Grain‐scale deformation and the fold test ‐ evaluation of syn‐folding remagnetization

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96427/1/grl3496.pd

Phyllosilicate fabric characterization by Low‐Temperature Anisotropy of Magnetic Susceptibility (LT‐AMS)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94895/1/grl16043.pd

Synchroneity of folding and crosscutting cleavage in the Newfoundland Appalachians?

Cleavage that cuts obliquely across folds is relatively common in the Appalachians/Caledonides and this has been interpreted as evidence for regional transpression, an interpretation which is only valid if contemporaneity of folding and cleavage formation can be demonstrated. Crosscutting cleavages in folds of the Early Silurian and older Exploits Group of the northeastern Newfoundland Appalachians are axial planar to rare, mesoscopic F2 fold in the unconformably overlying Botwood Group on Change Islands. As an alternative to transected folds, it is argued that crosscutting cleavage relationships in the Exploits units are composite D1-D2 structures that represent fold superimposition.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28935/1/0000772.pd

Marble mylonites in the Bancroft shear zone, Ontario, Canada: microstructures and deformation mechanisms

Mylonitization of medium-grade marbles in the Bancroft shear zone, Ontario, Canada, is characterized by decreasing grain-size of both calcite and graphite, and a variety of textures. Calcite grain-sizes vary from several millimeters in the protolith, to 50-200 [mu]m in mylonite, to m in ultramylonite. Corresponding calcite grain shapes are equant in the protolith, elongate in protomylonite (first-developed dimensional preferred orientation), equant in coarse mylonite, elongate in fine mylonite (second-developed dimensional preferred orientation) and generally equant in ultramylonite, which suggests that external energy (applied stress) that tends to elongate grains competed with internal energy sources (e.g. distortional strain) that favor equant shapes. Graphite grain-size changes from several millimeters to centimeters in the protolith to submicroscopic in ultramylonite. In the mylonitic stages, graphite is present as dark bands, while in the ultramylonitic stage it is preserved as a fine coating on calcite grains.Based on textural evidence, twinning (exponential creep; regime I), dynamic recrystallization (power law creep; regime II) and possibly grain boundary sliding superplasticity (regime III) are considered the dominant deformation mechanisms with increasing intensity of mylonitization; their activity is largely controlled by calcite grain-size. Calcite grain-size reduction occurred predominantly by the process of rotation recrystallization during the early stages of mylonitization, as indicated by the occurrence of core and mantle or mortar structures, and by the grain-size of subgrains and recrystallized grains. Grain elongation in S-C structures indicates the activity of migration recrystallization; these structures are not the result of flattening of originally equant grains. Differential stress estimates in coarse mylonites and ultramylonites, based on recrystallized grain-size, are 2-5 and 14-38 MPa, respectively. Initial grain-size reduction of graphite occurred by progressive separation along basal planes, analogous to mica fish formation in quartzo-feldspathic mylonites.Calcite-graphite thermometry on mylonitic and ultramylonitic samples shows that the metamorphic conditions during mylonitization were 475 +/- 50[deg]C, which, combined with a differential stress value of 26 MPa, gives a strain rate of 1.2 x 10-10s-1 based on constitutive equations; corresponding displacement rates are mm yr-1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29662/1/0000751.pd

The 40 Ar‐ 39 Ar laser analysis of K‐feldspar: Constraints on the uplift history of the Grenville Province in Ontario and New York

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95104/1/jgrb13261.pd

Kinematic analysis of an en echelon--continuous vein complex

An array of sigmoidal tension gashes from the Idaho--Wyoming thrust belt changes laterally into a continuous vein. Detailed mechanical twin analysis was used to determine the strain variation in the optically and chemically homogeneous blocky calcite filling. In the continuous portion of the vein complex, the shortening axes are parallel to the vein boundary. However, the orientation of the shortening axes in the tip areas of the sigmoidal gashes are at an angle of approximately 35[deg] to the vein boundary, and are parallel to the trend of the tips. Twinning patterns in the central portions of the gashes record two principal strain axes of shortening of nearly equal magnitude with the maximum perpendicular to the vein trend. Everywhere in the vein complex the orientation of the maximum extension axis is parallel to the twist axis of the gashes. The petrofabric strain results show that the vein filling has largely recorded local strains. The pattern of variation in orientation of the principal strains in the vein complex is in close agreement with the theoretically determined stress distribution in similar structures. Our results show that the sigmoidal gashes were formed at the leading edge of a propagating vein and that the sigmoidal shape reflects changes in the local strain field rather than a remote shear. The orientation of these local strains closely corresponds to the orientation of the local stresses.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27567/1/0000611.pd

Avalonian proximity of the Ordovician Miramichi Terrane, northern New Brunswick, northern Appalachians: Paleomagnetic evidence for rifting and back-arc basin formation at the southern margin of Iapetus

A paleomagnetic investigation of the Middle Ordovician Tetagouche Group in northern New Brunswick was undertaken to determine the paleogeographic position of the Miramichi Terrane. Stepwise thermal demagnetization of pillow basalts reveals a high-temperature characteristic magnetization with a mean direction of D = 060[deg], I = +69[deg], k = 22, [alpha]95 = 13[deg] (tilt-corrected, N = 7 sites; 73 samples). A positive fold test and the presence of antipodal normal and reversed polarity directions indicate that this ancient direction is Ordovician in age, with a paleopole position of 52[deg]N, 352[deg]E. The corresponding paleolatitude of 53[deg]S places these volcanic rocks near the southern margin of the lapetus Ocean, at paleolatitudes similar to those revealed by Avalon for the Middle to Late Ordovician. The mafic and felsic volcanic rocks and marine sedimentary rocks of the Tetagouche and Fournier groups have been interpreted to be remnants of a rifted continental margin and a Middle Ordovician back-arc basin. Our results show that the process of rifting and back-arc basin formation occurred at the Avalonian margin of lapetus, which implies that Ordovician subduction was not restricted to the Laurentian margin, but also marks the southern margin of lapetus.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30440/1/0000063.pd

Acadian and Alleghenian remagnetization of the Jim Pond Formation, central western Maine, northern Appalachians

Samples were collected from ten sites of the Late Cambrian-Early Ordovician Jim Pond Formation for paleomagnetic study. Stepwise thermal demagnetization reveals three separable components of magnetization. Component I is typically removed by 350[deg]C; it is subparallel to the present day field (354[deg]/ + 76[deg] vs. 342[deg]/ + 72[deg]) at the site location (45.3[deg]N, 289.4[deg]E) and is considered to be a recent partial overprint. Component II, without tilt-correction, is a south-southeasterly and shallow direction (mean: 165[deg]/0[deg], k = 31.4, a95 = 8.6[deg]) that is removed over an intermediate temperature range (350-600[deg]C). Component III, without tilt-correction, is a northeasterly and shallow, upward direction (mean: 10[deg]/-24[deg], k = 21.5, a95 = 7.3[deg]) and is removed over the highest temperature range (480[deg] to 690[deg]C). Though not statistically significant, for Components II and III the precision parameter, k, decreases and the [alpha]95 increases when tilt-correction is applied, suggesting that both are post-folding magnetizations.Component II, without tilt correction, has a corresponding paleomagnetic pole located at 43[deg]N, 130[deg]E (dp, dm = 4.3[deg], 8.6[deg]), which falls near the Late Carboniferous segment of the Laurentian Apparent Polar Wander Path (APWP). Component III, without tilt correction, has a corresponding pole located at 32[deg]N, 98[deg]E (dp, dm = 4.7[deg], 7.8[deg]), which falls near the Lower-Middle Devonian segment of the APWP. We conclude that the Jim Pond Formation has undergone two Paleozoic remagnetization events, one in the Early to Middle Devonian and a second one in the Late Paleozoic. The ages of these remagnetizations coincide with the timing of major orogenic activity in the area i.e. the Acadian and Alleghenian, respectively. The remagnetization event associated with the Acadian pulse can be recognized in other paleomagnetic investigations in the northern Appalachians.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29457/1/0000539.pd

Discordant Silurian paleolatitudes for central Newfoundland: New paleomagnetic evidence from the Springdale Group

Ancient remanences are retained by the Early Silurian (429 + 6/-5 Ma) mafic volcanics of the Springdale Caldera (five sites) and the overlying red beds (seven sites). Dual polarity magnetizations are obtained by thermal demagnetization of samples from the red beds, whereas single polarity directions are observed in the volcanics. High unblocking temperatures indicate hematite as the remanence carrier in both the volcanics and sediments. These high-temperature, characteristic remanences are easily isolated and pass both the tilt and conglomerate tests; they are likely to be of primary Silurian age. Characteristic declinations are predominantly northerly and northeasterly, and indicate significant structural rotations on a local scale. When the results of the red beds and the volcanics are combined they show characteristic inclinations that are shallower than those of the correlative Botwood Group (ca. 36[deg] vs. 43[deg]) but not nearly as shallow as those reported from the King George IV Lake area (0.5[deg]). Mean inclinations obtained from the Springdale red beds are, however, significantly shallower than those of the Springdale volcanics. The same difference can be seen in other previous Silurian paleomagnetic studies of central Newfoundland. We infer that an inclination error affects the red bed magnetizations of the Springdale Group, Botwood Group (Wigwam Formation) and rocks of the King George IV Lake area. Therefore, the results from Silurian red beds should not be used to determine paleolatitudes for central Newfoundland. The mean paleolatitude of the Springdale Group volcanics is 30[deg]. The mean paleolatitudes for both the Springdale volcanics and Botwood volcanics (Lawrenceton Formation) are indistinguishable within paleomagnetic error limits from the predicted paleolatitude of Newfoundland on the northeast-trending North American margin. Thus, no detectable post-Silurian displacement is shown by the volcanics of the Springdale Group with respect to cratonic North America.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30456/1/0000084.pd

The quantification of crystallographic preferred orientation using magnetic anisotropy

Magnetic anisotropy analysis presents an alternative and fast method for obtaining and quantifying crystallographic preferred orientations in rocks, using relatively simple equipment. Two natural examples and numerical modeling demonstrate that magnetic anisotropy increases with increasing degree of crystallographic preferred orientation. The normalized magnetic parameters Mi = ln(ki/(kmax * kint * kmin)1/3) (kmax >= kint >= kmin are the principal magnetic susceptibilities) correlate directly with March `strains' obtained from X-ray texture goniometry. The additional advantage of our method is that the preferred fabrics are determined from large sample volumes (typically about 11 cm3) rather than the essentially two-dimensional slice used in optical and X-ray methods. Thus, magnetic anisotropy provides a reliable measure of bulk crystallographic preferred orientation in rocks.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31050/1/0000727.pd