摘要:

A major, abrupt, southward decline in elevation (∼800 m) with an accompanying increase in Bouguer gravity anomaly (∼80 mGal) crosses a seismically active region of southern Nevada at about 37°N. The steepness of the gravity gradient requires significant crustal density contrast, some of which is probably caused by the plutonic roots of voluminous Tertiary volcanism to the north, but the amplitude of the anomaly requires additional contrast within the mantle. The topographic step probably developed in mid‐Miocene time, coincident with the arrival of the starting head of the Yellowstone thermal plume at the base of the lithosphere. A plausible combination of crustal and deep buoyancy sources, related to the heat and melt input of an anomalously hot asthenospheric source, is consistent with gravity, seismic, heat flow, and isotopic observations and explains the origin of the topographi

ISSN:0278-7407    

DOI:10.1029/95TC02288

年代:1995

数据来源: WILEY

摘要:

The trans‐Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench. The central part of the belt is being deformed by the Chapala‐Tula fault zone, an approximately 450‐km‐long and 50‐km‐wide zone of active extension. The volcanic arc and the arc‐parallel Chapala‐Tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the Mexican Basin and Range. The Acambay graben, about 40 km long and 15 km wide, is located approximately 100 km northwest of Mexico City and is one of the major troughs within the Chapala‐Tula fault zone. The border faults of the Acambay graben, Acambay‐Tixmadejé in the north and Pastores in the south, are separated in the west by stepovers from range‐bounding faults of similar orientation, Epitacio Huerta in the north and Venta de Bravo in the south. The stepovers occur at the intersection of these faults with an older system of Basin and Range faults. An early‐stage right‐lateral component of motion along the Venta de Bravo and Pastores faults is inferred on a map scale from a left‐stepping en echelon array of normal fault segments. The divergence of the en echelon segments from the general fault trend decreases gradually from west to east, suggesting that the early extension was rotational. The present relative displacement along the southern margin of the system, on the other hand, results in a left‐lateral strike‐slip component. This is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left‐oblique slip. The striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component. This is supported on a map scale by the structure of the right stepover between the Acambay‐Tixmadejé and Epitacio Huerta faults, which shows no evidence of local extension or shortening. The divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation. This could explain why no active extension has been observed to the east of the Chapala‐Tula fault zone, in the eastern part of the trans‐Mexican volcanic belt. During theMs= 6.9 Acambay earthquake of November 19, 1912, surface rupture occurred along both margins of the graben at the base of multiple‐event scarps. Along the Acambay‐Tixmadejé fault, the coseismic rupture is 41 km long. The vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum. Furthermore, the change in the vertical surface offset along the fault is approximately proportional to the change in height of the Acambay‐Tixmadejé multiple‐event fault scarp. The easternmost part of the ground rupture passes through a plain and not at the base of a multiple‐event scarp as farther west. It may therefore correspond partly to an increase in length of the Acambay‐Tixmadejé fault during the 1912 earthquake. The slip rate along the southern border of the Acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a40Ar/39Ar age of 0.4±0.1 Ma. This basalt may be displaced up to 15 m by the Pastores fault, which indicates a middle‐late Quaternary slip rate of ≤0.04 mm/yr. Furthermore, based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake, we estimate a recurrence interval of ≥5000 yea

ISSN:0278-7407    

DOI:10.1029/95TC01930

年代:1995

数据来源: WILEY

摘要:

Through an integrated interpretation of gravity, magnetic, seismic reflection, and well data we are able to define the geometry, lateral extent, and tectonic setting of late Middle Proterozoic mafic rocks (the Pecos mafic intrusive suite) in the basement of the Permian basin of west Texas and eastern New Mexico. In this region, seismic reflection data are characterized by subhorizontal layered reflectivity within basement that can be tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram. In places the intrusion forms a sill, and the base of the sill is imaged in the reflection data. Integration of the seismic interpretation with gravity models shows that the Pecos mafic intrusive suite is 3 – 10 km thick and 50 – 100 km wide. The overall structure of the suite is that of a sill with a locally dikelike keel. North ‐ south trending gravity and magnetic anomalies associated with the Pecos mafic intrusive suite suggest that it is composed of four large intrusive centers which cover a minimum area of 9200 km2, making it one‐seventh the size of the Bushveld Complex and one of the largest layered mafic intrusions in the world. We interpret this massive intrusion to be associated with collisional rifting in the foreland of the Grenville

ISSN:0278-7407    

DOI:10.1029/95TC02317

年代:1995

数据来源: WILEY

摘要:

Systematic changes in fabrics of melanges have been detected in rocks of the Shimanto Belt, Japan. Several shear indicators, including S‐C structures, Riedel shears, and folds, in the melanges indicate consistent sense of shear both in outcrop and microscopic scales. The deformation mechanisms, independent particulate flow and pressure solution, suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening. After lithification, cataclastic deformation occurred locally, but most of the earlier deformation features are preserved. This process appears to have taken place along the decollement beneath the off‐scraped accretionary prism. Sinistral reverse sense of shear is observed in the Campanian melange, whereas a dextral reverse sense of shear is preserved in the early Eocene melange in the Shimanto Belt. This change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates. Three different plate models have been proposed for the Late Cretaceous to Early Tertiary western Pacific margin: (1) subduction of the Kula‐Pacific ridge along the continental margin, (2) subduction of the Pacific Plate since 85 Ma, and (3) subduction of the aborted Kula‐Pacific ridge after 43 Ma. The observed change in fabric of melange is consistent only with the third model, which predicts a change in relative convergence from sinistral to dextral at the appropria

ISSN:0278-7407    

DOI:10.1029/95TC01929

年代:1995

数据来源: WILEY

摘要:

The relatively high‐T, high‐P/T, subduction‐related basement in the Southern California Continental Borderland preserves important information regarding the late Mesozoic tectonic evolution of coastal southern California. Integrated petrologic, geochemical, and geochronological study of the subduction‐related Catalina Schist exposed on Santa Catalina Island documents the formation of lawsonite‐blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling, newly initiated subduction zone. Petrologic and40Ar/39Ar thermochronologic data demonstrate similar, perhaps shared, retrograde PT histories for all but the lawsonite‐albite facies rocks under pumpellyite‐actinolite facies conditions as early as ∼95–100 Ma. The40Ar/39Ar data, together with the abundance of pumpellyite‐actinolite facies vein assemblages and lack of higher‐P/T (epidote‐ or lawsonite‐blueschist) over‐printing in high‐grade units, are consistent with the rapid emplacement of the Catalina Schist into the shallow‐ to mid‐crust by ∼ 95 Ma by combined underplating and tectonic/erosional removal of overlying rocks in the accretionary complex (cf. Platt, 1986). The Catalina Schist and the adjacent Peninsular Ranges Batholith potentially represent paired metamorphic belts formed along southwest North America at 120 to 90 Ma. Combining the evidence for the evolution of the Catalina Schist with the previously advanced hypotheses regarding tectonic evolution of the batholith, suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone, at ∼ 120–115 Ma, in which the Catalin

ISSN:0278-7407    

DOI:10.1029/95TC01931

年代:1995

数据来源: WILEY

摘要:

The Oregon margin near 45°N spans a regional transition in structural style from seaward vergence in the south to landward vergence in the north. This variation probably reflects a regional change in both sediment type and rate of deposition that affects the potential for overpressure in the sediments. Structural style within the survey area shows a gradual northward transition from seaward to landward vergence and to lower slopes within the landward vergent area, suggesting a northward decrease in basal shear stress. Superimposed on this gradational variation are abrupt changes in structural style that correlate with NW striking strike‐slip faults in the Cascadia Basin. Because sediments thicken toward the east, translation along the strike‐slip faults results in juxtaposition of sediments with different physical properties and loading histories. In addition, the faults themselves may act as fluid conduits, resulting in stepwise changes in pore pressure on the décollement and concomitant change in structural style across the faults. Although the Oregon‐Washington margin is dominated by landward vergence, landward vergence has not been adequately explained by theoretical models or replicated in experimental models because of a simple omission in the boundary conditions of the underlying conceptual model. Landward vergence requires not only low basal shear stress but also an arcward dipping décollement (and to a lesser degree, a relatively strong wedge). In order for landward vergence to predominate, these three factors must combine in such a way that the backward verging thrust planes are

ISSN:0278-7407    

DOI:10.1029/95TC02320

年代:1995

数据来源: WILEY

摘要:

Several publications propose that main‐phase fold‐thrust development on Spitsbergen was Late Cretaceous and not Tertiary as previously thought. The question of timing is crucial to models for crustal response to transpressive plate motions. Involvement of Tertiary strata in fold‐thrust structures, the sedimentology of the Tertiary basin strata, and studies of paleo‐stress field evolution all indicate Paleocene to Eocene fold‐thrust development during opening of the Norwegian‐Greenland oceanic basin. A regional angular unconformity of<1° between Paleocene and Early Cretaceous strata is consistently disconformable to the eye and precludes any significant older deformation in the immediate area. Pre‐unconformity deformation was likely strike slip in character and concentrated in the west. The proposal for Late Cretaceous fold‐thrust belt formation is inconsistent

ISSN:0278-7407    

DOI:10.1029/95TC01257

年代:1995

数据来源: WILEY

摘要:

The Québec Appalachians have been shaped by two major orogenies: the Middle to Late Ordovician Taconian and the Middle Devonian Acadian. Thrust faults and nappe structures characterized the Taconian deformation throughout the Québec Appalachians, whereas structural styles pertaining to the Acadian orogeny differ from southern Québec to the Gaspé Peninsula. Structural analysis of Late Ordovician to Middle Devonian supracrustal rocks shows two different Acadian deformational regimes: strike‐slip tectonics in the Gaspé Peninsula and thrust or dip‐slip tectonics in southern Québec. In southern Québec, Acadian regional deformation produced folds which vary from upright to overturned and tight to isoclinal from the NW to the SE. Major faults (e.g., La Guadeloupe) are northwestward directed thrust faults marked by highly ductile shear zones outlined by mylonite. In the Gaspé Peninsula, structural trend is NE and major E–W dextral strike‐slip faults transect this trend. Folds are open and upright, inclined and tighter near the major faults where they have a clockwise rotation like the regional cleavage. Major faults (e.g., Grand Pabos) follow corridors delimiting high‐strain zones where the fabrics developed are indicative of a ductilebrittle regime of deformation. In the Témiscouata region, major structural features of both southern Québec and the Gaspé Peninsula are recognized. Structures of this region consist of ENE trending dextral strike‐slip faults and high‐angle WNW verging reverse faults, parallel to major NNE folds. These structures reflect the transition from purely horizontal movement along major strike‐slip faults of the Gaspé Peninsula to vertical movement along thrust faults in southern Québec. Acadian structural variations within the Québec Appalachians are interpreted in terms of a continental collision of Gondwana along the irregular margin of Laurentian and i

ISSN:0278-7407    

DOI:10.1029/95TC01449

年代:1995

数据来源: WILEY

摘要:

The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic‐Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large‐scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well‐defined along‐coast NW–SE decreasing granitoid intrusion age trend (∼1.2 cm/yr in the 100 Ma‐40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW–SSE trending Cretaceous‐Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America‐Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (∼7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco‐Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40–25 Ma, with a velocity of 6.5–4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America‐Farallon‐Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large‐scale right‐lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more im

ISSN:0278-7407    

DOI:10.1029/95TC01928

年代:1995

数据来源: WILEY

摘要:

The mantle reflector pattern beneath the Scottish and Irish Caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle. Previous investigations interpreted the reflector as a thrust related to the Caledonian orogeny (Silurian‐Early Devonian) or as a normal fault or shear zone active during post‐Caledonian (e.g., Devonian or Permo‐Triassic) extension. This study uses all available deep seismic reflection data to map mantle reflector structure along the Caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology. The distribution of the mantle reflector shows that it is not spatially related to either Caledonian compressional or subsequent basin extensional structures in a simple way. A structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform‐synform pair beneath the West Orkney basin (north of the Scottish mainland) and the Færoe‐Shetland basin west of Shetland, respectively, separated by an inferred zone of structural disruption. A working hypothesis for this study is that these two surfaces may have once been part of a continuous, more or less planar surface that has been offset and deformed by either (or perhaps a combination) of two processes: (1) differential extension of crust that was at least partially coupled to the uppermost mantle beneath the West Orkney and Færoe‐Shetland basins; (2) a broad, approximately orthogonal (i.e., east‐west trending) zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust. Our results also suggest that the mantle reflector structure may be inherited from Neoproterozoic events associated with rifting of the Laurentia‐Balti

ISSN:0278-7407    

DOI:10.1029/95TC01202

年代:1995

数据来源: WILEY