Graben Fill Research Articles

Stratigraphic and structural evolution of the middle Miocene synvolcanic Oregon-Idaho graben

The Oregon-Idaho graben is a newly identified north-south–trending synvolcanic graben in southeastern Oregon and southwestern Idaho within the middle Miocene backarc rift system that extends 1100 km from southern Nevada to southeastern Washington. The graben formed along the western margin of the North American craton shortly after the largest volumes of tholeiitic flood basalt erupted (Columbia River Basalt Group, Steens-Pueblo Basalt, basalt of Malheur Gorge, basalt and latite unit of [Ekren et al., 1981][1]). Rhyolite flows and ash-flow tuffs (16.1–14.0 Ma) erupted from northeastern Oregon (Dooley Volcanics) to northern Nevada (McDermitt volcanic field) shortly after the flood basalt was emplaced. Subsidence of the Oregon-Idaho graben (15.5–15.3 Ma) coincides with eruption of rhyolite flows and caldera-related ash-flow tuffs from vents along the margins and within the graben. Mafic and silicic intragraben volcanism accompanied sedimentation from about 15.3 to 10.5 Ma. Sedimentary and volcanic rocks from extrabasinal sources, especially southwestern Idaho, were introduced periodically. After initial subsidence, the evolution of the Oregon-Idaho graben is divided into three stages. Stage 1 (15.3–14.3 Ma) followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben. Stage 2 (14.3–12.6 Ma) movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc-alkalic volcanism. Fine-grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli-fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence. Synsedimentary hot-spring alteration and precious-metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits. During stage 3 (12.6–10.5 Ma) the subbasins were filled, and graben-wide fluviatile and lacustrine sedimentation resumed. At about the same time, renewed rhyolitic volcanism occurred on both flanks, and tholeiitic volcanism resumed within the Oregon-Idaho graben. Subsidence in the Oregon-Idaho graben ceased as west-northwest–striking faults related to the formation of the western Snake River plain became active. [1]: #ref-28

Fracturing and fluid migration during Palaeogene compression and Neogene extension in the Catalan Coastal Ranges, Spain

The Catalan Coastal Ranges (NE part of the Eastern Iberian Margin) correspond to a system of grabens formed at the north‐western margin of the Valencia Trough. Extensional activity in the Catalan Coastal Ranges occurred at least from early to late Miocene and reactivated earlier transpressive faults related to the Palaeogene compression. In the central part of the Catalan Coastal Ranges, tectonic micro and macrostructures (faults, joints, stylolites) are well developed in the Mesozoic (mainly Cretaceous) limestones as well as in the Miocene graben fill deposits. In these rocks, seven generations of fractures, which formed during different tectonic phases, have been distinguished. Type 1–4 fractures affect only the Cretaceous limestones, type 5 and 6 fractures the Cretaceous and Miocene deposits, and type 7 fractures only the Miocene sediments. The fractures are filled with calcites, and locally with an internal sediment or a dolomite cement. The Cretaceous host‐rock has an average δ18O value of –4·3‰ PDB (Peedee Belemnite), an average δ13C value of +0·6‰ PDB, a 87Sr/86Sr ratio of 0·70741, up to 5630 p.p.m. of Mg2+, up to 2615 p.p.m. of Sr2+, and up to 1560 p.p.m. of Fe2+. Type 2 fractures are related to Palaeogene compression. The calcite cement filling this type of fracture has an average δ18O value of –8·2‰ PDB, an average δ13C value of –0·6‰ PDB, a 87Sr/86Sr ratio of 0·70714, up to 4560 p.p.m. of Mg2+, up to 3275 p.p.m. of Sr2+, and up to 3540 p.p.m. of Fe2+. These results indicate a fluid characterized by a high rock–fluid interaction approaching a closed system equilibrium.Type 5 fractures are related to the syn‐rift stage. The calcite cement filling this type of fracture has an average δ18O value of –6·9‰ PDB, an average δ13C value of –4·3‰ PDB, a 87Sr/86Sr ratio of 0·70787, up to 5375 p.p.m. of Mg2+, up to 1750 p.p.m. of Sr2+ and up to 2855 p.p.m. of Fe2+. These results indicate a fluid characterized by a low rock–fluid interaction and an open hydrogeological system. The cements filling the compressional fractures are characterized by undulose extinction, subgrain formation and deformed mechanical twin planes indicating formation under stressed conditions. In contrast, cements filling extensional fractures are characterized by translucent crystals with uniform extinction, indicating free growth not subjected to stress.

Crustal deformation of the Lhasa terrane, Tibet plateau from Project INDEPTH deep seismic reflection profiles

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep reflection data in the Yangbajain‐Damxung graben of southern Tibet yield evidence of magmatism and deformation beneath the southern Lhasa terrane and Yarlung‐Zangbo suture. Shallow reflections and low‐velocity first arrivals indicate a thin, Quaternary graben fill of generally less than a few hundred meters thickness. Underlying stratified reflections, extending to a maximum of about 11.5 km depth, likely originate from deformed Paleozoic‐Cenozoic supracrustal strata of the Lhasa terrane. A prominent, undulatory band of reflections within the crystalline basement extends beneath the length of the Yangbajain‐Damxung graben (depth ranges from ∼12 km to ∼18 km). This horizon has been interpreted to mark the top of a midcrustal partial‐melt zone underlying southern Tibet. The undulatory character of this horizon additionally suggests that it may be tectonically deformed. A somewhat deeper, subhorizontal, wide‐angle reflection extends southward beneath the outcrop of the Yarlung‐Zangbo suture, indicating that the suture is cut off or superposed by a younger structure at depth. Three gently north dipping reflections at 19 km, 24 km, and 27 km depth beneath the Gangdese batholith are suggestive of a midcrustal duplex and may mark the northward (downdip) extension of the late Oligocene‐early Miocene Gangdese thrust system. A prominent ∼40° north dipping reflection imaged in the deep crust beneath the Gangdese batholith, between 40 and 60 km depth, might mark the downdip expression of the Yarlung‐Zangbo suture or, alternatively, a younger reverse fault in the lower crust (or both). Viewed in aggregate, the reflection data are suggestive of moderate, postcollisional shortening of the upper crust of the Lhasa terrane, accompanied by melting of the middle crust. Although the data are permissive of wholesale underthrusting or fluid injection of Indian continental crust beneath the Lhasa terrane, they show no direct evidence for this having occurred.

Gypsum karstification induced subsidence: effects on alluvial systems and derived geohazards (Calatayud Graben, Iberian Range, Spain)

The karstification of the tertiary gypsum formations in the Calatayud Graben has given rise to synsedimentary and postsedimentary subsidence phenomena in both the Neogene sediments of the endorheic graben fill and in the Quaternary alluvial sediments deposited under exorheic conditions. In the so-called Maluenda and Perejiles areas, covering 4.4 and 12 km 2 respectively, the Neogene sedimentary units stratigraphically above the gypsum have subsided due to the karstification of the underlying evaporites. The maximum subsidence in both areas has reached at least 200 m. The Jalón-Jiloca-Perejiles alluvial system has been affected by karstic subsidence during its Quaternary evolution. Ten stepped alluvial levels (pediment-terrace) have been identified. As a consequence of synsedimentary karstic subsidence the deposit of an alluvial level can be locally thickened, changing in a short distance from less than 10 m to more than 100 m in thickness. Thus, the deposit of an individual terrace can be locally superimposed on the deposits of older terraces. The palaeokarst recorded in the sediments of the different alluvial levels shows how the karstic subsidence has controlled the dynamics of the sedimentary system. The alluvial karstic subsidence is currently active in the flood plains affecting fluvial channels, buildings, communication ways, irrigation ditches and the Calatayud gypsum scarp.

The Sub‐Balkan graben system of central Bulgaria

The Sub‐Balkan graben system in central Bulgaria forms the present northern boundary of the Aegean extensional region. This east‐trending graben system lies along the southern flank of the Stara Planina range and consists mainly of half‐grabens. The sedimentary fill in the grabens ranges in age from late Miocene to Recent and records the initiation and evolution of the graben system. The sedimentary fill in the grabens is oldest in the central graben and becomes progressively younger to the west and east, indicating a diachronous development of the grabens. Grabens are formed in the hangingwalls of south‐dipping low‐angle normal faults which have been displaced by younger higher angle normal faults along the foot of the Stara Planina. Hangingwall rocks have been complexly faulted and rotated such that some graben fill has been rotated down‐to‐the‐north. The Sredna Gora range south of the grabens is part of a complexly faulted and rotated hangingwall block bounded on the south by south‐dipping normal faults forming the northern boundary of the Thracian Basin. The Stara Planina range has been formed by uplift and rotation due to footwall unloading along the low‐angle normal faults and forms the northern margin of the graben system. Most of the topography of Bulgaria south of the Sub‐Balkan graben system is the result of late Miocene to Recent extensional processes linked to the Aegean region that have been superposed on convergent features and earlier extensional features that extend back to late Eocene time.

Geology and the Tectonics of the Gediz Graben

The Gediz Graben is a part of a Basin and Range type graben system of the western Anatolia. In this study, a 40 km lenght of the Gediz Graben between Sart Mustafa (Salihli) and Dereköy (Alaşehir) is examined and especially the stratigraphic and the structural characteristics of the region are discussed. The Salihli Alaşehir plane forms the infill of the Gediz Graben and in the center of the study area, the plane extends WNW-ESE direction. The northern and southern borders of the Gediz Graben lithologically, morphologically and structurally are dfferent. Along the southern border, the topography between the Salihli-Alaşehir plane and the basement rocks of the Menderes metamorphics is very rugged with high elevations. On the other hand, the northern border of the graben is characteristically nearly flat or it has a very undluting surface. The contrasting morphology between the northern and southern boundaries reflects directly the structural differences in these two areas. Along the southern border of graben, the Salihli group unconformably overlies the Menderes metamorphics and the granadioritic stocks that cut this metamorphic sucsession . The Salihli group is dominantly represented by coarse continental detritic rocks and is nearly 2000 m in thikness. Three different formations are separated in this group and they are named as the Acıder, Göbekli and Asartepe formations. The beds of the Salihli group generally dip southward againist the basement rocks and they are cut by a series of high-angle normal faults. In only one area along the southern border, Gastropoda fossils were found in the Göbekli formation and they yielded an age of the Early- Pliocene (Dasien). Along the northern border of the graben, The Adala group crops out and it is composed of river and lake sediments of upto 400 m in thickness. Two different formations are separated in this group and they are named as the Filiztepe and Mevlütlü formations. The Adala group rests uncformably above the Menderes metamorphics along a low-angle to flat erosional surface. There is no foult along that border, and the layers of the sedimentary rocks are either flat or dip with a low-angle. The Gediz Graben started from by a regional tensional forces and first the low-angle boundary fault (Karadut faults) was formed. This fault has characteristics of a detachment fault and in the fault zone cataclastic rocks were formed. the thickness of the cataclastic zone ranges from 10 to 60 m and the lithology of the rocks inside this zone depends upon the ubderlying Menderes Metamorphics. Away from the fault zone, there is a gradation from gauge breccia to mylonite-ultramylonite. On the other hand, the mylonitic rocks also grade into undeformed granodioritic or metamorphic rocks. Just after and during the movement og the detachment fault, along the southern border of the graben, a series of high angle normal faults were formed. These high-angle faults were active during rhe deposition of the graben fill and some of them are even presently active. However, along the northern border, only the Mevlütlü fault and the Pleistocene-Holocene Kula volcanism were formed by the tectonic activity. The Gediz Graben has been evolved as s half graben and seismicity along this graben is still going on.

Crustal structure and dynamics in the Rhine Graben and the Alpine foreland

SUMMARY The Rhine Graben rift stretches for 300 km from Basel to Frankfurt. The axis of maximum subsidence switches from west in the southern graben, to east in the northern graben. Subsidence in the south began in the upper Eocene and was interrupted in the mid-Miocene by a broad uplift of 1–1.5 km in the Vosges-Schwarzwald (Black Forest) region, whereas subsidence in the north proceeded continuously from the lower Oligocene to the present. A combined geophysical and geological interpretation of the Rhine Graben is presented in the form of a 3-D gravity model and 2-D flexural plate modelling from the Alps across the Alpine foreland to the Rhenish Massif. Modelling is based on crustal structure revealed by geologic, gravity and seismic data, including ECORS, DEKORP and EGT profiles. The 3-D gravity modelling supports a modest density contrast of 0.3 g cm−3 at the crust-mantle boundary. Since there is no evidence of a large-scale upper-mantle anomaly, a dense lower crust (2.9-3.0 g cm−3) is proposed as the cause of this modest density contrast. The presence of a dense, mafic, lower crust in south-west Germany is corroborated by lower-crustal xenoliths, elevated P-wave velocities (6.7-7.2 km s−1) and high Poisson's ratios (0.26-0.29). Residual gravity and magnetic data reveal strong crustal heterogeneities, particularly from the dense, magnetic Saxothuringian Terrane to the light, non-magnetic Moldanubian Terrane. The polarity of the asymmetric Rhine Graben reverses at this Variscan tectonic boundary. Light graben fill can account for nearly the entire 20–40 mGal, short-wavelength gravity minimum, but the presence of intrusive bodies ranging from granitic to mafic composition can also strongly influence the local gravity field. 2-D modelling of the flexure of the European lithosphere along seven 1000 km long profiles indicates a flexural bulge of 1–1.5 km in the Alpine foreland of Southern Germany, which corresponds to an ENE-trending Moho uplift of the same magnitude and to a band of free-air gravity highs. Longer flexural wavelengths in the east indicate increasing lithospheric rigidity and increasing elastic thickness towards the Bohemian Massif. The Vosges-Schwarzwald dome is interpreted as the intersection of the Oligocene Rhine Graben rift and associated rift flank uplift with the mid-Miocene flexural bulge.

Polyphase structural evolution of the southwestern Argentine Precordillera

The western and southwestern parts of the Argentine Precordillera display complex geometries which are not consistent with those of a typical high-level fold-and-thrust belt. They are the result of a polyphase structural evolution which spans the Early Paleozoic to Late Tertiary period. After an Early Paleozoic folding and shearing event under a greenschist facies metamorphism, uplift, erosion, and deposition of Late Carboniferous to Early Permian clastics were accompanied by extensional faulting. This was followed by a Permian folding and faulting event which led to a partial inversion of the Late Carboniferous-Early Permian graben fill. Permian to Triassic crustal extension was combined with block faulting and the deposition of a thick volcanic sequence. The subsequent Late Tertiary crustal shortening partly reactivated older fault lines. Excluding folds, a few thrusts, and reverse faults, the crustal shortening within the older blocks was accommodated by a dominant sinistral strike-slip faulting under a W-E compressive regime. Above a major décollement, the entire sequence of faulted and folded blocks was carried from west to east towards its present position. The regional situation indicates that this southern part of the orogen was transferred further to the east with respect to the central thin-skinned parts. The movements are interpreted to be related to an important thrust fault which obliquely cuts through the fold-and-thrust belt.

Inversion of the Broad Fourteens Basin (offshore Netherlands), a small-scale model investigation

The southern North Sea exhibits spectacular examples of graben inversion which have been extensively documented by oil industry exploration. The Broad Fourteens Basin, which is located at the southern limit of the Southern Permian Salt Basin, displays numerous structural patterns of positive inversion, which are related to the inversion, which are related to the presence and thickness of Zechstein salts. In the northern part of the Broad Fourteens Basin, the Zechstein salt is rather thick (> 500 m), allowing decollement of the Mesozoic series during both extension and compression. Salt is missing to the southern part of this graben, and both Mesozoic series and Palaeozoic ‘basement’ display similar patterns of deformatiqn. The mechanical significance of inversion patterns was investigated through laboratory experiments on brittleductile models. Models were built with sand and silicone putty that represent, respectively, the frictional behaviour of the Mesozoic cover as well as the Palaeozoic basement, and the viscous behaviour of a decollement layer, mainly composed of Zechstein salts, which is intercalated between them. The models were scaled to fit natural configurations observed in the Broad Fourteens Basin. All small-scale experiments were carried out in two steps: (1) graben formation with synkinematic sedimentation, and (2) compression at various angles of obliquity to the graben. Experimental results show that the structures generated by or reactivated during inversion are strongly dependent on: (a) the strength of the decollement layer, which is a function of the layer thickness and of the displacement velocity applied at the model boundaries; and (b) the strength of the sedimentary overburden, which is a function only of its thickness (for a given lithology). Reactivation of normal faults occurs when the angle between the compression and the direction of the normal faults is smaller than 45. The ductile layer, which can be a source for diapirs during extension, plays during compression an important rô1e in the thrusting of the graben fill onto the adjacent platforms.

Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore‐offshore large‐aperture seismic and gravity data. High velocities obtained within a highly reflective half‐graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide‐angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high‐velocity lower crust interpreted to result from syn‐rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half‐graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre‐rift Moho is probably within or above the high‐velocity lower crust, whereas the post‐rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

Analysis of conductive and convective heat transfer in a sedimentary basin, demonstrated for the Rheingraben

The identification and quantification of conductive and convective components in the heat transfer of a sedimentary basin is demonstrated for the Rheingraben. Three different methods of varying complexity as well as three independent data sets are employed: (1) energy budget considerations based on hydraulically perturbed thermal data from shallow boreholes (< 500 m), (2) 1-D vertical Peclet number analysis of thermal data from 22 deep boreholes (> 1000 m), and (3) 2-D finite difference modelling of the fully coupled fluid flow and heat transport equations on a vertical cross-section of the entire Rheingraben. Energy budget considerations yield a conductive basal heat flow density of 84 + 40/-10 mW m-2, and in good agreement with this Peclet number analysis, gives median values in the range 90 ± 35 mW m-2. In the first case, the basement is formed by low permeable, tertiary sediments at about 500 m depth, and in the second by the transition from the sedimentary graben fill to the crystalline basement at depths of between 2000 and 4000 m. It is shown how results from numerical modelling support the flow field assumptions made by methods (1) and (2), as well as the value of 80 ± 10 mW m-2 for average basal heat flow density entering the graben from below. Conversely, the Peclet number range Pe ≤ 1.2 inferred from method (2) can be applied for a (at least partial) calibration of the fully coupled hydrothermal model calculations. This technique is suggested as a potentially interesting thermal method for constraining regional-scale permeability. An interpretation of heat transport is presented that satisfies the experimentally established patterns of both temperature and heat flow density in the Rheingraben. Moreover, it is demonstrated that the thermal anomalies along the western rim of the graben (such as Pechelbronn, France or Landau, Germany) can be convincingly explained by a basin-wide, deep rooted E–W groundwater circulation that locally enhances a background basal heat flow density of about 80 mW m-1 on average by 50 per cent and at individual sites by as much as 120 per cent.

Middle Eocene dike swarms and their relation to contemporaneous plutonism, volcanism, core-complex mylonitization, and graben subsidence, Okanogan Highlands, Washington

Research Article| November 01, 1990 Middle Eocene dike swarms and their relation to contemporaneous plutonism, volcanism, core-complex mylonitization, and graben subsidence, Okanogan Highlands, Washington Grace A. McCarley Holder; Grace A. McCarley Holder 1Department of Geology, Washington State University, Pullman, Washington 99165 Search for other works by this author on: GSW Google Scholar R. Wade Holder; R. Wade Holder 2Department of Geology and Geography, Georgia Southern College, Statesboro, Georgia 30460 Search for other works by this author on: GSW Google Scholar Diane H. Carlson Diane H. Carlson 3Department of Geology, California State University, Sacramento, California 95819 Search for other works by this author on: GSW Google Scholar Author and Article Information Grace A. McCarley Holder 1Department of Geology, Washington State University, Pullman, Washington 99165 R. Wade Holder 2Department of Geology and Geography, Georgia Southern College, Statesboro, Georgia 30460 Diane H. Carlson 3Department of Geology, California State University, Sacramento, California 95819 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (11): 1082–1085. https://doi.org/10.1130/0091-7613(1990)018<1082:MEDSAT>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Grace A. McCarley Holder, R. Wade Holder, Diane H. Carlson; Middle Eocene dike swarms and their relation to contemporaneous plutonism, volcanism, core-complex mylonitization, and graben subsidence, Okanogan Highlands, Washington. Geology 1990;; 18 (11): 1082–1085. doi: https://doi.org/10.1130/0091-7613(1990)018<1082:MEDSAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sheeted porphyry dike swarms are concentrated along and parallel to the margins of the Republic and Keller grabens in northeast Washington. They were emplaced during graben subsidence and, in part, during mylonitization of the Kettle and Okanogan domes (core complexes) above the transition into the brittle deformation zone during Eocene regional extension. The swarms consist of older high-K biotite rhyolite and younger high-K hornblende dacite and andesite dikes. The biotite rhyoGte is correlative with the basal O'Brien Creek Formation now exposed in the grabens but has no documented plutonic equivalent in the adjacent Colville batholith. The hornblende dacite and andesite dikes (Scatter Creek Formation) are divided into (1) a low-Zr, low-K2O type that fed the lower part of the Sanpoil Volcanics and (2) a high-Zr, high-K2O type that fed the upper part of the Sanpoil Volcanics, respectively. The Scatter Creek Formation dikes are the hypabyssal equivalents of both the Devils Elbow intrusive suite and the earlier phases of the Herron Creek intrusive suite in the Colville batholith. The occurrence of the dike swarms both within and adjacent to grabens filled with comagmatic volcanic rocks demonstrates that the graben fill was deposited in situ during regional extension and is not an allochthonous fault block transported to its present location during regional compression. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Plate boundary tectonics, global sea-level changes and the development of the eastern South Island continental margin, New Zealand, Southwest Pacific: ‘The existence of the widely acclaimed Vail (sea-level) curves seems to be encouraging a circularity in arguments about local correlation … and is certainly discouraging attempts to investigate local departures from the so-called global standard’. (Miall, 1986, p.131)

The Great South Basin is a mid-Cretaceous continental margin rift related to the break-up of the New Zealand sector of Gondwanaland. Syn-rift Cretaceous-Paleogene sediment accumulated up to a thickness of 8 km, followed by post-rift subsidence of the northwest and southeast basin margins to form the Canterbury Basin and Campbell Plateau respectively. The sedimentary evolution of the eastern South Island continental margin is described within the context of four megasequences containing eight regionally recognizable seismic sequences: Syn-rift (Phases 1–2, non-marine graben fill), Post-rift (Phases 3–6, epeiric marine transgression), Paraconformity (phase 7, mid-Oligocene maximum transgression), and Syn-Orogenic (Phase 8, plate-boundary driven terrigenoclastic regression). The successive phases of this overall transgressive-regressive cycle can be equated with the natural and progressive development of the sedimentary sequence within known regional factors. The eastern South Island margin sequence forms an appropriate southern hemisphere test of the global sea-level curves of Vail et al. (1977) and Haq et al. (1986). Although eustatic sea-level cycles may have played some part in the differentiation of transgressive phases 3–7, the stratigraphic sequence is adequately interpreted in terms of intrinsic basin evolution within a local tectonic context.

Constraints on the thermal history of the Dead-Sea Graben as revealed by coal ranks in deep boreholes

Abstract Coal ranks have been studied by vitrinite reflectance measurements in three boreholes; Zuk Tamrur 1 and Zohar 8 on the Judea Desert plateau (adjacent to the graben margin) and Amiaz 1 on a down-faulted block at the southwest Dead-Sea Graben. In the plateau boreholes, time-temperature index (TTI) calculations reveal that the coalification profile probably evolved under a Late Cretaceous thermal event with a thermal gradient of 35°–38°C/km followed by gradual decay to the present level c. 20°C/km by Miocene time. In Amiaz 1, the highest coal rank encountered, 0.5% Ro, is marginal for independent TTI calculations, however, some constraints on thermal history can be inferred. Stratigraphic and structural relationships between the 0.5% Ro isoreflectance in Amiaz 1 and the plateau boreholes reveals that most of the coalification measured in Amiaz 1 pre dates the formation of the graben and that an additional 3.4 km of Miocene-Holocene graben fill barely affected the coalification. Cessation of coalification despite increasing burial indicates that the post-Miocene thermal gradient in the Amiaz block could not have exceeded 20°–23°C/km. This finding is consistent with present day heat flow and other geophysical information. Tectonic models for the evolution of the Dead Sea Graben requiring a high thermal regime are inconsistent with the presented data. This raises questions as to the mechanism involved in the formation of rhomb-shaped grabens in general, on the one hand and for the hypothesis of the “leaky” nature of the Dead Sea transform north of the Gulf of Elat, on the other.

Processes of allochthonous terrane evolution, Mindoro Island, Philippines

On Mindoro, two tectonostratigraphic terranes, the North Palawan block and the Mindoro block, are separated by a steeply dipping shear zone. The North Palawan block was rifted off southern China in the Oligocene, and comprises most of the shallow southern end of the South China Sea. The eastern part of this terrane, exposed on Mindoro, includes Jurassic and Paleogene strata, probable rift basalts of Oligocene age, and Oligo‐Miocene graben fill deposits. In the late Oligocene and early Miocene, the eastern North Palawan block underwent crustal stretching, with grabens oriented at a high angle to synchronous spreading anomalies in the South China Basin. We hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening South China Basin. The Mindoro block consists of a pre‐Upper Cretaceous low‐grade metamorphic basement, overlain by Upper Cretaceous and upper Eocene strata. The Mindoro block records several episodes of intense deformation of Mesozoic and Tertiary age, and contrasts with the North Palawan block both in terms of stratigraphy and structural evolution. The Mindoro Suture, which separates the Mindoro and North Palawan blocks, is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping, anastomosing faults. Deformed tectonic slices in the suture zone include serpentinized ultramafic rock, amphibolite, and rocks derived from both of the bounding terranes. We interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid‐Tertiary time. Paleogeographic considerations for early and middle Miocene time support the premise that the west side of the central Philippine archipelago was a left‐lateral transform boundary. The Mindoro and North Palawan blocks were initially juxtaposed along the transcurrent Mindoro Suture by late Miocene time. Subsequent deformation on Mindoro includes crustal shortening associated with the southern end of the Manila Trench, which projects onshore on southwest Mindoro. The onset of shortening here probably reflects a late Miocene change in Philippine‐Eurasian plate motion. The Mindoro Suture Zone is now locked, and it acts as a rigid buttress against which strata of the North Palawan block have been structurally stacked. The Mindoro Suture Zone is similar to other terrane boundaries in the northern Philippines. The history of terrane transport in this region was apparently dominated by strike‐parallel shuffling of terranes along major transcurrent boundaries within the Philippine arc. Transcurrent motion has taken place in extensional and compressional regimes, and it is ongoing today. This may be a dominant process by which complex island arc systems are disrupted, prior to accretion onto a continental margin.

Nature of the eastern boundary of the Rio Grande rift from COCORP surveys in the Albuquerque basin, New Mexico

Reprocessing and synthetic seismic modeling of COCORP profiles across the eastern margin of the Rio Grande rift at Abo Pass, New Mexico, suggest that synthetic and antithetic Cenozoic normal faults sole into or are truncated by a northwest dipping listric master fault which bounds the eastern side of the basin and reaches a depth of at least 10 km beneath the southern Albuquerque basin. This fault appears to project toward the surface position of the Los Pinos Cenozoic normal fault, which lies basinward of and trends parallel to a mapped Laramide duplex thrust zone. The Los Pinos fault may sole into the older thrust, reactivating this preexisting compressional structure at depth. A lateral change in reflection amplitude and frequency is conspicuous across the eastern rift margin. Analysis demonstrates that source coupling is the main cause of a much deteriorated signal below the rift basin, although complex ray paths and attenuation by the graben fill may contribute.

Biological marker characteristics of oils and asphalts from carbonate source rocks in a rapidly subsiding graben, Dead Sea, Israel

A detailed GC/MS study of biological marker compounds in the saturated and aromatic hydrocarbon fractions of oils and asphalts from the Dead Sea area, Israel, provided decisive information to the solution of a long-lasting controversy by showing that the asphalts are products of early generation in an immature stage from the same type of carbonate source rock which generated more mature oils. The asphalts are not biodegraded residues of the oils. Oils from six different wells, and asphalts from wells, outcrops, and a floating block from the Dead Sea all have very similar sterane and triterpane patterns. They all lack rearranged steranes (diasteranes) indicating a carbonate source matrix and compare reasonably well with a sample of Upper Cretaceous bituminous chalk from Nebi Musa. The main difference between the oils and the asphalts is a significantly higher triaromatic to mono- plus triaromatic steroid hydrocarbon ratio in the former. This is explained as a result of rapid subsidence and heating of their source rock close to the deep parts of the Dead Sea graben. The oils thus were generated in the more deeply buried source rock blocks under the graben fill, whereas the asphalts either originate from an immature source rock section closer to the graben rims or represent an earlier phase of generation and expulsion. This study also provides general information on the evolution of biological markers in carbonate source rocks. Low-activation-energy processes, like isomerisation of steranes, appear to occur much faster at low temperatures than in shales. The high sulfur content and less cross-linking of the biogenic organic matter into a complex kerogen structure are suggested to be responsible for this. Care should be taken when using only sterane isomerisation to assess the maturity of hydrocarbons from carbonate rocks and of carbonate-derived oils.

High-Resolution Seismic Data of Atlantic Margin Basement: ABSTRACT

Reflection seismic data for petroleum exploration have improved resolution of the rift structure of the United States Atlantic margin. Previously conjectural features are not clearly seen and additional features have been recognized. Our best data are from the Long Island-George's Bank area. Generally, the geologic elements visible, looking landward, are: (1) a belt of block-faulted oceanic crust that merges into a wedge of synrift sediments, (2) a basement ridge 10-15 km wide with maximum relief of 1.5 km, at a depth generally below 9 km, under the East Coast Magnetic Anomaly (ECMA), (3) a very wide zone of rising basement, structurally bland except for a major hinge near the updip edge of the ECMA, (4) an updip zone of complicated rift-stage faults with basinward-dippin listric faults, which visibly sole at ~14 km, and common listric counter-regional faults, and (5) a major unconformity, the base of the drift sequence, with rift-stage structures extensively pene-planed. Graben fill is 2-stage and includes earlier highly rotated beds separated by an unconformity from mildly deformed sediments. Downdip, the major unconformity at the base of the drift sequence becomes a strong, continuous reflector that downlaps the landward edge of oceanic crust. End_of_Article - Last_Page 271------------

Graben Hydrocarbon Occurrences and Structural Style

Major hydrocarbon occurrences, types of traps, and structural styles have been synthesized from the Sirte basin, the Suez and Viking grabens, and from other normal faulted regions. Hydrocarbons occur in a stacked succession of one or more basins: pregraben, graben fill, and interior sag. Preservation of pregraben accumulations depends on late initiation of crustal arching and limitation of uplift to the graben shoulders. Furthermore, a stable pregraben tectonic environment is required to ensure internal continuity of subsequent fault-block closures. Hydrocarbon traps in the pregraben and graben-fill sediments are primarily dependent on the multidirectional orientation of normal faults, tilting of fault blocks, and, in many cases, flexing or erosion parallel with block edges. The fault pattern is dominated by longitudinal faults subparallel with the graben axis and, secondarily, by oblique faults. The direction and degree of block rotation are influenced by fault profile (planar or listric), degree of extension, fault pattern (doglegs, junctions, terminations), downwarping of the subsequent sag basin, and isostatic adjustments between large blocks. Fold closures result from the upward termination of faults into forced folds, which may be accentuated by drag during renewed faulting. Passive drape and differential compaction extend he folds to shallower depths. Because they form along block edges, the flexures have trends that mimic the multidirectional fault patterns. Truncation traps are usually developed from the erosional retreat of fault scarps and they can also mimic the fault patterns. Where the sedimentary succession is complete, an interior sag basin is superimposed across the graben. Its lower part is structured mostly by drape and differential compaction across the underlying graben fault blocks and fault-block topography. The upper part is usually an unsegmented syncline, and hydrocarbon traps at this level include depositional buildups (reefs, turbidite mounds), and salt diapirs or other structures caused by superimposed deformation unrelated to graben tectonics.

The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia

A new type of stratabound sediment-hosted ore deposit. The deposit is estimated to contain in excess of 2,000 million metric tons of mineralized material with an average grade of 1.6 percent copper, 0.06 percent uranium oxide, and 0.6 g/metric ton gold. Proterozoic. Graben fill sediments were deposited in an arid subaerial environment during rifting or strike-slip faulting. The stratabound sulfide mineralization is syngenetic or syndiagenetic and is probably related to local volcanism. Uranium and rare earths were deposited during and after the sulfide mineralizing phase. Unusual features are the association of copper, uranium, rare earths, and gold; the association of reduced sulfur species such as chalcocite and bornite with high hematite concentrations; and the occurrence of stratabound copper and uranium minerals in sedimentary rocks deposited in a very high-energy environment.--Modified journal abstract.