David M. Fountain

Constitution of the Lower Continental Crust Based on Experimental Studies of Seismic Velocities in Granulite

Rocks of the granulite facies have been proposed as major constituents of the lower continental crust. To evaluate this possibility, compressional and shear wave velocities have been determined to pressures of 10 kb for 10 granulite samples, thus enabling comparisons of seismic data for the lower crust with the velocities and elastic properties of granulite rocks. The samples selected for this study range in composition from granitic to basaltic, with bulk densities of 2.68 to 3.09 g/cm 3 . At 6 kb, compressional ( V p ) and shear ( V s ) wave velocities range from 6.39 to 7.49 km/sec and from 3.36 to 4.25 km/sec, respectively. Velocities in granulite rocks are shown to vary systematically with variations in mineralogical constitution. Both V p and V s , increase with increasing pyroxene, amphibole, and garnet. Velocities increase with an increasing ratio of pyroxene to amphibole in hornblende-granulite subfacies rocks of approximately equivalent chemical compositions. Decreasing quartz content in granulite rocks produces an increase in V p and an accompanying decrease in V s , thereby significantly changing Poisson9s ratio. The range of velocities measured for the granulite samples is similar to the range of seismic velocities reported for the lower continental crust; thus, the hypothesis that granulite rocks are major lower crustal constituents is further strengthened. Furthermore, it is shown that lower crustal composition is extremely variable, and therefore valid discussions of composition must be limited to specific regions where seismic velocities are well known. The use of seismic velocities in estimating lower crustal composition is illustrated for the Canadian Shield in Ontario and Manitoba.

The Ivrea—Verbano and Strona-Ceneri Zones, Northern Italy: A cross-section of the continental crust—New evidence from seismic velocities of rock samples

Abstract Seismic velocities under confining pressures to 10 kbar have been measured for rocks of the Ivrea—Verbano and Strona—Ceneri Zones of northern Italy, a metamorphic complex thought to represent a cross-section of the continental crust and crust—mantle boundary. Laboratory-determined compressional wave velocities for schists and gneisses of the amphibolite facies found in the upper levels of the section (having an average density of 2.74 g/cm3) average 6.45 km/sec at pressures between 6 and 10 kbar. These increase with depth to values greater than 7.1 km/sec for amphibolites and rocks of the amphibolite—granulite facies transition and to 7.5 km/sec. (average density 3.06 g/cm3) in intermediate and mafic granulite facies rocks near the base of the section. Compressional wave velocities then abruptly increase to 8.5 km/sec in ultramafic complexes near the Insubric Line. Regional geophysical surveys show that P g is 6.0 km/sec (density of 2.7 g/cm3), P∗ is 7.2–7.4 km/sec (density of 3.1 g/cm3) and P n is 8.1 km/sec, values which are in agreement with the laboratory data when effects of temperature are taken into consideration. Estimated thicknesses of exposed rock units are in reasonable agreement with thicknesses determined for crustal layers in seismic refraction experiments. The agreement between the regional crustal structure and the laboratory-determined values of velocity and density provides strong evidence for the hypothesis that the rocks of this metamorphic complex represent a cross-section of the continental crust of the Po Basin. Using the Ivrea—Verbano and Strona—Ceneri sequence as a model of the continental crust, the crust of northern Italy is shown to consist of a thick series of metamorphic rocks with greenschist facies rocks occupying the uppermost levels. These grade downward into amphibolite facies gneisses and schists with occasional granitic intrusives. The Conrad discontinuity is marked by a change from silicic and intermediate amphibolite facies gneisses to intermediate and mafic granulite facies rocks in which hydrous minerals diminish in abundance and thus represents a distinct transition in terms of both composition and metamorphic grade. The lower crust is dominated by a heterogeneous series of mafic and metapelitic rocks in the granulite facies. Importantly, metasedimentary rocks of intermediate silica content found in the complex can have compressional wave velocities equivalent to velocities in mafic rocks suggesting that the lower continental crust everywhere is not necessarily mafic in composition. Ultramafic complexes near the Insubric Line may represent the upper mantle of the continent and their setting suggests that the continental crust-upper mantle boundary is sharp and is not isochemical.

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.

OCEANIC CRUSTAL BASEMENT: A COMPARISON OF SEISMIC PROPERTIES OF D.S.D.P. BASALTS AND CONSOLIDATED SEDIMENTS

Christensen, N.l., Fountain, D. M. and Stewart, R. J., 1973. Oceanic crustal basement: a comparison of seismic properties of D.S.D.P. basalts and consolidated sediments. Mar. Geol., 15: 215-226. Compressional (V p) and shear (V s) wave velocities have been measured to 1.0 kbar for 14 cores of well-consolidated sedimentary rock from Atlantic and Pacific sites of the Deep Sea Drilling Project. The range of Vp (2.05-5.38 km/sec at 0.5 kbar) shows significant overlap with the range of oceanic layer-2 seismic velocities determined by marine refraction surveys, suggesting that sedimentary rocks may, in some regions, constitute the upper portion of layer 2. Differing linear relationships between Vp and Vs for basalts and sedimentary rocks, however, may provide a method of resolving layer-2 composition. This is illustrated for a refraction survey site on the flank of the Mid-Atlantic Ridge where layer-2 velocities agree with basalt, and two sites on the Saya de Mallia Bank in the Indian Ocean where layer-2 velocities appear to represent sedimentary rock.

Possible lower crustal rocks recovered on Leg 31 by deep-sea drilling in the Philippine Sea

Abstract Compressional wave velocities measured in gabbroic rocks and metabasites recovered from Site 293 of Leg 31 in the Philippine Sea (on the Central Basin Fault) are correlative with seismic velocities determined for Layer 3. The lower crustal origin for these rocks suggested by this data is further supported by the similarity between these samples, dredge haul samples from fracture zones in the main ocean basins and rocks found in ophiolite complexes. These plutonic rocks were possibly introduced to the sea floor by movements along the Central Basin Fault, a major tectonic feature in the Philippine Sea, or formed as part of new ocean crust within a leaky transform fault.