Ophiolitic Basement Research Articles

Geology and tectonic evolution of the Bear Mountains Fault Zone, Foothills Terrane, central Sierra Nevada, California

The Late Jurassic‐Early Cretaceous Bear Mountains fault zone (BMFZ) is the westernmost strand of the Foothills fault system in the Western Metamorphic belt of the Sierra Nevada. The tectonic significance of this 300‐km‐long fault zone has been downplayed in the past, but we contend that it is a major discontinuity within the Foothills terrane. The BMFZ is an ˜ 5‐km‐wide shear zone consisting of slate‐metagraywacke‐matrix melange that experienced intense polyphase deformation and metamorphism. Small‐scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone. A host of lithologically diverse tectonic blocks are enclosed in the BMFZ. Blocks of sandstone, metavolcanic rocks, and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the Foothills terrane, whereas other block types are exotic. Exotic blocks include ultramafites that locally contain pods of gabbro, garnet amphibolite, greenschist, sedimentary breccia, and volcaniclastic rocks. The breccia is composed primarily of clasts of amphibolite and minor garnet‐bearing impure quartzite (metachert?); it probably accumulated at the base of fault scarps. The exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the Foothills terrane, and they record a deformation prior to incorporation in the slate‐metagraywacke matrix. Amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the Tuolumne ophiolite east of the BMFZ. The presence of exotic blocks implies large‐scale mixing and large displacements in the BMFZ, as do differences in stratigraphy across the fault zone. Metamorphic grade does not vary significantly across the BMFZ, however, suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose. There is no direct evidence for major strike slip along the BMFZ as has been proposed by others; any such slip must have occurred before the reverse‐slip related structures of the zone. We thus interpret the BMFZ as an intra‐arc reverse fault of moderate to large displacement, with possible earlier strike slip, that incorporates ophiolitic basement showing an older, complicated history.

The geology and tectonic evolution of Waigeo Island, NE Indonesia

Waigeo occupies a critical position between the Halmahera-Philippine arcs to the northwest and Australia-New Guinea to the southeast. The island consists of a deformed ophiolitic basement of supra-subduction zone type overlain by probable Paleogene forearc sedimentary rocks. The forerarc basement is cut by mylonite shear zones and the basement and sedimentary cover rocks were deformed by southward-directed thrusts and associated folds during the Oligocene. The deformed sequences are intruded by basic dykes, thought to be coeval with basalts and andesites of Late Oligocene age. The entire Paleogene sequence is overlain unconformably by a thick (up to 2000 m) sequence of Miocene limestones which accumulated during a tectonically quiet period. A final phase of deformation occurred during the Pliocene which caused the development of two very large anticlines and an intervening syncline, associated with left-lateral wrench faults. Waigeo is interpreted to have been situated in a forearc position in an intra-oceanic island arc during the early Paleogene, forming part of the East Halmahera-Waigeo forearc terrane. The Waigeo arc terrane collided with a continental block in about the Middle Oligocene, contemporaneous with similar arc-continent collision in northern New Guinea. A period of tectonic quiescence during the Miocene was followed by Pliocene deformation in Waigeo related to a left-lateral wrench faulting on splays of the Sorong Fault in northern New Guinea. The Pliocene deformation is interpreted as resulting from compression on a right-stepping restraining bend in this wrench fault system.

The Maliau Basin, Sabah: Geology and tectonic setting

A field survey of the eastern half of the Maliau Basin established the presence of several good quality coal seams in the dominant mudstone sequence of the Early to Middle Miocene Tanjung Formation. Although volumetrically minor, the several metres thick sublitharenite beds dominate the landscape forming high waterfalls, concen­ tric cuesta and other ridges with dipslopes inclined into the basin. Sandstone channel fills are common. Sedimentary current indicators including current ripple marks, tabular and trough cross beds, and lineation of carbonised plant fragments indicate that the Il).ajority of currents was towards northeast. The presence of coal, dominant mudstone, marine body and trace fossils suggest that the Tanjung Formation developed in low-energy environ­ ment, probably in wide tidal flats. Except for one locality exposing open folds, the beds dip uniformly and gently inward forming an eccentric basinal structure associated with normal faulting. These phenomena could have only been caused by gravitational subsi­ dence without tectonic compression. The northern boundary strike ridge intercepts internal strike ridges at oblique angles. This pattern and the off-centre position of the youngest beds suggest that the Maliau Basin during the deposition of the Tanjung Formation became progressively narrower while its depocentre migrated towards the south rim. The structural basins contairiingthe Tanjung Formation and time-equivalent Kapilit Formation are arranged in northeast and southeast rows; the Maliau basin occupying the junction of these two directions. Geologically the Maliau Basin is the landward extension of the Tarakan Trough and is at least partially floored by melange rocks of the Kinabalu Suture Zone. Subsidence ofthis basin and other early-middle Miocene basins of east Sabah began when the East Sabah Terrane stopped drifting south and isostatic readjustment sank the denser ophiolitic basement of the suture zone.

The southern termination of the Philippine Trench

Recent studies of the southern end of the Philippine Trench using GLORIA sidescan sonar and single channel seismic data and geological studies on the island of Halmahera suggest that the trench is in the process of propagating south and that some of the ESE-WNW convergence is transferred via a broad NE-SW zone of dextral strike-slip across northern Halmahera into the Molucca Sea Collision Zone. NE-SW ridges and lineaments on the seafloor can be traced into major faults and structural trends on land. The bathymetric expression of the Philippine Trench is lost where it meets an oceanic plateau on the Philippine Sea Plate. This elevated and probably thickened plateau appears to have inhibited any further propagation of the trench southwards. To the north of the plateau there is a well-developed accretionary prism, but to the south deformation of sediments on the seafloor is less intense. A prominent ridge, with sediments passively banked up against it marks the southern limit of deformation associated with the Philippine Trench. To the south lies the eastern Halmahera-Waigeo Ophiolite Terrane, an area of shallow water and islands underlain by ophiolitic basement between Halmahera and the Sorong Fault Zone. There are no bathymetric or structural features to indicate any form of active link between the Philippine Trench and the Sorong Fault through or along the northern side of this ophiolite terrane. There is no earthquake evidence for any form of fault linking the Philippine Trench to either the Palau Trench or the Sorong Fault, but there are numerous earthquake epicentres reported between the southern end of the trench and the Molucca Sea. The island of Halmahera is considered to lie in a diffuse boundary zone at the margin of the Philippine Sea Plate. Continued collision of the opposing arcs in the Molucca Sea will ultimately prevent further E-W convergence between Halmahera and Sulawesi. It is predicted that either the Philippine Trench will propagate south along its present line to meet the Sorong Fault or that another trench will develop further to the east. In either case the arc and ophiolite terrane of Halmahera might then be accreted onto the Eurasian margin.

Structure and geological history of the Lepanto-Cervantes releasing bend on the Abra river fault, Luzon Central Cordillera, Philippines

The Luzon Central Cordillera corresponds to the uplifted and tectonized magmatic arc associated with the Manila Trench subduction zone. It is cut into N-S trending strips by the horsetail-like overprint of the Philippine strike-slip fault. The median branch, herein named the Abra River Fault, splits away to the west from the Dalton Fault and runs north-northwestward for 240 km through the range. Trending N150E in its southern part, it undergoes a sinistral bend in the Lepanto-Cervantes area, and then strikes N-S, makes a second bend and runs north-northeast up to the northwestern tip of the island. In the bend area, the compartment east of the fault has subsided, forming a narrow strip along the southern Lepanto segment and a wider trough parallel to the fault which stretches 25 km from Cervantes. The basement of the strip is position dependent, corresponding either to the Oligo-Miocene arc succession (volcanoclastic sediments and dioritic intrusive), or to the pre-Late Eocene ophiolitic basement of the arc. In the Lepanto area, two volcanic formations are preserved: the oldest one, the Malaya Formation is latest Miocene in age (6.1 ± 0.3 and 5.7 ± 0.28 Ma by K/Ar) and represents the infill of the trough, the youngest, the Mount Pudso volcanics, is Holocene in age (0.5 Ma by K/Ar). The Cervantes trough contains a thick (reaching 1700 m) volcanoclastic sequence dated as latest Miocene-Early Pliocene (3.7 ± 0.18−6.2 ± 0.28 Ma by K/Ar (the Malaya Formation). On the western and southern sides, the Abra River Fault separates the very deformed infill from the strongly uplifted pre-Late Miocene basement. On the east side, the basement, whose elevation increases gradually eastward, is disconformably overlain by the Mio-Pliocene sequence. To the north, the trough is slightly deformed. The structural analysis of the Lepanto-Cervantes strip, together with the local and regional geological and geochronological data, allow us to propose a three-stage history. The first stage during the early Late Miocene, is the major left-lateral wrench displacement of the Abra River Fault and the appearance of the bend under a transpressive regime. The second stage from latest Miocene to Early Pliocene, and marked by strong volcanic activity, comes with and/or immediately follows the subsidence of the Lepanto-Cervantes strip. The last stage, starting in the Late Pliocene, is characterized by the E-W foreshortening of the Cervantes Trough, and the beginning of its erosion.

The Ordovician evolution of the South Mayo Trough, western Ireland

The South Mayo Trough, a broad synclinorium containing a 10‐km sequence of Early Ordovician turbidites passing up into shallow water sandstones, is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north‐facing arc. The arc collided with the Laurentian margin in Early Llanvirn times to deform the Dalradian Supergroup rifted margin clastic sequence. Prior to the late Llandovery, sinistral transcurrent faulting transposed the Connemara Dalradian terrane to its present position on the “outboard” ocean ward side of the South Mayo Trough. The South Mayo Trough was fed principally by arc and ophiolite‐derived elastics during the Arenig with an increasing metamorphic component during the Llanvirn. The Late Ordovician transpressional “docking” of Connemara provided the southerly source for high‐grade metamorphic clasts in the Deny veeny Formation.

Basement Rocks of Halmahera, Eastern Indonesia: Implications for the Early History of the Philippine Sea: ABSTRACT

The oldest rocks known on Halmahera, eastern Indonesia, are petrologically and chemically similar to supra-subduction ophiolites and include boninitic volcanics resembling those dredged from the Marianas forearc. The age of the ophiolitic rocks is unknown; in east Halmahera they are overlain by Late Cretaceous and Eocene volcanics and associated sediments. Similar volcanics form the basement of western Halmahera. Plutonic rocks intruding the ophiolite and associated metamorphic rocks also yield Late Cretaceous to Eocene radiometric ages. The petrology and chemistry of the igneous rocks indicate an island arc origin. These rocks are locally overlain by shallow-water Eocene limestones and all are overlain unconformably by Neogene sediments. The Halmahera basement rocks have many structural, petrological, and stratigraphic similarities to submarine plateaus of the southern and northern Philippine Sea and basement terranes of the eastern Philippines. The authors suggest that these similarities indicate the existence of an extensive region of Late Cretaceous and Eocene volcanism built upon probable Mesozoic ophiolitic basement. The resultant thickened crust was later fragmented by spreading in the West Philippine Sea Central Basin and backarc spreading in the Eastern Philippine Sea. It is difficult to reconcile the present distribution of these crustal fragments with a linear arc, but equally more » difficult to propose a simple alternative. A proto-Philippine archipelago, with short-lived arcs separated by small oceanic basins, may be the closest modern analog. The development of younger subduction zones has been influenced by the distribution of thickened crustal fragments as they have re-amalgamated since the Miocene. « less

The structure of the Bracco Ophiolite complex (Ligurian Apennines, Italy): a change from Alpine to Apennine polarity

The Bracco ophiolite complex, exposed in the area studied as the Bracco Subunit of the Melange Nappe, is tectonically intercalated between the underlying Canetolo Nappe and the overlying Lavagna Nappe. Detailed structural analysis and mapping in these units reveals a multi-phase deformation history. In the Bracco Subunit and the Lavagna Nappe, this history is characterized by a change, from SW (i.e. ‘Alpine’) directed folding of the ophiolitic basement and its sedimentary cover, to NE (i.e. ‘Apennine’) directed folds and thrusts. The Canetolo Nappe only shows Apennine vergent structures. The Alpine vergent structures are considered to be developed during the Late Cretaceous to Palaeocene in an accretionary wedge associated with an east-dipping subduction zone. The first Apennine directed structures are low-angle extensional faults and cascading folds, indicating that the change to Apennine polarity resulted from gravitational collapse of the accretionary wedge during the Palaeocene to Eocene. Gravitational instability is considered to have resulted from a drastic decrease of subduction rate during the Palaeocene, which has led to extension and uplift of the wedge. The rapid decompression under isothermal to slightly decreasing temperature conditions of the high-pressure terrains in the Voltri Massif concurs well with this tectonic scenario.

A preliminary Neogene paleomagnetic data set from Leyte and its relation to motion on the Philippine fault

Leyte Island is bisected by the left-lateral Philippine fault. Paleomagnetic samples were collected from nineteen lava flows in Leyte and nearby islands to the north. The data collected from these sites were combined with previously reported results from northeastern Mindanao. The paleomagnetic data from Neogene rocks within or very near the Philippine fault zone on Leyte, Mindanao, Biliran, Maripipi, and Genuruan Islands fall into two populations. Seventeen late Neogene sites (fourteen normal polarity and three reversed polarity) yield a mean direction D = 358.9°, I = 20.1° ( α 95 = 6.6°, k = 30.1) and a paleomagnetic pole λ = 88.9° N, φ =12.5° E ( A 95 = 4.8°, K=56.1). Eight early Neogene sites (three normal polarity and five reversed polarity) give a mean direction D = 23.2°, I = 13.9° ( α 95 = 9.3°, k = 36.2) and a paleomagnetic pole λ = 66.5° N, φ = 220.5° E ( A 95 = 7.1°, K= 62.0). The late Neogene pole is indistinguishable from Plio-Pleistocene poles from the Philippines, Taiwan, and Vietnam, and the late Miocene pole of the central Philippines suggesting there have been no significant rotations or translations of this region with respect to other Southeast Asian regions during the past 5 M.y. The early Neogene paleomagnetic pole is statistically indistinguishable from the middle to early Miocene pole of the central Philippines. The early Neogene pole is statistically distinguishable from early Miocene pole of Marinduque Island (Philippines) as well as the late Miocene pole of the central Philippines and the Plio-Pleistocene pole of the entire Philippines. This preliminary paleomagnetic data set suggests that Leyte and adjacent islands to the north have been a portion of the same tectonic block as northeastern Mindanao, Cebu, and western Panay since early Neogene. There are also no paleomagnetically detectable fault-related rotations associated with the Philippine fault given the resolution of this data set. No consensus exists as to the amount of offset or the age of inception of the Philippine fault. The 110 km offset between ophiolitic basement in northeast Leyte and that in southwest Leyte suggests this amount of left-lateral displacement has occurred along the Philippine fault since the middle Tertiary. In contrast to data from continental regions, no shear-related rotations are found associated with the Philippine fault. This observation suggests that the hot, immature, transitional island arc/continental crust of the Philippine region deforms differently in response to shear than well developed continental crust.

Structural chronology, oroclinal deformation, and tectonic evolution of the southeastern Klamath Mountains, California

Devonian to Jurassic volcanic, sedimentary, and minor intrusive rocks of the Redding section (RS) accumulated in an intermittently active island arc setting. General stratigraphic concordancy and the downsection continuity of metamorphic facies series indicate that these rocks shared a common postdepositional geologic history. Burial metamorphism produced secondary mineral assemblages of the zeolite, prehnite‐pumpellyite, pumpellyite‐actinolite, and greenschist facies. Close correspondence between pressures calculated from existing stratigraphic overburden and those predicted from the observed metamorphic facies' series indicates that less than 3 km of strata have been eroded from the presently exposed top of the Early‐Middle Jurassic Potem Formation. Shortly after deposition of the Potem Formation, compressional deformation (F1) produced open to isoclinal folds, foliations, and small thrust faults. K‐Ar dates from post‐tectonic intrusions indicate that F1 deformation was completed before 168 Ma. This minimum age of F1 deformation is corroborated by a postfolding paleomagnetic overprint acquired at about 171 Ma. A second suite of intrusions dated at 120–130 Ma provided a thermal pulse that variably reset the K‐Ar system in F1 fabric‐defining celadonite and other relatively Ar‐unretentive materials. The Nevadan orogeny (Late Jurassic) did not significantly deform rocks of the RS, if at all, nor is evidence for Permo‐Triassic (Sonoman) deformation apparent. The geometry of F1 structures varies systematically with location: in the western Bollibokka Quadrangle (BQ), F1 folds are upright, with subvertical axial planes and associated foliations. In the eastern BQ, F1 folds are overturned to the east, with axial planes and associated foliations dipping as little as 40° to the west. Regional relationships suggest that F1 may have been a response to coeval tectonism in the western Klamaths, wherein crustal thickening during regional westward thrusting induced gravitational detachment of the RS from its partly serpentinized ophiolitic basement. F1 structures subsequently were broadly warped about vertical axes, producing variable paleomagnetic declination anomalies that range from 0 to greater than 100° clockwise. The timing and cause of this F2 deformation event are as yet relatively unconstrained but were almost certainly post‐Jurassic and may have been associated with development of the Columbia Embayment. Paleomagnetic data indicate North American paleolatitudes for the RS since at least the Permian, and apparently weigh against an allochthonous or “exotic” origin for this terrane. Paleontologic data indicate strong faunal similarities with North America and to a lesser extent with a Tethyan province. The significance of this apparent dual provinciality remains uncertain, but extreme endemism of Tethyan fusulinids, corals, brachiopods and pelecypods is questionable.

Late Palaeogene–Quaternary geology of Halmahera, Eastern Indonesia: initiation of a volcanic island arc

The Late Palaeogene–Quaternary stratigraphy of Halmahera is described, and new formation names are proposed, based on recent field investigations of the NE and central part of the island. This stratigraphic information provides new insights into the Neogene history of Halmahera and the development of the present island arc. The Late Palaeogene and younger rocks rest unconformably on an ophiolitic Basement Complex which formed part of a Late Cretaceous–Early Tertiary fore-arc. After volcanic arc activity ceased in the Eocene the former fore-arc terrane was uplifted and deeply eroded in the Late Palaeogene. Some of the Late Palaeogene–Early Miocene river valleys are currently being re-excavated by the present rivers. Slow subsidence began in the mid–late Oligocene and by the end of the Miocene all eastern Halmahera was the site of shallow-water carbonate deposition. There is no evidence for arc volcanism in central Halmahera at this time and the reported Oligo-Miocene volcanism in nearby regions is interpreted as volcanism related to the Sorong Fault system. The Miocene shallow water region subsided rapidly in the Early Pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites, with increasing amounts of calc-alkaline volcanic debris from a Pliocene volcanic arc built on the western arms of Halmahera, probably on the eroded Early Tertiary arc. This phase of rapid subsidence in the Pliocene back-arc region resulted from the initiation of subduction of the Molucca Sea lithosphere eastwards beneath Halmahera. Differential subsidence on NW–SE and NE–SW sets of faults in the region immediately behind the active arc led to the formation of deep sediment-filled basins adjacent to the eastern arms. A major deformation event in the Pleistocene resulted in folding and local thrusting at the junction between eastern and western Halmahera and volcanism ceased in the Pliocene arc. The third Halmahera arc, the Quaternary arc, currently active in the northern part of the islands, began activity within the last 1 Ma and is built upon the deformed and partly eroded Pliocene arc. The Pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward-dipping Molucca Sea plate with adjacent plates, either with a fragment of the Australian continent in the Sorong Fault zone and/or with the Philippine Sea plate beneath northern Halmahera.

Felsic igneous rocks within the 3.3‐ to 3.5‐Ga Barberton Greenstone Belt: High crustal level equivalents of the surrounding Tonalite‐Trondhjemite Terrain, emplaced during thrusting

Felsic igneous rocks within the circa 3.5‐Ga ophiolitic Onverwacht Group of the Barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession. These felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession. Our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic‐ultramafic rocks of the Onverwacht Group. Rather, the data indicate that these felsic units are high‐level equivalents of the widespread gneissic to isotropic trondhjemite‐tonalite plutons which surround and intrude the greenstone belt. The felsic igneous rocks are predominantly shallow‐level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the Onverwacht Group by 40–50 Ma. The felsic intrusions were preferentially emplaced along thrust zones, cutting the ophiolite basement and its overlying sedimentary cover. Structural and chemical data imply that the hydrated ophiolitic rocks of the Barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting. Such structural imbrication of simatic crust can be best incorporated in a model advocating Archean intraoceanic obduction associated with shallow‐angle subduction, similar to that documented for the neotectonics of the Western Aleutians.

Evolution of Subsidence Styles in Forearc Basin: Example from Cretaceous of Southern Vizcaino Peninsula, Baja California, Mexico: ABSTRACT

Late Jurassic to Early Cretaceous arc magmatism is represented by volcaniclastic rocks of the Eugenia Formation in the northern Vizcaino Peninsula and by the metamorphosed Cedros-San Andres volcanoplutonic complex, with a dismembered ophiolitic basement, in the southern peninsula. The Vizcaino Peninsula became the site of forearc sedimentation by the Aptian-Albian (late Early Cretaceous), when arc magmatism moved abruptly eastward to the present-day Peninsular Range. On the southern Vizcaino Peninsula, a conformable stratigraphic section, complicated by later faulting, records a gradual transition from a ridged forearc, broken by basement uplifts and grabens (the Aptian-Albian Asunction Formation), to a broadly subsiding, deep marine forearc basin (the Cenomanian Valle Formation). The basal contact of the Asunction formation has irregular relief caused by brecciated basement rocks and talus accumulated along fault zones. An upward-fining sequence several hundred meters thick records abrupt uplift and gradual denudation of adjacent metamorphic basement. Contemporaneous andesite arc volcanism to the east supplied ash and fresh volcanic detritus to the grabens. Angular sand to boulder-size detritus of the Asunction Formation was derived locally, and includes basic to intermediate meta-igneous rock fragments, with epidote, actinolite, and chlorite, as well as serpentine. Abundant calcareous fossils are commonly unbroken, suggesting local sources formore » these as well. Angular to subrounded, sand to cobble-sized, intermediate to mafic volcanic rock fragments were derived from a more distant island arc to the east, which occasionally provided intermediate to felsic tuffs to the basin. This source is probably represented by the Aptian-Albian Alisitos Group, which forms much of the western wall of the Late Cretaceous Peninsular Range batholith.« less

A transform margin in the Mesozoic Tethys: evidence from the Swiss Alps

Remnants of the Liguria-Piemont Ocean with its Jurassic ophiolitic basement are preserved in the South Pennine thrust nappes of eastern Switzerland. Analysis of South Pennine stratigraphy and comparison with sequences from the adjacent continental margin units suggest that South Pennine nappes are relics of a transform fault system. This interpretation is based on three arguments: (1) In the highly dismembered ophiolite suite preserved, Middle to Late Jurassic pelagic sediments are found in stratigraphic contact not only with pillow basalts but also with serpentinites indicating the occurrence of serpentinite protrusions along fracture zones. (2) Ophiolite breccias (»ophicalcites«) occurring along distinct zones within peridotite-serpentinite host rocks are comparable with breccias from present-day oceanic fracture zones. They originated from a combination of tectonic and sedimentary processes: i.e. the fragmentation of oceanic basement on the seafloor and the filling of a network of neptunian dikes by pelagic sediment with locally superimposed hydrothermal activity and gravitational collapse. (3) The overlying Middle to Late Jurassic radiolarian chert contains repeated intercalations of massflow conglomerates mainly comprising components of oceanic basement but clasts of acidic basement rocks and oolitic limestone also exist. This indicates a close proximity between continental and oceanic basement. The rugged morphology manifested in the mass-flow deposits intercalated with the radiolarites is draped by pelagic sediments of Early Cretaceous age.

Brooks Range and Eastern Alps: A Tectonic Comparison: ABSTRACT

A comparison of the tectonic evolution of the Brooks Range (BR) and the Eastern Alps (EA) reveals a remarkable parallelism. Both of these Mesozoic-Cenozoic orogenic belts are underlain by sialic crust formed in an earlier Paleozoic orogenic cycle. The old basement is revealed in major tectonic windows: the Tauern Fenster (EA) and the Doonerak Window-Schwatka Mountains (BR)--which are unconformably overlapped by transgressive, neritic marine clastic to carbonate successions--the Permo-Triassic through Hochstegenkalk sequence (EA), and the Kekiktuk-Kayak-Lisburne sequence (BR). These successions are passive-margin sequences that pass southward, in palinspastically restored cross sections, to synchronous deep-water facies deposited on ophiolitic basement--Bunderschiefer on T iassic-Jurassic ophiolites (EA) and Kuna facies or Etivluk sequence on upper Paleozoic ophiolites (BR). Onset of subduction-collision is marked by olistostromal facies--Cretaceous wildflysch (EA) and Jura-Cretaceous Okpikruak Formation (BR)--and the development of major flysch-molasse successions in the foreland basins of the collisional fold and thrust belts. Important major contrasts between these two mountain ranges reside in their colliding blocks and their post-orogenic histories. Alpine orogenesis was driven by continent-continent collision, closing out a young, narrow ocean, whereas Brooks Range deformation appears to have originated by arc-continent collision, closing out an older, broad (?)ocean. Younger Cenozoic deformation is extensional and strike-slip in the Eastern Alps, producing disjunctive basins, but Cenozoic deformation in the Brooks Range is diverse and includes compression in the east and extension in the far west. By means of numerous stratigraphic and structural analogs in the better known Eastern Alps, the comparison of these two mountain belts provides interpretive insight into the Brooks Range. End_of_Article - Last_Page 666------------

Facies History of the Upper Jurassic-Lower Cretaceous Great Valley Sequence: Response to Structural Development of an Outer-Arc Basin

ABSTRACT The facies history of the Great Valley sequence is closely linked to structural events in an arc-trench system. Upper Jurassic-Lower Cretaceous marine rocks, as much as 8,500 m thick along the western margin of the Sacramento Valley, northern California, were deposited on slopes and submarine fans that extended over an unstable basement floored by ophiolitic rocks. Paleocurrent and paleoslope analyses indicate that eastward subduction, initiated as early as Kimmeridgian, created an outer-arc ridge which bounded the west side of the basin. Debris was shed dominantly from the north and east, but significant contributions were ridge-derived. Sediment fabrics, structures, and bedding styles indicate that earliest deposition on basin plains and on eastward facing slopes was followed, at le st until the Valanginian, by a stable sedimentological framework of prograding and avulsing submarine fans characterized by broad, shallow midfan channels, depositional lobes, and abundant, thick deposits of interchannel mud. The Great Valley rocks are transected by northwest-trending faults that produced strike separation of the east-dipping strata. Smooth facies transitions, but with abrupt thickness variations across faults, suggest that faulting with some normal component of motion was contemporaneous with deposition during latest Jurassic and earliest Cretaceous. Later movement, either strike-slip or reverse, brought coarse, inner-fan facies that were deposited on Klamath basement into fault contact with basin-plain facies that were deposited on ophiolitic basement.

Isotopic geochemistry of burial-metamorphosed volcanogenic sediments, Great Valley sequence, northern California

Isotopic and mineralogic data from an 8500-m thick section of the Great Valley sequence, northern California, indicate that changes in the δ 18O values of authigenic minerals resulted from the conversion of smectite to a 10 Å clay-mineral as temperature increased with burial in the Jurassic- Cretaceous outer-arc basin. The clay-mineral assemblage in mudstone is characterized by a proportional increase of the 10 Å clay-mineral with increasing stratigraphic depth, and by a depletion in the δ 18O value of the mixed-layer smectite/10 Å clay-mineral with descending stratigraphic position from +21.9 to + 15.5%. SMOW. Modeling of the oxygen isotopic data from authigenic phases, based on equilibrium fractionation during clay-mineral diagenesis, indicates that δ 18O values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25°C/km during maximum burial to about 6–7 km. δD values of the mixed-layer smectite/10 Å clay-mineral range between −69 to −44%. SMOW. Using temperatures calculated from the oxygen isotopic data, the deuterium and oxygen isotopic data indicate that the smectite underwent late-stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of δ 18O ≈ +8%. SMOW and δD ≈ −25%. SMOW. The δ 13C values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved HCO t- 3 was continuously enriched in 13C as temperature increased with burial. At the base of the sequence and immediately overlying the ophiolitic basement rocks, several hundred meters of strata were altered by more oxygen-depleted (δ 18O ≅ +4 to +5%.) hydrothermal fluids emanating from the ophiolitic rocks, probably at maximum burial depth.

Accretionary Growth of Western North America: Examples from Alaska: ABSTRACT

It is now recognized that much of the Cordillera of western North America was assembled during Mesozoic time by accretion of allochthonous terranes. These terranes include: oceanic island arcs, scraps of ocean basins with ophiolitic basement, and fragments of continental margins, some with Precambrian basement, as well as composite terranes composed of two or more unlike elements joined together before accretion. Approximately 200 separate terranes have now been recognized in the Cordillera, each of which contains a distinctive stratigraphic record that differs strongly from that of its neighbor. Combined geologic, paleontologic, and physical (paleomagnetic) data substantiate that some of these terranes have been displaced thousands of kilometers northward from low paleol titudes. As a result of these movements, paleobiogeographic provinces have been disrupted, and nonindigenous faunal elements transported to North America. A similar pattern of accretionary terranes characterizes much of the circum-Pacific margin, including China, eastern Australia, New Zealand, and parts of Antarctica. The great number and diversity of the displaced terranes found throughout this region suggest that the paleogeography of Panthalassa was extremely complex. End_of_Article - Last_Page 2156------------

Stratigraphy and sedimentology of Jurassic bedded chert overlying ophiolites in the North Apennines, Italy

ABSTRACTIn the North Apennines of Italy, Upper Jurassic bedded chert stratigraphically overlies ophiolitic rocks and is overlain by Lower to Middle Cretaceous pelagic limestone and shale, and Upper Cretaceous flysch. The bedded chert, best exposed in East Liguria and on Elba, is typically 30–80 m thick, but occasionally reaches 150–200 m thickness. It consists of two main alternating lithologïes: siliceous mudstone (SM) and radiolarite (R).Chert sections commonly show characteristic stratigraphic changes. Lower cherts display a striking rhythmic alternation of R and ferruginous SM beds. In middle cherts, SM beds are much less ferruginous and shalier intercalations are locally present. In upper cherts, R beds are less frequent and SM beds are essentially non‐ferruginous.R beds are generally 1–4 cm thick, and consist of 80–90% quartz, 5–15% clays and usually < 1% hematite. They are commonly parallel‐laminated, and rarely size‐graded. In size‐graded beds, large radiolaria are more abundant near the bed base (commonly together with ophiolitic or SM clasts) and small radiolaria more abundant near the bed top. Sorting is poor throughout most R beds. R beds are interpreted as turbidites (cf. Nisbet & Price, 1974). Model calculations suggest that typical settling velocities of radiolaria during redeposition are < 1 cm sec−1, which is low and of restricted range relative to the 1–10 cm sec−1 settling velocities of clastic grains of comparable size range. Radiolaria therefore should have only a limited tendency to grade and sort during deposition from a turbulent current.SM beds are commonly 1–7 cm thick, although much thicker ones occur near the base of sections, and consist mainly of 50–70% quartz, 15–35% clays and 0–15% hematite. Microscopic clay‐silica aggregates and highly corroded remnants of radiolaria are common. SM beds are interpreted as mainly ambient pelagic sediment which accumulated slowly in topographic lows, and which was modified by near‐surface dissolution of biogenic silica. In SM beds which contain two texturally different layers, the lower one is interpreted as the top of the underlying radiolarian turbidite.North Apennine cherts represent the first sediment deposited on oceanic crust formed during the opening of the North Apennine part of the Tethys. The ophiolitic basement had a rugged topography which favoured the redeposition of siliceous sediment. Hematite and local Mn enrichments in SM beds in the lower chert sections represent hydrothermal precipitates inferred to have originated at a spreading axis. During seafloor spreading, accumulation of siliceous sediments progressively reduced the topography. Deposition of ophiolitic detritus within the sediments phased out during early chert sedimentation, and the hydrothermal contribution during early‐middle chert sedimentation. As local basins filled, during late chert sedimentation, radiolarian turbidites became less frequent.The first limestones at the top of chert sections are calcareous ooze turbidites derived from above the CCD and deposited slightly below it. Gradual descent of the CCD to ocean floor depths at the end of the Jurassic (Bosellini & Winterer, 1975) led to the replacement of siliceous by carbonate sedimentation.

The South Lake Igneous Complex, Newfoundland: a marginal basin – island arc association

The South Lake Igneous Complex, located south of Notre Dame Bay in the Exploits Zone of central Newfoundland, consists of ophiolitic (layered gabbro and sheeted dike) fragments intruded by a quartz–hornblende diorite and a later tonalite. All units are cut by swarms of basalt dikes. The sheeted dikes of the ophiolitic fragments are depleted in incompatible elements relative to mid-ocean ridge basalts, but to a lesser degree than the depleted lavas of the Bett's Cove Ophiolite in the Notre Dame Bay Zone to the west, and are interpreted as being components of oceanic crust formed in a back-arc basin. The intruding plutonic phases are rich in hydrous minerals and are severely depleted in incompatible elements, and are considered to be related to the formation of an island-arc volcanic pile upon the ophiolitic basement, with the rising magmas massively intruding and disrupting the basement rocks. The late dikes are enriched in incompatible elements and have a transitional tholeiitic–alkalic character. How much younger these dikes are than the rocks they intrude is unknown. Assuming that they are similar in age, an origin of these basalts from a subducted oceanic ridge is speculated.