摘要:

A belt of northwardly younging Neogene and Quaternary volcanic rocks and hydrothermal vein systems, together with a distinctive Cretaceous terrane of the Franciscan Complex (the Permanente terrane), exhibits about 160 to 170 km of cumulative dextral offset across faults of the East San Francisco Bay Region (ESFBR) fault system. The offset hydrothermal veins and volcanic rocks range in age from .01 Ma at the northwest end to about 17.6 Ma at the southeast end. In the fault block between the San Andreas and ESFBR fault systems, where volcanic rocks are scarce, hydrothermal vein system ages clearly indicate that the northward younging thermal overprint affected these rocks beginning about 18 Ma. The age progression of these volcanic rocks and hydrothermal vein systems is consistent with previously proposed models that relate northward propagation of the San Andreas transform to the opening of an asthenospheric window beneath the North American plate margin in the wake of subducting lithosphere. The similarity in the amount of offset of the Permanente terrane across the ESFBR fault system to that derived by restoring continuity in the northward younging age progression of volcanic rocks and hydrothermal veins suggests a model in which 80–110 km of offset are taken up 8 to 6 Ma on a fault aligned with the Bloomfield‐Tolay‐Franklin‐Concord‐Sunol‐Calaveras faults. An additional 50–70 km of cumulative slip are taken up ≤ 6 Ma by the Rogers Creek‐Hayward and Concord‐Franklin‐Sunol‐Calaveras faults. An alternative model in which the Permanente terrane is offset about 80 km by pre‐Miocene faults does not adequately restore the distribution of 8–12 Ma volcanic rocks and hydrothermal veins to a single northwardly younging age trend. If 80–110 km of slip was taken up by the ESFBR fault system between 8 and 6 Ma, dextral slip rates were 40–55 mm/yr. Such high rates might occur if the ESFBR fault system rather than the San Andreas fault acted as the transform margin at this time. Major transpression across the boundary between the Pacific and North American plates at about 3 to 5 Ma would have resulted in the transfer of significant slip back to the San Francisco Peninsula segment of the San Andreas fault. Since that time, the ESFBR fault system has continued to slip at rates of 11–14 mm/yr. If this interpretation is valid, the ESFBR fault system was the Pacific‐North American plate boundary between 8 and 6 Ma, and this boundary has migrated both eastward and westward with time, in response to changing plat

ISSN:0278-7407    

DOI:10.1029/95TC02347

年代:1996

数据来源: WILEY

摘要:

Results from Ocean Drilling Program Leg 112 indicated tectonic erosion of the Peruvian convergent margin during subduction of the Nazca Ridge followed by renewed accretion against the erosional scar. Seismic images and swath mapping morphology show a regional geology shaped as subduction of Nazca Ridge migrated 800 km along the Peru Trench. The accretionary prism is imaged at two stages of development since upper Miocene time with depth‐migrated seismic records in which the “back stop” is well defined. From these data, material flux can be quantified. After the ridge crest subducted and accretion dominated over erosion, the prism grew rapidly to 10‐ to 15‐km width. During rapid growth, the margin taper was large and about 60 percent of the sediment supply accreted. As the prism growth slowed, only 30 percent accreted despite an increased trench sediment supply. Since the convergence rate changed little, the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted. The decreased prism growth may be partially controlled by varying the structure of the back stop. The force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary. Thus the shear stress transmitted across the plate boundary is insufficient to drive “out‐of‐sequence” thrust faults that would thicken the back stop or raise it beyond a limited height. A constant back stop height limits accretion, and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop. Nazca Ridge subduction affected the convergent margin mass flux for about 8 Ma. Mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest. When growth of the accretionary prism slowed, a greater terrigenous input through the trench was accommodated by sediment subduction. The resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which ma

ISSN:0278-7407    

DOI:10.1029/95TC02618

年代:1996

数据来源: WILEY

摘要:

New deep seismic reflection profiles across the Banda Arc of Indonesia reveal reflectors in the uppermost 50 km of the lithosphere. Combined with existing earthquake hypocenter locations and focal mechanisms, the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m.y. and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge. A clearly defined Wadati‐Benioff zone, and recent deformation of shelf sediments observed on older shallow seismic profiles, indicated to previous workers that substantial convergence occurred at the Timor Trough. A few focal mechanisms, seafloor escarpments, and recent geodetic surveying indicate that convergence at ∼7 cm yr−1currently (last 200 kyr?) occurs at the northern margin of the now inactive volcanic arc, the Wetar Thrust zone. Reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the Timor Trough and Wetar Thrust. Specifically, basement reflectors beneath the toe of the accretionary complex have reverse‐sense offsets that imply blind thrusts. The whole crust is horizontally shortened, not only the sedimentary cover rocks that previously deformed into duplexes above a decollément. Reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting. Thus each arc represents a doubly vergent fold‐and‐thrust belt, but only the northern one is currently active. Crustal thicknesses inferred from seismic velocities, reflectors, and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3–4 km of topographic relief in the region of

ISSN:0278-7407    

DOI:10.1029/95TC02352

年代:1996

数据来源: WILEY

摘要:

The Middle America Trench between the Cocos Ridge and a well‐studied corridor off the Nicoya Peninsula has a more varied morphology and structure than previously reported. The morphological positive features on the lower plate significantly affect the upper plate structure. The Cocos Ridge has uplifted the margin opposite the Osa Peninsula. Northwest of Cocos Ridge, numerous seamounts on the oceanic crust sculptured the margin as they subducted. A seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted. The greater portion of the trench sediment is subducted beneath a lower slope accretionary mass. Beneath the middle and upper slope is a margin wedge consisting of a high‐velocity rock with few internal reflections. Its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area. Overlying this surface is a low‐velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred Eocene section beneath the shelf. The shelf basement is composed of Nicoya complex (ophiolite) with the same acoustic velocity, similar structure, and no apparent dividing geologic boundary with the margin wedge. We favor a seaward continuation of the Nicoya complex to the middle slope and emphasize the evidence for a non‐steady state Tertiary tectonic

ISSN:0278-7407    

DOI:10.1029/95TC02355

年代:1996

数据来源: WILEY

摘要:

The Central Chain ophiolites in New Caledonia are fragments of a supra‐subduction zone (SSZ) ophiolite, now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick Middle Triassic to Upper Jurassic volcaniclastic sequence. Most of the fragments were formed by a single tholeiitic magmatic episode, but one of these, the Koh ophiolite, was formed by two tholeiitic magmatic episodes separated by boninites. The first event in the Koh ophiolite formed cumulate gabbros, dolerites, plagiogranites, and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements (LREE) and abnormally low TiO2(0.5% at Mg#=60). Shortly after their eruption, these tholeiitic lavas were overlain by a high‐Ca boninitic unit with a basal section of boninite pillows, flows, and breccias and an upper section of boninitic dacites and tuffs. The last magmatic phase involved eruption of evolved tholeiitic basalts, as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite. This second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts (BABB) erupted during the early rifting history of modern back arc basins. The boninitic volcanics belong to a high‐Ca series with slightly lower SiO2, Al2O3, and TiO2compared to those from modern island arc systems, and they lack the positive Zr spike relative to adjacent rare earth elements (REE) in normalised element variation patterns. These boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40–60% extension of the initial basin crust and eruption of the upper tholeiites. The dominance of BABB‐like tholeiites throughout the Central Chain ophiolites in New Caledonia, the restricted occurrence of boninites, and the stratigraphy and chemistry of the Koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation. We suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites, during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre. The occurrence of a thick blanket of calc‐alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the Koh boninites were not associated with the initiation of subduction. A close modern analogy for the Koh ophiolite exists on the Hunter Ridge protoisland arc between southernmost Vanuatu (New Hebrides island arc) and the Fijian islands; there, high‐Ca boninites lacking positive Zr spikes occur together with low‐Ti tholeiites and more typical BABB tholeiites where the southern spreading centre of the North Fiji Basin is propagating into the protoarc crust of t

ISSN:0278-7407    

DOI:10.1029/95TC02316

年代:1996

数据来源: WILEY

摘要:

Classic ophiolites, as exemplified by the Troodos Massif in Cyprus and the Papuan Ultramafic Belt in eastern New Guinea, are large overthrust masses which are generally associated with large positive gravity anomalies. However, similar rocks occurring in extensive fragmented terranes which have also been described as ophiolitic do not produce large gravity effects. The eastern part of the island of Halmahera, in northeastern Indonesia, is an ophiolite of this latter type. On the two eastern arms of the island, a Mesozoic ophiolitic basement is overlain by, and imbricated with, Upper Cretaceous and Paleogene arc volcanic and sedimentary rocks. Bouguer gravity values are generally in the range +50 to +150 mGal and are characterised by steep local gradients indicative of shallow sources. The Bouguer gravity average suggests that the crust is at least 20 km thick, and it must be even thicker if a significant part of the anomalous gravity field is due to the presence of a cold and therefore dense, lithospheric slab within the asthenosphere, associated with the present‐day subduction beneath Halmahera. The absence of any exposures of continental basement rocks or of quartzose sediments in eastern Halmahera suggests that these ophiolites have not been overthrust onto continental crust and that the thickening occurred in an intraoceanic island arc. The Paleogene arc was evidently characterised by volcanism occurring over an unusually wide area. In this it resembles the Izu‐Bonin volcanic arc, which, like Halmahera, has been situated at the margin of the Philippine Sea Plate throughout its history. The gravity field of the Halmahera ophiolite is comparable with that of the Bonin volcanic arc, but there is no Halmahera parallel to the very high gravity fields recorded over the Bonin Islands forearc ridge. The equivalents of this part of the Paleogene arc may be represented by the ophiolitic complexes now distributed along the northern margin of the orogenic belt in New Gui

ISSN:0278-7407    

DOI:10.1029/95TC02353

年代:1996

数据来源: WILEY

摘要:

Silurian imbricate thrusting, Early Devonian high‐grade metamorphic nappe emplacement, and Devonian‐Carboniferous extensional denudation characterize deformation in the Appalachian hinterland of Cape Breton Island. Compressional deformation following Early Silurian arc volcanism features imbrication of Cambrian‐Precambrian basement rocks of Gondwana derivation with Ordovician‐Silurian cover sequences across thick zones of mylonite during south directed transport. High grade metamorphism and gneissic rocks of late Silurian age in the region indicate that significant tectonic burial and crustal thickening occurred as a result of the thrusting. Partial denudation of the high grade assemblages occurred during Early Devonian thrust emplacement of the Cabot nappe toward the northwest, along the Highlands Shear Zone. The nappe is characterized by an amphibolitic gneiss and high‐grade schist complex defining a large folded klippe above Silurian units. Kyanite is widespread within the nappe, and a distinctive feature of the thrust sheet is the dynamothermal metamorphism of cooler greenschist‐grade footwall rocks producing inverted isograds; staurolite is regionally distributed and occurs in pelitic units in the immediate footwall of the Highlands Shear Zone forming a discontinuous halo around the klippe. Greenschist‐grade footwall rocks are exposed in structural windows as a result of folding and faulting. Shear sense indicators along the margins of the Cabot nappe have been rotated into their present positions due to superposed folding, providing apparent movement directions for the nappe. Complete exhumation to surface occurred during Late Devonian extension along the low‐angle Marg

ISSN:0278-7407    

DOI:10.1029/95TC02289

年代:1996

数据来源: WILEY

摘要:

A structural and metamorphic study was carried out in the basal units of the Ordenes Complex in Spain, thought to represent a subducted part of the Paleozoic margin of Gondwana. According to their metamorphic evolution, this part of the margin was subducted at the onset of the Variscan Orogeny, becoming part of an accretionary complex developed below a colliding element built previously. Variations in the PT conditions of the first high‐pressure metamorphic event along the units indicate a polarity of the subduction to the west. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units, whereas the convergence continued. Recumbent folds and thrusts developed along with successive normal detachments. Compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading. The unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted unit

ISSN:0278-7407    

DOI:10.1029/95TC02617

年代:1996

数据来源: WILEY

摘要:

A paleomagnetic study of Paleozoic and Mesozoic crystalline rocks in the northern Ford Ranges of Marie Byrd Land, West Antarctica, has determined a middle Cretaceous (circa 100 Ma) paleomagnetic pole and provided constraints on possible clockwise rotation of these ranges and on the rifting of east Gondwana. The40Ar/39Ar thermochronology data from the Fosdick Mountains record a period of rapid cooling from ∼700°C beginning at ∼100 Ma. We relate this to extension, intrusion, and uplift associated with the beginning of rifting between Campbell Plateau and Marie Byrd Land. All rocks from the Fosdick and Chester Mountains are normally polarized. We interpret thermochronology and paleomagnetic data to infer that the region was extensively remagnetized in middle Cretaceous time. Inclinations in samples from the Chester Mountains are less steep than those from the Fosdick Mountains, which we interpret as ∼25° of south tilting of the Chesters. We interpret cooling age data for the time of magnetization to infer that the tilting began after 105 Ma and ended prior to 103 Ma. We further interpret this as constraining the beginning of extension between the Campbell Plateau and western Marie Byrd Land to the interval 105 to 103 Ma. Virtual geomagnetic poles from samples of Early Carboniferous age granodiorite from the western Phillips Mountains lie on the late Paleozoic apparent polar wander path for Australia transferred to Antarctica. Directions from 29 sites in the central and eastern Phillips and Fosdick Mountains give a Middle Cretaceous paleomagnetic pole at 222.3° E, 70.5° S (A95 6.1°, KAPPA 20.0). This pole is indistinguishable from other Middle Cretaceous poles for studies further east in Marie Byrd Land. Combining middle Cretaceous poles determined for three other studies of the Antarctic Peninsula, Thurston Island, and the Ruppert‐Hobbs coasts with ours gives a Pacific West Antarctic pole at 215.2° E, 73.5° S (A95 4.0°, KAPPA 528.9). This pole is discordant by 5° to 10° from synthetic mid‐Cretaceous East Antarctic reference poles, but the degree of discordance is very sensitive to the choice of the specific reference pole. The lack of native East Antarctic reference poles leaves this analysis inconclusive. Accepting 10° of discordance, we favor an interpretation where Pacific West Antarctic crustal domains or microplates have rotated clockwise 40° to 90° and translated a few degrees away from East Antarctica during Late Cretaceous time. An electronic supplement of this material may be obtained on a diskette or Anonymous FFP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the user name and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC02524, Paleomagnetic study of the northern Ford Ranges, western Marie Byrd Land, West Antarctica: Motion between West and East Antarctica, Bruce Luyendyk, Stan Cisowski, Christine Smith, Steve Richard, and David Kimbrough). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Paymen

ISSN:0278-7407    

DOI:10.1029/95TC02524

年代:1996

数据来源: WILEY

摘要:

We present results from the first detailed geological transect across the Mongolian Western Altai using modern methods of structural geology and fault kinematic analysis. Our purpose was to document the structures responsible for Cenozoic uplift of the range in order to better understand processes of intracontinental mountain building. Historical right‐lateral strike‐slip and oblique‐slip earthquakes have previously been documented from the Western Altai, and many mountain fronts are marked by active fault scarps indicating current tectonic activity and uplift. The dominant structures in the range are long (>200 km) NNW trending right‐lateral strike‐slip faults. Our transect can be divided into three separate domains that contain active, right‐lateral strike‐slip master faults and thrust faults with opposing vergence. The current deformation regime is thus transpressional. Each domain has an asymmetric flower structure cross‐sectional geometry, and the transect as a whole is interpreted as three separate large flower structures. The mechanism of uplift along the transect appears to be horizontal and vertical growth of flower structures rooted into the dominant right‐lateral strike‐slip faults. The major Bulgan Fault forms the southern structural boundary to the range and is a 3.5‐km‐wide brittle‐ductile zone that has accommodated reverse and left‐lateral strike‐slip displacements. It appears to be linked to the North Gobi Fault Zone to the east and Irtysh Fault zone to the west and thus may be over 900 km in length. Two major ductile left‐lateral extensional shear zones were identified in the interior of the range that appear to be preserved structures related to a regional Paleozoic or Mesozoic extensional event. Basement rocks along the transect are dominantly metavolcanic, metasedimentary, or intrusive units probably representing a Paleozoic accretionary prism and arc complex. The extent to which Cenozoic uplift has been accommodated by reactivation of older structures and inversion of older basins is unknown and will require further study. As previously suggested by others, Cenozoic uplift of the Altai is interpreted to be due to NE‐SW directed compressional stress resulting from the Indo‐Eura

ISSN:0278-7407    

DOI:10.1029/95TC02354

年代:1996

数据来源: WILEY