摘要:

Paleomagnetic studies from the Marianas, central Philippines, Ryukyus, Sulawesi, Fiji‐New Hebrides all show major differences in the declination values observed from individual islands within the same arc. Comparable differences are not observed in the inclination values. A simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis. Bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain, rifted continental fragment, or an island arc. The coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of Vogt [1973] that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate. As a result of this interaction, the upper plate boundary is locally deformed. In addition to the paleomagnetic evidence, other geologic and geophysical data from these areas suggest such interactions. For example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported. As this deformation proceeds, eventually the stresses resulting from the collision may result in any of the following tectonic processes: 1) slip‐line style strike‐slip faults allowing for the sideward extrusion of portions of the upper plate [Tapponnier et al. 1983]; 2) changes in the polarity of the subduction zone; 3) development of strike‐slip faults around the margin of the indenter; or 4) reorganization of the entire plate margin. In addition, data from the Marianas, southern end of the RyuKyu arc, and the Lau Basin suggest that back arc basin development may result from the collision related defo

ISSN:0278-7407    

DOI:10.1029/TC003i004p00409

年代:1984

数据来源: WILEY

摘要:

A simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between Archaean and Phanerozoic terrains in terms of plate tectonics. The model has been derived using the following parameters: (1) the spreading rate at mid‐ocean ridges; (2) the age of the oceanic lithosphere at the time of subduction; (3) the area‐age distribution of the seafloor; (4) the continental surface area as a fraction of the total surface area of the earth; and (5) the erosion rate of continents as a function of continental surface area and the total number of continental masses. Observations in Phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate (that is, either continental or oceanic). The subduction of young oceanic lithosphere (less than 50 m.y. old) which is thermally buoyant appears to result in a reduced volume of igneous activity. Most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic‐rhyolitic volcanism. When very young lithosphere is being subducted (50 m.y. old) appears to result in greater volumes of igneous activity, including the eruption of andesitic magmas. Thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted. Our model predicts that as the earth aged and as heat flow from the interior of the earth diminished, the proportion of old oceanic lithosphere being subducted increased, fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion. If the subduction of old oceanic lithosphere results in an 8–10 times greater volume of subduction zone magmatism, our model predicts or explains all of the following observed features of earth history: (1) Archaean terranes appear to record two periods of rapid continental accretion, between 3.8 and 3.5 b.y. ago and between 3.1 and 2.6 b.y. ago; (2) there are very few differences and many marked similarities between rocks from Archaean terranes and equivalent rocks from Phanerozoic terranes; (3) the total continental area appears to have remained essentially constant for the past 2 b.y. (4) Archaean andesites are comparatively rare, and the relative abundances of mafic and siliceous rocks appear to change during the Archaean and the Proterozoic, with siliceous volcanics becoming proportionately more abundant in the geologic record with time; (5) plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the Archaean. Plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere, meaning that Archaean tectonics and Phanerozoic tectonics are but two points on an evolutionary co

ISSN:0278-7407    

DOI:10.1029/TC003i004p00429

年代:1984

数据来源: WILEY

摘要:

Tilted and uplifted marine terraces in southern Oregon show progressive landward tilting of the coastal ranges at about 5–16 × 10−8rad. yr−1for the last 0.25 m.y. Tide gauges in Washington and British Columbia, and ten resurveyed leveling lines running inland from the coast, indicate contemporary landward (down‐to‐the‐east) tilt rates of about 1–12 × 10−8rad. yr−1averaged over periods of from 10 to 50 years. The leveling lines traverse, and the terraces cut across, dipping Cenozoic strata: Pleistocene (dips to 3°), Mio‐Pliocene (dips to 30°) and Eocene (dips to 60°). Southern Oregon from Cape Blanco to the Siletz River shows geodetic or terrace tilting in the same direction as the underlying stratal dips. Hence present‐day deformation continues past deformation of the coastal ranges and is most likely related to active subduction of the Juan de Fuca plate. The steep stratal dips, lack of major active faults and historic earthquakes, and presence of very young bedding‐plane faults suggest that much of the onshore deformation and shortening within the overlying North American plate is taken up by folding rather than thrust faulting. Present shortening rates across north‐trending folds near the coast are between 0.02 and 1.9 × 10−7yr−1. The rate of shortening decreases rapidly eastwards from the Juan de Fuca ‐ North American plate boundary. A total of about 25 mm yr−1of permanent shortening could be occurring within the North American plate; most of it in the westernmost 40 km. The landward tilt and shortening rates are similar to those above many other subduction zones that have experienced great thrust earthquakes. While a high strain rate measured near Seattle, Washington, has been interpreted as elastic strain accumulation before a thrust earthquake, the low level of historic seismicity and the similarity of short‐ and long‐term deformation rates suggest alternatively that the subduction beneath Washington is aseismic. The issue has considerable implications for seismic hazard evaluation in the Pacific Northwest and could be resolved by a search for the effects (or lack o

ISSN:0278-7407    

DOI:10.1029/TC003i004p00449

年代:1984

数据来源: WILEY

摘要:

Chronostratigraphic and paleoclimatic comparisons of microfossils from deep‐sea cores, from samples of an exploratory drill hole, and from dredged rock of the Gulf of Alaska with coeval microfossil assemblages on the North American continent provide constraints on the northward migration of the Yakutat block, the Prince William terrane and the Pacific plate during Tertiary time. The comparative paleolatitudes of microfauna and flora provide three main constraints. (1) The Prince William terrane was in its present position with respect to North America (at high latitudes, 50° ± 5°N) by middle Eocene time (40–42 Ma), consistent with models derived from paleomagnetic data. (2) The adjacent Yakutat block was 30° ± 5° south of its present position in early Eocene (50 Ma), 20° ± 5° south in middle Eocene (40–44 Ma), and 15° ± 5° south in late Eocene time (37–40 Ma), thus requiring a northward motion of about 30° since 50 Ma. Moreover, the Yakutat block was at least 10° south of the Prince William terrane during Eocene time. These data are consistent with migration of the Yakutat block with the Pacific and Kula plates for at least the last 50 Ma. (3) site 192 on the Pacific plate was at about 15° ± 5°N latitude in the late Cretaceous (68 Ma), at 30° ± 5°N in early Eocene (50 Ma), at 40° ± 5°N in middle Eocene (40–44 Ma), at 45° ± 5°N in late Eocene (37–40 Ma), and north of 50° ± 5°N in latest Eocene to early Oligocene time (34–37 Ma). These paleolatitudes, based on planktonic foraminiferal assemblages, indicate northward drift consistent with the North America‐Pacific plate reconstructions from about 68 Ma to 40 Ma (Engebretson, 1982). However, from Cretaceous to early Eocené time, faunal data indicate significantly lower latitudinal positions, and from Oligocwne to early Miocene time, significantly higher latitudinal positions. These discrepancies can be explained by the northward expansion of tropical faunas during the globally warm early Tertiary and southward expansion of cold subarctic faunas as a r

ISSN:0278-7407    

DOI:10.1029/TC003i004p00473

年代:1984

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/TC003i004p00497

年代:1984

数据来源: WILEY