Ben A. van der Pluijm

Significance of the Nova Brasilândia metasedimentary belt in western Brazil: Redefining the Mesoproterozoic boundary of the Amazon craton

(� 910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguapeo´ belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The ‘‘Grenvillian’’ deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguapeo´ belt, in contrast, seems to record only limited deformation internal to the Paragua craton. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 3660 Mineralogy and Petrology: Metamorphic petrology; 1035 Geochemistry: Geochronology; 8025 Structural Geology: Mesoscopic fabrics; 9360 Information Related to Geographic Region: South America; KEYWORDS: Rodinia, Grenville mobile belt, Amazon craton, Paragua craton, geochronology, P-T path. Citation: Tohver, E., B. van der Pluijm, K. Mezger, E. Essene, J. Scandolara, and G. Rizzotto (2004), Significance of the Nova ...

Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)

Abstract The anisotropy of magnetic susceptibility (AMS) of Eocene mudrocks from the Southern Pyrenean Foreland Basin documents the progressive development of tectonic fabrics in sediments that mesoscopically show no evidence for deformation. Two end members are observed: (1) a strong oblate fabric, with strong clustering of the minimum axes of susceptibility that is characteristic of depositional and compaction processes; and (2) a prolate magnetic ellipsoid with moderate to strong minimum axes girdle that reflects weak cleavage in the mudrocks. Low-temperature experiments and anhysteretic remanent magnetization reveal that the paramagnetic fraction dominates the AMS signal and thus, represent the preferred orientation of phyllosilicates. Characteristic stages of magnetic fabrics in mudrocks can be established and thus the AMS can be used as an indicator of cleavage development and intensity in mudrocks. The three stages of fabric development (early deformation, pencil structure and weak cleavage) reflect a process of increasingly preferred crystallographic orientation of the phyllosilicates that is evidenced by the change of distribution of the minimum susceptibility axes. This is particularly evident in the eigenvalues, which we plot on a Woodcock diagram that describes both shape and fabric strength, and allows different stages of deformation to be distinguished. Because appreciable depth during deformation (elevated temperatures and high confining pressures) can be ruled out, we propose that the sediments were wet and only partly lithified when the phyllosilicates became progressively reoriented. Thus, the observed fabric is explained as a result of the close interaction of deformation and diagenetic processes. Tectonically induced deformational fabrics formed while diagenesis progressed. Although the study area is a foreland basin, it offers a valuable analogue to active accretionary wedges. The application of AMS in these settings, where sediments are progressively dewatered through the development of fabrics, permits a similar quantification of fabric development and, eventually, of deformation.

Oroclinal bending and evidence against the Pangea megashear: The Cantabria-Asturias arc (northern Spain)

The Cantabria-Asturias arc of southwestern Europe is a highly curved Variscan belt that formed along the ancient plate boundary between Gondwana and Laurussia during the assembly of Pangea. New paleomagnetic data from 59 sites in the southern limb of the arc were combined with previously published data from 109 sites to determine the evolution of the arc. A previously unrecognized postrotation magnetization is found in the southern limb, refuting earlier models of arc formation that concluded secondary rotation of only 50% of present-day arc curvature. The new data show that the arc underwent true (100%) oroclinal bending of an originally linear belt in a two-stage tectonic history. This history represents two regional compression phases: (1) east-west in the late Carboniferous (Pennsylvanian) and (2) north-south in the Permian (both in present-day coordinates). The north-south compression phase coincides with the northward movement of Gondwana and its final collision with Laurussia. This tectonic scenario argues against an indentor scenario, and does not support a 3500 km dextral megashear proposed in earlier reconstructions.

Separation of paramagnetic and ferrimagnetic susceptibilities using low temperature magnetic susceptibilities and comparison with high field methods

Magnetic susceptibility of rocks can be dominated by diamagnetic and paramagnetic matrix minerals, ferrimagnetic and antiferromagnetic trace minerals, or a combination. The interpretation of magnetic fabric data (anisotropy of magnetic susceptibility, AMS) hinges on the qualitative and quantitative analysis of the sources of magnetic susceptibility. We discuss two methods that quantify the contribution of the different groups to the AMS: (1) comparative measurements of the magnetic susceptibility in low fields and high fields and (2) heating curves from 77 K to room temperature (low temperature magnetic susceptibility, LTMS). Method 1 measures paramagnetic, diamagnetic, and antiferromagnetic susceptibilities above the saturation magnetization of the ferrimagnetic minerals and method 2 interprets heating curves based on the fact that only the paramagnetic susceptibility is a function of temperature (Curie—Weiss law). Curie constants, paramagnetic Curie temperatures, and phase transitions (Verwey at 118 K: magnetite; Morin at 263 K: hematite) are diagnostic for specific minerals and provide further information about the contributing minerals of the sample. The relative contribution of the ferrimagnetic and paramagnetic minerals to the total susceptibility can be estimated from both methods with the same precision, if antiferromagnetic and diamagnetic contributions are insignificant. However, the LTMS method requires only simple equipment and procedures. The low temperature method can be extended to the three-dimensional case to decompose the total susceptibility tensor into its paramagnetic and ferrimagnetic sub-tensors (low temperature AMS, LTAMS). L1’MS and LTAMS are powerful additions to the group of magnetic fabric methods that allow the quantification of mineral preferred orientation in natural samples.

INFLUENCE OF MECHANICAL COMPACTION AND CLAY MINERAL DIAGENESIS ON THE MICROFABRIC AND PORE-SCALE PROPERTIES OF DEEP-WATER GULF OF MEXICO MUDSTONES

We report on how the effects of mechanical compaction and clay mineral diagenesis have affected the alignment of phyllosilicates in a suite of Miocene-Pliocene mudstones buried to sub-seabed depths of between 1.8 and 5.8 km in the deep-water Gulf of Mexico. Mechanical compaction has reduced the porosity of the samples to 15% at 5 km, with modal pore sizes between 10 and 20 nm. High-resolution X-ray texture goniometry data show that the intense mechanical compaction has not resulted in a strongly aligned phyllosilicate fabric. The muds were apparently deposited with a weak or isotropic phyllosilicate fabric which was not substantially realigned by mechanical compaction. Unusually, X-ray diffraction of <0.2 µm separates shows that: (1) there is no illitization trend between 90 and 120°C; and (2) discrete smectite persists to ∼120°C, coexisting with R1 I-S or R0 I-S with 30–40% expandable layers. Between 120 and 130°C, discrete smectite disappears and the expandability of I-S decreases to ∼25–30%. We propose a two-stage diagenetic process involving (1) the alteration of volcanic glass to smectite and (2) the illitization of smectite and I-S; the alteration of glass results in smectite without a preferred orientation and retards the illitization reaction. We suggest that the lack of a strongly aligned phyllosilicate fabric reflects the apparently limited extent of illitization, and thus recrystallization, to which these mudstones have been subjected.

Evaluating magnetic lineations (AMS) in deformed rocks

Magnetic lineation in rocks is given by a cluster of the principal axes of maximum susceptibility (Kmax) of the Anisotropy of Magnetic Susceptibility (AMS) tensor. In deformed rocks, magnetic lineations are generally considered to be the result of either bedding and cleavage intersection or they parallel the tectonic extension direction in high strain zones. Our AMS determinations, based on a variety of samples that were taken from mudstones, slates and schists from the Pyrenees and Appalachians, show that strain is not the only factor controlling the development of magnetic lineation. We find that the development and extent to which the magnetic lineation parallels the tectonic extension direction depends on both the original AMS tensor, which in turn depends on the lithology, and the deformation intensity. Rocks having a weak pre-deformational fabric will develop magnetic lineations that more readily will track the tectonic extension.

Ordovician paleogeography and the evolution of the Iapetus ocean

Paleomagnetic data from northern Appalachian terranes identify several arcs within the Iapetus ocean in the Early to Middle Ordovician, including a peri-Laurentian arc at ~10°‐20°S, a peri-Avalonian arc at ~50°‐60°S, and an intra-oceanic arc (called the Exploits arc) at ~30°S. The peri-Avalonian and Exploits arcs are characterized by Arenigian to Llanvirnian Celtic fauna that are distinct from similarly aged Toquima‐Table Head fauna of the Laurentian margin, and periLaurentian arc. The Precordillera terrane of Argentina is also characterized by an increasing proportion of Celtic fauna from Arenig to Llanvirn time, which implies (1) that it was in reproductive communication with the peri-Avalonian and Exploits arcs, and (2) that it must have been separate from Laurentia and the peri-Laurentian arc well before it collided with Gondwana. Collectively, the paleomagnetic and faunal data require an open Ordovician ocean adjacent to the Appalachian margin and argue against a Taconic-Famatinian collision between North and South America.

Composite magnetic anisotropy fabrics: experiments, numerical models and implications for the quantification of rock fabrics

Housen, B.A., Richter, C. and van der Pluijm, B.A., 1993. Composite magnetic anisotropy fabrics: experiments, numerical models, and implications for the quantification of rock fabrics. Tectonophysics, 220: 1-12. Magnetic fabrics from rocks with multiple mineral-preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics. Our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and/or orientations of the individual mineral fabrics, and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks. These criteria include: (1) the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics, and (2) the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases, as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases. Composite magnetic fabrics are observed in the shales and slates of the Martinsburg Formation, Lehigh Gap, Pennsylvania. Modeling of the AMS (anisotropy of magnetic susceptibility) and ARMA (anhysteretic remanent magnetization anisotropy) behavior constrains the relative degree of anisotropy of the bedding-parallel and cleavage-parallel fabrics. In particular, ARMA model results allow a good estimate of magnetite fabric strength. We conclude that, in the presence of composite magnetic fabrics, quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric. Such determinations require knowledge of the mineral(s) that are responsible for the measured magnetic fabric and their behavior during deformation.

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18-1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilandia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15-30 km) exposed in North America to shallower levels (5- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed.

PREFERRED ORIENTATION OF PHYLLOSILICATES IN GULF COAST MUDSTONES AND RELATION TO THE SMECTITE-ILLITE TRANSITION

Development of preferred orientations of illite-smectite (I-S) has been studied using X-ray diffraction (XRD) texture goniometry to produce pole figures for clay minerals of a suite of 16 mudstone samples from a core from the Gulf Coast. Samples represent a compaction-loading environment in which the smectite-to-illite (S-I) transition occurs. In five shallow, pre-transition samples, there is no significant preferred orientation for smectite-rich I-S. Development of preferred orientation of I-S, although weak, was first detected at depths slightly less than that of the S-I transition. The degree of preferred orientation, which is always bedding-parallel, increases rather abruptly, but continuously, over a narrow interval corresponding to the onset of the S-I transition, then continues to strengthen only slightly with increasing depth. The degree of post-transition preferred orientation is also dependent on lithology, where the preferred orientation is less well-defined for quartz-rich samples.Previously obtained transmission electron microscope (TEM) data define textures consistent with the change in orientation over many crystallites. The smectite in pre-transition rocks consists largely of anastomosing, “wavy” layers with variable orientation and whose mean orientation is parallel to bedding, but which deviate continuously from that orientation. This results in broad, poorly defined peaks in pole figures. Post-transition illite, by contrast, consists of thin, straight packets, with most individual crystallites being parallel or nearly parallel to bedding. This results in pole figures with sharply defined maxima. By analogy with development of slaty cleavage in response to tectonic stress during metamorphism, the S-I transition is marked by dissolution of smectite and neocrystallization of illite or I-S locally within the continuous “megacrystals” of smectite. The transition is inferred to have some component of mechanical rotation of coherent illite crystals within a pliant matrix of smectite. The data suggest that change in orientation and coalescence of clay packets plays an important role in the formation of the hydraulic seal required for overpressure generation.