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Samantha R. Geosphere ;; 11 5 : — Growth of magnetite has been variably linked to fluid-bearing events or clay diagenesis, and the development of a chemical remagnetization as a result of such events. In this study we examine remagnetized carbonate rocks from the central Sierra Madre Oriental the Mexican fold-thrust belt in order to develop a method for dating synfolding remagnetizations.

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We find that the history of the central Sierra Madre Oriental involved two separate remagnetization events in our study area; synfolding remanence acquisition ca. The growth of magnetite leading to chemical remagnetization detected in these limestones is interpreted as the result of rock interactions with an Fe-bearing fluid. Carbonates that contain interlayered shales with radiometrically datable authigenic illite present an opportunity to directly determine the age of synfolding paleomagnetic remagnetizations. Prior to this study, the common way to date a remagnetization event was qualitatively, through comparison of a determined magnetic direction to the apparent polar wander path of the region.

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In deformed areas, the age range of the remagnetization episode can be estimated relative to folding, through application of the paleomagnetic fold test Facer, Tohver et al. The clays were associated with limestone layers that had undergone three remagnetization events Weil et al. Remagnetization in carbonates typically occurs by the growth of superparamagnetic to stable single-domain magnetite from the release of iron Fe during mineral reactions or from the introduction of an Fe-bearing fluid Elmore et al.

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For example, during illitization, smectite transforms to illite, releasing Fe as temperatures increase with burial Altaner and Ylagan, The growth of magnetite into a stable single-domain structure allows a remanent magnetization to be acquired, resulting in a chemically remagnetized unit Hirt et al. Katz et al.

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This remagnetization is only present where there is also evidence of clay diagenesis. This hypothesis was further supported by Katz et al.

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The occurrence of illite is associated with chemically remagnetized rocks in these studies, and the presence of smectite is associated with primary magnetizations or comparatively weaker chemical remagnetizations. Elmore et al. Thus, while the growth of magnetite has been well studied, dating of remagnetization events remains a challenge; the latter provided the motivation for this study. Illitization from smectite or illite precursors is common in naturally deformed rocks e.

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Scanning electron microscopy, X-ray diffraction, stable isotope, and geochronological analyses of samples from several folds showed that illite grew along shear-related horizons during folding. The studied clay samples in central Mexico were collected along the same Aptian—Albian shale horizon and in all cases these samples provided well-defined Ar-Ar illite ages that were younger than deposition.

Extensive work has shown that many carbonates around the world have been remagnetized Jackson and Swanson-Hysell, ; McCabe and Elmore, ; Van der Voo and Torsvik,highlighting the potentially wide-scale application of this approach. The specimens analyzed for this study are carbonate successions from the Tamaulipas Formation in the Sierra Madre Oriental central Mexico.

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From the structural point of view, the Sierra Madre Oriental is an east-northeast—verging thin-skinned fold-thrust belt, also known as the Mexican fold-thrust belt. The highs and lows related to the carbonate basins and platforms were created in the Jurassic with the opening of the Gulf of Mexico, resulting in basin-and-range—type extension Carrillo-Martinez et al.

The carbonates were deposited in the Barremian—Cenomanian and the basinal deposits are characterized by deep-water muddy carbonates, whereas the platform rocks are fossiliferous shallow bank deposits Imlay, ; Suter, The ability to successfully date mesoscopic folds made this an ideal area to test the feasibility of absolute dating of synfolding remagnetizations. The Tamaulipas Formation is the focus of this study because of the accessibility and large-scale areal extent of this unit. Local-scale folds were targeted and 6—10 samples were collected per site using a portable Pomeroy EZ Core Drill.

A Brunton compass and inclinometer were used to determine the azimuth and plunge of the core samples and the orientation of the beds.

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Cored samples were cut to 2. Broken samples were glued back together with alumina cement and all specimens were labeled with Velvet underglaze nonmagnetic temperature-resistant paint. Remanent magnetizations were measured using a three-axis 2G superconducting magnetometer.

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Specimens were thermally demagnetized using an ASC TD demagnetizer after trials revealed that a separate magnetic vector did not always become apparent when using the alternating field demagnetization technique. Thermal treatment revealed two magnetic components, whereas alternating field demagnetization revealed only one.

Principal component analysis was used to analyze the demagnetization data and the fold test was applied in order to determine the relative timing of magnetizations; i. The fold test proportionally untilts the fold limbs with different dips, utilizing the fold axis to anchor the rotations.

Magnetite is the principal carrier dating the magnetization as determined by three-dimensional 3D isothermal remanent magnetizations IRMs oriental, performed with an ASC Scientific Impulse Magnetizer followed by thermal demagnetization of the samples Fig. Throughout the study area, 23 sites generated interpretable including 10 fold tests Valley to paired sites, as labeled in Figure 2.

Directions in the Tampico-Misantla Basin are reversed and cluster in the southeastern quadrant Fig. The characteristic direction in fold is anomalous due to the very shallow directions of site 6. Figure 2 shows the fold test for two paired sites from each of the sampled folds in the study area. Eight site pairs yielded a synfolding or late synfolding remagnetization and two site pairs produced postfolding remagnetizations Fig. Three folds in the eastern side of the Tampico-Misantla Basin produced synfolding ; one fold in Valley western side of the Tampico-Misantla Basin is very late synfolding and the other two produced postfolding magnetizations.

In this study, remagnetizations from synfolding remanence in carbonates were examined and dated in conjunction with folded clay-bearing units. Titanohematite or titanomagnetite are unlikely as an authigenic chemical remanent magnetization or depositional remanent magnetization carrier in these rocks. The remaining possibilities are titanium-free magnetite and hematite. The high coercivity component shown in the Lowrie test is probably a result of anisotropy in the sample and does not reflect hematite as the magnetic carrier.

Alternating field demagnetization of another sample within the same site reveals a median destructive field of 30 mT and a nearly complete elimination of the remanence at 80 mT, which does not support the presence of hematite Fig. The presence of superparamagnetic grains is supported by a spike in bulk susceptibility values at liquid nitrogen temperatures as compared to room temperature measurements before and after Tauxe, According to blocking curves for magnetite as shown dating Pullaiah et al.

Since the basins did not reach temperatures necessary for a TVRM, we conclude that the remanent magnetization in the Tampico-Misantla Basin is a chemical remagnetization. We infer that during chemical remagnetization there is growth of magnetite from a superparamagnetic state to a stable single-domain structure.

For samples in which magnetite grains have grown to the stable single-domain range, a magnetic remanence is preserved during remagnetization Butler, The origin of this chemical remagnetization connects with folding and illitization, likely reflecting tectonically related fluid pulses. This sulfide is most likely present in low concentrations, but concentrated enough to cause a spike in magnetization upon heating and alteration. Sites with oriental decay of the magnetization were not used in the analysis of this area.


Among many explanations of this behavior e. The coeval remagnetization is of normal polarity and its directions are concentrated to the northwest and down. Sitesand show synfolding remagnetizations in folds that are Late Cretaceous The Tampico-Misantla Basin is farthest to the east with the youngest deformation ages for this study area. The sites in this area are dominated by southeast and upward magnetization directions, which are likely to have been acquired during one of the latest Cretaceous to Eocene reversed polarity intervals.

Sites, and produce synfolding with a corresponding age for these folds of Very late synfolding magnetization is seen in sites and postfolding remagnetizations are observed in sites and of this area.

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Site 5 is near the other sites and is of reversed polarity, but the fold test result of sites is not as reliable due to anomalous from site 6. Folds in the study area have different ages, despite their geographic proximity, which indicates that regional metasomatism was not the cause of illitization. Illitization occurred primarily within bentonitic shale layers, while remagnetization was studied in neighboring limestone layers. Illitization occurred during folding and magnetite remagnetization is synfolding; therefore, the chemical remagnetization and illitization are of the same geologic age.

Using ages obtained from sampled folds and paleomagnetic from this study, a progressive deformation-magnetization history from west to east is recognized in the central Sierra Madre Oriental.

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In the Tampico-Misantla Basin, the westernmost side shows postfolding magnetizations, suggesting a scenario of early folding in the west followed by regional remagnetization as the eastern portion of each basin is deforming. A regional history of deformation and remagnetization emerges.

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The basins and platforms were formed in the Jurassic during opening of the Gulf of Mexico, followed by deposition of the Tamaulipas Formation in the Barremian—Cenomanian mid-Cretaceous. From the Late Cretaceous to Paleogene, deformation developed a fold-thrust belt in the area that progressed from west to east, both regionally and within each basin. This remagnetization event occurred near the end of the long Cretaceous Normal Superchron.

Folding continued into the Tampico-Misantla Basin in the Paleogene and during a reversed geomagnetic field, the Tampico-Misantla Basin was remagnetized ca. Remagnetization patterns are rarely of mixed polarity, but this study is an exception. Deformation occurred synchronously with remagnetization in several places synfolding remagnetizationsbut also occurred without simultaneous remagnetization in the Tampico-Misantla Basin postfolding remagnetizations ; the latter indicates that crystallization of illite does not a priori result in magnetite growth and remagnetization.

If the illitization process was the single cause of remagnetization in this scenario, then all of the studied folds should show synfolding remagnetization. Therefore, a parallel process is involved to produce synfolding remagnetization in some deforming rocks, but not others, that we speculate is the regional infiltration of Fe bearing fluids.

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Thus, a spatiotemporal link between regional deformation and remagnetization exists, but not one in which deformation and illitization alone result in remagnetization. By providing a new approach and demonstrating a robust correlation between the age of regional-scale deformation and remagnetization, we show that fold dating can also be used to determine the timing of synfolding remagnetizations, using rock types that are common in many foreland deformation belts. Our approach, therefore, has potential application to many orogenic systems around the world. A General tectonic map for western North America with location of study area.

B Field photograph showing example of dating studied fold sites 24 and Each marker corresponds to a fold two sites in the study area with the corresponding fold test. Four unpaired sites are also shown 1, 2, 30, and The x axis and y axis of the fold test plots are percent unfolding and k proxy for kapparespectively Fisher, Red markers indicate a fold or Valley that was not interpretable due to chaotic behavior during demagnetization. B Representative thermal demagnetization plots of the Tamaulipas Formation limestones in geographic coordinates IS, in situ.

Black symbols represent vector endpoints plotted in the horizontal plane; white symbols oriental vector endpoints plotted in the vertical plane. CSM is an example showing chaotic decay; samples showing this behavior are not used in the analysis.