Paleomagnetism of Silver Island, Keweenaw Peninsula, Michigan: Additional Support (?) for the Primary Curvature of the MCR
Silver Island lies between Eagle Harbor and Copper Harbor on the NW coastline of the Keweenaw Peninsula, Michigan. The island consists of a series of basaltic lava flows which dip to the north-northwest (356°) at an angle of approximately 38°. These flows constitute part of the Lake Shore Traps (LST), a series of interbedded lava flows within the Copper Harbor Conglomerate (CHC). The LST represent the youngest eruptive material associated with the 1.1 Ga Mid-Continent Rift (MCR). The most recent paleomagnetic study (now 15 years old) on the Lakeshore Traps (LST) defined three distinct directional clusters. Each cluster of directions corresponded to a different stratigraphic package of the LST within the CHC and all have extremely low between-site dispersion (s ∼ 4°) suggesting rather rapid emplacement of the LST packages. Consequently, each cluster has its own unique direction of magnetization. Since the lower two LST packages crop out along the coast line of the Peninsula with different structural trends, an opportunity was presented to test the conclusions of Hnat et al. (2006) that the curvature of the MCR was primary. To that end nine lava flows were sampled on Silver Island and their mean direction compared to the equivalent mean from lava flows sampled by Diehl and Haig (1994) from the tip of the Keweenaw Peninsula (upper lava flows of the Middle LST) which have an entirely different structural trend. Characteristic directions of magnetization of the Silver Island lava flows were isolated either using alternating field, thermal or a combination of both. The mean direction of magnetization for the nine sites is: D = 277.7°, I = 46.9°, α95 = 3.0°, k = 292.5. The mean direction recalculated from the Diehl and Haig study is: D = 277.8°, I = 40.6°, α95 = 2.9°, k = 315.7. Although the declinations of the two means are identical, interestingly the two means are statistically distinct at the 95% confidence level. Fold tests were inconclusive. Nevertheless, the nearly identical declinations from two locations with different structural trends suggest the curvature of the MCR on the Keweenaw Peninsula is primary. Differences in inclinations at this writing remain unresolved.
DSA Analysis of IRM Curves for Hydrocarbon Microseepage Characterization in Oil Fields From Eastern and Western Venezuela
During the last few years we have performed surface reservoir characterization at some Venezuelan oil fields using rock magnetic properties. We have tried to identify, at shallow levels, the "oil magnetic signature" of subjacent reservoirs. Recent data obtained from eastern Venezuela (San Juan field) emphasizes the differences between rock magnetic data from eastern and western oil fields. These results support the hypothesis of different authigenic processes. To better characterize hydrocarbon microseepage in both cases, we apply a new method to analyze IRM curves in order to find out the main magnetic phases responsible for the observed magnetic susceptibility (MS) anomalies. This alternative method is based on a Direct Signal Analysis (DSA) of the IRM in order to identify the number and type of magnetic components. According to this method, the IRM curve is decomposed as the sum of N elementary curves (modeled using the expression proposed by Robertson and France, 1994) whose mean coercivities vary in the interval of the measured magnetic field. The result is an adjusted spectral histogram from which the number of main contributions, their widths and mean coercivities, associated with the number and type of magnetic minerals, can be obtained. This analysis indicates that in western fields the main magnetic mineralogy is magnetite. Conversely in eastern fields, the MS anomalies are mainly caused by the presence of Fe sulphides (i.e. greigite). These results support the hypothesis of two different processes. In western fields a net electron transfer from the organic matter, degraded by hydrocarbon gas leakage, should occur precipitating Fe(II) magnetic minerals (e.g. magnetite). On the other hand, high concentrations of H2S at shallow depth levels, might allow the formation of secondary Fe-sulphides in eastern fields.
Multiple Magnetization Events Revealed in a Single Core in Upper Ordovician to Upper Devonian Carbonates of the Williston Basin, Manitoba (Canada)
A paleomagnetic and rock magnetic study was conducted in well 16-33-5-24W1 from the southwestern corner of Manitoba, in the Williston Basin. The samples were taken from the Upper Ordovician to Upper Devonian carbonates of the Birdbear, Souris River, Winnepegosis, Interlake, and Red River formations. The strongest magnetization was found in the lower samples of the Red River carbonates, whereas Upper Birdbear, Winnepegosis and Interlake have significantly lower natural remanent magnetization (NRM) values. As revealed by the partial anhysteretic remanent magnetization (pARM) spectra, saturation isothermal remanent magnetization (SIRM) crossover plots, S-ratios, and unblocking temperatures, the magnetization is carried by hematite in the uppermost samples of the Birdbear Formation, whereas magnetite carries the magnetization in Souris River and Red River samples. In all other samples the magnetic signal is lost after 330C, therefore the magnetic carrier could be either magnetite or pyrrhotite. The pARM spectra and SIRM crossover plots and points reveal magnetite in the pseudosingle and single domain ranges. The Souris River and Red River samples show the smaller magnetite grain size through their higher S100 ratios, ARM/SIRM values, and SIRM cross over points. Two of the formations, the Souris River and Red River, carry similar paleomagnetic components, with reversed directions and inclination-only means of I = -49.1 +/- 4.5 degrees (n = 10 specimens/ 6 plugs, k = 104.6) and I = -48.1 +/- 2.4 (n = 21 specimens/ 11 plugs, k = 194.7) respectively. In addition, three specimens (2 plugs) from the Birdbear show a similar or slightly steeper reversed magnetization, with inclinations of -45.0 to -68.4. The similarity in some of their magnetic parameters (same inclination values and higher ARM/SIRM, S100 and SIRM crossover point values) leads to the conclusion that their magnetization is probably due to the same fluid- flow event. Since the Birdbear and Souris River carbonates were deposited during the Devonian, the timing of this fluid-flow event is placed between Lower and Middle Jurassic. Some of the magnetic properties found in the Red River carbonates (higher NRM and Hcr, lower S100) might indicate that the magnetizing fluid originated in the subbasement rocks of the Williston Basin. The remainder of the Birdbear, Winnepegosis and Interlake formations have poor paleomagnetic results. Three hematite-bearing specimens from the top of the Birdbear have very weak NRMs, and shallower positive inclinations (-0.1 to 44.1). The Winnepegosis has 5 specimens (3 plugs) giving an inclination mean of I = -35.5 +/- 10.6 (k = 37.2) that points to a primary or post-depositional magnetization. Interlake paleomagnetic directions were obtained from 3 specimens that show positive inclinations (12.5- 47.4).
Paleomagnetic Study of a Miocene Deformation in a Region Close to the Camargo Volcanic Field, Chihuahua, Mexico
The Sierra the Aguachile is a Miocene volcanic sequence located in the SE of Chihuahua State NW of the Camargo volcanic field and belongs to the Agua Mayo Group, which unconformably overlays Mesozoic calcareous units. The Sierra de Aguachile sequence defines a structure that may be interpreted as a plunging fold, which could be the result of a reactivation of the San Marcos Fault. This major fault is well known more to the east but may extend into the study area where it would be covered by the younger volcanic sequences; its main activity has been reported to be during the the Neocomian with reactivation phases in the Paleogene and Miocene. To test if the observed structure is the result of a tectonic deformation that happened after the emplacement of the volcanic sequence, a paleomagnetic study was carried out. A total of 14 sites were sampled from different parts of the structure, all in the capping ignimbrite layers. Site mean directions were determined using AF demagnetization. The fold test was applied to analyze if the remanence was acquired in situ or before the proposed folding. Precision parameters k before and after application of the tectonic corrections are 25.38 and 31.43, respectively. This indicates that the Sierra de Aguachile indeed was folded after emplacement of the ignimbrites, which restricts the age of the corresponding tectonic event to be younger than 31.3 +/- 0.7 Ma. Due to the gentle folding though, the difference in precision parameters is not significant at the 95% probability level.
Alternative Pangea Reconstructions: A Matter of Flawed Data? Implications of a new Early Triassic Paleopole from Argentina
Despite widespread agreement about the paleogeographic configuration of Pangea just prior to its Jurassic dispersal, the relative positioning of Gondwana and Laurasia during the Permian and Early Triassic has been a long-standing question that has generated a lengthy history of debate. Assuming a classical Pangea geometry, Permo-Triassic (260-240 Ma, P/T) paleopoles from these landmasses do not statistically coincide, unless heavy filtering is applied to the paleomagnetic database or non-dipole field elements are considered. Several alternative paleogeographic models have been proposed to account for this paleomagnetic discrepancy, but most are considered to be geologically untenable. Thus, the problem is rooted in our data, our paleogeographic model, or our understanding of the paleomagnetic field (or some combination therein). In order to eliminate the possibility of flawed data, a compilation of high-quality results from multiple cratons needs to be amassed, but a review of the global paleomagnetic database reveals a scarcity of reliable data from cratonic Gondwana during P/T time. Gondwana's P/T APWP is often built from older results that lack the rigor of modern standards (routine demagnetization, principal component analysis, etc.) and/or have poor age constraints. Here we report new high-quality results from a combined paleomagnetic and geochronological study of the Early Triassic Puesto Viejo Formation of Argentina. Paleomagnetic sample collecting was focused in volcanics (dominantly ignimbrites and lava flows) as these are impervious to the low-latitude biasing phenomenon of sedimentary inclination shallowing. Our paleomagnetic analysis yields steep, antipodal directions, which we argue to be an early/primary magnetization on the basis of a positive fold-test and a positive conglomerate test. This new result supersedes an earlier Puesto Viejo pole and much improves the Triassic segment of the South American APWP relative to the greater-Gondwana APWP. This serves as an indication that other old paleomagnetic results may need to be re-examined or considered suspect and that additional high-quality, well-dated results may alleviate the Pangea conundrum without requiring a reconsideration of the geocentric axial dipole (GAD) field model or a dramatic restructuring of the classical Pangea geometry.
A paleomagnetic study of the Antarctic Peninsula
In the Paleozoic, South America, South Africa and Antarctica were part of Gondwana. The Weddell Sea began to form at about 146 Ma, after rifting between the Antarctic Peninsula and southernmost South America. Much uncertainty still exists about the geometrical fit and subsequent drift history between Patagonia and Antarctica. Geophysical and geological data which describe the tectonic history are sparsely distributed and often of poor quality. During the last two years we have collected more than 1000 paleomagnetic samples from 70 sites at several localities (King George Island, Robert Island, Yankee Bay, Half Moon Island, Byers Peninsula and Snow Island) from the South Shetland Islands and Anderson Island in the northern tip of Antarctic Peninsula. Our main objective was to provide first-order constraints on latitudinal displacements and the amount of tectonic rotations as an essential test of published tectonic models. Paleomagnetic results were obtained from 50 sites. All samples from sites in volcanic and intrusive rocks have well-defined univectorial magnetizations. Unfortunately, all sites in late Paleozoic sediments have been remagnetized and the magnetizations are often unstable upon thermal demagnetization. Cretaceous and Cenozoic units display very little apparent polar wander. Results from intrusive rocks of expected Jurassic age do not confirm the expected relative rotation betwen the Antarctic Peninsula and East Antarctica. Further radiometric dating are needed to confirm the age of these units.