摘要:

Propagating rifts may not be incompatible with plate tectonic tenets which are considered at the appropriate scale. Propagation of a rift is a natural consequence of relative motion about a pole of rotation. It is not necessarily valid that (1) rifts propagate in one direction or (2) crustal stretching increases with progressive propagation of a rift.

ISSN:0278-7407    

DOI:10.1029/TC003i006p00611

年代:1984

数据来源: WILEY

摘要:

A growing body of deep seismic reflection profiles across the Appalachian‐Ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting, of later thrusting, or of some combination of rifting and thrusting. The subthrust features can be placed into three general categories: (1) small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading, perhaps reactivating earlier rifting faults, (2) moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland, interpreted in part as strata deposited during the earlier rifting, and (3) hinterland‐dipping reflection sequences beneath more interior portions of the mountain belt, previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin. These interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings. The comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults, the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process. These dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the Appalachian foreland. The comparisons also suggest an alternative to interpretations of the hinterland‐dipping sequences as thrust‐related, in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins. The implication of this new interpretation is that the actual continent/ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the Appalachians and Ou

ISSN:0278-7407    

DOI:10.1029/TC003i006p00619

年代:1984

数据来源: WILEY

摘要:

Geological and geophysical constraints suggest that the Snake Range décollement of east‐central Nevada is a major Tertiary low‐angle normal fault zone. This interpretation is consistent with all existing data, and alleviates problems that result if large displacement across the décollement is excluded [Miller et al., 1983]. Based on published surface geology and COCORP seismic reflection profiling, we have constructed cross sections that suggest approximately 60 km of normal displacement on the décollement. Advantages of this interpretation over models that exclude large displacement are that it provides for overburden consistent with kyanite‐grade metamorphism of footwall rocks, predicts reasonable crustal thicknesses before and after extension without invoking unexposed mantle‐derived Tertiary intrusions at depth, and explains contrasting metamorphism and structural styles of hanging wall and footwall without requiring an extreme geothermal gradient during regional metamorphism and extensional strain. The present domal shape of the décollement is interpreted to result at least partly from reverse drag above an underlying, younger detachment related to much of the extension in ranges to the west of the

ISSN:0278-7407    

DOI:10.1029/TC003i006p00647

年代:1984

数据来源: WILEY

摘要:

The late Mesozoic‐Cenozoic history of south central California is punctuated by at least five major tectonic events. The oldest of these involves Late Cretaceous drag folding and thrusting of epizonal shelf sediments westward over and to the north along the rising eastern margin of the Peninsular Ranges Batholith. These overthrusts, with associated drag folds and nappes, form imbricate stacks of metasediments, slices of plutons, and mylonites which overlie a regional core of Cretaceous granitic plutons of the Peninsular Ranges Batholith. A second, distinct kind of epizonal, low‐temperature crustal displacement includes an episode of mid‐Tertiary detachment faulting and folding. This represents the westerly extension of the detachment terrane of eastern California and western Arizona and Utah, superimposed upon the Cretaceous structural features, The mid‐Tertiary motions involve major E‐W extension and N‐S compressive strain, with formation of east‐trending antiformal “core complexes,” remarkably parallel to a pervasive set of fold axes and B‐lineations formed in the Cretaceous thrusting and folding. All of these features are repeatedly segmented and displaced by step‐faulting as sial in the Salton Trough is fragmented and depressed 5 to 10 km below sea level at intervals throughout the Cenozoic. The irregular step‐faulted interface of the Peninsular Ranges Batholith and Salton Trough is accentuated by a series of NE to SE striking left‐lateral and dipslip faults with displacements of several to 10 kilometers. These appear to both predate and occur during movements along the San Andreas system. Pliocene‐Pleistocene right‐lateral faulting, a younger extension of the San Andreas displacements north of the Transverse Ranges, has broken and displaced all older structural features, with total net slip in south central California of perhaps 300 km. Total right‐lateral slip along the several strands of the San Andreas system are measurable via reconstruction of the mosaic of numerous older, predominantly E‐W trending linear elements, mid‐Tertiary antiforms, and Cretaceous and older rocks. Right‐lateral strain on the San Andreas appears to either alternate with or to be a byproduct of N‐S compressional strain from the Pleistocene to the present. A resultant effect is a large family of E‐W trending folds and f

ISSN:0278-7407    

DOI:10.1029/TC003i006p00659

年代:1984

数据来源: WILEY

摘要:

The narrow zone of mid‐Cenozoic detachment terrane in the southwestern United States can be widened and traced across the southern strands of the San Andreas fault zone into the eastern margin of the Peninsular Ranges Batholith. The Yaqui Ridge core complex and detachment fault, in southern Borrego Valley, California, exemplifies the nature of detached terranes in southcentral California. The detachment fault dips 10°–40° to the northeast, and separates a lower core of gneissic Late Cretaceous granodiorite from an unconsolidated, unmetamorphosed megabreccia of Eocene to Miocene age. Foliations in the lower plate generally conform to the strike and dip of the overlying detachment fault, and become less distinct away from the fault. The megabreccia which forms the upper plate is composed of unsorted, fairly well rounded clasts characteristic of batholithic and metasedimentary rocks of the region. Plio‐Pleistocene lacustrine sediments unconformably overlie the megabreccia in some areas. The detachment fault and upper plate are expressed as a series of klippen which parallel Yaqui Ridge along the northeast and southeast flanks. The fault is marked by a typically narrow (∼6–10 cm) band of intensely sheared cataclasite. A chlorite‐breccia zone occurs below the cataclasite, and gradually grades into the less deformed, regionally foliated gneissic granodiorite. On the southeast flank, what appears to be an extension of the cataclasite and detached upper plate is exposed only locally, near the southeast nose of the antiform. Here, the upper plate consists of blocks of pale gray to tan, aphanitic fault gouge. Superimposed upon and cutting through the detachment‐related features is a broad zone of left‐oblique‐slip faulting termed the Yaqui Ridge shear zone. This fault zone trends WNW‐ESE, parallel to Yaqui Ridge, and separates the detachment fault from the ridge, merging with the detachment fault near the eastern margin. Further east, the shear zone widens into a massive zone of gouge>400 m wide and disappears under the sediments of Borrego Valley and the Salton Trough. To the west, the shear zone slices into the eastern margin of the Peninsular Ranges Batholith and extends the marginal plutons in an E–W direction. Four dominant tectonic episodes can be recognized from exposures at and adjacent to Yaqui Ridge: (1) A Late Cretaceous synkinematic cataclasis and metamorphism accompanying and following emplacement of the Yaqui Ridge and related plutons, and forming a pervasive regional NW‐trending foliation and NE‐trending lineation. (2) A subsequent mid‐Cenozoic, shallow, low temperature event of detachment faulting and folding which produces localized brittle cataclasis which overprints the earlier NW‐trending fabrics. (3) The Late Miocene‐Early Pliocene development of Yaqui Ridge shear zone along the northeast margin of Yaqui Ridge, shearing, rotating and overprinting the fabrics formed during the Late Cretaceous plutonism and regional deformation. (4) A Pleistocene episode of high‐angle faulting and folding which produce a series of E‐W‐trending anticlines and synclines in Upper Pliocene to Pleistocene sediments. Linear elements which have formed throughout the several Late Cretaceous

ISSN:0278-7407    

DOI:10.1029/TC003i006p00677

年代:1984

数据来源: WILEY