Matthew H. Salisbury

Exposed cross-sections through the continental crust: implications for crustal structure, petrology, and evolution

Abstract The continental crust is exposed in cross-section at numerous sites on the earths surface. These exposures, which appear to have formed by obduction along great faults during continental collision, may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive Bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle. From an examination of five complexes which meet these criteria, it is concluded that the most prominent layering in the crust is not compositional but metamorphic. The lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks, respectively. Post-metamorphic granitic intrusions are common at intermediate to shallow levels. Position of discontinuities in refraction velocity, where present, commonly correspond to changes in composition or metamorphic grade with depth. The continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by COCORP. Field observations, coupled with geochemical data, indicate a complex evolution of the lower crust which can include anatexis, multiple deformation, polymetamorphism and reworking of older crustal material. The complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments.

Exposed Cross-Sections of the Continental Crust

Phanerozoic Sections.- The Ivrea crustal cross-section (northern Italy and southern Switzerland).- The exposed crustal cross section of southern Calabria, Italy: structure and evolution of a segment of Hercynian crust.- An exposed cross-section of continental crust, Doubtful Sound Fiordland, New Zealand geophysical and geological setting.- Uplift and exhumation of middle and lower crustal rocks in an extensional tectonic setting, Fiordland, New Zealand.- A crustal cross-section for a terrain of superimposed shortening and extension: Ruby Mountains-East Humboldt Range metamorphic core complex, Nevada.- Progress in tectonic and petrogenetic studies in an exposed cross- section of young (~100 Ma) continental crust, southern Sierra Nevada, California.- Characteristics of a continental margin magmatic arc as a function of depth: the Skagit-Methow crustal section.- The evolution of the Kamila Shear Zone, Kohistan, Pakistan.- Crustal formation at depth during continental collision.- Precambrian Sections.- A Field Guide to the Kapuskasing Uplift, a cross section through the Archean Superior province.- Major thrust faults and the vertical zonation of the middle to upper Proterozoic crust in central Australia.- The late Archean high-grade terrain of south India and the deep structure of the Dharwar Craton.- An oblique cross section of Archean continental crust at the northwestern margin of Superior Province, Manitoba, Canada.- Two transects across the Grenville Front, Killarney and Tyson Lake areas, Ontario.- Crustal Composition and the Role of Fluids.- Basaltic composition xenoliths and the formation, modification and preservation of lower crust.- Average composition of lower and intermediate continental crust, Kapuskasing structural zone, Ontario.- Fluid-rock interactions in the Ivrea Zone and the origin of high lower crustal conductivities.- Electrical conductivity: the story of an elusive parameter, and of how it possibly relates to the Kapuskasing Uplift (Lithoprobe, Canada).- Faults, Crustal Deformation and Emplacement Mechanisms.- Deformation sequence in the southeastern Kapuskasing structural zone, Ivanhoe Lake, Ontario, Canada.- The exhumation of cross sections of the continental crust: structure, kinematics and rheology.- The fluid crustal layer and its implications for continental dynamics.- Geophysical Structure and Properties of the Crust.- Geophysical interpretation of astrogravimetric data in the Ivrea Zone.- The nature of the Kapuskasing Structural Zone: results from the 1984 seismic refraction experiment.- Exposed continental crust: seismic results to be tested.- The structure of the crust and uppermost mantle offshore Britain: deep seismic reflection profiling and crustal cross-sections.- Intracrustal detachment and wedging along a detailed cross section in Central Europe.- Overview.- Strategy for exploration of the buried continental crust.- Exposed cross sections of the continental crust - synopsis.

Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites

Abstract Ji, S., Salisbury, M.H. and Hanmer, S., 1993. Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites. Tectonophysics, 222: 195–226. In order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones, we have undertaken experimental measurements of P-wave velocities up to 600 MPa for 20 granulite and upper amphibolite facies mylonites from the Snowbird tectonic zone (Canada). The rocks, whose composition ranges from ultramafic to felsic, have experienced extremely large ductile strain. It is found that the amphibolitic mylonites exhibit significant Vp anisotropy (6–13% at 600 MPa), whereas the pyroxene-bearing granulite-facies mylonites and the quartz or feldspathic (tonalitic, granitic and diatexitic) mylonites are quasi-isotropic. In order to constrain interpretation of the measured Vp properties, microstructural analyses have been systematically performed in the samples studied. It is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low. The amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong LPO ([001]∥lineation, and (100)∥foliation). The anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic: Vp(X) >Vp(Y) >Vp(Z), where X is parallel to stretching lineation, Y is normal to the lineation in the foliation plane and Z is normal to the foliation plane. But the anisotropy pattern in the mica-rich mylonites is transversely isotropic: Vp(X) = Vp(Y) >Vp(Z). If sufficiently large volumes of the crust display such patterns, this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques. The study demonstrates that while seismic reflectivity is strongly lithology-controlled, fabric-induced anisotropy can both enhance and decrease seismic reflectivity. Hence, the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones, but may indicate changes in composition or metamorphic grade.

Velocities, elastic moduli and weathering-age relations for pacific layer 2 basalts

Abstract Compressional (V p ) and shear (V s ) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both V p andV s show wide ranges (3.70to6.38km/sec forV p and1.77to3.40km/sec forV s at0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of −1.89and−1.35km/sec per100my forV p andV s respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.

Physical properties and seismic imaging of massive sulfides

Laboratory studies show that the acoustic impedances of massive sulfides can be predicted from the physical properties (Vp, density) and modal abundances of common sulfide minerals using simple mixing relations. Most sulfides have significantly higher impedances than silicate rocks, implying that seismic reflection techniques can be used directly for base metals exploration, provided the deposits meet the geometric constraints required for detection. To test this concept, a series of 1-, 2-, and 3-D seismic experiments were conducted to image known ore bodies in central and eastern Canada. In one recent test, conducted at the Halfmile Lake copper‐nickel deposit in the Bathurst camp, laboratory measurements on representative samples of ore and country rock demonstrated that the ores should make strong reflectors at the site, while velocity and density logging confirmed that these reflectors should persist at formation scales. These predictions have been confirmed by the detection of strong reflections from...

Shear-wave velocities, anisotropy and splitting, in high-grade mylonites

Shear-wave velocities have been measured for 10 high-grade, granulite- and upper amphibolite-facies mylonites from the Snowbird tectonic zone (Canada) at various pressures up to 600 MPa in order to explore the kinematic significance of shear-wave splitting in the lower crust. At 600 MPa, Vs anisotropy ranges from 11% in amphibolitic mylonites to 2% in granulite-facies pyroxene-bearing and tonalitic mylonites. Hornblende-bearing granitic mylonites display intermediate Vs anisotropy (6–7%). Vs anisotropy is lower than Vp anisotropy in hornblende-rich mylonites, whereas Vs anisotropy is higher than Vp anisotropy in feldspar-rich mafic, tonalitic, granitic and diatexitic mylonites. Shear-wave splitting is pronounced in rocks displaying strong S-wave anisotropy. In most of the high-grade mylonites studied, the polarization direction of the first arrival, Vs1, corresponds to the extension lineation (X) when propagation is parallel to Y (perpendicular to lineation and parallel to foliation), or to the Y-direction when propagation is parallel to X. For all other propagation-directions, the polarization direction of Vs1 is oblique to the X-, Y- and Z-directions. Therefore, it may be difficult to infer the structural implications of shear-wave splitting in lower continental crustal shear zones if one does not know (1) the orientation of the propagation and fast vibration-directions with respect to in-situ lineation and foliation, and (2) the lattice preferred orientation of in-situ rock-forming minerals.

Sea floor spreading, progressive alteration of layer 2 basalts, and associated changes in seismic velocities

Compressional (V p) and shear (V) wave velocities and the dependen t elastic constants have been determined by the pulse transmission technique to 10 kb for basalts from six Atlantic sites of the Deep Sea Drilling Project. Vp and Vsare found to vary linearly with density,V ranging at 0.5 kb from 4.53 km/sec for the lowest density sample (2.40 g/cm3) to 6.37 km/sec for the highest [2.92 g/cm3), while V~ranges similarly from 2.09 to 3.38 km/sec. This range of Vp is consistent with the wide variability in layer 2 velocities found from refraction studies. Petrographic and X-ray analyses demonstrate that the wide range in densities observed for these basalts is the result of progressive clay alteration and low-grade metamorphism; elastica\1y this can be noted as an increase at 10 kb in d Vp/dP from 0.01 to 0.05 km/sec kb and an increase at 0.5 kb of compressibility from 1.38 to 2.58 Mb-3 with decreasing density. Of particular interest, in the ridge traverse of Leg 3 where the basalt ages are we\1 known, density and velocity decrease linearly with age from the ridge crest to the abyssal plain. At least provincially, where uniformity of rock type may be suspected, it may be possible to derive submarine weathering rates and approximate layer 2 ages from careful refraction surveys.

Seismic reflectivity of a finely layered, granulite-facies ductile shear zone in the southern Grenville Province (Quebec)

Abstract Lithoprobe reflection profile 54 across the granulite-facies Morin shear zone (MSZ) in the Grenville Tectonic Province (Quebec, Canada) reveals a west-dipping zone of strong reflections. The origin of these reflections has been investigated by detailed field mapping, laboratory measurements of P-wave velocities and densities of representative rock samples from the main lithological units, and forward synthetic modeling. Petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties. Synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin-layer clusters, likely resulted from intensive tectonic transposition and metamorphic differentiation. Lattice-preferred-orientation (LPO)-induced anisotropy of granulite-facies mylonites (except for those containing abundant sillimanite, biotite or amphibole) is very low, and hence unlikely to be the primary cause for reflections. Confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 MPa. Since composite reflections of thin-layer clusters can cause high reflectivity, the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin-layer, mafic clusters within dominantly felsic rocks.

Listric faults imaged in oceanic crust

Deep seismic reflection profiling has provided exceptional images of listric faulting in oceanic basement of Jurassic age off the coast of Nova Scotia. The faults dip toward the Mid-Atlantic Ridge and sole near the base of the crust at a common depth of about 4 km, presumably the brittle-ductile transition at the time of deformation. At least five rotated fault blocks, with dips ranging from 0° at the surface to 45° at the base, are observed along a 36 km flow-line segment, suggesting a stretching factor, β, of 1.5-2.5. The faults can be traced upward with small displacement in the overlying sedimentary strata to approximately the middle Cretaceous strata, implying continued movement or consolidation for about 70 m.y. after crustal formation.

Magnitude and symmetry of seismic anisotropy in mica- and amphibole-bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau

We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica- or amphibole-bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually assumed in the interpretation of seismic data. The average bulk Vp anisotropy at 600 MPa for the chlorite schists, mica schists, phyllites, sillimanite-mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show Vp anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite-mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite, and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation, forming orthorhombic anisotropy with Vp(X) > Vp(Y) > Vp(Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica- or amphibole-bearing schists and mylonites are approximately transversely isotropic in terms of S wave velocities and splitting although their P wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan Plateau. The N-S to NW-SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica- and amphibole-bearing rocks deformed by predominantly compressional folding and subordinate strike-slip shear. These blocks have been rotated clockwise 70–90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia.