Keith Benn

Overprinting of magnetic fabrics in granites by small strains: numerical modelling

Abstract The anisotropy of magnetic susceptibility (AMS) is used to determine weakly defined mineral preferred orientation fabrics in granites. However, the effects of small strains superimposed on the low-intensity AMS fabrics typical of these rocks is not well understood. Numerical simulations are presented which investigate the effects of small superimposed strains on the shapes, magnitudes and orientations of pre-existing AMS fabrics, which are similar to those reported in the literature for undeformed granites. A computer-generated biotite petrofabric is subjected to progressive strain, using Marchs strain response model, for pure shear (plane strain), axially symmetric shortening and simple shear regimes. Results indicate that a pre-existing AMS fabric might be significantly modified or overprinted by small strains (≈10–15% strain in the pure shear and axially symmetric shortening regimes, 0.5 ⩽ γ ⩽ 1.0 for simple shear). A new AMS fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric. At low strains, the evolution of the shape of the AMS ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework; the AMS ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain. This modelling suggests that small strains affecting a pluton following its emplacement could mask AMS fabrics developed earlier. The results also suggest that, in simple shear, the orientation of the AMS ellipsoid may serve as a shear-sense indicator for small strains even if the earlier fabric is not isotropic.

Structure of the high-grade Opatica Belt and adjacent low-grade Abitibi Subprovince, Canada: an Archaean mountain front☆

Plutonic gneisses in the Archaean Opatica Belt in Canada record crustal-scale, WSW-vergent (i.e. longitudinal), high-temperature D1 ductile shearing. Similar structures occur in adjacent greenstone belts, but only in narrow amphibolite facies zones near the base; metavolcanics in the greenschist facies portions do not record penetrative D1 structures. The greenstone belts are interpreted as allochthonous on the gneisses; they form the uppermost D1 thrust sheets. SSE-vergent (i.e. approximately transverse) D2 thrusting imbricated the Opatica Belt and formed a large antiform—either a culmination or an antiformal stack—in the southern Opatica. The imbricated Opatica Belt wedged under the northern part of the Abitibi Subprovince and created a tight syncline with NNW-vergent D2 thrusts (interpreted as backthrusts) on its northern limb. The Opatica Belt-Abitibi Subprovince contact, therefore, resembles some Phanerozic mountain fronts. Further southwards-propagating imbrication of the crust beneath the Abitibi Subprovince created a S-vergent foreland fold and thrust belt in the overlying greenstones. High-grade Opatica gneisses and adjacent low-grade Abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an Archaean mountain belt, similar in many respects to Phanerozoic examples.

Syn-Acadian emplacement model for the South Mountain batholith, Meguma Terrane, Nova Scotia: Magnetic fabric and structural analyses

The anisotropy of magnetic susceptibility and the structural geology of the Late Devonian South Mountain batholith (Meguma Terrane, Nova Scotia) are used to characterize its emplacement and structural evolution. The South Mountain batholith is the largest peraluminous granitoid complex within the Appalachian orogen. It is a composite batholith made up of stage 1 granodiorites and monzogranites and stage 2 leucomonzogranites and leucogranites. The anisotropy of magnetic susceptibility was used to map the biotite petrofabric at the scale of the batholith. The biotite fabric pattern is similar to the pattern of folding and shearing of the feldspar megacryst foliation, documented at the outcrop scale. In the stage 1 plutons, the magnetic foliation is deformed into a girdle about the horizontal, northeast-southwest—trending magnetic lineation, which is parallel to regional fold axes and extension lineations in the country rocks. In the stage 2 plutons, a horizontal northeast-southwest– to east-west–trending magnetic lineation is also present, but the horizontal orientation of the magnetic foliation is widely preserved. The folding and shearing of the foliations and the horizontal northeast-southwest–trending lineation that is pervasive throughout the South Mountain batholith are interpreted to record Acadian tectonic deformation of the plutons as they crystallized. Acadian deformation continued during the latest stages of crystallization, resulting in the localization of mineral deposits and greisens along shear zones, faults, and joints. A model is presented for the syntectonic emplacement of the South Mountain batholith as a laccolithic complex.

Emplacement and deformation of granites during transpression: magnetic fabrics of the Archean Sparrow pluton, Slave Province, Canada

Abstract The Sparrow pluton is part of the Prosperous Suite of two-mica granites that crop out within amphibolite grade meta-greywackes of the southern Yellowknife Domain, in the Slave Province of the Canadian Shield. The magnetic susceptibility ( K ) and the anisotropy of magnetic susceptibility (AMS) were used to systematically map the structural patterns in the pluton and to establish the relationship between plutonism and Late Archean tectonics in the region. Paramagnetic Fe-phyllosilicates (biotite, chlorite) and very fine-grained magnetite contribute to K and to the AMS. The magnetic foliation and the magnetic lineation are predominantly controlled by the biotite fabrics and their orientations are consistent with the regional D 2 strain field. The horizontal magnetic lineations in the Sparrow pluton suggest a horizontal stretching component associated with the regional D 2 event. The zonation defined by K values is compatible with a fold pattern trending parallel to the regional F 2 folds and S 2 foliation and to the magnetic fabric trends in the pluton. The intensities and symmetries of the AMS also define map patterns that are consistent with D 2 deformation. Microstructural study indicates the pluton recorded D 2 strain as it crystallized and cooled from the solidus, demonstrating syn- D 2 emplacement. The results indicate the pervasive structural patterns in the Sparrow pluton are an integral part of the regional strain field, and that they are kinematically consistent with a transpressive D 2 strain regime. Mapping the fabric patterns within syntectonic plutons provides a useful approach to the kinematic analysis of synemplacement deformation events in multiply deformed metamorphic terranes.

Magnetic fabric of the Barrington Passage pluton, Meguma Terrane, Nova Scotia: a two-stage fabric history of syntectonic emplacement

Abstract Structural analysis and the anisotropy of magnetic susceptibility (AMS) are used to document the internal, magmatic to high-temperature subsolidus fabrics of the tonalitic Late Devonian Barrington Passage pluton. The AMS fabric orientations compare favorably with mesoscopic-scale fabrics. In general, the K1–K2 principal plane of the AMS is close to the observed mineral foliation defined by biotite and plagioclase, and the K1 axis is aligned parallel to a mineral lineation defined by the same minerals, and to a locally developed, weak extension lineation defined by quartz. Measurement of the anisotropy of anhysteretic remanence (AAR), and image analysis of biotite preferred orientations confirm that the orientation of the AMS is principally controlled by biotite. Locally anomalous AMS orientations are attributed to the influence of fine-grained magnetite crystals, the presence of which is revealed by measurements of hysteresis and coercivity properties. A two-stage fabric history is revealed. First, horizontal fabrics were formed (foliation, compositional banding) during emplacement of a laccolith-like pluton. The second stage involved synmagmatic folding of the horizontal foliation and banding in response to regional contraction associated with the Acadian Orogeny, with development of a lineation that records stretching parallel to the fold axis. The results demonstrate that the Barrington Passage pluton was emplaced during the main phase of the Acadian Orogeny. A model is proposed that depicts the Barrington Passage pluton as the folded base of a larger syntectonic pluton, that is now largely eroded off.

Magnetic susceptibility, magnetic mineralogy and magnetic fabrics in a late Archean granitoid-gneiss belt

Abstract The Opatica belt is a late Archean granitoid-gneiss terrane situated within the northern Abitibi Subprovince, along the northern margin of the Abitibi greenstone belt, Quebec. This belt is composed of several suites of tonalite-granodiorite-granite composition, variably deformed under epidote-amphibolite to upper-amphibolite grade conditions. Specimens from 87 sampling sites within the Opatica belt show a wide range in variation of magnetic susceptibility ( K ); the large variations are due essentially to varying proportions of ferromagnetic trace minerals. Detailed study of the opaque mineralogy by reflected light petrography, electron microprobe analysis and SEM observations, as well as alternating field and thermal demagnetization experiments confirm that multi-domain, very pure magnetite is the dominant ferromagnetic phase which, along with ferrosilicates, contribute to the magnetic susceptibility. In some cases, hematite may also make a significant contribution to K . In rocks with low K , the susceptibility may be dominated by the ferrosilicates hornblende and biotite. Intensity of anisotropy of magnetic susceptibility (AMS) is also highly variable, and appears to be controlled by the mineralogy of the rock rather than by strain intensity or deformation state. The shapes of AMS ellipsoids are also controlled by mineralogy. Detailed comparisons are made of AMS fabrics with field measurements of magmatic and metamorphic foliations and mineral and stretching lineations, and with mineral lineations determined by image analysis of polished slabs. There is very good agreement between the average orientations of poles to structural foliations, and the average orientation of the K3 principal axes of the susceptibility ellipsoids in all outcrops. In most cases, there is also good agreement between average structural lineations and the average K1 axis. In quartz-monzodiorites, disagreement between the average lineation and average K1 may be due to complex textural occurrences of magnetite, and the presence of abundant hematite. In gneisses, broad scatter of the orientations of K1 for individual samples from an outcrop (quantified by statistical confidence regions about the principal axes) locally occurs within the foliation plane, and may be attributed to the partial overprinting of earlier fabrics by subsequent deformation.

SPATIAL RELATIONSHIPS BETWEEN NATURAL SEISMICITY AND FAULTS, SOUTHEASTERN ONTARIO AND NORTH-CENTRAL NEW YORK STATE

Abstract A statistical analysis was carried out to investigate spatial associations between natural seismicity and faults in southeastern Ontario and north-central New York State (between 73°18′ and 77°00′W and 43°30′ and 45°18′N). The study area is situated to the west of the seismically active St. Lawrence fault zone, and to the east of the Lake Ontario basin where recently documented geological and geophysical evidence points to possible neotectonic faulting. The weights of evidence method was used to judge the spatial associations between seismic events and populations of faults in eight arbitrarily defined orientation groups. Spatial analysis of data sets for seismic events in the periods 1930–1970 and post-1970 suggest stronger spatial associations between earthquake epicentres and faults with strikes that lie in the NW–SE quadrants, and weaker spatial associations of epicentres with faults that have strikes in the NE–SW quadrants. The strongest spatial associations were determined for groups of faults with strikes between 101° and 146°. The results suggest that faults striking broadly NW–SE, at high angles to the regional maximum horizontal compressive stress, are statistically more likely to be spatially associated with seismic events than faults striking broadly NE–SW. If the positive spatial associations can be interpreted as indicating genetic relationships between earthquakes and mapped faults, then the results may suggest that, as a population, NW–SE trending faults are more likely to be seismically active than NE–SW striking faults. Detailed geological studies of faults in the study area would be required to determine possible neotectonic displacements and the kinematics of the displacements.

Tectonic Delamination of the Lower Crust During Late Archean Collision of the Abitibi–Opatica and Pontiac Terranes, Superior Province, Canada

The southeastern Superior Province of the Canadian Shield preserves a very complete record of the accretion and deformation of juvenile crust during a 100 Ma period, 2750 Ma through 2650 Ma. The region includes, from north to south, the Opatica granite-gneiss belt, the Abitibi granite-greenstone Subprovince, and the Pontiac metasedimentary Subprovince. The nature of the terrane suture and the crustal structure in the southeastern Superior Province are evaluated based on a synthesis of lithological, structural, geochronological, and geophysical data, including a reinterpretation of Lithoprobe deep seismic reflection profiles. All geological and geophysical data are consistent with the Opatica belt being contiguous with middle crust that underlies greenstones of the Abitibi Subprovince. The Opatica belt can now be considered part of the Abitibi Subprovince, which represents one large tectonic terrane, the Abitibi-Opatica terrane. The boundary between the greenstones of the Abitibi Subprovince and the metasedimentary rocks of the Pontiac Subprovince is a Late Archean terrane suture. The reinterpretation of the seismic reflection profiles suggests the Pontiac-Abitibi terrane suture involved wedging of older crust, underlying the Pontiac Subprovince, into the middle crust of the younger Abitibi Subprovince, resulting in delamination of the Abitibi lower crust. Deformation related to collision resulted in folding of the upper and middle crust of the Abitibi-Opatica plate for 250 km inboard of the terrane suture. The crustal deformation style, delamination, and thrusting of the stronger lower crust and the large-scale folding of the softer upper-middle crust are compatible with calculated strength profiles that include a lower crust composed of mafic granulite. The rheological profile of the Abitibi-Opatica plate, and the folding of the upper and middle crust during plate collision, may have resulted from a combination of radiogenic heating due to the abundance of granitoids in the middle crust and syncollisional plutonism.

Late Archaean Kenogamissi complex, Abitibi Subprovince, Ontario, Canada: doming, folding and deformation-assisted melt remobilisation during syntectonic batholith emplacement

The Kenogamissi complex represents a large exposure of folded Late Archaean crystalline crust exposed within the Abitibi Subprovince, Ontario, Canada. It is composed of an heterogeneous amphibolite-grade orthogneiss unit, and several generations of batholiths and plutons of tonalite, granodiorite and granite composition. Together, the various units represent granitic magmatism during the period from 2740 Ma to 2660 Ma. Structural mapping and petrographic studies were focused on the orthogneiss unit (2723 Ma), on the newly defined Roblin tonalitegranodiorite batholith (ca. 2713 Ma) and on the highly strained metavolcanic rocks within the deformation aureole that surrounds the Kenogamissi complex. Structural analysis indicates that the Kenogamissi complex was emplaced into the greenstones as a dome that caused severe flattening and recumbent F2 refolding of earlier F1 folds in the deformation aureole. Doming is interpreted to be caused by the emplacement and inflation of tonalite-granodiorite batholiths, such as the Roblin Batholith, into the actively folding Swayze greenstone belt. Continued regional folding resulted in F3 refolding of F1 and F2 in the deformation aureole. Continued regional folding also deformed and folded the Kenogamissi complex and resulted in further uplift and emplacement of the complex into the greenstone belt. The early-formed magmatic foliation and compositional layering in the Roblin Batholith were folded by F3 while the batholith was still a crystal mush, and an F3 axial-surface magmatic foliation was locally formed. Folding of the partially molten Roblin Batholith also resulted in the remobilisation of fractionated liquids into shear zones which formed on the limbs of the F3 magmatic folds. Similar structures are present in the orthogneiss unit and are interpreted to represent remobilisation of melts which intruded the orthogneiss at the time of emplacement of the Roblin Batholith. The formation of the dykes on sheared fold limbs may be attributed to increased dilatancy during localised shearing of the crystal mush. Deformation-assisted remobilisation and extraction of fractionated liquids, and the possible transport of the fractionated liquids to higher levels in the crystallising Roblin Batholith, may have played a role in its magmatic differentiation.

Electromagnetic characteristics of Cambodian soil: implication for land mine detection in soil containing ferromagnetic minerals

Electromagnetic (EM: Magnetic Susceptibility [MS], Electrical Conductivity) and soil texture characteristics were determined for a Cambodian soil from an area where landmine detection interference has been experienced. The purpose was to collect information for developing techniques to discriminate between EM signals from small metallic particles in landmines and from iron-oxides or ferromagnetic mineral grains in soil. Ferromagnetic minerals are iron-oxides with strong MS characteristics. Results indicate that this soil consisted of four textural components: clasts (2-10 mm), medium-coarse-sand (<2.0 mm), fine-sand (<0.25 mm) and clay-silt (<0.063 mm). The coarse-sand had high MS values (~550x10-8 SI/kg) due to high ferromagnetic mineral content (~20 wt.%). Some large rounded clasts, however, had considerably higher MS values (~11000x10-8 SI/kg) due to high ferromagnetic mineral concentrations (30-60 wt.%), a likely source of significant landmine detection interference. The finer components had smaller MS values and iron-oxide contents. Complex electrical conductivity (1-106 Hz) of iron-oxides showed significant frequency dependence due to capacitance effects of electrochemical double layers on their surfaces in contact with soil moisture. This frequency dependence of iron-oxides may provide opportunities for potential EM systems design to discriminate between soil and landmine responses.