Victor A. Schmidt

Hysteresis properties of titanomagnetites: Grain-size and compositional dependence

Sized fractions of x = 0.6, 0.4, 0.2 and 0.0 titanomagnetites were studied with a vibration magnetometer. In the course particles (d > 150 μm), no compositional dependence of hysteresis parameters was found. HC was less than 50 Oe, HR/HC > 4 and JR/JS < 10−2, reflecting multi-domain behaviour. In contrast, fine particles (d ⋍ 0.1 μm) revealed systematic grain-size dependence of parameters with coercive force as high as 2,000 Oe in x = 0.6 titanomagnetite. Grain-size dependence studies revealed broad transition sizes for the onset of true multi-domain behaviour depending upon which factor is chosen. In magnetite it varies from 10 to 20 μm. The experimental critical size for single-domain behaviour for magnetite is about 0.1 μm and for x = 0.6 titanomagnetite 1–2 μm.

The evolution of Middle America and the Gulf of Mexico–Caribbean Sea region during Mesozoic time

A plate-tectonic model for the evolution of Middle America and the Gulf of Mexico-Caribbean Sea region is presented. The model, which is based upon the existence of the Mojave-Sonora megashear, incorporates into the Triassic Pangea reconstruction three microplates between North and South America, thus avoiding the overlap of the Bullard fit. These plates are the Yaqui, bounded on the north by the Mojave-Sonora megashear; the east and west Maya plates, bounded on the north by the Mexican volcanic zone and on the south by a predecessor of the Motagua fault zone; and the Chortis plate (parts of Guatemala and Honduras). During Late Jurassic time, as North America split away from Europe, Africa, and South America, shear, with left-lateral sense of displacement, occurred along the transform faults that bounded the micro-plates. If ∼800 km of left-lateral displacement along the Mojave-Sonora megashear, ∼300 km along the Mexican volcanic belt, and ∼1,300 km along a proto-Motagua megashear are restored, and if Yucatan and Cuba are rotated to fit against northern South America, then (1) a curvilinear belt of late Paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in Texas, eastern Mexico, nuclear Central America, and Colombia; (2) a Mediterranean-like sea is delineated that was a precursor of most of the present Gulf of Mexico; (3) correlation is implied between the distinctive quartzose San Cayetano Formation of Cuba and the Caracas and Juan Griego Groups of Venezuela. Geometric constraints suggest that probably shear initially occurred along the Mexican volcanic zone near the end of the Middle Jurassic. Subsequently, probably about 160 m.y. ago, displacements that total ∼800 km began along the Mojave-Sonora megashear. Contemporaneously, Yucatan and fragments of pre-Cretaceous rocks that compose parts of central and western Cuba migrated northward toward their present positions. Rotation of Yucatan was facilitated by considerable displacement along the proto-Motagua zone and along a zone that is probably coincident with the modern Salina Cruz fault. Accumulation of widespread major salt units of Late Jurassic (Callovian to early Oxfordian) age in the Gulf Basin probably occurred contemporaneously with the arrival of these blocks at their present positions. Clastic units that interfinger with some of the youngest salt units and rim the Gulf of Mexico have not recorded major recognized translations since their accumulation. Clockwise rotation of South America and the Chortis plate occurred during Early Cretaceous time. This movement, which was manifested by subduction of Jurassic ocean floor against the previously rifted precursor of the island of Cuba and under parts of Hispaniola and Puerto Rico, is recorded by circum-Caribbean orogeny. Abrupt changes in the relative motions between North and South America during Late Cretaceous time may have resulted in extension and outpourings of basalt upon the Jurassic rocks of the ocean floor of the Venezuelan Basin. West of Beata Ridge, sea-floor spreading formed the Colombian Basin. Related subduction occurred as the Chortis plate (including part of Central America, the Nicaraguan Rise, and southeastern Cuba) was sutured against the Maya East plate along the present Motagua fault and Cayman Trench. Our model is constrained by published geologic data, the relative positions of North and South America from Atlantic sea-floor magnetic anomalies, and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between North and South America. Paleomagnetic data from Middle America are sparse but do not conflict with the predicted motions of some of the microplates, especially Chortis.

Paleomagnetic Study of a Reversal of the Earth's Magnetic Field

A detailed record of a field reversal has been obtained from the natural remanent magnetization of the Tatoosh intrusion in Mount Rainier National Park, Washington. The reversal took place at 14.7 � 1 million years and is interpreted to be from reverse to normal. A decrease in the intensity of the field of about an order of magnitude occurs immediately before the reversal, while its orientation remains substantially unchanged. The onset of the reversal is marked by abrupt swinging of the virtual geomagnetic pole along an arc of a great circle. During the reversal the pole traces a path across the Pacific. In the last stage of the process recorded in the sections, the succession of virtual geomagnetic poles is very similar to those generated by secular variation in the recent past. Although the cooling rate of the intrusion is not sufficiently well known to permit a useful calculation of the duration of the reversal process, an estimate based on the length of the supposed secular variation cycles gives 1 to 4 x 103 years for the reversal of field direction and approximately 1 x 104 years for the time scale of the intensity changes.

Magnetostratigraphy of sediments in mammoth cave, kentucky.

Clastic sediment deposits found within the caves of Mammoth Cave National Park have yielded a magnetostratigraphic pattern of magnetic polarity reversals which indicates-that they were deposited over a range of at least 1 million and most likely 2 million years.

Determination of stream-incision rate in the Appalachian plateaus by using cave-sediment magnetostratigraphy

Paleomagnetic dating of clastic fluvial sediments contained in caves within the walls of a steeply incised gorge allowed calculation of a maximum incision rate for the East Fork Obey River. The maximum incision rate for this major stream on the western margin of the Cumberland Plateau, north-central Tennessee, was found to be 0.06 m/ka. This rate was determined on the basis of the paleohydraulic relation between the caves and the surface stream, the presence of a normal-to-reverse polarity transition in clastic fluvial sediments deposited within the caves, and the vertical distribution of polarity found in sediments throughout the gorge. The dating results indicate that this highly developed fluviokarst, containing several of the longest known caves in the United States, developed wholly within the Pleistocene and Holocene.

Paleomagnetic poles for the Carboniferous Brush Creek limestone and Buffalo siltstone from southwestern Pennsylvania

Abstract We report partial results of a larger project being undertaken in the Appalachian Basin to determine the character of the geomagnetic field in the Carboniferous. The Brush Creek limestone, which contains abundant terrigenous matter, yields reliable results from three different sites, while the overlying Buffalo siltstone appears at one locality. AF demagnetization curves, IRM acquisition curves, thermomagnetic analysis, and optical reflection microscopy indicate that the remanence carriers are magnetites or titanomagnetites, which are probably primary in origin. Stepwise AF demagnetization reveals that the best demagnetizing fields are 15–20 and 20–30 mT for the limestone and siltstone units, respectively. A total of 93 specimens from 20 samples (oriented hand samples and field-drilled cores) yields a paleopole at S 36.1, W 55.7, with an alpha-95 of 4.2° and a k of 13.1 for the Brush Creek limestone. The Buffalo siltstone paleopole is located at S 27.4, W 57.0, with alpha-95 = 6.1° and k = 13.0 using 45 specimens from 31 oriented hand samples. These results agree fairly well with the mean reversed Carboniferous paleopole and Noltimiers coal pole, but are sufficiently different to raise the possibility of doing magnetostratigraphic correlations.

Circularity of paleomagnetic data sets : an aid in the recognition of contaminating secondary overprints

Abstract A simple method is presented for determining in a statistical sense whether or not a paleomagnetic data set is circularly distributed about its mean. The method uses the same principal component analysis that is commonly used in paleomagnetic data processing and yields a ratio a/b, where a and b refer to the intermediate and minimum eigenvectors obtained from a principal component analysis in which the analysis is constrained to the origin of the coordinate system. Tables and graphs are presented that provide a/b ratios at 95% and 99% confidence limits for data sets with differing numbers of specimens. If the a/b ratio has a value greater than the tabulated threshold values, then the parent distribution may be taken to be elliptical rather than circular at the corresponding confidence limit. The method should see application in distinguishing data sets contaminated by a small secondary overprint, even after demagnetization, from data sets that are not significantly contaminated by components other than random noise.

A paleomagnetic test of the proposed Mojave-Sonora megashear in Northwestern Mexico

Abstract Three Upper Triassic-Lower Jurassic formations in southwestern United States and northwestern Mexico have been investigated paleomagnetically as a means of testing for displacement along the proposed Mojave-Sonora megashear. Luning Formation of west-central Nevada and Sil Nakya Formation of southern Arizona lie north of the megashear whereas, in northwestern Sonora, Antimonio Formation lies south of the megashear. Previously published results from the Nazas Formation of presumed Lower Jurassic age from north-central Mexico were incorporated into the interpretations. These four units compose two sets of correlative formations on opposite sides of the proposed megashear. Restoration of 800 km of left-lateral displacement along the megashear about the pole of rotation determined by Anderson and Schmidt (1983), improves the grouping of the two sets of paleomagnetic poles noticeably, with the improvement having statistical significance at the 95% confidence level. We feel that the data support, but do not prove, the postulated displacement along the megashear. All four poles are displaced from Irvings (1979) smoothed apparent polar wander path for cratonic North America in the intervals 190 and 200 Ma. The displacement needed to bring all of these poles into coincidence is a pure rotation of about 8° counterclockwise about the sampling area. Two probable Cretaceous poles were obtained from the Sand Wells Formation of southern Arizona and from unnamed volcanics in northwest Sonora. These poles are essentially identical and suggest that displacement along the megashear had ceased by the Cretaceous. Data from the Early Jurassic Mulberry Wash and Pitoikam formations in southern Arizona appear to have been remagnetized in a later Cretaceous event and were not used in this study.

The measurement of anisotropy of magnetic susceptibility using a cryogenic (SQUID) magnetometer and a comparison with results obtained from a torsion-fiber magnetometer

Abstract We present a new set of orientations for use in the measurement of anisotropy of magnetic susceptibility (AMS) on a cryogenic magnetometer that provides a number of advantages over previously described methods. These include simplicity of analysis, a means of providing immediate internal consistency checks for detection of sample misorientations and other problems, a fixed sample holder that minimizes effects external to the sample, and ease of measurement for both cubic and cylindrical samples. Seven samples were measured five times each for AMS using this method. For comparison, the same replications for each sample were also carried out on a torsion-fiber magnetometer. The cryogenic magnetometer yielded determinations of the principal susceptibility axes that were in excellent agreement with the torsion-fiber measurements, except when the combination of two conditions occurred: (a) per cent anisotropy was low, and (b) the ratio of remanence to induced moment was ⩾ 1.0. Scatter increased with either of these two conditions and, in addition, the directions of the apparent susceptibility axes were significantly affected when the remanence exceeded the induced moment. We also found significant discrepancies in the magnitude of susceptibility determined using the two methods, though these are dependent on measurements not inherent to either method. We conclude that measurement of AMS using a cryogenic magnetometer is feasible for many samples if a torsion-fiber instrument is not available, but that the latter remains the instrument of choice.

Dating of cave sediments at Wee Jasper, New South Wales, by magnetostratigraphy

Previously Punchbowl, Signature and Dogleg Caves low down in the Goodradigbee valley at Wee Jasper, NSW, could only be inferred to be younger than Early Miocene basalts on the plateau to the west. Five main quasi‐horizontal cave levels, with discrete bodies of fluvial sediment at more frequent intervals, invited application of magnetostratigraphy to this sequence of passages, which are successively younger downwards, in the hope of more precise dating. Thirteen sites at 10 altitudes over a range of 31 m were sampled at 2–8 points each. The lower third of the system yielded normal polarity and so formed since 0.73 Ma. Treating the palaeomagnetic data conservatively, the upper two thirds, yielding dominantly reversed polarities, can be regarded as forming between the Brunhes‐Matuyama boundary and the Jaramillo. However, this implies a drastic change in rate of lowering of local karst base level and valley incision for which there is no morphological support. Assuming a linear rate of vertical cave developme...