Randolph J. Enkin

Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to the present

Paleomagnetic study of China and its environs has been the center of a major international effort for the last 10 years. In this paper, we critically review all available paleomagnetic poles of Upper Permian to Tertiary age from the main blocks of China with the goal of placing constraints on models of the formation and the subsequent deformation of the region. After selecting “reliable” poles by applying objective criteria, we divide our analysis into first-order (motions of blocks) and second order (deformation within blocks). For first order analysis, apparent polar wander paths are constructed for the major blocks. We discuss the compatibilities and contradictions between the geological and paleomagnetic records. A sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented. In general, the major blocks were probably in contact throughout the Permian and Triassic, but the Jurassic was the key age during which most of the movement toward Chinas present configuration took place. Our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events. During the Cretaceous, Chinese poles agree with poles from other continents transferred onto Eurasia. At the second order, we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region, indicating that much of China has been affected by small (< 20°) differential rotations. This we interpret as deformation caused in part by the extrusion of the Chinese blocks away from the Indian collision. The complete annotated list of poles is given as an appendix.

The fold test in paleomagnetism as a parameter estimation problem

Most proposed fold test formulations use significance tests to try pre-tilting and post-tilting remanence hypotheses. We suggest that it is better to consider the fold test as a parameter estimation problem. Making the usual assumption that the distribution of remanence vectors was originally roughly parallel, we propose, using a monte carlo simulation technique, to estimate the amount of tectonic tilting at the time of magnetization along with a 95% confidence interval. If, for example, this confidence interval included 100% then one could not rule out pre-tilting remanence. In the older terminology, the fold test is positive. The k-ratio test of McElhinny [1964] is often said to be conservative in that if a study passes the k-ratio test then it certainly passes a more rigorous test. We show with a typical counter-example that this assertion is incorrect. Observational uncertainty of bedding directions is easily included in this formulation.

The direction–correction tilt test: an all-purpose tilt/fold test for paleomagnetic studies

Abstract The tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting. While several tilt test formulations have been proposed, none fully satisfies the requirements of statistical validity, applicability to all bedding geometries, and ease of use. This paper introduces the direction–correction (DC) test, which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections. The DC test is similar to McFadden’s correlation test because both test whether or not the site directions contain information about the bedding tilts, however the DC test gives greater weight to sites with greater counter-bedding. The DC test is similar to Watson’s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions, however the DC test uses analytical rather than numerical methods. Graphical output aids the researcher in recognizing problem sites. Using both real and simulated data, the DC test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries. The simulations show that the power of the tilt test is inversely proportional to the 95% confidence interval (α95) of the overall mean. As a rule of thumb, paleomagnetists should attempt to sample sufficient sites to obtain an α95 less than 1/6 of the bedding attitude difference.

Ancient Permafrost and a Future, Warmer Arctic

Climate models predict extensive and severe degradation of permafrost in response to global warming, with a potential for release of large volumes of stored carbon. However, the accuracy of these models is difficult to evaluate because little is known of the history of permafrost and its response to past warm intervals of climate. We report the presence of relict ground ice in subarctic Canada that is greater than 700,000 years old, with the implication that ground ice in this area has survived past interglaciations that were warmer and of longer duration than the present interglaciation.

Three‐dimensional micromagnetic analysis of stability in fine magnetic grains

We examine the stability of magnetic remanences in grains of magnetite near the critical single-domain grain size (d0). The magnetic structure of a grain is followed during a 180° magnetization reversal, and it is shown that noncoherent reversals can occur even in grains smaller than d0. The energy barrier to noncoherent revesal is much lower than that of coherent rotation, so that pseudo-single-domain grain behavior will be exhibited by grains significantly smaller than d0.

Yellowstone in Yukon: The Late Cretaceous Carmacks Group

The Late Cretaceous Carmacks Group, a thick subaerial volcanic succession that once covered much of southwest Yukon, was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit. Coeval hydrothermal activity resulted in widespread alteration and gold mineralization. The lavas are shoshonites, enriched in large ion lithophile and light rare earth elements, but depleted in high field strength elements. Ankaramitic absarokite flows in the upper Carmacks Group range up to 15 wt% MgO, requiring a high liquidus temperature (1400 °C at 1 bar, dry). High K 2 O contents (>3%) of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle. Although previously interpreted as subduction related, the Carmacks Group was erupted during a Cordilleran-wide magmatic lull and lacks coeval calc-alkalic batholiths. The lavas are petrologically similar to plume-related Eocene to Pliocene potassic lavas of the western United States. New paleomagnetic collections, combined with previous work, place the Carmacks Group 17.2° ± 6.5° (1900 ± 700 km) south of its present position relative to the craton during deposition, near the paleolocation of the Yellowstone hotspot. The spatial coincidence, similarity of tectonic setting, and lithologic similarity of the Carmacks Group and Yellowstone volcanic successions suggest that the Carmacks Group is the 70 Ma effusion of the Yellowstone hotspot. Subsequent northward displacement of the Carmacks Group is attributed to coupling with the Kula plate. Correlation of the Carmacks Group and the Yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern Intermontane belt at 70 Ma.

Paleomagnetic and tephra evidence for tens of Missoula floods in southern Washington

Paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from Glacial Lake Missoula, Montana, entered Washington’s Yakima and Walla Walla Valleys during the last glaciation. In these valleys, the field evidence for hiatuses between floods is commonly subtle. However, paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at Zillah, Touchet, and Burlingame Canyon display consistent secular variation that correlates serially both within and between sections. The secular variation may further correlate with paleomagnetic data from Fish Lake, Oregon, and Mono Lake, California, for the interval 12,000‐17,000 14 C yr B.P. Deposits of two successive floods are separated by two tephras derived from Mount St. Helens, Washington. The tephras differ in age by decades, indicating that a period at least this long separated two successive floods. The beds produced by these two floods are similar to all of the 40 beds in the slack-water sediment sequence, suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years.

Simulated annealing of three-dimensional micromagnetic structures and simulated thermoremanent magnetization

This paper deals with the application of simulated annealing to rock magnetism. The implementation of the algorithm is discussed in detail. It is shown that the technique can be used to determine stable micromagnetic structures by finding very low energy states which are not easily found using other optimization techniques. A modified simulated annealing algorithm is used to incorporate thermal fluctuations into a micromagnetic model in order to simulate the acquisition of a thermoremanent magnetization (TRM). The results obtained agree with Neels single domain (SD) TRM theory when the magnetization of the model is constrained to rotate coherently. When this constraint is removed, a decrease in the blocking temperature of the model is observed above ≈0.07 μm showing Neels SD TRM theory to be inaccurate for grains above this size.

Transdomain thermoremanent magnetization

Transdomain thermoremanent magnetization (TRM) is produced when thermally activated transitions between different domain structures become blocked during cooling. This paper investigates transdomain remanence in the pseudo-single-domain size range ( 6000kT for SD ↔ 2D transitions. Transdomain viscous remanent magnetization (VRM) will not occur, even on geological timescales, by a one-dimensional excitation such as edge nucleation of a domain wall. Transdomain blocking temperatures, at which energy barriers fall to 25kT–60kT, are ≥553°C for SD ↔ 2D and ≥574°C for 2D ↔ 3D transitions. There are two separate blocking temperatures, e.g. TB1 for SD ↔ 2D and TB2 for 2D ↔ SD transitions. Usually, only the higher of the two has practical significance because the favored (lower energy) state is already 100% populated at this temperature. Our theory is the first to make quantitative predictions of transition paths, relaxation times, and blocking temperatures for transdomain TRM. It is also quite robust. Relaxing the one-dimensional constraint and introducing crystal defects would make it easier to nucleate domains, but energy barriers and blocking temperatures would not be reduced greatly. Our principal conclusion, that only the lowest energy state at blocking is significantly populated, is a fundamental consequence of the Boltzmann statistics of equilibrium states and is unaffected by the details of transitions between states. Grain interactions may be responsible for the multiplicity of states observed in large titanomagnetite grains following replicate TRM experiments.

Palaeomagnetic dating of the earliest continental Himalayan foredeep sediments: Implications for Himalayan evolution

Abstract Between the time of the India-Eurasia collision (50–45 Ma) [1] and the climax of crustal shortening and thrust stacking in the Himalaya when the Main Central Thrust (MCT) was active (21 Ma) [1,2] there is a ca. 30 My gap about which little is known. This paper aims to shed light on this period by dating the initiation of major erosion from the rising Himalaya and the probable start of uplift, a significant event in the orogens history. This was achieved by accurately dating, for the first time, the Dagshai Formation sediments of northern India, which are interpreted as early Himalayan foreland basin deposits that record initial large-scale erosion of the orogen [3]. Oriented hand samples were collected from six sites and analysed, using palaeomagnetic techniques. Both polarities are represented and the remanence passes a fold test. Fitting the measured palaeolatitude to that expected for the Indian plate dates the Dagshai Formation at 35.5 Ma ± 6.7 Ma, and this is taken as the time when the embryonic Himalaya began to be strongly eroded and regionally uplifted.