摘要:

A convenient representation of triple junctions that involve only ridges (R) and transform faults (F) is proposed: this representation combines in a simple way information from geographic and velocity spaces. The velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates. A discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that, in general, there are several triple junction configurations that are compatible with a given triangle. This discussion stresses the importance of oblique and asymmetric spreading. Pairs of triple junction configurations that are of particular interest are RRR‐RRF and RRR‐FFR. When the triple junction lies outside of the triangle, at least one of the ridges will shorten, leading to yet another type of potentially unstable configuration. The present configurations of the Bouvet, Galapagos, and Indian Ocean triple junctions are reviewed. A method is proposed to reconstruct past configurations of the Indian Ocean junction at the time of anomalies 23 and 28. The main parameters that influence triple junction evolution are the lengths of transform faults, the availability of magma and related connectivity of magma chambers and the (also related) spreading velocities. This study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes: the effusive and tectonic modes, corresponding here to RRR and to RRF‐RFF configurations respectively. These modes apparently alternate in episodes of typically 1 Ma duration. The significance of this time constant and consequences for lithospheric mechanics are briefly disc

ISSN:0278-7407    

DOI:10.1029/TC003i003p00317

年代:1984

数据来源: WILEY

摘要:

Paleomagnetic results are presented for early Tertiary and Cretaceous igneous and volcanic units from the northern Antarctic Peninsula. When combined with previously published data, they permit the construction of a polar wander path that is very similar to one derived for the East Antarctic craton by rotating Tertiary and Cretaceous poles from India and Australia into the Antarctic frame of reference. When considered in this context, the paleomagnetic data from the Orville Coast, which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern Antarctic Peninsula with respect to the northern part, may be considered as an in situ resulte The implications of the new data are as follows: (1) The Antarctic Peninsula down to the Orville Coast has been a part of the Antarctic plate since 100 m.y. ago. (2) Movement of the Antarctic Peninsula required to avoid overlap in reconstructions of Gondwanaland must have occurred before 100 m.y. ago.

ISSN:0278-7407    

DOI:10.1029/TC003i003p00333

年代:1984

数据来源: WILEY

摘要:

The front of the Caledonian orogenic belt in NW Scotland is marked by a zone of foreland thrusting which contains imbricates of Lewisian basement, Proterozoic cover and Cambrian shelf sediments. Faults within this Moine thrust belt, in most localities, propagated in a foreland directed sequence. Therefore the earliest of these thrusts was the Moine thrust, which carried a basement assemblage of Moine and Lewisian rocks over the Cambrian shelf. The development of large duplex and imbricate structures in the footwall postadted movement on this thrust. Hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the Cambrian shelf. No synsedimentary or prethrust extensional features are recognised within the Cambrian succession which displays a remarkable stratigraphic and thickness consistency. This implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to Caledonian thrusting. A balanced cross section constructed across the northern part of the Moine thrust belt has given a restored width for the Cambrian shelf of 54 km. Geophysical data suggest that the present foreland crustal thickness is 28 km. If this is taken to be the crustal thickness beneath the Cambrian shelf during Caledonian thrusting, a minimum cross sectional area of 1512 km² of Lewisian rocks must remain beneath the present outcrop of the Moine thrust sheet. This foreland basement wedge may have been imbricated subsequent to the development of the Cambrian imbricates. Caledonian structures within the Moine thrust sheet are interpreted as forming an imbricate stack with the Moine thrust acting as a floor thrust. The ductile imbricate thrusts possibly roofed into the Naver slide. Late open folds, which warp foliation, slides, and metamorphic isograds within the Moine sheet, may represent culminations in the foreland Lewisian imbricates beneath the Moine thrust. These postulated culminations may have been sufficient to initiate gravity‐driven structures. This may explain late movements on high‐level faults within the Moine thrust

ISSN:0278-7407    

DOI:10.1029/TC003i003p00347

年代:1984

数据来源: WILEY

摘要:

COCORP profiling across the midcontinent geophysical anomaly in northeastern Kansas reveals structural basins and other features of the Precambrian Keweenawan rift buried beneath the Phanerozoic cover. The 40‐km‐wide main basin is asymmetric, with a maximum depth of 3 km on the east and 8 km on the west. The basin fill is characterized by a lower layered sequence of strong continuous west dipping reflectors which may be correlated with Middle Keweenawan interbedded volcanic and clastic rocks exposed along the MGA in the Lake Superior region. Overlying this layered sequence is a zone of weak, discontinuous reflectors correlated here with the predominantly clastic rocks characteristic of the Upper Keweenawan sequence near Lake Superior. A second tilted but shallower basin lies to the east of the main basin and appears to be filled predominantly with clastic sedimentary rocks. The character of the seismic data, the seismic velocity distribution, and gravity modeling suggest that mafic intrusions lie beneath the main rift basin. Normal faults associated with the rift dip at moderate angles to the east. Palinspastic reconstruction indicates that the rift basin formed by the rotation of fault bounded blocks during crustal extension. Although reactivation of preexisting structures appears to have occurred in many other rifts profiled by COCORP, the evidence is inconclusive on this point in the case of the Kansas data. The structures mapped by COCORP surveys in Kansas and elsewhere suggest that asymmetric sequences of layered reflectors are characteristic, and perhaps diagnostic, of rift basin deposits in gene

ISSN:0278-7407    

DOI:10.1029/TC003i003p00367

年代:1984

数据来源: WILEY

摘要:

Major episodes of Tertiary low‐angle normal faulting in the southwestern United States south of latitude 37°N moved northward with time, occurring about 25 million years ago in southern Arizona, 20 million years ago in the Mojave Desert region, and 10 to 15 million years ago in the Las Vegas area. In most areas, faulting and associated volcanism occurred when the Mendocino triple junction lay to the west. This correspondence probably resulted from the unstable configuration of the triple junction. East‐west to northeast‐southwest horizontal stretching, as plates pulled apart at the triple junction, prevented development of a hole in the lithosphere at the triple junction. The regional strain history predicted by this model is consistent with observed Tertiary low‐angle faults, strike‐slip faults,

ISSN:0278-7407    

DOI:10.1029/TC003i003p00385

年代:1984

数据来源: WILEY

摘要:

Anomalous paleomagnetic directions have been determined for 17 sites in the Frenchmans Springs member of the Wanapum basalt formation, Columbia River basalt group. These sites are located in the Ginkgo flows from near Vantage, Washington, to Portland, Oregon, a distance of approximately 300 km. The average paleomagnetic direction for six of these sites, centered around Vantage is D = 147°, I = 41°, α95= 4.5°. The expected Miocene field direction is D = 355°, I = 65°. At some localities there are two distinct Ginkgo flows, in direct stratigraphic succession, with statistically identical anomalous directions. Their anomalous paleomagnetic direction makes these flows a valuable marker horizon in the Columbia River basalt group. The nondipole field direction of the Ginkgo flows correlates well with available results from the Miocene Cape Foulweather basalts of Oregon. This correlation strongly supports the hypothesis that these coastal basalts of Oregon are the distal ends of Columbia Plateau derived basalt flows. The spatial distribution of these anomalous field directions suggests about 14° of clockwise rotation between Vantage and Portland. Combining these data with data from the Oregon Coast basalts allows a maximum declination difference of about 35°. The increase in declination can be best explained by clockwise rotation, about nearby vertical axes, increasing to the southwest across the Columbia Plateau and Orego

ISSN:0278-7407    

DOI:10.1029/TC003i003p00397

年代:1984

数据来源: WILEY