Synfolding Magnetization Research Articles

Kinematics of the crust around the Tanggula Shan in North–Central Tibet: Constraints from paleomagnetic data

We have conducted a paleomagnetic investigation on the Middle–Upper Jurassic marine strata exposed in the hanging wall of the Tanggula Thrust system near the Yanshiping area, northern Tibet. Progressive demagnetization experiments successfully isolated stable magnetization over a broad spectrum of demagnetization temperatures. The mean direction of the characteristic remanent magnetizations for the Middle–Late Jurassic Yanshiping Group in stratigraphic coordinates (D/I (Declination/Inclination)=5.6°/60.3°, k=22.9, α95=12.9°, N=7s) is much more clustered than the mean direction in geographic coordinates (D/I=345.5°/37.2°, k=2.5, α95=48.4°), indicating magnetization was not acquired after folding. Although the conventional fold test is positive, incremental untilting test on the characteristic remanent magnetization reveals that a maximum value of precision parameter k occurs at 82.1±4.6% untilting (D/I=3.3°/57.8°, k=43.9, α95=9.2°), which indicates the ChRMs are probably acquired during Late Cretaceous folding. This synfolding magnetization component is therefore secondary. The corresponding pole position (84.4°N, 119.4°E with dp/dm=13.5/9.9°) is inconsistent with Jurassic–Early Cretaceous paleopoles of the region, but the paleolatitude is consistent with the Late Cretaceous paleolatitude observed in the Qiangtang terrane and its periphery. The synfolding component is carried by both magnetite and hematite, which were identified by isothermal remnant magnetization acquisition experiments, unblocking temperatures of stable magnetic components, and Curie temperature determination and correlated with observed hydrothermal veins. Available geological evidences indicate that the synfolding magnetization is probably the result of chemical remagnetization caused by orogenic fluids or hydrothermal sources during the early uplift of the Tibetan Plateau.

Tectonic rotations in the Late Palaeozoic continental margin of southern South America determined and dated by palaeomagnetism

SUMMARY Detailed palaeomagnetic studies of Late Palaeozoic rocks exposed in the UspallataCalingasta Valley region, located at about 32s 69.5W in the Argentine Andean Chain, have been recently accomplished. Previous results are discussed in Valencio & Vilas (1985) and Rapalini et al. (1989). The most recent studies were carried out mainly on the thick sequences of late Early Permian to Late Permian rhyolites and ignimbrites assigned to the Tambillos and Horcajo Formations. Both formations yielded palaeomagnetic pole positions (P15: 319.6E, 78.9S7 N = 16, k = 33.5, AgS = 6.5 and P17: 264.8E, 72.4S, N = 26, k = 6.4, Ag5 = 12, respectively) concordant with the Late-Palaeozoic path of South America. Preliminary palaeomagnetic results from the Middle Carboniferous Hoyada Verde Formation, exposed in the same region, suggest that these rocks carry a synfolding magnetization acquired during the Late Carboniferous, as the palaeomagnetic pole position computed from partially corrected remanence directions (C6: 356.2E, 41.9S, D1 = 8.3, 02 = 6.0) agrees with the Late Carboniferous poles from cratonic areas of South America. This concordant position of C6 definitely rules out the possibility of a Late Palaeozoic allochthony of this section of the Argentine Andean Chain. The discordant positions previously found in Late Carboniferous and early Early Permian rocks of this region are interpreted as caused by large clockwise crustal block rotations that occurred not later than the late Early Permian. A tectonic model is proposed to explain these rotations which suggests that they were associated with strike-slip displacements parallel to the western continental margin of South America caused by an oblique subduction of the Proto-Pacific Plate during the Early Permian. Many tectonic and geologic observations fit this model. The concordant position of C6 also suggests that a complex pattern of crustal block rotations should have developed in this continental margin. Other palaeomagnetic data suggest that similar rotations may have taken place in other areas of the Argentine Andean Chain in the Early Permian.

Paleomagnetism of Vendian rocks in the southwest of the Siberian Platform

[1] Presented in this paper are paleomagnetic data for the Vendian sedimentary rocks of the southwestern region of the Siberian Platform, obtained during the study of the reference rock sequences of the Central and Biryusa areas of the Sayan region and of the Yenisei mountain range in the lower reaches of the Angara River and its tributaries. This study proved the wide development of metachronous pre- and synfolding magnetization components which originated after the accumulation of the sedimentary rock sequences, yet, obviously not later than the Early Cambrian. All of the study rocks of the NemakitDaldynian age show two clearly distinguished paleomagnetic trends which seem to have formed during or soon after the accumulation of the rocks. Earlier, we got similar results for the Late Vendian rocks of the Southwest Baikal and East Sayan regions and also for the transitional Vendian-Lower Cambrian rock sequences of the Siberian Platform. This allowed us to infer the anomalous behavior of the geomagnetic field at the end of the Vendian to the beginning of the Lower Cambrian. The results obtained in the study reported here prove the actual basis of this hypothesis. During our study of the older Ediacarian and Ediacarian-Nemakit-Daldynian rocks, we managed to distinguish stable high-temperature magnetization components, obviously reflecting the trend of the geomagnetic field that existed during the accumulation of these rocks. The paleomagnetic poles corresponding to these components allowed us to reconstruct the late Vendian trend of the apparent migration of the pole and evaluate the character of the Siberian Platform movements during that time, which allowed us to chose the polarity for the Riphean paleomagnetic trends of Siberia. INDEX TERMS: 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1525

Using syntectonic remagnetizations for fold geometry and vertical axis rotation: the example of the Cévennes border (France)

Classical progressive unfolding makes a distinction between pre-folding, post-folding or synfolding magnetization. It yields the direction of magnetization only in the two first cases. To determine the direction of synfolding magnetization, possible different unfolding percentages during remagnetization have to be assumed according to the site. The small circle approach of Surmont et al. not only leads to this direction, but also to the dip value at each site during remagnetization. In the Cévennes area, extensive palaeomagnetic study reveals syntectonic remagnetization enabling the reconstruction of the shape of a fold for the time of the magnetic overprinting as well as analysis of the rotations undergone by some sites after the remagnetization.

Synfolding remagnetization and deformation: results from Palaeozoic sedimentary rocks in West Virginia

SUMMARY Palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent Palaeozoic units along two transects (Meldey and Rig) of the Patterson Creek anticline in the Appalachian orogen of West Virginia to better understand the relationship between deformation and remagnetization. While no primary directions were observed, we isolated identical, well-defined, secondary Late Palaeozoic magnetizations in carbonates from the Late Silurian Tonoloway Formation and the Early Devonian Helderberg Group. Standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the Helderberg Group and pre-folding to early synfolding in the Tonoloway Formation. A fold test on all data combined indicates a synfolding magnetization (best grouping at 60–65 per cent unfolding), but the resulting distributions are not circular. An alternative interpretation, based on optimal differential unfolding at the site mean level yields one group with a nearly pre-folding, or early synfolding, magnetization (>70 per cent unfolding) that is both circular and concordant with North America's apparent polar wander path. The second group has a ‘variable synfolding’ magnetization that is elliptical at the optimum unfolding level and falls off the path. Differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result. The sites falling into the pre-folding group are biased towards the Tonoloway Formation, the Medley Transect, the northwest limb, away from small folds and lower dip angle relative to the sites in the synfolding group. The two groups defined by the fold test results also have a relationship with both total strain and strain partitioning. The rocks containing the early synfolding remanence exhibit less total strain and, in particular, less pressure solution strain relative to grain boundary sliding and calcite twinning. It is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions. The variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization. In any event, the relationship between deformation and magnetization is not simple. Apparently identical magnetizations from adjacent sites may have very different histories and/or responses to deformation. While the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale.

Thrust Ramp Geometry and Spurious Rotations of Paleomagnetic Vectors

Interaction of deformation axes during pure translation of a hanging-wall over a footwall composed by frontal and oblique ramps is carefully evaluated together with the evolution of associated paleomagnetic vectors. Four different cases are distinguished on the basis of the deflection on the paleomagnetic vectors when the bedding correction is applied during the restoration process. Two cases (frontal and oblique ramp without mutual interaction) do not produce any deflection. But two cases in the transition zone between both ramps will undergo non-coaxial axis of tilting during progressive deformation. One of them will produce spurious rotation if the bedding correction is applied. These errors will affect the oroclinal bending diagram as well as the fold test producing an apparent oroclinality and an apparent syn-folding magnetization respectively. A well-known geometry and kinematics of the thrust system is needed to properly restore the beds (and vectors) and to avoid the spurious rotations and its collateral effects in paleomagnetic investigations. A paleomagnetic study in the Pyrenean External Sierras is shown as an example. Primary Eocene vectors underwent a clockwise rotation (40° about) during the emplacement of the South Pyrenean sole thrust, however the Rasal-Gabardiella system of oblique ramps display spurious rotations ranging from −8° up to 13° if the inappropriate bedding correction is performed.

Examining the relationship between remagnetization and orogenic fluids: central Appalachians

Preliminary work in the central Appalachians shows that the relationship between orogenic fluids and remagnetization is not as simple as many workers have assumed. Fluid inclusion and stable isotope data from veins show that the Paleozoic section in the central Appalachian Valley and Ridge province contained multiple hydrostratigraphic intervals during the Late Paleozoic Alleghanian orogeny with ‘warm’ migrating fluids restricted to the Middle to Upper Devonian interval. Paleomagnetic core samples from throughout the entire stratigraphic section give a similar syn-folding magnetization with nearly identical paleopoles. Therefore, the relatively homogeneous remagnetization of the rock section does not reflect the stratification of fluids. Consequently, the stratigraphically restricted ‘warm’ migrating fluids are apparently not directly related to the overall syn-folding remagnetization.

Synfolding magnetization: detection, testing and geological applications

A correct method for determining the direction of a synfolding remanence is described, and a new test, which in a number of cases can discriminate between a true synfolding remanence and a remanence that is the sum of pre- and post-folding components, is suggested. It is shown that it is possible to reveal the folding evolution of a local object under an assumption that the acquisition of the synfolding remanence was synchronous over this object.

Apparent synfolding magnetization as a result of overlap of pre‐ and post‐folding magnetizations

In most rocks, Natural Remanent Magnetization (NRM) can involve two or more superimposed components. Demagnetizations hopefully result in the separation of these components, which may then be resolved using least‐squares regression methods. In a study of Cretaceous redbed sites from Qaidam (China), thermal demagnetization of most specimens revealed 3 rectilinear segments in orthogonal plots, giving the appearance of a 3‐component magnetization. In order to help deciding if the intermediate straight segments could have resulted from overlap between post‐folding low temperature (LTC) and pre‐folding high temperature components (HTC), we modeled the overlap using a priori information on the directions and relative intensities of LTC and HTC derived from the actual observations. We constructed typical synthetic Zijderveld (1967) diagrams for each site, which could be compared with those obtained from the real samples. We conclude for the Qaidam samples that the intermediate temperature component (ITC) is most likely an artifact resulting from overlap between the LTC and HTC.

Remagnetization of the folded Belden Formation, northwest Colorado

Paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in Belden Formation (Pennsylvanian) limestones on an asymmetric fold in Colorado. The natural remanent magnetizations contain three components: a modern viscous magnetization removed at low temperatures, a reversed Tertiary magnetization (declination (D) = 186.7°, inclination (I) = −54.6°) of thermoviscous origin removed at intermediate temperatures, and an apparently synfolding Late Cretaceous/early Tertiary characteristic magnetization (D = 338.7°E, I = 62.5°, precision parameter k = 54.9, radius of 95% probability cone ∝95 = 5.9°, 65% unfolding) removed by 580°C. Hysteresis properties suggest that the characteristic magnetization resides in single‐domain magnetite. Low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization. The 87Sr/86Sr values (0.7088–0.7100) show the limestones to be radiogenic, with 87Sr enriched halos around calcite‐cemented tectonic fractures. There is no control on the distribution and intensity of the magnetization by the veins or structural position (e.g., highly mineralized versus less mineralized rock). Lithology, rather than fluid alteration, controls the magnetization. Fecal pellets contain pyrite, with alteration rims identified as magnetite by scanning transmission electron microscope X ray diffraction patterns. Selective demagnetization has established that the fecal pellets carry the characteristic magnetization. It is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding. A synfolding magnetization caused by the syndeformational mineralizing fluids, however, can be ruled out. Structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition. Overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result. The replacement of pyrite by magnetite is interpreted as prefolding in origin, perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons.

Paleomagnetism of the Sassamansville diabase, Newark Basin, southeastern Pennsylvania: Support for Middle Jurassic high-latitude paleopoles for North America

A paleomagnetic study of the Sassamansville diabase, which intrudes the Passaic Formation red beds of the Newark Basin in southeastern Pennsylvania, was conducted to constrain the age of the diabase9s magnetization. Nine sites were collected from the diabase around the Sassamansville syncline. A tenth site was collected at an associated dike 12 km to the north. Eight to ten individually oriented cores were drilled at each site. Alternating field (af) and thermal demagnetization isolated a stable, internally consistent characteristic remanence at eight sites. Using the bedding tilt of the nearest baked or unbaked Passaic Formation sedimentary rocks for a fold test, the site means showed best clustering at 65% unfolding. This magnetization is a statistically significant (at 93% confidence) synfolding magnetization. The diabase9s magnetization fails a baked contact test conducted at one of the sites, indicating that the magnetization is secondary and not a primary thermal remanent magnetization (TRM). Isothermal remanent magnetization (IRM) and partial anhysteretic remanent magnetization (pARM) acquisition experiments and of demagnetization results indicate that the magnetization is carried by high-coercivity magnetite. Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) results indicate that this magnetite has a different fabric from the northeast-southwest horizontally lineated fabric carried by low-coercivity magnetite grains that do not carry the remanence. The lineated fabric may have been caused by flow of the diabase from the north-east to the southwest during emplacement. When the rock magnetic results are considered in light of previous petrographic observations (Davidson and Wyllie, 1968) of these rocks, the rock magnetics suggest that the magnetization is carried by secondary magnetite, which has grown as rims on primary magnetite. The secondary magnetite may have formed during a hydrothermal event proposed by Sutter (1988) based on geochronologic evidence and dated at 175 Ma. If the diabase magnetization is considered to be coeval to a widespread remagnetization (B component) of the Newark Basin sedimentary rocks (Witte and Kent, 1991), the diabase magnetization indicates 15° of counter-clockwise rotation for the Sassamansville syncline. This is consistent with either left-lateral motion along the border fault or along the intrabasinal Chalfont Fault. The presence of counterclockwise block rotations along the border fault could weaken previously reported evidence that the Newark Basin (B component) remagnetization occurred after the Newark Basin strata had acquired most of their north-westerly tilt (Witte and Kent, 1991). Correc-tion for the tilt would move the Newark (B component) remagnetization paleopole to lower paleolatitudes but necessitate significant post-175 Matectonic activity in the Newark Basin. This would contradict the long sedimentary record of rifting and border fault activity for the preceding 50 m.y. A partial tilt correction may be the best geologic and paleomagnetic interpretation because it would minimize the amount of post-175 Ma tectonic activity required while bringing the Newark (B component) remagnetization paleopole into paleolatitudinal agreement with a paleopole derived from the rotation of European, South American, and African data into North American coordinates (Van der Voo, 1992).

Late Paleozoic remagnetization in limestones of the Craven Basin (northern England) and the rock magnetic fingerprint of remagnetized sedimentary carbonates

Samples of Lower Carboniferous Chatburn and Pendleside limestones collected from 27 sites in the Craven Basin (northern England) carry a syn‐Hercynian remagnetization residing in magnetite. The paleomagnetic directions are all of reversed polarity consistent with a Kiaman Superchron remagnetization. The partially (62%) tilt corrected mean direction has the minimum correlation statistic implying a synfolding magnetization. The resulting pole appears to be Late Carboniferous in age, and there is no indication of any magnetization component that predates the remagnetization event. The magnetization appears to be of the same type as the synfolding remagnetizations documented in several Appalachian carbonate units. Jackson (1990) reported that remagnetized Appalachian carbonates have distinctive hysteresis properties, and we have obtained similar hysteresis loops from remagnetized Paleozoic carbonates from the Craven Basin (England), the Great Basin (Nevada), Alaska, and New York State. The hysteresis loops are all “wasp‐waisted” and yield highly anomalous hysteresis ratios, compared to data from synthetic or “typical” magnetite‐bearing samples. In contrast, unremagnetized limestones from Italy show hysteresis behavior which is more typical for magnetite‐bearing rocks (Channell and McCabe, this issue). These results suggest that remagnetization in magnetite‐bearing carbonates can be readily detected on the basis of hysteresis properties, prior to a full‐scale paleomagnetic investigation.

Remagnetization by basinal fluids: Testing the hypothesis in the Viola limestone, southern Oklahoma

Migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late Paleozoic secondary magnetizations in the rocks of North America. Paleomagnetic and geochemical results from the Ordovician Viola Limestone in southern Oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit. The Viola Limestone contains what we interpret to be a pervasive Pennsylvanian synfolding magnetization residing in magnetite and a localized Permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins. Both secondary magnetizations are interpreted as chemical remanent magnetizations (CRM) based on low burial temperatures and the presence of authigenic magnetic phases. The relative proportion of the Permian CRM in hematite gradually decreases whereas the magnetite CRM increases with distance from the veins. Fluid inclusion and Sr isotope studies indicate that the vein mineralization (calcite with Mississippi‐Valley‐type oxides and sulfides) precipitated from basinal fluids which were warm, saline, and radiogenic. The radiogenic 87Sr/86Sr values of the limestones in the alteration zones, and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones. The coincidence of the geochemical and remagnetization trends suggests that the Permian CRM dates the migration of basinal fluids through the veins. Geochemical results from the Viola Limestone containing the pervasive CRM indicate that it is relatively unaltered with no evidence for basinal fluids. The lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive CRM need to be tested. The results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone.

Permian paleomagnetism of the northern Tien Shan and its tectonic implications

As a part of structural and paleomagnetic investigations of the late Paleozoic and younger tectonism in the Tien Shan, Permian sedimentary and volcanic rocks were sampled at four localities within the Northern Tien Shan west of the Issyk Kyl Lake. After complete thermal demagnetization, prefolding characteristic components were isolated for four localities, one of which also yielded a synfolding magnetization. When compared with the European reference data, the inclinations of these Permian rocks appear to be close to the reference ones. Mean declination for the northernmost locality also agrees with the European data, while the results for the southern parts of the area point to counterclockwise rotations. Comparison with data from an independent study (Thomas et al., 1992) shows that the rotations pre‐date Alpine deformation but probably postdate the initial stage of the folding which affected the Permian rocks. Analysis of paleomagnetic data from different parts of Kazakhstan together with our results from the Northern Tien Shan has led to the conclusion that these blocks had already been welded with the European plate in Permian time.

The effects of grain‐scale deformation on the Bloomsburg Formation Pole

Previous paleomagnetic results from the Silurian Bloomsburg Formation in the central Appalachian Valley and Ridge Province suggest that the Bloomsburg's characteristic magnetization is a synfolding magnetization acquired during the Acadian Orogeny. However, there is little geological evidence to support extensive Acadian folding in the central Appalachians. A more favorable explanation is that the characteristic magnetization originated as a prefolding Silurian magnetization and has been altered by grain‐scale deformation to mimic a synfolding Devonian remagnetization. In order to investigate whether penetrative deformation has altered the Bloomsburg Formation's characteristic signal, the relationship between strain and remanence was examined around three central Appalachian Valley and Ridge folds. Structural analysis of mesoscopic and microscopic features along with center‐to‐center finite strain analysis at these three folds indicates a deformation sequence that includes prefolding layer‐parallel shortening overprinted by top‐to‐the‐foreland, bedding‐parallel shear and flexural slip/flow folding. Inclination of the characteristic magnetization varies systematically with the magnitude of finite strains. This relationship between strain and remanence suggests that the characteristic magnetization has been progressively rotated toward shallower inclinations by penetrative deformation analogous to rigid particle rotation in a viscously deforming matrix. Using directions from sites with relatively low finite strain values, a new Bloomsburg Formation pole is calculated at 18.7°N, 107.4°E (K = 58.4, A95 = 8.2°). This new pole falls on the Silurian track of the North American apparent polar wander path between the Silurian Wabash Reef pole and the Silurian‐Devonian Andreas pole. In addition, these results suggest that previously reported 20°–30° difference in declination of site mean directions north arid south of the Pennsylvania salient may also be the result of grain‐scale reorientation of the remanence‐carrying grains and not oroclinal bending.

Synfolding magnetization in the Jurassic Preuss Sandstone, Wyoming‐Idaho‐Utah Thrust Belt

The Jurassic Preuss Sandstone, exposed in five thrust plates of the Wyoming‐Idaho‐Utah thrust belt, carries directions of remanent magnetization that group most tightly after only partial unfolding. Field, petrographic, and rock magnetic evidence indicates that the carrier of this magnetization is detrital, low‐Ti titanomagnetite. The detrital titanomagnetite was remagnetized at low temperatures (75°–150°C) probably completely during folding. Anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain. We suggest that low‐temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization, the acquisition of which was enhanced by stress, or a piezoremanent magnetization that involved stress‐induced movement of domain walls during intracrystalline strain, or was a combination of the two mechanisms. Stress may promote remagnetization at temperatures much lower than predicted by current theoretical models. Other mechanisms, such as acquisition of chemical remanent magnetization during folding, deflection of a prefolding magnetization by internal strain, or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the Preuss. The locus of acquisition of synfolding magnetization in the Preuss migrated in conjunction with deformation in the thrust belt. A model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps. A lack of reverse‐polarity directions remains a puzzling feature of the remanence. The remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in Late Cretaceous and early Tertiary (?) periods of predominantly normal polarity.

Cretaceous paleomagnetism of the Methow‐Pasayten Belt, Washington

Detailed demagnetization experiments isolated a characteristic remanent magnetization in ten stable sites from the upper Cretaceous Winthrop and Midnight Peak Formations in the Methow‐Pasayten belt of north‐central Washington. This remanence agrees best between opposite limbs of a fold (the Goat Peak syncline) when corrected for 46% of tilt. This is consistent with magnetization acquired during deformation. Synfolding magnetization may have been facilitated by a thermo‐chemical event associated with synkinematic intrusions along the axis of folding. The mean direction (D=12.0°, I=61.1°, Alpha‐95=4.8°) is highly discordant with respect to the expected direction for north‐central Washington. This discordance points to about 1,400 km of poleward transport and 48° of clockwise rotation between 93 and 45‐48 Ma.