Archean regional transpression and paleomagnetism in northwestern Ontario, Canada

The Archean metamorphic rocks of the Superior province of the Canadian Shield occur in lithologically defined belts or subprovinces. The tectonically more stable interiors of belts possess consistent primary components of magnetic remanence. In the case of the Quetico belt, these stable directions are tightly grouped about 005°/55° with some minor dispersion and most were acquired during the cooling that followed syntectonic recrystallisation.

This study examines the directions of primary remanence components for rocks along the margins of the Quetico belt, within 4 km of the strongly deformed vertical, ENE-trending boundaries. The boundaries are known to have experienced dextral transpression involving penetrative single-phase deformation which out-lasted metamorphism. Within a few kilometres of the belt boundaries, the primary remanence components are re-distributed along a vertical ENE-trending, great-circle girdle which is nearly parallel to the plane of transpressive shear and regional schistosity. It is suggested that the effects of transpression have mechanically deflected the components of primary remanence toward this plane.     

Sources of magnetic susceptibility in a slate

Magnetic susceptibility and its anisotropy in the Borrowdale Volcanic slates at Kentmere in the English Lake District are attributed largely to preferred orientation of a paramagnetic chlorite of diabantite-ripidolite composition. In units of 10⁻⁶ cgs/g, the principal susceptibilities for the slates are 9.61; 9.42; 8.69 and for the chlorite grains the minimum anisotropy is represented by principal susceptibilities of 11.57; 11.22 and 9.15. Because the magnetic susceptibility is carried by a tightly packed, matrix-forming mineral that has recrystallised during the deformation it is not possible to imagine simple grain rotation as being responsible for the anisotropy of susceptibility.     

Installation age of limestone masonry determined from its viscous remagnetization

Many rocks passively acquire some time‐dependent or “viscous” remanent magnetism (VRM) at ambient temperatures, without any extraordinary energetic intervention. This magnetization overprints existing remanent magnetization so that it is effectively a remagnetization subparallel to the contemporary geomagnetic field, averaging the geomagnetic field orientation. Certain limestone masonry remagnetizes viscously over an archaeologically useful interval (100 to 8000 Ka) so that the degree of remagnetization is monotonically (but not linearly) related to the construction age. The laboratory unblocking temperature (T_UB) that removes the viscous magnetization is a simple monotonic measure of relative age. The longer a piece of masonry remained stabilized in a certain orientation, the greater is its viscous remagnetization and the higher is its T_UB. Monuments of known age with a similar limestone source permit us to establish a calibration curve of T UB against historical ages. The resulting calibration curve may then be used to predict the ages of otherwise‐undated masonry. Viscous remanent magnetism dating provides precision of <50a in medieval monuments in England and <150a precision for classical to Neolithic monuments in Cyprus; precision depends on the remagnetization rate of the limestone in question. Our calibration curves, for the Jurassic Oolitic Limestone of England and for the Lefkara‐Pakhna Chalks of Cyprus, allowed us to investigate the authenticity of a medieval English synagogue in Lincoln, England, and of a medieval house in Cyprus. Multiple archaeologic VRMs show that masonry was recycled in historical times. © 2006 Wiley Periodicals, Inc.     

Correlation of strain with anisotropy of magnetic susceptibility (AMS)

Existing correlations between strain and anisotropy of low-field magnetic susceptibility (AMS) have been re-assessed using a single parameter to express both anisotropies. TheP parameter (Hrouda, 1982) shows potential as a powerful single expression of the intensity of strain and of AMS. Previous correlations are improved by use of this parameter. Cautious optimism is justified for correlations between strain and susceptibility in a certain strain window between a lower limit (excluding the incomplete overprint of predeformation anisotropy) and an upper limit (excluding the effects of saturation anisotropy). For successful correlations the influence of stress-controlled recrystallisation should be minimal and the mineralogical sources of susceptibility must predate deformation.     

Comparison of Archean structural styles in two belts of the Canadian Superior Province

In the Wabigoon granite-greenstone belt, volcanic strata are folded isoclinally about vertical axes, and the strata and folds wrap around mushrooming intrusions of tonalitic gneiss. The vertical fold plunges of the volcanic strata may result from diapirism deflecting the pre-existing ESE verging and facing recumbent folds, Steep, post-tectonic metamorphic gradients cross-cut the axial surfaces. There is no consistent tectonic trend on the regional scale.In the adjacent Quetico slate and gneiss belt tectonic trends are consistently east-west. Metagreywackes are isoclinally folded with vertical, east-west axial planes. The folds have strongly curving periclinal axes so that the ‘structural facing’ of the strata varies from up-through sideways- to down-facing. Isograds are parallel to the east-west axial traces.These structural contrasts may be valid for much of the length of the Quetico and Wabigoon belts along their interface.     

Ophiolite Tectonics,Rock Magnetism and Palaeomagnetism,Cyprus

Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron–titanium oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance, strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover, mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation. The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia terrane boundary and the Arakapas-Transform fault form an approximate E–W composite boundary that we term the Troodos Tectonic Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor metamorphism, perhaps 7–15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal circulation. The gabbros and mantle rocks commonly bear young (<12 Ma) remanences probably acquired (or re-acquired) during uplift of the Troodos terrane. In the cover of pre-Messinian deformed limestone (>8 Ma), the remagnetizing effects of penetrative strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in present-day coordinates, Troodos rocks moved ~1,000 km South; then ~4,500 km East and finally ~900 km North at an approximate rate of 75 km/Ma [1 km/Ma = 1 mm/a]. This true motion path commenced ~88 Ma ago and rates of motion since 65 Ma may be too high due to the limited precision of strain-corrections of the ChRM orientations in limestone. This true motion path is compatible with the eastward and then northward rotation of Africa relative to Europe although other workers show relative motion paths.     

Minimum strain from conglomerates with ductility contrast

An approximate estimate of the minimum strain ellipsoid for rigid clasts in a ductile matrix may be made using the shapes of the most eccentric and least eccentric clast. Further, a minimum strain estimate for the matrix may be made using the curvature of strain shadows of cleavage.