摘要:

The apparent polar wander path for a plate is determined from paleomagnetic data by plotting a time sequence of paleomagnetic poles, each representing the location of the earth's spin axis as seen from the plate. Apparent polar wander paths consist of long, gently curved segments termed tracks linked by short segments with sharp curvature termed cusps. The tracks correspond to time intervals when the direction of plate motion was constant, and the cusps correspond to time intervals when the direction of plate motion was changing. Apparent polar wander tracks, like hot spot tracks, tend to lie along small circles. The center of a circle is called a hot spot Euler pole in the case of hot spot tracks and a paleomagnetic Euler pole in the case of paleomagnetic apparent polar wander paths. Both types of tracks mark the motion of a plate with respect to a point, a rising mantle plume in the case of hot spot tracks and the earth's paleomagnetic axis in the case of apparent polar wander paths. Unlike approaches uced in previous studies, paleomagnetic Euler pole analysis yields all three components of motion—including the east‐west motion—of a plate with respect to the paleomagnetic axis. A new method for analyzing paleomagnetic poles along a track by using a maximum likelihood criterion gives the best fit paleomagnetic Euler pole and an ellipsoid of 95% confidence about the paleomagnetic Euler pole. In analyzing synthetic and real data, we found that the ellipsoids are elongate, the long axes being aligned with a great circle drawn from the paleomagnetic Euler pole to the center of the apparent polar wander track. This elongation is caused by the azimuths of circular tracks being better defined than their radii of curvature. A Jurassic‐Cretaceous paleomagnetic Euler pole for North America was determined from 13 paleomagnetic poles. This track begins with the Wingate and Kayenta formations (about 200 Ma) and ends with the Niobrara Formation (about 87 Ma). Morgan's hot spot Euler pole for 200–90 Ma lies only 15° outside the 95% confidence ellipsoid of the paleomagnetic Euler pole. The good but not perfect agreement reflects displacement between the hot spot and paleomagnetic reference frames at an average rate that is smaller by an order of magnitude than the rate at which the faster plates are moving. The angular velocity of North America about the Jurassic‐Cretaceous paleomagnetic Euler pole was determined by plotting the angular positions of paleomagnetic poles along the track as a function of age. For the Cretaceous the angular velocity was too small to measure. During the Jurassic the angular velocity was high, corresponding to a root‐mean‐square velocity of 70 km/m.y. for the North American plate. A short time interval of even more rapid movement during the Middle and Late Jurassic, possibly corresponding to the beginning of rapid displacement between North America and Africa, is suggested by the data. The direction of absolute motion of North America during the Jurassic was toward the northwest. A Carboniferous‐Permian‐Triassic paleomagnetic Euler pole was determined from 26 paleomagnetic poles. The progression of poles along this track is consistent with known ages and stratigraphy, except for some systematic differences between poles from Triassic rocks on the Colorado Plateau and poles from Triassic rocks off the Colorado Plateau. These differences could be due to a small clockwise rotation of the Colorado Plateau with respect to cratonal North America, or to miscorrelations between Triassic rocks on the Colorado Plateau and off the Colorado Plateau, or to large lag times between the deposition and magnetization of some rock units, or to some combination of these possibilities. Despite these ambiguities in interpreting paleomagnetic data from Triassic rocks, the general pattern of apparent polar wander and plate motion during the Carboniferous through Triassic is clear: The root‐mean‐square velocity of North America was slow (about 20 km/m.y.) during the Carboniferous, probably slow (about 20 km/m.y.) during the Permian, but rapid (60–100 km/m.y.) during the Triassic. Paleomagnetic Euler pole analysis establishes that the present slow (less than 30 km/m.y.) velocity of large continental plates like North America is not an intrinsic property of the plates. Occasionally these plates have, for intervals of 50 ± 20 m.y., moved as rapidly as the oceanic plates are moving today. In our interpretation, during times of rapid motion the continents were attached along a passive margin to oceanic lithosphere that was being subducted at some distance from the continent. Rapid motion stopped when the oceanic lithosphere had been consumed by subduction. If North America, Greenland, and Eurasia were joined as a single land mass during the Jurassic, then a likely location for the subducting oceanic plate attached to this landmass is along the southern margin of the cratonal core of Asia with the oceanic plate extending into Tethys. At the cusp between the Carboniferous‐Permian‐Triassic track and the Jurassic‐Cretaceous track, the trend of the path changes by 160°. The western point of the cusp, which is delineated by paleomagnetic poles from the Chinle, Wingate, and Kayenta formations, is 13° farther west in our analysis than it is in commonly accepted apparent polar wander paths for North America. An implication for terrane analysis is that northward displacements found by using our Late Triassic and Early Jurassic poles are up to 2000 km smaller than are those found by using previously published Late Triassi

ISSN:0278-7407    

DOI:10.1029/TC003i005p00499

年代:1984

数据来源: WILEY

摘要:

Prior to formation of the Tasman Sea, the Late Paleozoic‐Mesozoic Rangitata Orogen of New Zealand and New Caledonia abutted the Paleozoic New England Orogen of eastern Australia. Comparison of the record of Permian‐Cretaceous igneous and deformational events from the two orogens suggests that their tectonic evolution was interrelated and is a consequence of convergent plate interaction along the southwest Pacific margin of Gondwana. The following relations are proposed: (1) termination of arc volcanism and widespread sedimentation in New England, together with the onset of regional deformation and crustal anatexis were synchronous with the commencement of volcanism and sedimentation within the Rangitata Orogen; (2) Early Permian andesitic volcanism in eastern New England represents an along‐strike extension of the Brook Street terrane of New Zealand; (3) Late Permian regional deformation in New England coincides with both a break in subduction‐related igneous activity in the New England and Rangitata Orogens and a shift in the locus of this activity; (4) Late Permian‐Triassic calc‐alkaline igneous activity in New England correlates with a phase of relatively continuous accumulation of pyroclastic material in the forearc basin of the Rangitata Orogen; (5) cessation of plutonism in New England corresponds with commencement of formation of the Esk Head Melange in New Zealand and the probable commencement of juxtaposition of the Te Anau and Alpine Assemblage; (6) Late Cretaceous epizonal plutons intruded into the New England Orogen are similar in character and age to those emplaced during the final phases of Rangitata orogenesis, and both appear to mark initial stages of rifting associated with formation of the Tasman Sea. The generation of Permian and Triassic igneous activity in eastern New England by convergent plate interaction results, on present reconstructions of the Gondwana margin, in an excessively wide arc‐trench gap succession, for the position of the trench is constrained to east of New Caledonia from the Permian onward. This suggests there may have been some rearrangement of tectonic elements along the margin resulting in widening of the Lord Howe Rise between Australia and New Caledonia in the period between termination of igneous activity in New England and formation of the Tasman Sea in the La

ISSN:0278-7407    

DOI:10.1029/TC003i005p00539

年代:1984

数据来源: WILEY

摘要:

In this study new instantaneous rotation vectors describing the motion of the Philippine Sea plate relative to surrounding plates are determined that most nearly satisfy all available geological, geophysical and seismological observations along the plate margins. Pacific‐Philippine and Caroline‐Pacific, poles are generated from a set of trial Eurasia‐Philippine poles of rotation, using published determinations of the Eurasia‐Pacific and Caroline‐Philippine poles. The Pacific‐Philippine, Caroline‐Pacific, and Eurasia‐Philippine poles are then checked for agreement with: (1) slip vectors determined from focal mechanisms of earthquakes along the Ryukyu trench and the Nankai trough, (2) convergence at the Sagami trough, (3) extension in the Mariana trough, (4) extension behind the Bonin trench, (5) extension in the western Sorol trough, and (6) convergence at the Mussau trench. Trial Eurasia‐Philippine poles are eliminated if predicted plate motions do not match observed plate motions. The locations and rotation rates of the three best fitting poles are Eurasia‐Philippine: 37.0°N, 141.0°E, 1.60°/Ma; Pacific‐Philippine: 3.8°N, 130.4°E, 1.68°/Ma; Caroline‐Pacific: 16.9°N, 141.4°E, 0.54°/Ma. The plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the Philippine, Pacific, and Caroline plates intersect. Intermediate and deep focus earthquake hypocenters along the southern Mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the Mariana trough disappears. The absence of volcanism and intermediate‐depth seismicity along the southernmost Mariana, Yap, and Palau sections of the plate boundary is attributed to very low convergence rates. A new focal mechanism solution along the trench south of Guam is the first clear underthrusting solution found along the Mariana trench and agrees with the convergence direction implied by the Pacific‐Philippine pole of rotation. No clear underthrusting focal mechanism solutions were found along the Yap and Palau trenches, although three high‐angle thrust solutions demonstrate that compression is occurring in the upper plat

ISSN:0278-7407    

DOI:10.1029/TC003i005p00555

年代:1984

数据来源: WILEY

摘要:

Rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust. Strain compatibility arguments indicate that a finite extension can be accommodated by displacements across (1) planar, nonrotating faults or ductile shear zones, (2) shear zones which rotate above a horizontal detachment (the domino model), or (3) shear zones which rotate as a result of a horizontally oriented, pure shear stretching component (the plastic model). Listric, normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component (in the plastic model) or the area loss component (in the planar shear zone model). Within the geometric framework of these various models, the effects of superposed simple shear, pure shear extension, and area change on the rock fabric are investigated. These displacement components, which may be superposed sequentially or simultaneously, determine the state of finite strain associated with a given magnitude of tectonic extension. The relationships between displacement and strain are expressed as graphs of foliation dip (or strain ratio) versus shear strain and as graphs of foliation dip (or strain ratio) versus tectonic extension. The shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone, whereas the tectonic extension graphs better illustrate these effects on the regional scale. The shear strain and tectonic extension graphs can be used by field geologists (1) to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation, (2) to distinguish between possible extension models from field data, and (3) to evaluate the influence of shear zone attitude and thickness upon the amount of displacement. Applications of these methods are illustrated for field areas in metamorphic core complexes of Arizona and in the Basin and Range province of the North America Cordillera.

ISSN:0278-7407    

DOI:10.1029/TC003i005p00577

年代:1984

数据来源: WILEY