Extensional Activity Research Articles

Intrusive rocks and tectono-metamorphic evolution of the Mako Paleoproterozoic belt (Eastern Senegal, West Africa)

The Kedougou Kenieba Inlier (KKI) (Paleoproterozoic of Eastern Senegal) is a portion of the West African Craton (WAC) containing a granite-greenstone terrain that experienced three distinct periods of magmatic activity, peaking at 2200, 2160–2130 and 2100–2070 Ma. In the Inlier, Paleoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Our structural data and deformation sequence indicate that the Mako Greenstone Belt and the adjacent granitoid complexes have undergone a multi-phase evolutionary history that is spread over a prolonged period. The available geochronological data and field studies allowed classification of the granitoids of the KKI into four generations GI, GII, GIII and GIV. The current data suggest that the oldest rocks in the KKI, the Badon granites (2198 ± 2 Ma) and the tonalitic gneisses from Tonkouto (2200–2198 Ma) (GI), could be correlated with an early Birimian magmatic event. The gneisses, crystallized at depth, record the earliest deformation and in contrast to other tonalites, do not appear to have intruded volcanic rocks. The second manifestation of magmatism was intrusion of mafic diorite – the Gabbro Sandikounda Layered Igneous Complex type (GII) and development of the Laminia Kaourou Plutonic Complex (LKPC) (2160–2130 Ma). These bodies pre-date or are sometimes synchronous with a major deformational episode, and may, therefore, have formed very early in convergent Birimian orogenesis. The third major peak of magmatic activity occurred after the above major episode with the development of the oval shaped Diombalou and Bouroumbourou plutons (GIII). The orientation of these plutons parallel to the regional strike of the schistosity indicates structural control on granite emplacement. Eburnean magmatism was terminated in the Mako Belt following compressional Eburnean deformation, with the emplacement of the Tinkoto, Mamakono plutons (GIV) in the east of the complex and continued in the Dialé–Daléma supergroup with the syntectonic emplacement of the Saraya batholith. Garnitiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2080 Ma. Field observations suggest the early plutons of the complex granitic (Kakadian) batholith intruded during convergent deformation whereas later igneous activity accompanied regional orogen-parallel extension, followed by exhumation. In the Mako Belt, thickening of the crust was proposed to have caused heating and the ‘apparent diapiric rise’ of the Diombalou and Bouroumbourou plutons.

Permian komatiites and associated basalts from the marine sediments of Chhongtash Formation, southeast Karakoram, Ladakh, India

The Karakoram micro-plate is the southern most sector of the Central Asian micro-plate mosaic which was separated by a narrow rift basin. A major rifting phase started during Permian time, which lead to drift of not only Karakoram but of the entire Eurasian (Asian) Plate from Gondwana land. This was at a time when a prominent sequence of black argillites occupied most part of the Karakoram Tethys basin. The geodynamic setting for this sequence may be interpreted as the evolution of a passive margin affected by extensional tectonics. The extensional activity is evident from the extrusion of basalts and komatiitic rocks in the region.

Thermal Evolution of the Tanlu Fault Zone on the Eastern Margin of the Dabie Mountains and Its Tectonic Implications

Abstract Five samples of muscovite from mylonites of the earlier Tanlu ductile shear zone on the eastern margin of the Dabie Mountains yield 40Ar/39Ar ages ranging from 178 Ma to 196 Ma. Three of them have reliable plateau ages of 188.7±0.7 Ma, 189.7±0.6 Ma and 192.5±0.7 Ma respectively, which indicates a syn‐orogenic, sinistral strike‐slip thermal event. This displacement movement derived from the continent‐continent collision of the North and South China blocks took place in the Early Jurassic and after uplifting of high‐pressure to ultrahigh‐pressure slabs to the mid‐crust. It is suggested that during the collision the Tanlu fault zone was an intracontinental transform fault caused by differential subduction speeds. The 40Ar/39Ar ages of mylonite whole‐rock and muscovite from the later Tanlu ductile shear zone suggest another sinistral strike‐slip cooling event at 128 Ma. During this strike‐slip faulting, large‐scale intrusion and doming uplift occurred in the eastern part of the Dabie orogenic belt. Data of K‐feldspar 40Ar/39Ar MDD and apatite fission‐track analysis from metamorphic rocks indicate two high‐speed cooling events experienced by the Tanlu fault zone, which took place at 90 Ma and 45–58 Ma respectively. They correspond respectively to two phases of extensional activities in the Late Cretaceous and Eogene as well as development of the Qianshan fault‐controlled basin to the east of the Tanlu fault zone. The cooling times recorded by K‐feldspar and apatite show that the uplifting in the Dabie orogenic belt occured earlier than that on the eastern margin occupied by the Tanlu fault zone. The above phenomena suggest that the uplifting of the Dabie orogenic belt during the Late Cretaceous to Eogene was not controlled by the Tanlu normal faulting, but as a result of the lithospheric delamination.

Characteristics of the mantle source region of sodium lamprophyres and petrogenetic tectonic setting in northeastern Hunan, China

A set of unique sodium lamprophyres is developed in the Cenozoic intraconti-nental extensional zone of northeastern Hunan. These lamprophyres are significantly different in major and trace elements and Sr, Nd isotopes from ordinary potassic lamprophyres. The rocks are characterized as being enriched in Na2O and high in TiO2 and weakly enriched in Nb, Ta, Nd and LREE with no negative Eu anomaly. The trace elements and Sr, Nd isotopic compositions are typical of the mantle source region of oceanic island basalts (OIB). The average initial 87Sr/86Sr ratio is 0.705332, and the average initial 143Nd/144Nd ratio is 0.512650, with ɛNd(t) being +3.5–+3.9, marking a mantle source region of unique sodium lamprophyres. The lamprophyres were formed by metasomatism of the primitive mantle at the bottom of lithosphere by vola-tile-containing fluids/melts from the asthenosphere. The measured Rb-Sr isochron age of sodium lamprophyre is 136.61 Ma, representing a period in which the tectonic setting changed from compressive to extensional. Sodium lamprophyres were formed in a mantle plume tectonic set-ting characterized by mantle upwelling from the asthenosphere within the continent. As-theno-spheric mantle upwelling is the principal geodynamic factor leading to the formation of sodium lamprophyres and constraining the Yanshanian intracontinental extensional activity in northeast-ern Hunan, China.

The Middle to Late Devonian Høybakken detachment, central Norway: 40Ar–39Ar evidence for prolonged late/post-Scandian extension and uplift

The regionally significant 0.5–2 km thick Høybakken detachment in central Norway bounds the southern margin of the Central Norway Basement Window and exhibits a well-developed top-to-the-WSW fabric characteristic of late Scandian, Devonian ductile extension. 40Ar–39Ar data obtained from hornblende, mica and K-feldspar mineral separates of rocks collected in a transect through the Høybakken detachment yield well-defined plateau and isochron mineral ages. Early Devonian exhumation and cooling of the Høybakken detachment footwall is recorded by hornblende ages of ∼ 400 Ma and mica ages of ∼ 390 Ma. The mylonitic fabric overlying the footwall records younger Middle Devonian mica crystallization ages of 384–381 Ma that are among the youngest extensional ductile fabrics dated in the Caledonides and suggest prolonged extensional activity on the Høybakken detachment. After inferred cessation of ductile extension at 381 Ma, the rate of uplift and cooling was reduced, and the footwall records Late Devonian to Early Carboniferous K-feldspar ages of 371–356 Ma. Prolonged extensional activity at Høybakken is compatible with recent U–Pb ages of deformed titanite crystals and established Rb–Sr ages of white mica in shear-related pegmatites, both from the southwestern part of the Fosen Peninsula, and 40Ar–39Ar ages of syn-tectonic mica overgrowth from the adjacent Hitra–Snåsa Fault. Together, these ages suggest the onset of ductile extension soon after ∼ 401 Ma, and with the Middle Devonian crystallization ages determined here, suggest that ductile extension on the Høybakken detachment had a duration of 11–20 Ma. The youngest age of 320 Ma was obtained from a K-feldspar in a cataclastic granite of the Høybakken detachment's hangingwall and is considered to date a phase of post-Scandian brittle deformation that overprinted the mylonitic shear fabric.

Structure and thermal regime beneath the South Pole region, East Antarctica, from magnetotelluric measurements

Ten tensor magnetotelluric (MT) soundings have been acquired in a 54 km long profile across the South Pole area, East Antarctica. The MT transect was offset from the South Pole station ∼5 km and oriented 210 grid north, approximately normal to the Trans-Antarctic Mountains. Surveying around South Pole station was pursued for four main reasons. First, we sought to illuminate first-order structure and physico-chemical state (temperatures, fluids, melts) of the crust and upper mantle of this part of East Antarctica. Secondly, conditions around the South Pole differ from those of previous MT experience at central West Antarctica, so that the project would help to define MT surveying feasibility over the entire continent. Thirdly, the results would provide a crustal response baseline for possible long-term MT monitoring to deep upper mantle depths at the South Pole. Fourthly, because Antarctic logistics are difficult, support facilities at the South Pole enable relatively efficient survey procedures. In making the MT measurements, the high electrical contact impedance at the electrode-firn interface was overcome using a custom-design electrode pre-amplifier at the electrode with low output impedance to the remainder of the recording electronics. Non-plane-wave effects in the data were suppressed using a robust jackknife procedure that emphasized outlier removal from the vertical magnetic field records. Good quality data were obtained, but the rate of collection was hampered by low geomagnetic activity and wind-generated, electrostatic noise induced in the ice. Profile data were inverted using a 2-D algorithm that damps model departures from an a priori structure, in this case a smooth 1-D profile obtained from inversion of an integral of the TM mode impedance along the profile. Inverse models show clear evidence for a pronounced (∼1 km thickness), conductive section below the ice tentatively correlated with porous sediments of the Beacon Supergroup. Substantial variations in sedimentary conductance are inferred, which may translate into commensurate variations in sediment thickness. Low resistivities below ∼30 km suggest thermal activity in the lower crust and upper mantle, and mantle support for this region of elevated East Antarctica. This contrasts with resistivity structure imaged previously in central West Antarctica, where resistivity remains high into the upper mantle consistent with a fossil state of extensional activity there.

Identification of Five Stages of Dike Swarms in the Shanxi‐Hebei‐Inner Mongolia Border Area and Its Tectonic Implications

Abstract Dike swarms are generally ascribed to intrusion of mantle‐source magma result from extension. Basic dike swarms around the Shanxi‐Hebei‐Inner Mogolia borders in the northern peripheral area of the North China Craton can be divided into five age groups according to isotopic dating: 1800–1700 Ma, 800–700 Ma, 230 Ma, 140–120 Ma, and 50–40 Ma. Geological, petrological and isotope geochemical features of the five groups is investigated in order to explore the variation of the mantle material composition in the concerned area with time. And the various extensional activities reflected by the five groups of dike swarms are compared with some important tectonic events within the North China Craton as well as around the world during the same period.

OVACIK GOLD DEPOSIT: AN EXAMPLE OF QUARTZ-ADULARIA–TYPE GOLD MINERALIZATION IN TURKEY

The Ovac1k gold deposit is located 100 km north of Izmir in western Turkey. It lies adjacent to the east-northeast–trending Bergama graben, and it consists of a series of high-grade gold-bearing epithermal quartz veins hosted by subaerial andesitic-dacitic lava dome facies of lower Miocene age. The region is underlain by Paleozoic metamorphic rocks and limestone, which are cut by medium- to high-level intrusions and overlain by subaqueous and subaerial andesitic to dacitic lava domes that host the Ovac1k gold deposit. Middle to late Miocene extensional tectonic activity was responsible for the formation of north-northeast–south-southwest to northeast-southwest–trending grabens. The extensional activity was accompanied by normal faulting with a later, variable sinistral strike-slip component oriented east-west and northwest-southeast. It is probable that these faults were critical in controlling the development of epithermal quartz veins, both mineralized and unmineralized. Two of four nearly east-west–trending epithermal veins at Ovac1k contain significant Au mineralized areas and display typical low-temperature epithermal textures, including crustiform banding, quartz pseudomorphs after bladed calcite, and multiphase hydrothermal breccias. Veins crop out throughout a maximum strike length of 400 m, and surface widths are up to 35 m. Mineralized areas extend down dip for at least 200 m. Gold appears to be related mainly to earlier-formed quartz-adularia veins and breccias. The gold occurs mainly in clasts of earlier vein material that has been transported from deeper levels in the vein system. Little gold is contained in the matrix of the breccias. The alteration associated with mineralization consists of secondary albite or adularia together with quartz and clays, such as smectite, interlayered illite/smectite and illite/chlorite, illite, and chlorite. Metallic minerals occur generally only in trace amounts and include chalcopyrite, pyrite, arsenopyrite, sphalerite, bornite, chalcocite, covellite, galena, and electrum. The mineralizing fluids had temperatures ranging from 150° to 250°C and salinities between 7.0 and 8.0 wt percent NaCl equiv. The veins have a distinct trace element signature, including high Au, Ag, As, and Sb. High Au and Ag with low As and Sb contents are found in the quartz-adularia veins, whereas low Au and Ag with high As and Sb are found in the silicified hanging-wall and footwall zones. Drilling has confirmed continuity of mineralized rock to a vertical depth of 175 m. To date a resource of 2,980,000 tonnes at 9.0 g/t Au containing 862,000 ounces has been delineated. Other deposits of this type should be sought where late Oligocene to early Miocene volcano-plutonic rocks are cut by major northeast-southeast–trending grabens with associated northwest-southeast– to east-west–oriented normal to sinistral strike-slip faults. Volcanic rocks with strong adularia-sericite alteration and silicification, accompanied by quartz veins with epithermal textures are primary targets.

Fluid transfers at a basement/cover interface: Part II. Large-scale introduction of chlorine into the basement by Mesozoic basinal brines

Significant fracture and porosity sealing characterizes the sedimentary cover-basement interface in the northwestern margin of the Aquitaine Basin (France). Dolomite and calcite (and sometimes fluorite, barite and quartz) constitute most of the fracture fillings. They contain primary inclusions of brines having chlorinities in the range of 3.3 to 5.5 mol Cl/kg solution, with total homogenization temperatures in between 65 and 130 °C for quartz and slightly lower for dolomite, barite, fluorite and calcite. Crush-leach analyses indicate that brines are characterized by Na/K ratios of 5 to 40, Na/Li ratios of 20 to 530, and Cl/Br ratios of 200 to 1000, which are rather typical of deep basinal brines. The fluid δ 18O signature is estimated to be ≈6.6±1.8‰ SMOW for a crystallization temperature of 100±20 °C between 80 and 120 °C and the δD value is −30±10‰ SMOW. The fluid source for the fracture filling mineral is interpreted as a deep sedimentary brine expelled during a period of maximum subsidence in the Aquitaine Basin, which migrated along the sediment cover/basement interface which is characterized by high permeabilities below the Toarcian shales. The sealing is likely to be linked to the mixing of the brines with dilute ascending, hot waters. These dilute waters infiltrated from emerged zones, convected and heated at depth, reaching temperatures of 100 °C (up to 150 °C on the basis of cation geothermometry). Extensional activity, of probable Cretaceous age, related to the Gascogne Gulf rifting could be considered as the most likely cause of a significant fluid migration event at the basement/cover interface all along the margins of the French Massif Central. These processes are large scale as shown by the similarities of mineral sequences, fluid types and general features of most of the F–Ba–Pb–Zn deposits located at the basement–sedimentary cover interface. The mass transfer of Cl linked to this stage is significant and accounts for the early introduction of large amounts of chlorine in the granitoid microporosity. This probably explains the significant chlorine concentrations of the present-day fluids recovered in the granitoid aquifer.

The role of granitic intrusions in the evolution of the Abitibi belt, Canada

The Abitibi greenstone belt, Canada, constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730–2725, 2718–2714 and 2705–2700 Ma. During all three volcanic episodes, which were generated by subduction-related processes, mantle plume-derived komatiitic volcanic rocks were also added to the composite Abitibi arc. Plume activity resulted in local regional extension, and restricted later volcanism within the belt. North–south shortening produced thrusts and folds, followed by SE-trending dextral strike-slip faults between 2700 and 2692 Ma, the principal belt-wide deformation. Subsequent E–W dextral strike-slip faulting, limited to the major faults, continued to 2690–2680 Ma (Destor–Porcupine fault) and even to 2680–2670 Ma (Cadillac–Larder fault). Extensional structures resulting from the thickened crust developed around 2660 Ma. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Their role in stabilizing, and adding to the volcanic crust is perhaps less well appreciated. Large synvolcanic tonalite-dominated intrusions of the tonalite–trondhjemite–granodiorite (TTG) suite are associated with the three major periods of volcanic activity of the Abitibi greenstone belt, and are genetically related to the volcanism. These synvolcanic intrusions occur both as high-level, flat, cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust. Tonalite gneiss, dated from 2714 to 2703 Ma, occurring in structural culminations, is considered to be arc-underplated material. The gneiss crystallized at depth, and, in contrast to the other tonalites, does not appear to have intruded volcanic rocks. The gneisses also record the earliest compressional deformation. Numerous intrusions of island arc affinity, both the TTG and monzodiorite (MZD) suites form stocks and small plutons emplaced during the late phases of major deformation. These are more numerous than voluminous, and most of the tonalite end members of the suites probably intruded deeper within the sequence, retarded by the now-thickened crust. Large tonalite intrusions dominate the uplifted deeper sections adjacent to the Abitibi belt in the Kapuskasing uplift and Grenville Parautochthon, and presumably make up much of the deep crust of the Abitibi belt imaged in Lithoprobe seismic profiles. Small stocks and dyke complexes characterize the late, localized strike-slip deformational phase. These intrusions are porphyritic syenites and related rocks, including some alkaline compositions. Garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 Ma. Thus, plutonic rocks are a volumetrically major overall component of the Abitibi belt and were implicated in every phase of its development, from early volcanic arc magmatic activity, through the various stages of deformation, to final stabilization as continental crust.

Late Quaternary fault movements in the Mt. Baldo-Lessini Mts sector of the Southalpine area (northern Italy)

AbstractPaleoseismological investigations have been performed at Mt. Baldo and in the Lessini Mts. in order to collect quantitative data on the activity of minor faults showing geomorphic evidence of recent activation. The 4.5-km-long, NNE-SSW trending Naole fault was responsible for the formation of a narrow depression at the top of Mt. Baldo, bordered by a continuous bedrock (carbonate) fault scarp to the west. The extensional activity along this minor fault is probably due to gravitational deformations (lateral spreading) in response to the warping of the Mt. Baldo anticline. A 1.5-km-long graben is instead related to the 2.5-km-long, NNW-SSE trending Orsara fault (Lessini Mts.) which was responsible for the formation of bedrock (carbonate) fault scarps. This minor fault is part of a complex structural framework made of few-km-long faults which show evidence of Quaternary activity. Two trenches have been excavated across the Naole fault which showed the occurrence of displacement events subsequent to 17435-16385 BP (cal. age) and probably prior to 5455-5385/5330-5295 BP (cal. age). Two other trenches have been excavated across the Orsara fault whose analysis indicated that the most recent displacement event occurred between 20630-19795 BP and 765-675 BP (cal. age). The upper chronological limits of the displacements give some indications about the minimum elapsed time since the last fault activation (about 5,300 years for the Naole fault and 5-8 centuries for the Orsara fault). Both 1) the maximum expected magnitude of the earthquakes which may originate along the Mt. Baldo thrust and 2) the identification of a main fault responsible for the displacements along the complex net of minor faults affecting the Lessini Mts. are still open questions. As for point 1 although historical earthquakes with magnitude 4.5-5 may be associated with the Mt. Baldo thrust, the investigations carried out in this area did not clarify whether larger magnitude earthquakes may be expected. As for point 2, the cause of the displacements along the Orsara (Lessini Mts.) fault may be related to the activity of a major blind fault (which, however, has never been identified), responsible for the uplift of the Lessini Mts. More generally, the obtained results demonstrate the limits of traditional paleoseismological analyses in Alpine areas whose erosional/depositional activity has been strongly conditioned by the Late Pleistocene glacial history. The lack of units younger than loess and colluvial sediments related to the Last Glacial Maximum makes it impossible to define narrower chronological constraints for the displacements and to estimate the number and size of the displacement events. Moreover, the rebound following the retreat of the thick glacial cover affecting the Alpine area may have induced stresses responsible for higher deformation rates after the Last Glacial Maximum. Higher surficial deformation rates could imply shorter recurrence intervals for faulting episodes and/or larger magnitude earthquakes. Therefore, paleoseismologically inferred data in Alpine areas may not correctly define the fault behaviour related to the present tectonic regime.

Tectonique quaternaire de la dépression d'Orava (Carpates occidentales) / Quaternary tectonics of the Orava Basin (Western Carpathians)

Abstract The fault-bounded Orava Basin was formed during the Neogene (Late Badenian to Pliocene) and enlarged during the Quaternary. It is a pull-apart basin filled with very fine-grained fluvial material reaching thickness of up to 930 m. Tectonic uplift in the Western Carpathians started in the Late Pliocene and affected the area of Orava Basin. Extensional activity manifested itself in the NE part of the present-day Orava Basin and the graben was filled with 112 m of Quaternary fluvial and glacio-fluvial deposits. The analysis of Quaternary terraces indicates that the SWpart of the Orava Basin floor was uplifted and dissected, thereby generating a system of 6 cut- and-fill terraces. The NE part of the Orava Basin consists of the large fluvial and glacio-fluvial fan of the Czarny Dunajec River. The Late-Wistulian and Holocenefan is related to active subsidence. Seismic refraction and reflection profiles document E-W Quaternary graben and horst structures. Recent earthquakes confirm ongoing tectonic activity. The opening of the Orava Basin started at ca 14 Ma and was synchronous with the onset of the uplift of the Tatra crystalline massif from depths of 5 km according to fission track data.

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.

Tectonic Significance of the Fen Province, S. Norway: Constraints From Geochronology and Paleomagnetism

The Fen Central Complex (FCC) and associated satellite dikes of the Fen Province in southern Norway record a magnetization dating to 583 Ma. The paleomagnetic pole calculated from these rocks falls at 56° N, 150° E (dp = 7°, dm = 10°) and compares favorably with two previous investigations. The mean inclination in our study is slightly steeper than that of one of the earlier studies and we attribute this to the fact that previous investigators inadvertently sampled younger (late Paleozoic/early Mesozoic) dikes in the area. The age of the Fen paleomagnetic pole is constrained by two consistent 40Ar/39Ar ages from these rocks, which average 583 ± 15 Ma, along with previously published ages ranging from 523–601 Ma. The Fen Central Complex, along with associated intrusions, were emplaced during minor extensional activity that occurred after continental separation between Baltica and Laurentia. This interpretation is consistent with our 580 Ma paleoreconstruction, which shows that Baltica had rifted and rotated from its 615 Ma position adjacent to Greenland. We interpret the ages and the tectonics to reflect opening of an ocean basin from north to south during the latest Neoproterozoic to earliest Paleozoic time (615–540 Ma). The polarity option for the Fen pole remains an important and open question due to recent suggestions of a true polar wander event in the mid‐Cambrian; however, we consider that the south polarity option offers the simplest tectonic model for the rifting of Rodinia and opening of the Iapetus Ocean between Baltica and Laurentia. Finally, we note the similarity of the Fen paleomagnetic pole to paleomagnetic poles of Permo‐Triassic age in Baltica and urge caution in the unequivocal use of the Fen pole as primary until further substantiated by coeval poles from Baltica.

Magmatic Underplating, Metamorphic Evolution and Ductile Shearing in A Mesozoic Lower Crustal‐Upper Mantle Unit (Tinaquillo, Venezuela) of the Caribbean Belt

The Tinaquillo orogenic spinel lherzolite is a piece of subcontinental mantle that emplaced in the Caribbean belt during Late Cretaceous. It was thrust with an overlying sequence of garnet‐sillimanite granulites and pyroxene‐garnet enderbites. This crustal formation also contains mafic and ultramafic granulitic rocks similar to those within the peridotite. The crust‐peridotite contact is a several‐hundred‐meter thick ductile shear zone parallel to the lithologic units and probably originally flat‐lying. The shearing was coeval with the emplacement of gabbronoritic sills Sm‐Nd dated at 90‐87 Ma. Macro‐ and microstructures and geothermobarometry show that this tectono‐magmatic event was preceded by gabbroic intrusions responsible for high‐temperature metamorphism (∼800°C) and anatexis in the preexisting crustal gneisses. U‐Pb zircon dating gives a ∼125 Ma age for the anatexis. Data suggest that nearly continuous magmatic activity occurred during Mid‐Cretaceous times. The early gabbroic bodies emplaced at low pressure (0.5–0.6 GPa) at Moho level, but the subsequent intrusions crystallized at higher pressure, suggesting a significant thickening of the crust above the peridotite‐gneiss contact. The P increase is estimated at about 0.15 GPa and thought to have been caused by magmatic intrusions and extrusions. This Cretaceous igneous, metamorphic, and extensional activity is interpreted as the deep expression of flood basalt plateau‐building. The Tinaquillo crust and peridotite also contain higher‐pressure relicts occurring as garnet mafic mylonites and dikes of sodic pyroxenites, hornblendites, and plagioclase‐amphibole felsic rocks, which seem to be contemporaneous with an older ductile shearing. U‐Pb dating of a felsic rock leads to a ∼150 Ma age for this tectono‐magmatic event. Geothermobarometry suggests that these rocks first equilibrated at pressures higher than 1–1.2 GPa but below the eclogite‐granulite facies transition, suggesting that the formations underwent severe extensional thinning prior to the Cretaceous events. We propose that the Tinaquillo massif is a fragment of a continental domain that was in backarc positions between the Farallon subduction and the active Protocaribbean oceanic rift during Upper Jurassic–Mid‐Cretaceous times. Subduction of the Protocaribbean ridge beneath this domain caused the formation of a volcanic plateau through a slab window process and the genesis, at Moho level, of the Tinaquillo Cretaceous granulites.

Structure and geological evolution of the island of Ponza, Italy: inferences from geological and gravimetric data

A geological and a geophysical survey have been carried out at Ponza Island, Tyrrhenian sea, Italy. Geological and structural data allowed to identify three main tectonic systems: NW-SE, NE-SW and E-W trending. The first one is related to a pre-volcanic tectonic event, probably linked to the Pliocene extensional activity of the Tyrrhenian evolution; the other two systems affected the volcanic units in two different stages of the Lower Pleistocene, the earlier one after the rhyolitic hyaloclastic formation (HF) emplacement and the later one after the emplacement of older trachytic pyroclastic deposits (Lower Pyroclastic Units—LPU). The latter event was followed by the emersion of the whole Ponza area, as testified by a marked erosional surface and marine terrace deposits cropping out at the top of LPU. The Upper Pyroclastic Units (UPU) represent the younger trachytic activity of the island (1.3 Myr) and do not show evidence of tectonic activity. The NW-SE-trending tectonic system probably assisted the rhyolitic magma rise, while the NE-SW- and E-W-trending systems mainly assisted the trachytic magma rise, responsible for the explosive and effusive activity in the southern area and for the hydrothermal fluids that caused alteration processes in the northern area. A 161-station gravimetric survey was carried out on the island and surrounding islets. The geological data and the gravimetric survey have been used to propose a 2.5 D model in which rhyolitic hyaloclastic deposits (ρ = 1.7 g cm −3) overlay an articulated Meso-Cenozoic sedimentary substratum (ρ = 2.6 g cm −3) laying at a depth to 300 m below sea level. Both formations are crossed by rhyolitic dykes (ρ = 2.4 g cm −3) which mark feeder fractures. In the M. Guardia area, where a maximum is present, this model accounts for the presence of a horst of the rigid basement, a shallow trachytic lava flow and its feeder (ρ = 2.8 g cm −3).

Modern tectonic regime in the Tyrrhenian area: observations and models

Summary The rapid opening, since 10 Ma, of the Tyrrhenian Sea in a context of convergence between Eurasia and Africa remains a puzzling geodynamic problem. One of the most plausible scenarios, proposed by Malinverno & Ryan (1986) attributes extension to southward and eastward migration of the trench, resulting from subduction of the African-Adriatic lithosphere below the European lithosphere. At present, subduction only continues along the Calabrian arc. A compilation of recent strain and stress indicators by Rebai, Philip & Taboada (1992) indicates that an extensional regime still prevails in the Tyrrhenian Sea, whereas compression dominates north and south of this area. In an earlier paper, Bassi & Sabadini (1994) used a numerical model to demonstrate the necessity of a ‘trench suction force’, acting along the subduction trench, to produce the present extension in the Tyrrhenian domain. This study, however, did not discuss in detail the tectonic regime in the different areas composing the domain, nor the match between the observations and the model. In this paper, we first review the constraints provided by the observations compiled by Rebaiet al. (1992) and by more recent measurements. We then investigate how much of the observed tectonic regime can be understood in terms of plate-interaction forces, using ‘thin-plate’ models (Bird 1989). The geometry and heterogeneous lithospheric properties of the models are constrained by observations. The best-constrained boundary conditions are held constant; the remaining are regarded as parameters that we tune in order to fit the observations. Our results indicate that the present-day tectonic regime in the Tyrrhenian area can be explained as a consequence of trench retreat associated with the subduction of Ionian oceanic lithosphere under the Calabrian Arc, combined with a rotation of the Apennines, possibly related to a rotation of the Adriatic microplate. This study supports, therefore, the model of Malinverno & Ryan (1986) and suggests, moreover, that rotation of the Apennines is an ongoing process, which is supported by seismological and geodetic data but remains to be understood in terms of geodynamics. A discrepancy between observations and models is observed in western Sicily, which can be attributed to modelling limitations. The weak extensional activity of the Corsica-Sardinia block, however, is not explained by this model.

Cretaceous and tertiary inversion in the Bjørnøyrenna Fault Complex, south-western Barents Sea

A system of half-flower-like structures, folds, reverse faults and footwall cut-offs is observed adjacent to the Bjornoyrenna Fault Complex in the south-western Barents Sea. Although the Bjørnoyrenna Fault Complex itself is principally an extensional feature with its period of maximum extensional activity in the early Cretaceous, these structures suggest that the subsidence along the fault complex was interrupted in the Hauterivian-Aptian by left-lateral (?)transtension, and in the late Cretaceous-early Tertiary by head-on inversion. Both these episodes caused relatively intense deformation, and seem to have regional significance. Consequently, the events need to be seen in relation to the plate tectonic development of the south-western Barents Sea.

Tectonic regime, granite emplacement and crustal structure in the inner zone of the Northern Apennines (Tuscany, Italy): A new hypothesis

Geological and geophysical data on the inner part of the Northern Apennines and the deep (4 km) geothermal wells contained therein have been reviewed in order to better characterize the tectonic regime of the region. The area is affected by Neogene–Quaternary igneous activity, reduced crustal thickness, high heat flow and active geothermal systems. The available information leads to the following conclusions: (1) at present, part of the chain, including its inner regions, are undergoing extensional activity, a tectonic situation in southern Tuscany dating back to the Late Tortonian; however, the simple model of solely extensional tectonics cannot exhaustively account for the phenomena observed in the area, as there is ample evidence of compressive events of Pliocene and Pleistocene ages. (2) The geophysical data indicate reduced lithosphere thickness and the uprising of the asthenosphere to shallow crust levels. The upper crust has been intruded by many granite bodies and the brittle–ductile transition is probably very shallow. An integrated geophysical model which accounts for heat-flow, seismic, gravimetric and magnetic data, indicates that a regional strong reflector, rather than coinciding with the beginning of the ductile crust, more likely indicates the presence of deep brines. Rather than a gneiss core complex and a generalized post-collision extensional model ( Cameli et al., 1993) we support a more complex geodynamic framework, including compressive phases of the internal zone and re-activation of the thrust of the Apennines as a consequence of crustal thrust emplacements from the Late Tortonian in the frame of a subducting delaminated lithosphere.

Fluid migration history in the north Irish Sea-North Channel region

Abstract Basins in the north Irish Sea region have experienced numerous stages of fluid migration. Carboniferous and Permo-Triassic sandstones both experienced rapid cementation due to high sedimentation/burial rates. Both successions also experienced syn-sedimentary faulting which focused cement precipitation and created permeability barriers. Further fracture-based diagenesis occurred post-lithification, and accompanying hydrothermal activity associated with Tertiary intrusions. The extensional activity which helped to define Permo-Triassic basins in the Southern Uplands terrane also channelled mineralizing fluids in the same terrane. These fluids included hydrocarbons in at least two districts. Along the North Solway Fault Zone, basinal fluids from the Carboniferous Solway Basin interacted with metalliferous groundwaters to precipitate metalliferous bitumens. The Solway Carboniferous outcrop also contains intergranular oil residues.