal thickness of 21 km, due primarily to a thicker lower-crustal layer with velocities of 6.5–7.0 km s � 1 . Crustal production clearly varies both temporally and spatially, with some crustal layers, including the igneous forearc crust, varying in thickness by a factor of up to 2 along strike. Average P wave velocities within the upper crust of the modern arc are 240–360 m s � 1 lower than in the Eocene arc but are 280 m s � 1 higher within the lower crust. Middle crust with velocities of 6.0–6.5 km s � 1 is best developed beneath the Eocene arc. These results suggest an evolution of arc structure with increasing age: We infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation. Although tonalitic rocks may predominate in the transition from upper to middle crust, the bulk of the crust is essentially basaltic.

Deep, high-amplitude reflections from a major shear zone above the subducting Juan de Fuca plate

Seismic reflection data show that a regionally extensive band of landward-dipping reflections exists above the subducting Juan de Fuca plate at the western Canadian continental margin. The reflections truncate at depth a major terrane boundary mapped near the surface, and must have a structural origin. Many reflections are very strong, having reflection coefficients locally as high as 10%-20%.These amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts, if the pores are very thin; however, seismic data from the accreted sedimentary melange farther seaward, from which much of the deep reflective zone is derived, suggest that significant porosity contrasts do not occur alone. Because higher reflection amplitudes are found in regions of inferred high strain, both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks, possibly originating close to the subduction decollement.

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plates descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, ∼110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.

Radio tomography and borehole radar delineation of the McConnell nickel sulfide deposit, Sudbury, Ontario, Canada

In an effort to reduce costs and increase revenues at mines, there is a strong incentive to develop high-resolution techniques both for near-mine exploration and for delineation of known orebodies To investigate the potential of high-frequency EM techniques for exploration and delineation of massive sulfide orebodies, radio frequency electromagnetic (RFEM) and ground-penetrating radar (GPR) surveys were conducted in boreholes through the McConnell massive nickel-copper sulfide body near Sudbury, Ontario, from 1993-1996. Crosshole RFEM data were acquired with a JW-4 electric dipole system between two boreholes on section 2720W. Ten frequencies between 0.5 and 5.0 MHz were recorded. Radio signals propagated through the Sudbury Breccia over ranges of at least 150 m at all frequencies. The resulting radio absorption tomogram clearly imaged the McConnell deposit over 110 m downdip. Signal was extinguished when either antenna entered the sulfide body. However, the expected radio shadow did not eventuate when transmitter and receiver were on opposite sides of the deposit. Two-dimensional modeling suggested that diffraction around the edges of the sulfide body could not account for the observed held amplitudes. It was concluded at the time that the sulfide body is discontinuous; according to modeling, a gap as small as 5 m could have explained the observations. Subsequent investigations by INCO established that pick-up in the metal-cored downhole cables was actually responsible for the elevated signal levels. Both single-hole reflection profiles and crosshole measurements were acquired using RAMAC borehole radar systems, operating at 60 MHz. Detection of radar reflections from the sulfide contact was problematic. One coherent reflection was observed from the hanging-wall contact in single-hole reflection mode. This reflection could be traced about 25 m uphole from the contact. In addition to unfavorable survey geometry, factors which may have suppressed reflections included host rock heterogeneity, disseminated sulfides, and contact irregularity. Velocity and absorption tomograms were generated in the Sudbury Breccia host rock from the crosshole radar. Radar velocity was variable, averaging 125 m/mus, while absorption was typically 0.8 dB/m at 60 MHz. Kirchhoff-style 2-D migration of later arrivals in the crosshole radargrams defined reflective zones that roughly parallel the inferred edge of the sulfide body. The McConnell high-frequency EM surveys established that radio tomography and simple radio shadowing are potentially valuable for near- and in-mine exploration and orebody delineation in the Sudbury Breccia. The effectiveness of borehole radar in this particular environment is less certain.

LITHOPROBE reflection studies of Archean and Proterozoic crust in Canada

Abstract LITHOPROBE, Canadas national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the North American continent, is a multidisciplinary program spearheaded by seismic reflection studies. Five recently recorded seismic lines, discussed in this paper, are located in Precambrian regions: the Mesoproterozoic Grenville Province, the Paleoproterozoic Trans-Hudson orogen (THO), the Paleoproterozoic/Archean basement of Alberta, and the Archean Superior Province. Data acquired across the Grenville orogen in eastern Quebec show strong reflectivity throughout the crust; upper crustal reflections can be correlated with exposed structural elements, including extensional shear zones and packages of deformed high-pressure rocks (eclogites). In a marine survey across the Grenville orogen off southeastern Labrador, seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly. Data acquired across central Alberta show crustal-scale thrust stacking and imbrication of the Archean Hearne craton. To the east across the Trans-Hudson orogen, images of similar collisional features are observed. Geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1.8 Ga, suggesting that collisional tectonic activity was coeval over a broad crustal region, ca. 1000 km across strike. In the Superior Province, seismic data across a collision zone involving the northern Abitibi greenstone belt and the arc-related Opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s (∼30 km) into the mantle. The latter feature is interpreted as representing a relict 2.69-Ga-old suture associated with subduction, providing the first direct evidence that plate tectonics was active in the late Archean. These five examples, supported by other LITHOPROBE results, refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies, combined with other geoscience studies, can lead to a better understanding of Precambrian tectonics. Reflectivity persists throughout the crust; there is no general separation into a poorly reflective upper crust and a reflective lower crust. Crustal reflectivity in Archean and Proterozoic regions is as pervasive as that in areas of more recent tectonism. The Precambrian reflection Moho is generally well defined but shows a range of characteristics. Relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity.