摘要:

This special issue contains papers that grew out of the American Geophysical Union Symposium “Models for motion of North America and plates of the Pacific basin, and the tectonic history of the western Cordillera—a search for correlations.” This symposium was held in San Francisco on December 13 and 14, 1982. Forty‐two papers were presented on such diverse topics as plate motion and microplate accretion histories, paleomagnetism of Cordilleran terranes and their displacement histories, the volcanic and plutonic history of the western Cordillera, and the timing of important geological episodes of regional extent (onset of extensional tectonics, onset and termination of episodes of strike slip faulting, etc.). Mechanisms of deformation within the Cordilleran continental margin were also discussed. The reader is referred to Eos Trans. AGU (vol. 63, no. 45, 1982) for the full set of ab

ISSN:0278-7407    

DOI:10.1029/TC003i002p00103

年代:1984

数据来源: WILEY

摘要:

High rates of subduction and near normal convergence of the Farallon plate correlate with the duration of the Laramide Orogeny in the Western Cordillera. The motion of the Farallon plate relative to North America is reconstructed back to the late Mesozoic using relative plate motion data. This reconstruction involves the use of a plate circuit in which each plate is linked to another with seafloor spreading information in order to relate converging plates. Two models for relative motions are considered. The preferred model predicts an increase in the subduction rate of the Farallon plate, exceeding 150 mm/yr, with a direction almost due east and perpendicular to the continental margin. The duration of this nonoblique (normal), high‐rate subduction correlates well with the duration of the Laramide Orogeny. This correlation must be regarded as only suggestive; the probable errors, when combined and propagated through the relative motion circuit, may be of the order of the effect observed. However, it is encouraging that these reconstructed Farallon/North American relative motions are similar to the published predictions of others, using the alternative, more direct approach of hot spot reconstruction

ISSN:0278-7407    

DOI:10.1029/TC003i002p00107

年代:1984

数据来源: WILEY

摘要:

Some of the major tectonic and magmatic events of the last 150 Ma in the Western Cordillera can be correlated with a new model for the displacement histories between western North America and adjacent oceanic plates. Sierra Nevada plutonism ended and Laramide compression began during increasingly rapid convergence of the Farallon plate with North America and during a moderate increase in the westward motion of North America in the hotspot reference frame. The end of the Laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions. The spectacular slowing of Faralion‐North America convergence is attributed to the decreasing age of the Farallon plate entering the trench and thus to a change from negative to positive buoyancy. The transition from widespread arc‐related magmatism and rapid extension to basaltic volcanism and moderate extension in the Basin and Range province finds no ready explanation in the plate motions of the Pacific basin. A change to oblique spreading in the Basin and Range province accompanied growth in length of the San Andreas fault as the Mendocino triple junction progressed northw

ISSN:0278-7407    

DOI:10.1029/TC003i002p00115

年代:1984

数据来源: WILEY

摘要:

We propose that an aseismic ridge of Late Jurassic and Early Cretaceous age on the Farallon plate was subducted beneath the Unites States Cordillera during the Laramide orogeny. The relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70–40 Ma in the existing near‐trench magmatic arc and Laramide uplift and faulting of crystalline basement 1000–1500 km inland from the trench. The timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the Laramide orogeny and the magmatic lull in the western Cordillera. Because the north–northeast trending aseismic ridge was moving northeast relative to the North American plate, the point of collision of the aseismic ridge with North America, and therefore the locus of shallow subduction, migrated southward with time, causing the well‐documented southward migration of the magm

ISSN:0278-7407    

DOI:10.1029/TC003i002p00121

年代:1984

数据来源: WILEY

摘要:

We have attempted to correlate geologic events in part of central California with computed motions of oceanic plates relative to cratonic North America. Three composite terranes are considered: (1) the Sierra Nevada, (2) the Coast Ranges northeast of the San Andreas fault, and (3) the Salinian block. In the Sierra Nevada, Jurassic plutonism (ending about 147 m.y. B.P.) and Cretaceous plutonism (120–80 m.y. B.P.) correlate with deduced Farallon‐North America (FA:NA) convergence, as does the Nevadan orogeny (158–153 m.y. B.P.). However, the gap in plutonism 146–121 m.y. B.P. outlasted by far an apparent minimum in convergence (145–135 m.y. B.P.). Magmatism ceased in the Sierra and uplift slowed during the Laramide orogeny 75–45 m.y. B.P. when tectonic activity had moved east to the site of the Rockies, presumably because of a low angle of subduction, but volcanisin resumed in the Sierra about 33 m.y. B.P., indicating that a steeper angle was reestablished. Apparently east‐west compression slackened when the Pacific‐North America (PA:NA) transform regime developed. Basalt was erupted in the southern Sierra 12–3 m.y. B.P., and during this same time, the range started its most rapid rise as a normal‐fault‐bounded block. The Coast Ranges north‐east of the San Andreas fault are underlain by the Franciscan Complex. The older parts of the Complex were probably assembled in a subduction zone far south of their present location, but we believe that the same FA:NA convergence that was cited for Sierran plutonism was responsible, despite the disparity in latitude, as the Farallon plate was probably in contact with most of the western margin of North and South America. A perceived strong pulse in convergence 100–85 m.y. B.P. evidently produced the Coast Range thrust which on geologic grounds alone, most likely originated between 96 and 88 m.y. B.P. Some of the northwesterly transport of Franciscan rocks relative to North America is ascribed to oblique Kula‐North America (KU:NA) convergence 85–43 m.y. B.P., during which period the Coast Range thrust was revived about 60 m.y. B.P. The Farallon plate again influenced the California margin after the KuIa plate had moved northward, and FA:NA convergence caused the accretion of the Coastal Belt and King Range terranes of the neo‐Franciscan. The advent of Pacific‐North America (PA:NA) interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the Mendocino triple junction (Dickinson and Snyder, 1978). In the Salinian block, plutons 107–82 m.y. old can be ascribed to the same FA:NA convergence as the contemporary Cretaceous plutons of the Sierra Nevada, although the Salinian intrusives originated much farther south. Paleomagnetic results (Champion et al., 1980) from the Salinian‐related Pigeon Point Formation, 75–71 (?) m.y. in age, suggest about 2500 km of poleward (essentially coastwise) movement. This postulated large movement during part (or all?) of the last 75 m.y. poses the greatest dilemma in our study. The KuIa is the only known plate whose (computed) obliquely convergent motion is appropriate for the Late Cretaceous‐Paleogene part of the migration, and even this motion is inadequate for the estimated transport within the most likely time window. Furthermore, the Farallon plate seems ideally suited for the Laramide orogeny 70–45 m.y. B.P. Both plates could not have been in contact with the same part of the margin at the same time. Oblique convergence of the Kula plate could possibly have caused large coastwise transport and also the Laramide orogeny. The Coast Ranges northeast of the San Andreas fault and the Salinian block both experienced strike slip faulting and en echelon folding correlative with PA:NA interaction, indicating that they were close together by Neogene time. Faulting and folding have been particularly marked since the inception of spreading in the Gulf of California and attachment of westernmost California to the Pacific plate about 4 m.y. B.P. The rise of individual coastal ranges, commencing 3–1 m.y. B.P., may possibly have been caused by slight convergence which appears in computed PA:NA rel

ISSN:0278-7407    

DOI:10.1029/TC003i002p00133

年代:1984

数据来源: WILEY

摘要:

Paleomagnetic results from three plutons of the central Sierra Nevada limit the motion of the Sierra Nevada relative to the North American craton to 5° ± 8° of poleward motion and 7° ± 11° of clockwise rotation since about 90 Ma. The indicated rotation and translation are not significant at the 95% confidence level. A new tectonic model is presented that is consistent with all available paleomagnetic data from the Sierra Nevada, Great Valley, Klamaths, Oregon Coast Range, Cascades, and western Nevada. The paleogeographic reconstructions of this model associate the displacement of major crustal blocks with Basin and Range extension starting at about 30 Ma. This model accounts for about 15° of clockwise rotation of the Klamaths and southern Cascades, about 2° of clockwise rotation of the Sierra Nevada and Great Valley, and 26° of clockwise rotation of the Oregon Coast Range since 30 Ma. The model is consistent with 40° of clockwise rotation of the Oregon Coast Range before 30 Ma, as proposed by Magill and Cox (1980). The amount of Basin and Range extension in this model at latitude 41°N is 225 km directed E‐W and at latitude 35°N is 245 km dir

ISSN:0278-7407    

DOI:10.1029/TC003i002p00157

年代:1984

数据来源: WILEY

摘要:

Abrupt westerly deflections of north‐south trending facies boundaries, the isopleth of (87sr/86Sr)° = 0.076, and arcuate structural trends of Paleozoic, Mesozoic, and Cenozoic age in the west‐central Basin and Range Province, have been explained by (1) post‐Early Jurassic oroflexural folding and shear, (2) the existence of a pre‐Late Jurassic resistant block along the western continental margin, possibly representing late Precambrian breakup, and (3) regional flexure (“Mina deflection”) prior to intrusion of Late Cretaceous plutons which crosscut flexure‐related structural trends. Paleomagnetic data gathered from Late Cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east‐west structural trends are not in support of significant oroflexural folding (and attending major clockwise rotation) since Late Cretaceous time. The data (Water Canyon pluton: D = 1°, I = 57°, α95= 4°, k = 9, N= 132; Gunmetal pluton: D = 354, I = 60, α95= 6, k = 21, N = 29; East Garfield Hills pluton: D = 349, I = 74, α95= 11, k = 14, N = 12; La Panta pluton: D = 22, I = 66, α95= 10, k = 16, N = 13; Gillis Canyon pluton: D = 354, I = 50, α95= 4, k = 20, N = 50; and folded units of the Excelsior Mountains: D = 309, I = 79, α95= 7, k = 12, N = 37) are dispersed from expected Cretaceous directions for the Mina deflection region, but they do not conform to a model of major systematic regional clockwise rotation. The generally small deflections of unit means from expected directions could be explained by irregular components of Late Cenozoic tilting and/or rotation. The paleomagnetic data do not deny the possibility of pre‐Late Cretaceous oroclinal bending. Nevertheless, regional structural analysis complimented by the paleomagnetic results indicate that the Mina deflection is in all likelihood a primary feature whose geometry reflects the late Precambrian fragmentation of western North America. Structures related to the accretion of allochthonous assemblages between Middle Devonian and Cretaceous time in this part of the Cordillera also reflect the pri

ISSN:0278-7407    

DOI:10.1029/TC003i002p00179

年代:1984

数据来源: WILEY

摘要:

Mesozoic structures in the western Cordillera of the United States yield important constraints for kinematic reconstruction of plate motions relative to North America. Structures in the Mesozoic arc(s) of the northern Sierra Nevada and of northeastern Oregon are remarkably uniform in timing, style, and with Late Cretaceous and/or Cenozoic rotations removed, orientation. Rocks of both regions experienced three major episodes of contraction occurring in the Triassic‐Early Jurassic, the Late Jurassic, and the Early Cretaceous. Cross folds were developed after the first two deformations but were not associated with significant shortening. The three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular (NE‐SW) to the regional trend of the arc system. In the back arc region of the northwestern Great Basin, deposition was continuous during the Triassic and Early Jurassic, and the region was shielded from tectonism in the arc to the west. Development of a regional fold‐thrust belt associated with major NW‐SE shortening was initiated in the back arc region during the Middle or Late Jurassic and continued through the Early Cretaceous. Corresponding deformation did not occur in the arc, and the arc and back arc were decoupled by a major left‐lateral fault system that subparallelled the arc axis. In the mid‐Cretaceous (approximately 100 m.y. B.P.), regional compression in the back arc abruptly changed to NE‐SW, and both the arc and back arc experienced major shortening. Development of homoaxial structures in the Sierran arc is probably analogous to shortening in present‐day arc systems undergoing oblique subduction, in which the shortening axis is parallel to the normal component of convergence. Thus the dominant structures in the Sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence. Strike‐slip faults and related compressional structures, such as the back arc left‐lateral fault and associated fold‐thrust belt, are probably the response to the strike‐slip component of convergence and indicate a period of left‐oblique subduction. Relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right‐lateral components of motion for the last 100 m.y. Before 100 m.y. B.P., relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left‐oblique convergence from the Middle or Late Jurassic to approximately 100 m.y. B.P. During the Triassic and Early Jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left‐oblique convergence occurred throughout the late Paleozoic and early Mesozoic (until 100 m.y. B.P.). Based on the available data, we cannot discount the possibility that a complex convergence history existed, however, with changes from right to left convergence in the Middle or Late Jurassic and from left to right co

ISSN:0278-7407    

DOI:10.1029/TC003i002p00201

年代:1984

数据来源: WILEY

摘要:

An “extensional orogeny” deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the “ignimbrite flareup,” massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1–2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid‐Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low‐K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high‐K rocks rest on an autochthonous sialic basement. Within the high‐K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K‐h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid‐Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab‐pull is likely to approach zero. Even prior to ridge‐trench collision, overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs. Subducted oceanic lithosphere no longer acted as a heat sink, which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting, extension, and metamorphism. Had these processes not died down, after ridge‐trench collision, the western segment of the Cordillera might have separated from North America to form a Japanlike archipelago, while the Basin and Range province foundered into an analog to the Sea of Japan. Instead of rupturing completely, the Basin and Range province

ISSN:0278-7407    

DOI:10.1029/TC003i002p00229

年代:1984

数据来源: WILEY

摘要:

The Pomona flow (12 m.y. B.P,) of the Columbia River Basalt Group was sampled for paleomagnetic analysis at 32 sites in the Yakima Fold Belt of the central Columbia Plateau to assess the pattern of deformation. Field directions from anticlinal ridges are found to be rotated clockwise relative to sites from synclinal areas. Two geographic patterns of rotation are present. The primary pattern is one in which the greatest amount of rotation occurs in the crest‐hinge area of the anticlines and decreases into synclines. A secondary pattern, in which the amount of rotation is consistent within a geometric segment but varies among them, is superimposed on the primary pattern. The pattern of clockwise rotation is interpreted to be the result of local tectonic rotation occurring along a closely spaced north‐northwest trending primary shear system and northeast trending secondary shear system. The shear system and concurrent rotation are interpreted to have developed during the growth of the Yakima folds. The relationship between the paleomagnetic data and the geologic structures can be used to constrain tectonic models for the Columbia Plateau. The data reported in this study indicate that clockwise rotation can not be attributed to rigid block or microplate rotation, but support models that predict localized dextral shear. Furthermore, it is postulated that this mechanism may contribute to clockwise rotation observed in other parts of the western United Sta

ISSN:0278-7407    

DOI:10.1029/TC003i002p00251

年代:1984

数据来源: WILEY