GP34A-01 INVITED
An Anisotropy-based Inclination Shallowing Correction: Paleomagnetism and Rock Magnetism of the Shepody Fm., New Brunswick and Nova Scotia, Canada
The North American apparent polar wander path (APWP) for the upper Paleozoic is dominated by red beds, which have been reported to suffer from deposition- or compaction- induced magnetic inclination shallowing. A series of different type anisotropy measurements have been performed on the Upper Mississippian Shepody Formation red beds of Maritime Canada. These measurements have been used in conjunction with standard paleomagnetic measurements to apply an anisotropy-based inclination correction. Paleomagnetic measurements on the Shepody Fm. revealed the presence of a primary component carried by hematite and a secondary, syn-folding component, carried by maghemite. Anisotropy of magnetic susceptibility (AMS) was used in conjunction with chemical demagnetization to calculate the anisotropy of the characteristic remanence-carrying grains. The resulting fabric is strongly bedding parallel with bedding perpendicular minimum axes. It has been interpreted as a primary depositional/ compactional fabric and was successfully used for an inclination shallowing correction: mean inclinations for the formation were corrected from 17.7° to 29.1°. The magnetic mineralogy of the Shepody Fm., as revealed by demagnetization intensity curves, isothermal remanence magnetization (IRM) acquisition experiments and Lowrie tests, is complex and composed of maghemite, hematite and goethite. In order to develop a less time-intensive technique for isolating the contribution of hematite to the magnetic fabric, a new protocol was devised. It consists of measuring the anisotropy by imparting IRM's in high fields (5 T) to activate the hematite, and following each magnetization step with a thermal demagnetization step at 120° and an alternating field demagnetization (AF) step at 100 mT to eliminate the contributions of goethite and maghemite, respectively. The fabric obtained through this procedure is similar to the AMS fabric; however, the orientation of the principal axes shows a slight imbrication that could indicate deposition from a current. A magnetic fabric was also measured for the secondary magnetization component carried by maghemite using anisotropy of anhysteretic remanence (AAR). Since the remanence was acquired at 70 percent unfolding the fabric was analyzed both in stratigraphic and 70 percent unfolded coordinates. In stratigraphic coordinates the fabric mimics the primary fabric determined with high field (hf) IRM anisotropy. At 70 percent unfolding the fabric is no longer discernible. This result suggests that secondary magnetic minerals can inherit a primary-appearing magnetic fabric. An inclination correction for the remanence carried by the primary hematite particles was performed using the hf-IRM anisotropy and an individual particle anisotropy value of a=1.34 estimated from a fit to the theoretical correction curves. It gave a result similar to the AMS-based correction with mean corrected inclinations of 28.8°. These values are consistent with results obtained from two other Lower Carboniferous rocks from North America and imply an approximately 6° increase in co-latitude for the average Lower Carboniferous paleopole.
GP34A-02 INVITED
The Acquisition of a Mineral Fabric During Simple Shear: Implications for the Measurements of Anisotropy of Magnetic Susceptibility of Tectonically Deformed and Igneous Rocks.
Other than their elongation ratio and the initial distribution of orientations of a set of particles, a key element controlling the acquisition of a stable fabric in simple shear is the amount of deformation expected for a rock: Small shear strains favor a stable fabric whereas large shear strain leads to a periodic behavior. Paradoxically, many workers interested in the AMS of tectonically deformed rocks have considered that under simple shear the fabric will behave rotationally whereas those workers interested in the AMS of dykes, lavas or other igneous rocks usually assume that the fabric is essentially stable. In this work I reexamine the conditions controlling the acquisition of a mineral fabric during simple shear, presenting threshold values that can be used as guidelines in the interpretation of AMS results obtained from a variety of rock types. Due to the many factors influencing fabric acquisition, it is also shown that unless some key parameters are determined from an independent technique, most models attempting to map the amount of deformation with a degree of anisotropy are extremely limited.
GP34A-03
Constraints on the Mechanics of Folding from Paleomagnetic Data
We analyzed published paleomagnetic data sets from fold belts upon actual fold geometry and the way how layers become tilted and rotated during deformation by folding. The analysis bases on the combination of remanence vectors and bedding poles to bipod-like pairs of vectors. These vector pairs have new geometrical properties that allow to study the path of tilting and rotation. From our analysis of more than 100 data sets, several general observations turn out: (1) Bedding strikes at the early stage of folding have a significant variation around a mean strike, with a standard deviation of about 30° on average. A concentric fold geometry as often presupposed in paleomagnetism is rather the exception than the rule. (2) The strike variations originate initially with folding, being a general feature of heterogeneity in the early stage of deformation. Until a layer dip of about 30°-50°, the layers are being tilted around (sub)horizontal axes parallel to respective bedding strike. These findings become evident from the distribution of the intersections of the remanence small circles. (3) When folding proceeds, layers with the dip azimuth parallel to the shortening direction (‘compatible layers') continue in the same way, i.e. the axes of tiling do not change orientation. ‘Incompatible' layers, with the dip azimuth not parallel to the shortening direction, however, become accomodated by rotation around (sub)vertical axes. Tilting and vertical-axis rotation perform simultaneously; the bedding poles follow curved paths in the stereoplot. This process of accomodation is referred to as ‘layer parallelisation‘. It can be observed in paleomagnetic data of strongly folded sediments in various fold belts.
GP34A-04
Structural Implications of the Paleomagnetic Data from the Aptian-Albian Sedimentary Formation in Lebanon
In order to estimate the block rotations around the central part of the Dead Sea Transform, 325 cores of Aptian and Albian rocks were sampled in 38 sites in Lebanon. Sites are situated in the cores and limbs of the Mount Lebanon and Mount Anti-Lebanon anticlines, with a widespread regional distribution. The intensity of the Natural Remanent Magnetisation is mostly low. Hysteresis loops indicates presence of at least two magnetic phases, one having high coercivity. In most samples, the NRM includes two components in addition to a viscous component A: component B carried by magnetite and component C carried by high coercive minerals. The mean direction of component B is slightly different before and after bedding correction, with a better clustering in inclination after the correction. Significant scattering in declination indicates effect of some local rotation. Progressive bedding correction for all the 37 sites gives the best clustering (maximum k value=17.8) for 85% of unfolding, but without significant difference with 100% unfolding (k=17.6). Positive reversal test in one site strongly suggests that component B is the primary magnetisation. The direction of component C has a coherent orientation before bedding correction. Its high clustering, its only normal polarity, and the significant fold test strongly argue for a single component acquired after folding. From comparison of the declination of the magnetisation component B with the African Apparent Polar Wander Path, a mean counter-clockwise rotation of 28.0 ± 6.4° affected all our sites since Lower Cretaceous period. According to the orientation of the component C, part of this rotation (17.7 ± 3.8°) occurred after folding, i.e. after the Late Miocene. A global rotation of all the Levant area being inconceivable, rotation affected small blocks limited by the numerous dextral presently E-W faults existing in Lebanon.
GP34A-05
Describing the 3-D kinematic history of the Wyoming-Idaho fold thrust belt: an integrated approach
The kinematic history of fold-thrust belts is recorded in their 3D displacement field (a combination of translation, internal strain and rotation). In an effort to describe the complete deformation field of the Wyoming-Idaho fold- thrust belt we have gathered an extensive amount of structural and paleomagnetic data from throughout the belt between Salt Lake City, Utah and Jackson, Wyoming. Paleomagnetic and anisotropy of magnetic susceptibility (AMS) data are reported here from 160 sites collected from the Triassic Ankareh Formation. The paleomagnetic data indicate that the Ankareh Formation provides a good record of the Earth's ancient magnetic field. At least three components of magnetization are revealed: a viscous magnetization acquired in a recent field, and two characteristic remanence directions that linearly decay towards the origin of demagnetization diagrams and are either post to syn-folding with a Tertiary-like inclination for Wyoming, or pre-folding with an inclination that agrees well with the expected local inclination value for the Middle to Late Triassic for Wyoming. Declinations of in situ remanent magnetization show considerable regional variation (up to 90 degrees), suggesting that significant relative rotations have occurred around the salient since the time of magnetization acquisition. The AMS data reveal several different AMS ellipsoid shapes, ranging from a primary fabric to a tectonic fabric that is geometrically correlated to the trend of regional fold axes. About 23% of the sites retain a primary sedimentary fabric. About two-thirds of the sites have an oblate fabric with a well defined magnetic lineation that is consistently parallel to regional fold axis trend. The general parallelism of the oblate fabric with bedding indicates a composite fabric between a primary sedimentary fabric and an early tectonic layer parallel shortening fabric in which the magnetic lineation represents either an intersection lineation or a stretching lineation. About 13% of the sites have a more developed tectonic fabric represented by a prolate AMS ellipsoid with its long axis parallel to the trend of regional fold axes. As a whole, the paleomagnetic declinations and magnetic susceptibility fabric orientations from the Wyoming-Idaho fold-thrust belt exhibit a fan pattern that is in close agreement with the plan- view arcuate shape of the belt. Where possible, strain data were collected in conjunction with magnetic data. These data are geometrically correlated with AMS fabric orientations, and a strike test reveals correlation between strain orientations and paleomagnetic vector trends. Integrated, these datasets indicate that the observed final curvature of structural trends in the Wyoming-Idaho fold-thrust belt reflects about 1/3 primary curvature and 2/3 secondary tectonic curvature. This pattern suggests a significant amount of vertical-axis rotation has taken place within individual thrust sheets subsequent to early shortening of the orogenic wedge, which must be taken into account in any robust 3-D kinematic model of thrust-belt formation.
GP34A-06
Paleomagnetism & Geobarometry of Cretaceous Plutons: Evidence that the Peninsular Ranges Batholith of Baja California is NOT Far-Traveled
Characteristic remanence directions, residing in single and pseudosingle domain magnetite, of D = 1°, I = 51°, A95 = 4° and D = 0°, I = 56°, A95 = 6° have been determined for 18 and 8 sites (311 specimens) in the 108 ± 1 Ma Alpine and 100 ± 1 Ma Ramona tonalite complexes, respectively, in the western zone of the Peninsular Ranges Batholith near San Diego, California. At 16 sites, Al-in-hornblende geothermobarometry gives crystallization depths of 8.8 to 14.1 km that, with previous depths and geologic mapping, indicate orogen-parallel faults with vertical displacements up to several kilometers that bound fault blocks. Paleomagnetic results from the eight available Cretaceous igneous collections, totaling 93 sites, indicate an 11 ± 3° ENE-side-up tilt of the blocks between ~125 and 89 ± 7 Ma. The tilt is attributed to torque from ENE shallow underthrusting of the Farallon Plate and resistive WSW overthrusting of the North American Plate. Also ~440 km of dextral displacement of the western zone of the batholith is predicted on an intra-arc fault. Excluding Neogene-opening of the Gulf of California, the eastern zone has not been displaced relative to North America. This fault block rotation model provides a likely tectonic solution to the Baja California debate between "fixist" geologists and "mobilist" paleomagnetists.
GP34A-07 INVITED
Rock Magnetism of Continental Sediments from Central Asia: Insights into Tectonic Exhumation and Erosion
We have carried out five detailed magnetostratigraphic and rock magnetic studies in 2 to 3 kilometer-thick continuous sections of continental sediments in central Asia. Sedimentation rates are established from magnetostratigraphy, while rock magnetic profiling helps to define changes in the source material or in the hydrodynamic regime acting during sedimentation. In particular, characterizing the temporal evolution in rock magnetic parameters via anhysteretic remanent magnetization and the anisotropy of magnetic susceptibility have proven invaluable to our work. They also help significantly in correlating the magnetostratigraphic column with the reference polarity time scale. Our presentation will focus on how rock magnetic profiling assists in the pattern matching process inherent in magnetostratigraphy, and how we interpret changes in the various parameters, including sedimentation rate, to glean information related to the exhumation and erosion of the nearby mountains.
GP34A-08
Inhomogeneous Shearing Around Major Shear Zone: Evidence From the Magnetic Fabric of Late-Panafrican Plutons in the Tuareg Shield (Algeria)
Teg Orak and Tihaliouine are Late-Panafrican granitic intrusions situated in the Tuareg shield, northwest of Tamanrasset. Both plutons are located close to the 4°50 accident, one of the N-S major shear zone cutting the Hoggar. They are constituted of several granitic facies, indicating differentiation and multiphase intrusion. They show only very locally preferred visible orientation of their minerals. Anisotropy of Magnetic Susceptibility (AMS) was determined on samples collected along cross-sections in the Teg Orak (349 samples for 43 sites) and Tihaliouine (160 samples for 21 sites) plutons. At Tihaliouine, the AMS presents relatively scattered directions, with magnetic foliation having often strike similar to that of the plutons border. At Teg Orak, samples collected in the north-western part of the pluton present also AMS rather in relation with orientation of the pluton border. For all the other samples, including those from an aplitic dyke crossing the granite and whatever the facies of the granite, the fabric is very coherent. Magnetic lineation is subhorizontal, striking NW-SE. Magnetic foliation plunges to the NE on the north-eastern part of the plutons, has a weak dip in the central part and plunges to the SW in the south-western part. This disposition indicates deformation of the eastern part of the granite during late magmatic stages in relation with dextral strike-slip movement along the shear zone. Tihaliouine and western Teg Orak (mainly with fabric related with magma flow) intruded within older granite while eastern part of Teg Orak (with fabric related to deformation) is located within diorites and metamorphic rocks. In the Teg Orak - Tihaliouine area, granitic host-rocks acted therefore as a rigid block, protecting the studied plutons from the effect of regional deformation, while metamorphic and more basic plutonic rocks had a more deformable behavior and allowed intrusion strain.