摘要:

Rates of latest Quaternary slip from stream terraces and moraines displaced by faults of the Alpine shear system in NE South Island, New Zealand, vary in space and time. Detailed histories of fault slip are obtained from combining displacement data with estimates of the age of the surface from weathering characteristics. Precision in surface ages is 5–20% using rock weathering rinds and 15–50% using soil properties. The oldest surfaces examined are 15–20 ka and have right‐lateral fault offsets up to 400–600 m. The main faults of the in the NE South Island (the Wairau, Awatere, Clarence, Hope, Kekerengu, and Porters Pass faults) have average latest Quaternary right‐lateral slip rates of 3–10, 5–10, 7–10, 25–40, 5–7, and 3–4 mm/yr respectively. Every fault has undergone a substantial decrease in lateral slip rate in the last 3–5 kyr. Summed across the plate boundary, the average latest Quaternary slip rates are comparable to long‐term rates of horizontal slip across the Australian‐Pacific plate boundary (around 40 mm/yr parallel to the boundary) and rates of geodetic strain and seismic moment release over the last 50–100 years (approximately the same). However, sums of lateral fault‐slip rates over the interval from 15 to 5 ka exceed the plate motions, whereas late Holocene lateral fault‐slip rates are less than half the long‐term average. The best explanation of these variations is slip across the plate boundary is episodic, varying over perhaps 5‐kyr intervals. This implies that 15–20 kyr is the time interval necessary to average out shorter, 5‐kyr ep

ISSN:0278-7407    

DOI:10.1029/91TC02890

年代:1992

数据来源: WILEY

摘要:

We have mapped a recently active left‐lateral strike‐slip fault (the Wecoma fault) on the floor of Cascadia Basin west of the Oregon convergent margin, using SeaMARC I sidescan sonar, Seabeam bathymetry and multichannel seismic and magnetic data. The fault intersects the base of the continental slope at 45°10′N and extends northwest (293°) for at least 18.5 km. The fault's western terminus was not identified, and the eastern end of the fault splays apart and disrupts the lower continental slope. The fault extends to the base of the 3.5‐km‐thick sedimentary section and overlies a basement discontinuity that may be related to movement along the Wecoma fault. Prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left‐lateral separation, allowing the general history of fault motion to be evaluated. The fault's average slip rate since 10–24 ka is inferred to be 5–12 mm/yr, based on the age of an offset submarine channel. Surficial structural relationships, in conjunction with the maximum inferred slip rate, indicate that fault movement initiated at least 210 ka and that the fault has been active du

ISSN:0278-7407    

DOI:10.1029/91TC02906

年代:1992

数据来源: WILEY

摘要:

A Cenozoic tectonic and sedimentary history is proposed for the Southern San Joaquin Basin (SSJB) and Tehachapi Mountains that evolved adjacent to the plate margin off central California. Seismic reflection, borehole, field, biostratigraphic, and paleomagnetic data are integrated into geologic and fault structure maps, cross sections, and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence, timing, and distribution of complex, tectonically linked events. The largely buried structures and strata in the SSJB preserve an unusually complete record of the mid‐Tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the Pliocene. Significant structural relief, existing across both extensional and contractile features, is preserved in the subsurface and an active fold‐thrust belt propagates basinward along the margin of the U‐shaped Tejon embayment The Cenozoic evolution of the SSJB reflects the regional deformation of central California as different tectonic events followed each other along the adjacent North American plate margin. Five Oligocene‐Miocene basin phases are identified in the SSJB: (1) late Oligocene/early Miocene extensional subsidence, with high‐ and low‐angle normal faulting, accompanied by volcanism and deposition of coarse syntectonic conglomerates; (2) middle Miocene uplift; (3) later mid‐Miocene transtensional subsidence to lower bathyal depths; (4) alternating subsidence and uplift until the late Miocene; and (5) flexural subsidence due to Pliocene to Recent contraction. Reconstructions of mid‐Tertiary California place the southern San Joaquin/Tehachapi extensional terrane as a paleotectonic block located between the Western Mojave terrane (then to the east) and the Western California terrane (then to the south and west). Regional extension occurred during a long transition period between convergent and transform boundaries along the North American plate margin. Significant slip along the San Andreas transform postdates this extensional event. Its origin apparently coincided with a regional middle Miocene uplift event, indicating that the San Andreas fault is younger than previously supposed. The Tertiary SSJB has subsided due to extension/transtension, crustal tilting, and thrust‐related loading. The Maricopa Subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the Sierra Nevada and locally has subsided beyond 12 km. In contrast, the relatively stable Tejon embayment, underlain by Sierran crystalline crust, achieved its maximum subsidence in the Miocene. Since Pliocene time, the SSJB has filled and continued to deepen after the basin was tectonically shut off from

ISSN:0278-7407    

DOI:10.1029/91TC02871

年代:1992

数据来源: WILEY

摘要:

Detailed field mapping of the central part of the area between the northern end of the Suez rift to the Nile Valley (Cairo‐Suez area) shows that the Eocene to Miocene rocks are affected by E‐W elongated belts of left‐stepped en echelon normal faults. These belts act as transfer zones between NW oriented normal faults synchronous with faults of the same trend in the Suez rift. Individual faults within the en echelon fault belts are oriented E‐W to WNW and dip in the same direction as the linked NW faults. Step faulting or horsts and grabens are two possible fault arrangements in the transfer zones depending on the geometry of the linked faults. The linked faults and the transfer zones are joined together in zigzag fault belts that extend northwest‐ward from the northern part of the Suez rift into the Cairo‐Suez area and possibly further northwestward. A substantial amount of the throw of the NW faults of the Suez rift is transferred northwestward by these zigzag fault belts. The throw generally decreases northwestward away from the rift. The en echelon fault belts were probably formed by right‐lateral divergent wrenching on E‐W oriented, deep‐seated, preexisting faults. This right‐lateral divergent wrenching is kinematically related to the dip‐slip movement on the linked, NW or

ISSN:0278-7407    

DOI:10.1029/91TC03184

年代:1992

数据来源: WILEY

摘要:

Unroofing of the Black Mountains, Death Valley, California, has resulted in the exposure of 1.7 Ga crystalline basement, late Precambrian amphibolite facies metasedimentary rocks, and a Tertiary magmatic complex. The40Ar/39Ar cooling ages, obtained from samples collected across the entire length of the range (>55 km), combined with geobarometric results from synextensional intrusions, provide time‐depth constraints on the Miocene intrusive history and extensional unroofing of the Black Mountains. Data from the southeastern Black Mountains and adjacent Greenwater Range suggest unroofing from shallow depths between 9 and 10 Ma. To the northwest in the crystalline core of the range, biotite plateau ages from ∼13 to 6.8 Ma from rocks making up the Death Valley turtlebacks indicate a midcrustal residence (with temperatures>300°C) prior to extensional unroofing. Biotite40Ar/39Ar ages from both Precambrian basement and Tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest, subparallel to the regional extension direction. Diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest. The cooling age pattern and geobarometric constraints in crystalline rocks of the Black Mountains suggest denudation of 10–15 km along a northwest directed detachment system, consistent with regional reconstructions of Tertiary extension and with unroofing of a northwest deepening crustal section. Mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment. The well‐known Amargosa chaos and perhaps the Badwater turtleback are examples of this “splaying” process.Considering the current distance of the structurally deepest samples away from moderately to steeply east tilted Tertiary strata in the southeastern Black Mountains, these data indicate an average initial dip of the detachment system of the order of 20°, similar to that determined for detachment faults in west central Arizona and southeastern California. Beginning with an initially listric geometry, a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward. This pattern may be explained by a model where migration of footwall flexures occur below a scoop‐shaped hanging wall block. One consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading. This process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle

ISSN:0278-7407    

DOI:10.1029/92TC00211

年代:1992

数据来源: WILEY

摘要:

The Subandean Zone of Bolivia is a foreland fold and thrust belt which forms the eastern edge of the central Andes mountains. Between 19°S and 22°S latitude, the construction of five balanced cross sections shows that the N‐S trending Subandean Zone is characterized by the existence of passive roof duplexes. These complex structures can be distinguished by the lithotectonic unit within which duplexing occurs. The five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes. The sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes. Apparently, these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units. While a passive roof duplex was developing, the sole thrust stuck and the major horizontal displacements were then transferred either to out‐of‐sequence thrusts or to a new sole thrust, in a deeper detachment horizon. Therefore each passive roof duplex would correspond to the orogenic front of the Andean range at one very particular time in the history of the Subandean Zone of southern Bolivia. From south to north, the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front. Available data do not permit us to explain completely this phe

ISSN:0278-7407    

DOI:10.1029/91TC03090

年代:1992

数据来源: WILEY

摘要:

Two major hypotheses have been advanced for the formation of the long wavelength (100–300 km) undulations of oceanic basement and overlying sediments developed in the central Indian Ocean basin: whole layer folding (buckling) and local thickening (inverse boudinage). Using appropriately scaled two‐layer analogue models for the oceanic lithosphere comprising a brittle layer above a ductile layer, we show that buckling of the entire brittle layer is likely to be the mode of deformation. However, the lithosphere‐asthenosphere boundary remains undisturbed. We find a relationship between the thickness of the brittle layer and the wavelength of folding such that the wavelength is 7 times the brittle layer thic

ISSN:0278-7407    

DOI:10.1029/91TC02908

年代:1992

数据来源: WILEY

摘要:

An Airy stress function solution is derived for the stress field in an elastic wedge‐shaped overthrust sheet subject to horizontal forces at the rear and at the front. From this stress solution the state of stress on the basal thrust (lower boundary of the sheet) is obtained. Both normal and shear stresses on the basal thrust are affected by the rear pushing and front buttressing forces and by the slopes of the upper and lower boundaries of the wedge‐shaped block. For (1) nonlinear components of external horizontal forces exerted on the vertical edges of the block and/or (2) nonzero slopes of the upper and lower boundaries the normal and shear stresses vary systematically with position along the basal thrust. The ratio of shear and normal stresses along this plane (the “frictional function” of the thrust) can be compared with the friction coefficient to determine where slippage is likely to occur. Since the frictional function is a function of position, the strength is exceeded only on part of the thrust (near the rear for most values of the parameters). The assumption of simultaneous displacement of the whole thrust sheet on the basal plane is therefore not realistic. Even simple static models such as the present one indicate that the motion on thrust faults must be envisaged in terms of progressive, consecutive “dislocation‐typ

ISSN:0278-7407    

DOI:10.1029/92TC00104

年代:1992

数据来源: WILEY

摘要:

Laboratory sand models simulating development of thrust belts indicate that (1) distance between the backstop and the first‐formed thrust is a linear function of initial sand layer thickness; (2) spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness; (3) initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and/or changes of mechanical properties of sand with increasing overburden; (4) faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth; and (5) new ramp initiation occurs to the hinterland of the basal detachment tip line. These observations can be interpreted both in context of area‐balance considerations and soil mechanics theory. If initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts (i.e., distance between the ground surface and the detachment), then our results suggest that locations of some thrust belt salients are controlled by along‐strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland‐tapering basin. Furthermore, our sand model observations suggest that the concept of a fixed, rigid backstop in the hinterland of thrust belts may be misleading. As a consequence of thickening, the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop

ISSN:0278-7407    

DOI:10.1029/92TC00175

年代:1992

数据来源: WILEY

摘要:

The western flank of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet is underlain primarily by Jura‐Cretaceous strata of the Gravina belt; pre‐Permian(?), Permian, and Triassic strata of the Taku terrane; and mid‐Proterozoic(?) to upper Paleozoic rocks of continental margin affinity. The continental margin rocks include mid‐Proterozoic(?) to lower Paleozoic(?) quartzite and marble of the Tracy Arm assemblage; mid‐Paleozoic metavolcanic and subordinate metasedimentary rocks of the Endicott Arm assemblage; and upper Paleozoic quartz‐rich metaturbidites and metaconglomerate of the Port Houghton assemblage. We suggest that these three assemblages are correlative with components of the Yukon‐Tanana terrane, which underlies a large region of Yukon and eastern Alaska. Rocks of the Gravina belt, Taku terrane, and Yukon‐Tanana terrane are juxtaposed along west‐vergent thrust faults of mid‐Cretaceous age and are internally deformed and disrupted along latest Cretaceous to early Eocene dip‐slip and possibly strike‐slip shear zones. These rocks and structures, together with mid‐Cretaceous to Eocene plutons of the Coast Mountains batholith, separate rocks of the Alexander‐Wrangellia terrane to the west from rocks of the Stikine terrane to the east. Mid‐Cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the Alexander‐Wrangellia terrane from central Alaska to northwestern Washington. The continental margin rocks in the northern Coast Mountains may be part of the in situ North American margin that has been overthrust by the Stikine and adjacent terranes. Alternatively, these rocks may have (1) rifted from and then returned to the North American margin, (2) moved>800 km along left‐lateral or right‐lateral faults from elsewhere along the North American margin, or (3) originated near a

ISSN:0278-7407    

DOI:10.1029/92TC00482

年代:1992

数据来源: WILEY