Dating synfolding remagnetization: Approach and field application (central Sierra Madre Oriental, Mexico)

Carbonates that contain interlayered shales with radiometrically datable authigenic illite present an opportunity to directly determine the age of synfolding paleomagnetic remagnetizations. By combining radiometric Ar/Ar dating of illitization in folds and thrusts with synfolding remagnetization, an absolute age for synfolding remagnetizations can be determined. 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. In deformed areas, the age range of the remagnetization episode can be estimated relative to folding, through application of the paleomagnetic fold test (Facer, 1983).

Tohver et al. (2008) dated remagnetization events in the Cantabrian-Asturian Arc (northwestern Spain) by applying Ar/Ar dating to clays collected from the area. The clays were associated with limestone layers that had undergone three remagnetization events (Weil et al., 2000). Tohver et al. (2008) obtained three different ages with this technique; however, one age was out of sequence with the predetermined order of remagnetization events.

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., 2012; Evans et al., 2000; Lewchuk et al., 2003). For example, during illitization, smectite transforms to illite, releasing Fe as temperatures increase with burial (Altaner and Ylagan, 1997). 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., 1993; McCabe et al., 1989).

Katz et al. (1998) presented evidence of a strong or detectable chemical remagnetization in the limestone-marl sequences of their study area, the Vocontian Trough (southeastern France). This remagnetization is only present where there is also evidence of clay diagenesis. This hypothesis was further supported by Katz et al. (2000), Gill et al. (2002), and Woods et al. (2002). 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. (2001), Evans et al. (2000), and Zegers et al. (2003) also presented evidence for the link between Fe-bearing fluid (Mississippi Valley–type and orogenic) movement and the growth of magnetite; they used fluid inclusions and stable isotope data to correlate the presence of remagnetized rocks with evidence of fluid migration. 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.g., Vrolijk and van der Pluijm, 1999), offering the potential for radiometric dating of deformation (van der Pluijm et al., 2001). Illitization associated with flexural folding of carbonate-shale successions in the study area (central Sierra Madre Oriental, Mexico) was examined (Fitz-Díaz and van der Pluijm, 2013; Fitz-Díaz et al., (2014). 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. This is in good agreement with textural observation showing only one generation of authigenic illite in these rocks (Fitz-Díaz et al., 2014).

By combining newly determined ages of illitization in folds with new paleomagnetic results in Mexico’s central Sierra Madre Oriental, this study demonstrates the ability to associate a radiometric age with synfolding remagnetizations. Extensive work has shown that many carbonates around the world have been remagnetized (Jackson and Swanson-Hysell, 2012; McCabe and Elmore, 1989; Van der Voo and Torsvik, 2012), 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). 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. It is ∼100–250 km wide, thinning to the southeast and dominated by Cretaceous carbonates (Fitz-Díaz et al., 2011a; Guzmán and de Cserna, 1963). The study area spans four Cretaceous paleogeographical areas: the Zimapán and Tampico-Misantla Basins and the Valles–San Luis Potosi and El Doctor Platforms. The El Doctor Platform is thrusted by the Tolimán Sequences on the western side of the study area (Fig. 1).

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., 2001; Gray et al., 2001). 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, 1944; Suter, 1987). Deformation of the area occurred during the Late Cretaceous to Paleogene, with the basins dominated by folding and intense water-rock interaction (see Fitz-Díaz et al., 2011b, for details); while the platforms were thrust dominated and showed much less fluid-rock interaction (Aranda-Gómez et al., 2000; Fitz-Díaz et al., 2012).

Deformation in the central Sierra Madre Oriental was dated (Fitz-Díaz et al., 2014) using the Ar/Ar illite dating technique; that study targeted fold and thrust dating in the region. The absolute age of folds was determined by clay grain-size separation, illite polytype characterization, and ⁴⁰Ar/³⁹Ar dating of multiple size fractions (Fitz-Díaz and van der Pluijm, 2013; Haines and van der Pluijm, 2008). Ages of thrusting were determined for the Tolimán Sequences (83.5 ± 1.5 Ma) on the western edge of the study area, the western and eastern Zimapán Basin (82 ± 0.5 Ma and 76.5 ± 1.0 Ma, respectively), and the western and eastern Tampico-Misantla Basin (64 ± 2.0 Ma and 43.5 ± 0.5 Ma, respectively); see Figure 2 for details (Fitz-Díaz et al., 2014). The ability to successfully date mesoscopic folds made this an ideal area to test the feasibility of absolute dating of synfolding remagnetizations.

Twenty-eight sites from the Barremian–Cenomanian Tamaulipas Formation were sampled in the central Sierra Madre Oriental (Mexican fold-thrust belt) (Fig. 2). 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.

Cored samples were cut to 2.2 cm length with a dual bladed saw at the University of Michigan. Broken samples were glued back together with alumina cement and all specimens were labeled with Velvet underglaze nonmagnetic temperature-resistant paint.

All specimens were measured and demagnetized in a magnetically shielded room, with a rest field of <200 nT, to minimize accumulation of any viscous magnetization. Remanent magnetizations were measured using a three-axis 2G superconducting magnetometer. Specimens were thermally demagnetized using an ASC TD-48 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. The specimens were heated to ∼420 °C, after which we typically observed a spike in magnetization intensity, due to growth of a new mineral. Results from the demagnetization process were analyzed with the Paleomac software by Cogné (2003) and graphed in orthogonal or stereographic projections (Zijderveld, 1967). 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.e., prefolding, synfolding, or postfolding (Kirschvink, 1980; Tauxe and Watson, 1994; Watson and Enkin, 1993). The fold test proportionally untilts the fold limbs with different dips, utilizing the fold axis to anchor the rotations.

Magnetic directions from 28 sites in the mid-Cretaceous section of the Tamaulipas Formation in the central Sierra Madre (CSM) are summarized in Table 1. Thermal demagnetization revealed a characteristic component below ∼420 °C, after which samples show a spike in magnetization (Fig. 3A). Magnetite is the principal carrier of the magnetization as determined by three-dimensional (3D) isothermal remanent magnetizations (IRMs), performed with an ASC Scientific Impulse Magnetizer followed by thermal demagnetization of the samples (Fig. 4A; Lowrie, 1990). Due to chaotic decay of the magnetization during demagnetization, 5 sites could not be interpreted; the remaining 23 sites show the removal of a present-day field component from 0 to ∼200 °C and a characteristic magnetization direction from 200 to ∼420 °C (Fig. 3B).

Throughout the study area, 23 sites generated interpretable results including 10 fold tests applied to paired sites, as labeled in Figure 2. Sites from the Valles–San Luis Potosi Platform, Tolimán Sequences, and El Doctor Platform were analyzed for comparison and did not provide a fold test option. Paleomagnetic directions in the Tolimán Sequences and Zimapán Basin are downward and northwesterly, which we interpret to be of normal polarity. Directions in the Tampico-Misantla Basin are reversed and cluster in the southeastern quadrant (Fig. 5). The characteristic direction in fold 5-6 is anomalous due to the very shallow directions of site 6.

Figure 2 shows the fold test results 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. 2). The Zimapán Basin yielded four folds with a synfolding magnetization (one being very late synfolding). Three folds in the eastern side of the Tampico-Misantla Basin produced synfolding results; one fold in the 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. Of the possible magnetic carriers in the studied samples, goethite and sulfides can be excluded as possible carriers since there is no decay at 120 °C or 320 °C. 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 Lowrie 3D IRM test (Fig. 4A) supports the identification of magnetite in site 11, and is typical of the carbonates sampled. 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. 4B). The lower blocking temperature (∼420 °C) as well as a combination of superparamagnetic and stable single-domain magnetite is a common occurrence in remagnetized carbonates (Jackson and Swanson-Hysell, 2012). 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, 2010).

According to blocking curves for magnetite (as shown in Pullaiah et al., 1975), with a laboratory blocking temperature of ∼420 °C and a relaxation time of 10–100 m.y., the required temperature for acquisition of a thermoviscous remanent magnetization (TVRM) is ∼250 °C. Based on microthermometry of fluid inclusions in syntectonic veins and vitrinite reflectance, temperatures within the Tampico-Misantla Basin ranged from 80 to 180 °C, and in the Zimapán Basin ranged from 220 to 250 °C during deformation (Gray et al., 2001; Fitz-Díaz et al., 2014). 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. Synfolding results in the Zimapán Basin indicate a relatively quick acquisition of a remanent magnetization instead of the required longer acquisition for thermoviscous remagnetization at these temperatures. 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, 1992). The origin of this chemical remagnetization connects with folding and illitization, likely reflecting tectonically related fluid pulses.

The peak in magnetization after ∼420 °C, illustrated in the magnetic moment plot of Figure 3A, suggests the presence of a nonmagnetic Fe sulfide component that oxidizes at higher temperatures. This sulfide is most likely present in low concentrations, but concentrated enough to cause a spike in magnetization upon heating and alteration. Sites with chaotic decay of the magnetization were not used in the analysis of this area. Among many explanations of this behavior (e.g., sample CSM8-1 in Fig. 3B), sparse or swamped magnetic mineral growth during remagnetization events may be a cause.

The Zimapán Basin and Tolimán Sequences, farthest to the west in the study area, have deformation ages ranging from 83.5 ± 1.5 to 76.5 ± 1.0 Ma (determined by Fitz-Díaz et al., 2014). The coeval remagnetization is of normal polarity and its directions are concentrated to the northwest and down. Folding progressed temporally from west to east with a late synfolding remagnetization in sites 20-21 in the Zimapán Basin. Sites 22-23, 24-25 and 28-29 show synfolding remagnetizations in folds that are Late Cretaceous (76.5 ± 1.0 Ma; Fitz-Díaz et al., 2014). Therefore, combining the synfolding nature of the magnetizations with the absolute ages of the folding, it is concluded that synfolding remagnetization in the Zimapán Basin is Late Cretaceous (ca. 77 Ma).

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 9-10, 11-12, and 16-17 produce synfolding results with a corresponding age for these folds of 43.5 ± 0.5 Ma (from Fitz-Díaz et al., 2014). Very late synfolding magnetization is seen in sites 18-19 and postfolding remagnetizations are observed in sites 3-4 and 5-6 of this area. Site 5 is near the other sites and is of reversed polarity, but the fold test result of sites 5-6 is not as reliable due to anomalous results from site 6. Thus, this area underwent deformation and/or folding in the west and, as folding proceeded to the eastern portion of the area, a younger remagnetization event occurred ca. 44 Ma (mid-Eocene).

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. These units within the same fold structures are connected through a pore fluid that was active during deformation, as demonstrated through comparative δ²H data measured in illite and in water of primary fluid inclusions trapped in syntectonic veins within the carbonate layers (Fitz-Díaz et al., 2014). Comparison of δ¹⁸O and δ¹³C in calcite from veins and host carbonates supports dissolution-precipitation within limestone layers during deformation, allowing for the formation of stylolites and veins (Fitz-Díaz et al., 2011b). 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 results from this study, a progressive deformation-magnetization history from west to east is recognized in the central Sierra Madre Oriental. The fold test results and radiometric dates support a sequence of sediment deposition, deformation (faulting and folding; D₁, D₂, D_2′, and D₃), synfolding and postfolding remagnetization events (with reversals; R₁ and R₂), ending with the acquisition of a present-day field magnetization (Fig. 6). 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. 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. Sites in the Tolimán Sequences and Zimapán Basin record Late Cretaceous deformation, and as folding progressed eastward, the area underwent a remagnetization event ca. 77 Ma. 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. 44 Ma. Remagnetization patterns are rarely of mixed polarity, but this study is an exception.

Absolute dating of synfolding remagnetizations can be obtained by integration with ⁴⁰Ar/³⁹Ar dating of deformation-induced illitization. Combining the ages from Fitz-Díaz et al. (2014) and paleomagnetic results from this study, the remagnetization history of the central Sierra Madre Oriental (Mexican fold-thrust belt) involved two events ca. 77 Ma and ca. 44 Ma. Deformation occurred synchronously with remagnetization in several places (synfolding remagnetizations), but 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. 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.

The study was partially supported by National Science Foundation grants EAR-0909288 (Van der Voo), EAR-1118704 (van der Pluijm), and HRD-1216750 (van der Pluijm), the Turner Fund at the University of Michigan (Nemkin), and CONACYT (Consejo Nacional de Ciencia y Tecnologia, México) grant 240662 (Fitz-Díaz).