Graben Segment Research Articles

Architecture and Kinematics of the Constance‐Frick Trough (Northern Switzerland): Implications for the Formation of Post‐Variscan Basins in the Foreland of the Alps and Scenarios of Their Neogene Reactivation

AbstractPost‐Variscan basins are commonly inferred to represent important precursor structures of later extensional and contractional tectonic events in the foreland of the European Alps. Nevertheless, their structural characteristics are typically poorly constrained. Our investigation sheds new light on the architecture and kinematics of the Constance‐Frick Trough (CFT) in Northern Switzerland. According to our analysis of an extensive and revised 2‐D reflection seismic data set, the ENE‐WSW striking CFT is divided into an eastern segment and a western segment characterized by opposing half‐graben geometries. The transition between the two segments coincides with the prolongation of a preexisting WNW‐ESE trending strike‐slip fault zone exposed in the adjacent Black Forest Massif to the north. Field‐based analysis of outcrop‐scale faults in the latter area implies that the minimum horizontal stress during the formation of the CFT was roughly oriented NE‐SW, which is oblique to the trough axis. While previously published analogue models imply that some characteristics of the CFT could be explained by lateral linkage of graben segments under oblique extension, the pronounced half‐graben geometry of the CFT suggests that low‐angle normal faulting may also have contributed to trough formation. The observed structural complexities of the CFT and uncertainties regarding its deep structure must be taken into account for interpretations of similar Late Paleozoic basins inferred in the subsurface of the northern Alpine foreland basin, in particular, concerning scenarios of their Neogene reactivation during Alpine orogeny.

3D numerical modelling of graben interaction and linkage: a case study of the Canyonlands grabens, Utah

AbstractGraben systems in extensional settings tend to be segmented with evidence of segment interaction. To gain a better understanding of the evolution of structures formed during graben growth and interaction, we here study the Grabens area of Canyonlands National Park, Utah, where a wide range of such structures is well exposed. With the aid of 3D numerical models, we attempt to reproduce structures observed in that region and to understand controls on the structural style of graben interaction by varying the spacing between pre‐existing structures. The sensitivity of the system to the thickness of the salt layer is also tested. Four distinct types of structures are observed when the spacing between inherited weak zones is varied: (1) grabens connecting in a relay zone divided by a narrow central horst; (2) graben segments interacting via a secondary stepover graben; (3) grabens propagating alongside each other with limited segment interaction; and (4) an abandoned graben segment in a system of multiple competing grabens. The presence of a basal salt layer (Paradox Member) promotes efficient graben propagation. A comparison between the observed structures and the numerical model results indicates that the detachment salt layer is relatively thin in the study area.

Neotectonics and paleoseismicity of the lower Katun’ valley (Gorny Altai)

The lower Katun’ area has a complex neotectonic framework, with the largest fault zone of Katun’ consisting of several en-echelon graben segments. Late Pleistocene sediments that fill the Katun’ Fault bear signature of earthquake-induced soft-sediment deformation (seismites). Deformation due to seismic triggers can be discriminated from nonseismic one on the basis of special features and be related to prehistoric earthquakes according to a number of criteria. The observed deformation inside and outside burial mounds of the Chultukov Log-1 group may result from an earthquake that occurred in the end of the first millennium BC. Fault scarps in Late Pleistocene sediments, as well as deformed Iron Age tomb patterns, indicate that the Katun’ lower reaches can have experienced past earthquakes of intensity at least 5 or 6 and magnitudes from 4.5 to 6.0.

Three-dimensional numerical modeling of upper crustal extensional systems

[1] We focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models. To date, few 3-D models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones. We use an arbitrary Lagrangian-Eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions. Plastic materials weaken with accumulating strain. To localize deformation, a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions. Controls on the geometry and spacing of three-dimensional plastic shear zones are investigated. The sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested. The model results indicate the primary controls of strain-dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction. The amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other, resulting in a trade-off effect between the amount of offset between the initial grabens and the strain weakening ratio. The three-dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions: (1) small-offset grabens with a single relay zone, (2) intermediate-offset grabens with one or more secondary step-over graben segments, and (3) large-offset grabens with limited to no significant segment interaction.

Evidence for intrusive activity on Mercury from the first MESSENGER flyby

Images from MESSENGER's first flyby of Mercury have shown convincing evidence for surface volcanism. Here we report on evidence in the new data for several features that are characterized by fractures and graben — rare features on a planet dominated by contractional deformation — that may be linked to intrusive activity. These features include: (1) A floor-fractured crater, interpreted to have been the site of laccolith-like sill intrusions; the feature is similar to some floor-fractured craters on the Moon and shows evidence for individual fractured dome-like uplifts on the floor. (2) A concentric complex of graben, observed inside the peak ring on the floor of the ~ 250-km-diameter Raditladi basin and associated with dark plains and possibly embayed by them; the feature may represent an unusual type of floor-fracturing associated with deeper intrusions and related ring dikes or cone sheets, or the graben may instead be the product of non-magmatic uplift of the basin floor. (3) A large radial graben swarm, Pantheon Fossae, located near the center of the Caloris basin, thus far unique on Mercury, and characterized by hundreds of individual graben segments ranging from ~ 5 km to ~ 110 km in length. In the nexus, graben crosscut one another and produce a local polygonal pattern; others curve away from the center as the nexus is approached. Two scales of graben length are observed; the radius of the dense radially symmetric plexus of graben is ~ 175 km, and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor. Two width scales of graben are observed; a large graben about 8 km wide emerges from the nexus and extends for ~ 100 km; most graben are less than half this width. Some graben walls appear cuspate, with convex-outward wall segments that resemble crater chain segments. One crater chain with distinctive raised rims parallels nearby graben. Locally, some graben appear in en echelon patterns, and smaller graben sometimes show cross-cutting (superposition) relationships. Abundant impact craters, the most prominent being Apollodorus, and secondary crater clusters and chains are superposed on the graben system; there is little evidence that craters greater than 5 km in diameter have been cut by a graben. This relation implies that the graben swarm formed soon after the emplacement of the Caloris floor plains. These graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir. Similar features, in which the dikes contribute to a near-surface stress field that favors radial graben, are known on the Earth, Venus, and Mars. The location of Pantheon Fossae in the center of the Caloris basin suggests that formation of the radial graben structure is linked to basin evolution.

Multiple fault reactivations within the intra-continental Rhine–Bresse Transfer Zone (La Serre Horst, eastern France)

Thermochronological, structural and kinematic analyses demonstrate the influence of Late Paleozoic basement structures on the localization and evolution of the Rhine–Bresse Transfer Zone (RBTZ), a central segment of the European Cenozoic Rift System. Zircon and apatite fission track data indicate that the Paleozoic crystalline basement and its Permo-Triassic cover, exposed in the La Serre Horst, experienced a Middle Jurassic to Early Cretaceous heating event which was followed by two distinct periods of cooling. A first cooling event occurred in the Early Cretaceous and is probably related to thermal relaxation. A second one, started at around 38 Ma, is interpreted to reflect exhumation that was induced by fault reactivation along the Late Paleozoic Burgundy Trough System during Eocene–Oligocene rifting and the formation of the RBTZ. This fault reactivation resulted in a complex fault pattern and local stress field perturbations in the surroundings of the pre-existing horst structure. Substantial extension was achieved by ENE–WSW striking normal faults that trend highly oblique to the dominant strike of the Rhine and Bresse Graben rifts. Seismic reflection data show that these major normal faults are typically associated with extensional flexures and oriented parallel to pre-existing Late Paleozoic basement faults of the Burgundy Trough. Paleostress analyses yield overall NW–SE extension during the Eo-Oligocene, slightly oblique to the structural trend of the RBTZ, and additionally, local stress field perturbations induced by the reactivation of pre-existing faults. Reactivation of Paleozoic structures in the RBTZ is of extensional rather than of strike-slip character indicating that the RBTZ is forming a separate oblique graben segment within the European Cenozoic Rift System rather than a sinistral strike slip transform zone supposedly connecting Rhine and Bresse graben structures.

The role of inherited extensional fault segmentation and linkage in contractional orogenesis: a reconstruction of Lower Cretaceous inverted rift basins in the Eastern Cordillera of Colombia

ABSTRACTLower Cretaceous early syn‐rift facies along the eastern flank of the Eastern Cordillera of Colombia, their provenance, and structural context, reveal the complex interactions between Cretaceous extension, spatio‐temporal trends in associated sedimentation, and subsequent inversion of the Cretaceous Guatiquía paleo‐rift. South of 4°30′N lat, early syn‐rift alluvial sequences in former extensional footwall areas were contemporaneous with fan‐delta deposits in shallow marine environments in adjacent hanging‐wall areas. In general, footwall erosion was more pronounced in the southern part of the paleorift. In contrast, early syn‐rift sequences in former footwall areas in the northern rift sectors mainly comprise shallow marine supratidal sabkha to intertidal strata, whereas hanging‐wall units display rapid transitions to open‐sea shales. In comparison with the southern paleo‐rift sector, fan‐delta deposits in the north are scarce, and provenance suggests negligible footwall erosion. The southern graben segment had longer, and less numerous normal faults, whereas the northern graben segment was characterized by shorter, rectilinear faults. To the east, the graben system was bounded by major basin‐margin faults with protracted activity and greater throw as compared with intrabasinal faults to the west. Intrabasinal structures grew through segment linkage and probably interacted kinematically with basin‐margin faults. Basin‐margin faults constitute a coherent fault system that was conditioned by pre‐existing basement fabrics. Structural mapping, analysis of present‐day topography, and balanced cross sections indicate that positive inversion of extensional structures was focused along basin‐bounding faults, whereas intrabasinal faults remained unaffected and were passively transported by motion along the basin‐bounding faults. Thus, zones of maximum subsidence in extension accommodated maximum elevation in contraction, and former topographic highs remained as elevated areas. This documents the role of basin‐bounding faults as multiphased, long‐lived features conditioned by basement discontinuities. Inversion of basin‐bounding faults was more efficient in the southern than in the northern graben segment, possibly documenting the inheritance and pivotal role of fault‐displacement gradients. Our observations highlight similarities between inversion features in orogenic belts and intra‐plate basins, emphasizing the importance of the observed phenomena as predictive tools in the spatiotemporal analysis of inversion histories in orogens, as well as in hydrocarbon and mineral deposits exploration.

Short-term episodes of carbonate productivity in a Cambrian uplifted rift shoulder of the Coastal Meseta, Morocco

This paper presents an analysis of the spatiotemporal distribution and composition of a volcanosedimentary complex, which has recorded a significant episode of carbonate productivity embedded in a thick and monotonous, turbiditic-dominated rift succession. The studied nucleation of a microbial–skeletal carbonate factory is middle Cambrian in age and situated at Sidi-Saïd-Maâchou, an uplifted rift shoulder of the Moroccan Coastal Meseta located at the junction of several graben segments, where an alkaline basaltic magma was extruded along fissures or intruded as sills and dikes. The geometry and facies of the skeletal calcarenite sheets, composed of hiatal shelled accumulations separated by microbial mats, is comparable to those of the Brèche à Micmacca recorded across the lower-middle Cambrian transition in the Anti-Atlas and High Atlas. The patchy and diachronous distribution of these ‘anomalous’ carbonate factories, spatially and temporally associated with volcanic episodes, marks the northward migration of active rifting structures throughout the Moroccan margin of West Gondwana.

Tectonic implications of transpression and transtension: Upper Rhine Graben

Seismic reflection data from the Upper Rhine Graben (URG) show a series of synclines along the linear eastern boundary faults from north of Karlsruhe to the Jura Mountains. Similar structures do not characterize the northern segment of the graben. The synclinal shape is portrayed by inversion related to transpression on the linear boundary faults, coupled with the conserved easterly dip of the sediments farther inside the tilted hanging block. The onset of transpression preceded the known early Burdigalian unconformity but was also an early Miocene event. It suggests that the regional compression axis (σ1) in early Miocene had a trend between 315° and 0°. The continuous transtension in the northern segment suggests that at no time during the history of the URG was the regional σ1 horizontal with a NE‐SW trend. The lack of transpressional structures in the younger Plio‐Pleistocene sediments along the entire eastern boundary of the URG suggests a late change in the regional stress field. The main eastern boundary faults and synclines north and south of Baden‐Baden are all continuous and have the same trend. Kinematically, they form a single graben segment and respond to the regional stress in the same way.

Volcanic rifting at Martian grabens

A large fraction of surface extension on Mars occurred at segmented grabens having width/length ratios akin to oceanic rifts on Earth. Association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes. A Martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery, high‐resolution digital elevation models (DEMs), scaled experimental modeling and three‐dimensional boundary element modeling of magmatic and tectonic processes, and a comparison with terrestrial rifts. The DEMs were obtained from Mars Observer Laser Altimeter, Viking Orbiter stereo images, or a combination of both. Comparison of terrestrial rifts included Afar, Iceland, and the East Pacific Rise. The ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly, resulting in emplacement of elongated magma reservoirs along the grabens. Injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of MPa and induces surface collapse. Each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces. The geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width. We conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic Martian grabens on the basis one graben‐one dike. Rather, every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the Icelandic fissure zones.

Basement structure in the southern North Sea, offshore Denmark, based on seismic interpretation

Abstract Seismic reflection data from the Danish North Sea are interpreted to map the structure of the Palaeozoic basement in the area of the MONA LISA deep seismic lines. Based on a characteristic near-basement reflection, the upper crystalline crust of Baltica of offshore Denmark is traced to the south into the southern Horn-Graben, and to the west to the eastern shoulder of the Central Graben. A two-way-traveltime map of the near-basement horizon and several interpreted seismic sections reveal that three main tectonic events influenced the topography of the basement: (1) a compressional event which could be Caledonian in age; (2) a Palaeozoic extensional event postdating the compressional deformation and expressed in a system of WSW-ESE to W-E striking Palaeozoic half-grabens; and (3) the Permo-Triassic rifting that led to the formation of NNW-SSE to NNE-SSW trending Mesozoic faults of the Horn Graben and the Central Graben which are oriented sub-perpendicular to the Palaeozoic system. Compressive deformation is localized in a narrow zone around and south of the hitherto interpreted Caledonian Deformation Front and foreland deformation on Baltica is suggested as its origin. The seismic image of the Palaeozoic halfgrabens indicates that the East North Sea High is an inverted Palaeozoic rift which subsequently was cut by younger Late Palaeozoic to Mesozoic rifts of the Horn and Central Grabens. The timing of the first extensional phase remains speculative, but it predates the Rotliegend unconformity. Some of the older Palaeozoic normal faults may have been reactivated as transfer zones between the different graben segments during the Permo-Mesozoic extension.

Upper Rhine Graben: Role of preexisting structures during rift evolution

The evolution of the Cenozoic Upper Rhine Graben was controlled by a repeatedly changing stress field and the reactivation of a complex set of crustal discontinuities that had come into evidence during Permo‐Carboniferous times. A comparison of the spatial and temporal thickness distribution of synrift deposits with preexisting fault patterns permits to infer a sequence of distinct basin subsidence phases that can be related to changes in the ambient stress field. Reactivation of a system of late Palaeozoic fault systems, outlining troughs and highs, controlled the nucleation of initially separated middle and late Eocene basins, the depocenters of which coincided with a preexisting WSW‐ENE trend. During Oligocene crustal extension the individual basins coalesced, resulting in the development of the SSW‐NNE striking Upper Rhine Graben. During the late Oligocene (Chattian) change in stress field, the Upper Rhine Graben was probably reactivated as a dextral strike‐slip system with the central graben segment forming a releasing bend. During the early Miocene (Aquitanian), a major reorientation of the regional stress field is held responsible for the main subsidence phase of the northern parts of the Upper Rhine Graben. This is reflected by a counterclockwise rotation and northeastward shift of the depocenter axis and later by the middle Miocene uplift and erosion of the southern parts of the Upper Rhine Graben. During the Plio‐Quaternary, the Upper Rhine Graben was reactivated as a sinistral strike‐slip system with the central graben segment forming a restraining bend.

Pre-Zechstein structures around the MONA LISA deep seismic lines in the southern Horn Graben area

Seismic reflection data from the Horn Graben area in the southeastern part of the North Sea, off-shore Denmark, have been interpreted to illustrate the upper crustal structures around the MONA LISA deep seismic lines. The study area comprises the southern Horn Graben area and the eastern part of East North Sea High, where the Caledonian collision suture between Baltica and Eastern Avalonia bends such that the strike direction changes from ESE in the south to NNW in the north. Integrated interpretation of normal-incidence reflection data and wide-angle refraction data reveals substantial occurrences of lower and upper Palaeozoic strata in the area, thickest below the Horn Graben. This may indicate that Horn Graben developed as a graben structure during late Palaeozoic in the former Caledonian foredeep. On the northern and eastern parts of the MONA LISA deep seismic reflection lines 1 and 3, the main E- dipping boundary fault of the southern Horn Graben segment appears to be listric at depth with a sub-horizon-tal detachment at the top of the reflective lower crust. We have mapped the lateral extent of the lower Permian, volcanic Rotliegend reflector in the study area on the basis of seismic lines from the RTD-81 survey. Dipping reflections observed in the sedimentary strata below the Rotliegend reflector are interpreted as Cal-edonian structures generated by folding and deformation in Lower Palaeozoic Baltica shelf sediments in the Caledonian foreland basin. A sequence of S- and W-dipping reflections above 4 s twt are interpreted as preserved Caledonian thrusts in the upper crustal frontal part of the SW-dipping Caledonian Deformation Front.

Interaction of Permo-Triassic and Jurassic extensional fault-blocks during the development of the northern North Sea

The main structural elements resulting from Late Permian-Early Triassic extension have been resolved across the total width of the northern North Sea. The northern segment of the basin (north of 61°N), bounded to the west by the Hutton Fault and to the east by the Norwegian mainland, was occupied by four westerly tilted half grabens, 25–35 km in width, prior to Jurassic extension. The central segment of the basin (60°20′–61°N) exhibits a full graben beneath the Jurassic Horda Platform, flanked by half grabens tilted towards the basin margins. The change in Permo-Triassic fault-block configuration, defining structural domains, is largely constrained by a basement discontinuity which runs diagonally (NE-SW) across the basin. Maximum Jurassic extension is represented by several centres of subsidence, aligned NNE-SSW across a N-S-oriented and broader Permo-Triassic basin. Jurassic extension was concentrated within the Viking Graben proper south of 60°N, which in this area is represented by single westerly-tilted, en échelon half graben segments 20–25 km wide. The area affected by Jurassic extension becomes progressively wider to the north, again governed by the basement discontinuity, eventually encompassing the total width of the basin ( c. 200 km) north of 61°N. The central segment of the northern North Sea displays the greatest contrast in Permo-Triassic and Jurassic structural styles and basin geometry. Jurassic stretching and faulting was negligible to the east (Horda Platform), precisely the area which had been the site of maximum Permo-Triassic stretching and fault activity. It has generally been assumed that Jurassic fault configuration along the eastern margin of the central segment of the Viking Graben, to a large extent reflects the reactivation of major Permo-Triassic faults. This study demonstrates the interference of Permo-Triassic and Jurassic basin geometries in this area, due to major faults of the two generations dipping in opposite directions. As a consequence, the E-tilted Oseberg Fault Block of Jurassic origin and c. 20km in width, is underlain by a wider ( c. 35 km) and W-tilted Permo-Triassic fault-block.

Further constraints on the temporal evolution of the Oslo Rift from precise U-Pb zircon dating in the Siljan-Skrim area

Precise U-Pb ages have been obtained from zircon and baddeleyite in plutonic rocks from the Siljan-Skrim area in the Vestfold Graben Segment in the south-western part of the Permian Oslo Rift. In addition, common lead data from feldspars in the same rocks are presented. The measured common Pb compositions of the feldspars are used to correct for initial common Pb in the zircons. The common Pb data indicate that U has been present in the feldspars since the time of crystallisation, probably in a late magmatic volatile phase in fluid inclusions. The Siljan-Skrim area comprises two intrusive complexes. The northern Skrim-Mykle complex consists of two large, composite intrusive bodies, the Skrim larvikite and the Mykle ekerite. The southern Siljan-Hvarnes complex consists of a series of monzonitic to syenitic intrusions, which are nested in a systematic way, indicating decreasing emplacement age from east to west. U-Pb zircon ages range from 281.2 ± 0.6 Ma (2σ) to 279.8 ± 0.7 Ma for three samples from the northern part of the Siljan-Skrim area and from 278.5 ± 0.8 Ma to 277.3 ± 0.8 Ma for four samples from the southern part of the area. The geochronological data suggest that large amounts of magma were emplaced within very short time spans, i.e. less than 2 Ma for each of the two complexes. The high precision of the U-Pb age results allows discrimination between intrusive activity in the northern and southern parts of the Siljan-Skrim area and suggests that emplacement of magmas was episodic.

Architecture and early evolution of the Oslo Rift

A revised assessment of architecture and pre-rift fabric connections of the Oslo Rift has been undertaken and linked to a new appraisal of observations and data related to the initial phase of the rift evolution. In addition to half-graben segmentation, accommodation zones and transfer faults are readily identified in the linking sectors between the two main grabens and between graben segments. Axial flexures are proposed between facing half-grabens. The accommodation zones were generally sites of volcanism during rifting. Pre-rift tectonic structures played an influential role in the rift location and development. The deviant N-S axis of the Vestfold graben segment is viewed as related to pre-rift structural control through faults and shear zones. This area was probably a site of Proterozoic/Palaeozoic crustal and lithospheric attenuation. Field evidence suggests that the rift started as a crustal sag with no apparent surface faulting in a flat and low-lying land at a time about 305–310 Ma. Volcanism, sub-surface sill intrusion and faulting started about simultaneously some time after the initial sag (300–305 Ma). Faulting and basaltic volcanism were initially localized to transfer faults along accommodation zones and a NNW-SSE transtensional zone along the eastern margin of the incipient Vestfold graben segment. This transtensional zone was probably created by right-lateral simple shear tracing pre-rift structures in response to a regional stress field with the tensional axis normal and the maximum compressional axis parallel to the NNE-SSW-trending rift axis.

Simple-shear deformation of the Skagerrak lithosphere during the formation of the Oslo Rift

Simple-shear deformation of the entire lithosphere has been postulated by Wernicke (1985) and others, but up to now unequivocal seismic evidence in support of this hypotheses has been lacking. Here we describe well-defined seismic reflectors below the Skagerrak Sea, one of which is interpreted as a low-angle fault underlying the Skagerrak Graben segment of the Permian Oslo Rift. This reflective lineament can be traced from the mid-crust through the lower crust, offsetting Moho and continuing downwards to ca. 50 km depth (16 s). A separate mantle reflection beneath the graben may be associated with an earlier period of thrusting. The 1730 km of deep seismic reflection data in Skagerrak indicate that the crust and mantle inherited a pronounced structural fabric from the Proterozoic Grenvillian-Sveconorwegian orogeny. During formation of the Oslo Rift, reactivation of these implied weak zones as localized detachment planes would explain the extensional deformation style of the non-magmatic Skagerrak Graben.

Crustal extension in the Oslo Graben, SE Norway: a method incorporating magmatism and erosion

Three methods for determining crustal extension were examined for use in the Late Paleozoic Oslo Graben of southeast Norway: 1. (1) analysis of faults and dikes, 2. (2) volume budget/isostasy, 3. (3) analysis of crustal thickness distribution assuming constant crustal volume during extension. These methods were found not to be suitable mainly due to rift age and magmatism. Instead, a new integrated model which combines the effect of Moho contours, volumes of mantle injecta and erosion, was used. Volumes of intracrustal mantle injecta are found from a model combining data on upper crustal felsic magmatic rocks with petrological and gravity inversion models. For the Vestfold Graben Segment in the south this implies twice the stretching estimate as compared to stretching derived from Moho contours alone, and two to three times the estimate from an (inadequate) set of fault and dike information. The batholiths mark zones of high crustal thinning, and magmatism was most likely caused by a combination of a thermal anomaly and lithospheric stretching.

Metallogeny and lead isotope data from the Oslo Paleorift

The Permian Oslo Paleorift, situated at the southwestern margin of the Baltic shield, includes the Oslo Region and its southward extension under Skagerak. The exposed part of the paleorift comprises two half-grabens (Vestfold and Akershus graben segments) filled with Permian sediments, volcanics and intrusive rocks as well as Cambrc-Silurian sedimentary rocks. The evolution of the Oslo Rift can be divided into a rifting stage, a caldera stage and a batholith stage. The rifting stage is characterized by weak mineralization of native copper in basalts. The caldera stage includes Fe-Ti cumulates in subvolcanic layered alkali gabbros, Nb-REE disseminations in aphyric trachytes, and epigenetic Mo-mineralizations. The batholith stage includes porphyry and intraplutonic vein deposits of Mo and contact metasomatic deposits of Fe, W, Mo, Be, Zn, Pb, Cu, Bi, As and F. Along the rift margin in Precambrian gneisses and amphibolites there are vein deposits of Fe-oxides, base metals and native silver. The relationship between the rift margin deposits and the evolution of the rift is still unclear, but many deposits are spatially related to Permian dolerite dykes. Lead isotope data from hydrothermal deposits in the Oslo Rift fall into two main groups. The first group comprises the rift margin deposits. They are very radiogenic and reflect an upper crustal source for the metals. The source of the silver deposits generally has a lower μ-value and a higher w- value than the base metal deposits. The rift margin deposits are therefore not a product of metal zonation from a single hydrothermal solution. The second group comprises deposits associated with granitic intrusions in the Oslo Graben and reflects a lower to intermediate crustal source for the metals. Deposits in the Vestfold graben segment are generally more radiogenic than deposits in the Akershus graben segment and this difference may be explained by a small contribution of highly radiogenic lead from underlying Precambrian granites. The data show that individual intrusions can be characterized by their lead isotope composition and thus confirm the relationship between intrusions and mineralizations. The data indicate a more radiogenic source for the peralkaline intrusions compared to the biotite granites, but more data are needed to confirm this suggestion.

Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling

This paper presents a compilation of 67 published and unpublished radiometric age determinations by the Rb-Sr method on magmatic rocks in the continental Oslo Rift in southeast Norway. The data include most major rock types and structural units in the subaerial part of the rift. The implications of these data with respect to the tectonomagmatic evolution of the rift, and processes in the mantle and crust, are discussed. The Oslo Rift was active for about 60 million years. The earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre-rift Lower Paleozoic sediments at about 300 Ma. Extrusion of plateau lavas of basaltic and intermediate composition (rhomb-porphyries) started about 295 Ma and lasted to about 275 Ma, with largest magma volumes and highest extrusion rate from 295 to 285 Ma in the Vestfold Graben Segment (GS) (one flow per 250,000 years and 0.30 km 3 per 10 3 years). The main faulting activity and graben formation occurred during this period. The plateau volcanism was succeeded by a stage of bimodal central volcanoes, most of which later underwent caldera collapses. Ring-dykes and central intrusions in these calderas give ages in the range 268-266 Ma in the Vestfold and 266-243 Ma in the Akershus GS. Large composite intrusive complexes were emplaced in the periods 27–268 and 273-241 Ma in the Vestfold and Akershus GS, respectively. A rate of propagation of the rift from SSW towards the NNE has been estimated to 1–2 cm y −1. On the basis of 87Sr 86Sr initial ratios and type of magmatic products, we estimate magmas to have been emplaced into the crust only during the first 20–25 Ma of the magmatic period. Some of the magmatism appears to be initiated by movement along Precambrian fault zones which were reactivated during the Oslo rifting event.