Transtensional Event Research Articles

Magnitudes and surface rupture lengths of paleo-earthquakes at the NW-part of the Peel Boundary fault zone, Roer Valley Rift System

The Peel Boundary Fault zone (PBFZ) is the 125 km long, seismically active, northern bounding fault zone of the Roer Valley Rift System (RVRS). The last damaging earthquake along the PBFZ was the Roermond earthquake of 1992. It had a magnitude of Mw 5.3 and no surface rupture. Previous results from two trenching studies located in the central and southeastern parts of the PBFZ provided evidence for two surface rupturing paleo-earthquakes. The largest earthquake had an estimated magnitude of Mw ∼6.8 and a surface rupture length of at least 35 km. As it took place around the Late Pleniglacial – Late Glacial transition a link to glacio-isostatic motions is likely. Results from a new trench situated at the northwestern part of the PBFZ shows evidence for three to four paleo-earthquakes, of which three were surface rupturing. These comprise two normal faulting- and one, younger trans-tensional displacement. The normal faulting events have ∼1 m vertical displacements each, which translate into magnitudes of Mw ∼7. Like the previous results, they occurred during the Late Pleniglacial-Late Glacial transition, at ∼15 ka and ∼ 14 ka. The younger trans-tensional event occurred sometime during the Holocene, pre-dating an unaffected, 13th century man-made paleo-channel on the hangingwall. The potential fourth, non-surface rupturing earthquake is indirectly evidenced by loading deformations of a sand layer and a collapsed brick-wall in the infill of the paleo-channel. Comparison of our trenching results to those from the two previous studies, which were located farther to the southeast along the PBFZ, shows that for one event a correlation is possible. The correlation would indicate a surface rupture length of at least 55 km. Combined, all trenching results indicate that the characteristic maximum rupturing displacement is ∼1 m, and thus that Mw ∼7 is the maximum magnitude of paleo-earthquakes along the PBFZ.

Structural and palynomorph evidence for Early Palaeogene‐Neogene conglomerate series in Peninsular Malaysia: Implication for basin development and tectonic history

Rifting initiation and development in Western Sundaland is spectacularly synchronous and diffuse spatially, with only one exception, the Malay Peninsula that appears as a large crustal horst, weakly deformed and thus isolated from the presently offshore stretched crust. Cenozoic stratigraphic records are limited onshore, with only isolated conglomeratic series that are keys to integrating the Malay Peninsula in the regional chronology of events. This study focuses on the Lawin and Chenderoh conglomeratic series, located in the Perak Valley, Peninsular Malaysia, that were previously interpreted as Late Tertiary to Quaternary in age, albeit without proper basin analysis and/or geochronological data. Through a GIS‐based kinematic analysis, we investigate the deformation affecting the basement rocks, while a field‐based study and palynomorph analysis were used to constrain the tectonic framework at the time of deposition of each conglomeratic series. Our findings reveal that the Lawin alluvial fan was deposited during an episode of E–W extension controlled by a N–S normal fault. The fan evolved from debris flow to sheet flooding depositional stages. The palynomorph study suggests a burial from the Pliocene onwards (<5 Ma) but older series are suspected at depth. From the Oligocene Chenderoh shale unit, a Miocene age of the conglomeratic series is inferred. Chenderoh conglomerate is believed to have developed within pre‐existing isolated depressions formed during regional transtensional events. Our study reveals that the Malay Peninsula was indeed affected by extension in the Late Palaeogene‐Early Neogene, but with low rates of subsidence as compared to rapidly subsiding offshore basins.

Geochemical and Nd isotopic signature of felsic volcanic rocks as a proxy of volcanic-hosted massive sulphide deposits in the Iberian Pyrite Belt (SW, Spain): The Paymogo Volcano-Sedimentary Alignment

The Iberian Pyrite Belt (IPB), located in the South Portuguese Zone of the Iberian Massif (Spain), is a world-class VHMS province containing numerous, huge mineral deposits hosted by bimodal volcanic sequences. The province can be subdivided in several domains, which differ from each other both in the nature and relative abundance of their volcanic sequences, and in the distribution of associated VHMS deposits. In spite of its geological and economic interest, information available for the northern part of the province is still relatively scarce. We present a new dataset of geochemical and Nd isotopic analyses on the Paymogo Volcano-Sedimentary Alignment (PVSA), an area that is considered to be representative of this northern segment of the IPB.Volcanic sequences in the PVSA contain very scarce mafic rocks. In contrast, felsic rocks are abundant and have been classified in two groups: (i) the Paymogo rhyolitic sequence, hosting most of the VHMS deposits and (ii) a dacitic sequence. Although approximately coeval, these groups of felsic rocks clearly differ in geochemical features and ƐNd signature. In view of these differences, it is suggested that felsic rocks were produced from different mid-crustal domains during a major event of partial melting related to a regional, late Devonian/Early Carboniferous transtensional event. It is further suggested that some volcanic rocks previously described as andesites, are better interpreted as dacites.The age (347.3 ± 0.8 Ma), geochemical features, (Zr < 200 ppm) and ƐNd signature (−3.2 to −2.9) of the Paymogo rhyolites are very similar to those in other parts of the IPB including those hosting the deposits in the Riotinto Mining District, where petrographic, geochemical features and ƐNd signature had already been proposed as a proxy of VHMS mineralization. These similarities between the Riotinto and Paymogo areas suggest that their geochemical features and Nd isotope signature could provide useful exploration criteria throughout the IPB, in order to distinguish potential VHMS-bearing units from barren volcanic successions.

Strain and vorticity analysis in the Zagros suture zone (W Iran): Implications for Neo-Tethys post-collision events

Shear zone development in the Zagros orogenic belt is a result of the oblique collision of the Arabian plate with the Iranian plateau. To distinguish areas deformed during the collisional phase from those deformed by post-collisional events, the coaxiality of deformation and the shape of the strain ellipsoid (SES) have been analyzed. The variation of strain intensity in each shear zone has been evaluated by calculating the strain ratio in the principal strain planes, the θ′ angle, and the kinematic vorticity number (Wk). In the post-collisional shear zones, Wk varies from 0.80 to 0.99 and θ′ from 16.7° to 44.01°; the strain component is mostly simple shear, though in some shear zones it is sub-simple shear. Outside of the shear zones, Wk is extremely low, varying from 0.00 to 0.26, and θ′ values are under 5°; a coaxial deformation regime is dominant and the strain component is pure shear. In most of the shear zones, SES is constricted and prolate, indicating that deformation occurred under transtensional conditions. However, in some regions, SES is flattened and oblate, as is common in transpressional zones. Shear zones that have evolved under dual deformation conditions suggest that transtensional events can be superimposed on transpressional structures.

Intrusion-Related Gold Deposits: New insights from gravity and hydrothermal integrated 3D modeling applied to the Tighza gold mineralization (Central Morocco)

The Tighza (or Jebel Aouam) district is one of the most important polymetallic districts in Morocco. It belongs to the Variscan Belt of Central Meseta, and includes W-Au, Pb-Zn-Ag, and Sb-Ba mineralization types that are spatially related to late-Carboniferous granitic stocks. One of the proposed hypotheses suggests that these granitic stocks are connected to a large intrusive body lying beneath them and that W-Au mineralization is directly related to this magmatism during a 287-285 Ma time span. A more recent model argues for a disconnection between the older barren outcropping magmatic stocks and a younger hidden magmatic complex responsible for the W-Au mineralization. Independently of the magmatic scenario, the W-Au mineralization is consensually recognized as of intrusion-related gold deposit (IRGD) type, W-rich. In addition to discrepancies between magmatic sceneries, the IRGD model does not account for published older age corresponding to a high-temperature hydrothermal event at ca. 291 Ma. Our study is based on gravity data inversion and hydro-thermal modeling, and aims to test this model of IRGD and its related magmatic geometries, with respect to subsurface geometries, favorable physical conditions for deposition and time record of hydrothermal processes. Combined inversion of geology and gravity data suggests that an intrusive body is rooted mainly at the Tighza fault in the north and that it spreads horizontally toward the south during a trans-tensional event (D2). Based on the numerical results, two types of mineralization can be distinguished: 1) the “Pre-Main” type appears during the emplacement of the magmatic body, and 2) the “Main” type appears during magma crystallization and the cooling phase. The time-lag between the two mineralization types depends on the cooling rate of magma. Although our numerical model of thermally-driven fluid flow around the Tighza pluton is simplified, as it does not take into account the chemical and deformation contributions, it provides evidence for abandoning the time-lag as an argument for invalidating the existence of a genetic link between older mineralizations and/or hydrothermal events spatially related to younger parent plutons.

Structural Controls on an Orogenic Gold System: The World-Class Siguiri Gold District, Siguiri Basin, Guinea, West Africa

Orebodies in the Siguiri district, a world-class Paleoproterozoic orogenic gold camp located in the Birimian of northeastern Guinea, are typically represented by cryptic subvertical damage zones that host a high density of mineralized veins. Although no large regional fault system was recognized, observations from five representative deposits of the Siguiri district (Sanu Tinti, Bidini, Kami, Kosise, and Sintroko PB1) show that these orebodies are locally controlled by incipient structures and spread across three distinct structural and lithostratigraphic domains. Two shale-dominated peripheral domains adjoin a central domain whose lithostratigraphy is dominated by medium- to coarse-grained graywacke and this domain hosts the bulk of the gold endowment of the district. The three domains exhibit similar structural elements that can be described within a four-stage deformation scheme. The first deformation event (D 1S ) is poorly constrained and interpreted to have been an N-S compressional event. It included development of minor folds with W- to WNW-gently plunging fold axes without a clear axial planar cleavage. The main and second deformation event (D 2S ) is interpreted to have been associated with E-W to ENE-WSW- directed compression. The D 2S event was responsible for forming the dominant N-trending structural grain of the district and creating interference patterns between F 1S and F 2S folds. The third event (D 3S ) developed progressively from D 2S compression into an early-D 3S E-W- to ENEWSW-directed transpression and a late-D 3S NNW-SSE-directed-transtension responsible for most of the gold mineralization in the Siguiri district. The fourth and last event (D 4S ) was an NW-SE-oriented compressional event responsible for the localized overprinting of veining by a steep to shallowly dipping NNE-SSW ductile cleavage. Late-D 3S gold-bearing mineral occurrences formed along subvertical N-S reverse faults, NE-trending dextral shear zones, WNW-trending sinistral faults, and E-trending normal relay faults developed or reactivated early-D 3S . Mineralization is expressed as mineralized shear zones or subvertical damage zones, characterized by a 10- to 15-m-wide zone of dense quartz-carbonate-sulfide veining, or disseminated gold-bearing sulfides. The mineralized veins consistently strike ENE-WSW, are steeply dipping, and commonly have a conjugate geometry at the mesoscale. Finite strain analysis of deformation, including analysis of folds, faults, and conjugate mineralized vein sets, is consistent with a stress switch from a compressional (D 2S ) to transpressional deformation (termed early-D 3S ). Results of paleostress analysis on conjugate mineralized vein sets that formed late during D 3S indicate that the stress field ranged from extensional to strike-slip, sometimes within the same vein locality. The late-D 3S deformation is interpreted to have been a transtensional event. The first change in the orientation of the principal stress axes is related to a switch from a far field-dominated to a body force-dominated stress field reducing the deviatoric component on the stress tensor. The second change in the orientation of the principal stress axes, from early-D 3S transpression to late-D 3S transtension, suggests that σ 1 and σ 2 were similar in magnitude, which facilitated localized stress switches. In the Siguiri district, the early-D 3S and late-D 3S stress switches, which occurred at both a local and regional scale, enhanced the fracture permeability and were critical for the establishment of active fluid pathways leading to the formation of a world-class gold system.

Serpentinization of mantle peridotites along an uplifted lithospheric section, Mid Atlantic Ridge at 11° N

Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during ~26Ma at a ~80km long Mid Atlantic Ridge segment (11° N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10Ma transtensional event along the Vema transform. Before the uplift residual mantle rocks were lying beneath a 0.8–1.3km thick basaltic crustal layer. The major and trace element compositions of the serpentinites, as well as their H, O, Sr, Cl and B isotopic compositions were interpreted based on thermal models of lithospheric spreading from ridge axis. The results suggest that serpentinization occurred mostly near the ridge axis. Serpentinization temperatures, estimated from stable isotopes, are consistent with resetting of the closure temperatures during the tectonic uplift of the lithospheric sliver, reflected by decreasing δ18O and increasing δ11B values. Modeling shows that the thermal influence of the transtensional event affected mainly the region close to the RTI (ridge–transform intersection). Petrological, elemental and isotopic data suggest that, when the ultramafic basal unit of the VLS was uplifted and exposed on the ocean floor, serpentinization became superseded by low temperature water–rock reactions, with Fe–Mn crust formation, which is still progressing, as recorded by δD. Ultramafic mylonites, prevalent in a short stretch of the VLS, show only a partial serpentinization process, together with pervasive contamination by low-temperature Fe–Mn crust.

Autochthonous versus allochthonous Upper Triassic evaporites in the Sbiba graben, central Tunisia

Two-dimensional seismic data and well data from the Sbiba graben in central Tunisia (North Africa) reveal its asymmetrical structural geometry affected by E–W, N–S, NW–SE, and NE–SW master strike-slip faults. These faults have been intruded by Upper Triassic evaporites, which can be classified into the following three types: (1) sealed evaporite anticlines and domes along folded structures; (2) extrusional wrinkles of “salt walls” at the borders of the graben related to along-strike displacement of the master fault zone; and (3) evaporite outpouring at the center of the graben interpreted as the result of fault and diapir movement. Detailed seismic-stratigraphic analysis highlights a complex interplay between fault growth and evaporite movement, which strongly controlled the evolution of the graben. During the Mesozoic, tectonic stresses induced multidirectional faulting along inherited faults. The vertical rising of the Upper Triassic evaporites was initiated locally at the intersection of master faults during regional extensional and transtensional events in the sub-evaporite basement. The interaction of normal faulting and evaporite diapirism caused lateral outpouring of surface-piercing evaporites. This phenomenon is well expressed in Upper Cretaceous (upper Cenomanian to lower Turonian) strata. The master faults, which cut the diapir at the border and the crest, have caused the extrusion of evaporites. The downward, gravity-driven flow of these evaporites across the slope surface and subsequent accumulation and preservation in Mesozoic sediments is related to the fault escarpment and the adjacent fault-induced depression. There is an apparent relation between tectonic inheritance and inversions of the graben, where interconnected deep faults and diapirs have been reactivated during extensional and contractional tectonic episodes, causing evolution of the structures with the associated sedimentary lapouts. Several autochthonous Triassic evaporites and associated fault escarpments have induced salt canopy structures.

The Geology and Mineralogy of the Loulo Mining District, Mali, West Africa: Evidence for Two Distinct Styles of Orogenic Gold Mineralization

Loulo is a world-class orogenic gold mining district in the Birimian terrane of western Mali. Orebodies are located along second or higher order shears associated with the Senegal-Mali shear zone, with gold mineralization largely linked to a transtensional event. The mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics. One group is typified by the Gara deposit, whereas the other by the Yalea deposit. Both deposit styles are hosted by similar rock types (calcareous graywackes and calciticdolomitic marbles). Gara-style orebodies occur as sulfide disseminations or ankerite-rich shear vein stockworks, hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the Senegal-Mali shear zone. These deposits are characterized by intense multi-stage albitization and tourmalinization (pre-, syn- and postmineralization). Gold lodes are Fe-rich (dominated by nickeloan pyrite), contain Cu-Ni ± Co minor and trace sulfides (e.g., chalcopyrite, gersdorffite, pentlandite, cobaltite, millerite), and show consistently high levels of P-REE-W–bearing phases (apatite, monazite, xenotime, and scheelite). Base metal concentrations show a marked increase in marble host rocks, with the formation of nickeloan pyrite-cobaltite-clausthalite ores. In contrast, Yalea-style deposits are associated with quartz ± ankerite vein lodes and disseminated sulfide stringer zones. Mineralization occurs along highly altered (tourmaline-absent), brittle-ductile, shears up to 8 km away from the Senegal-Mali shear zone. Wall-rock alteration is characterized by addition of K 2 O, CaO, CO 2 , H 2 O, and SiO 2 , with mineral assemblages consisting of chlorite-sericite-carbonate-quartz ± albite. Ore paragenesis is enriched in As, mainly as multistage growth of arsenopyrite and arsenian pyrite. Base metal sulfides, scheelite, and (REE)-phosphates are extremely rare. The diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region. The As-rich Yalea-style deposits have characteristics typical of Birimian gold mineralization in Ghana, and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism. On the other hand, the polymetallic, Fe-B–rich, Gara-style orebodies show atypical features for Birimian gold mineralization. Instead, field relations and the mineralogy and geochemistry of the Gara-style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks, with possible links between gold mineralization and iron oxide skarn development in the region. The data collected at Loulo highlight the diverse nature of orogenic gold deposits, especially in West Africa. This style of ore deposit can form from a variety of fluid sources, both metamorphic and magmatic.

Statistical analysis of Neogene sediment thickness deposited during the first transtensional and first transpressional evolutionary stages in the Bjelovar Sub-basin, Northern Croatia

Abstract The Bjelovar Sub-basin is situated in Northern Croatia and constitutes the southwestern branch of the Drava Basin. The Neogene-Quaternary sedimentary section can be subdivided into three megacycles, which are separated by regional unconformities. The data analyzed in this study are related to the first megacycle, which is characterized by a tectonic-erosional unconformity above the Neogene basement (electric log, abbr. e-log marker Pt or Tg); its top is the regional e-log marker Rs7. It is generally assumed that the Rs7 marker separates Sarmatian and Pannonian sediments and indicates the period just after the first transtensional and first transpressional events. The youngest elog marker, Rs5, represents the Lower/Upper Pannonian boundary (in Croatian stratigraphic nomenclature) and more generally the early period of the second transtensional event. The lithology of the Lower and Middle Badenian is characterized by coarse and medium-grained sediments, while the Upper Badenian to Lower Pannonian s...

Petrology and geodynamic significance of the post-collisional Pan-African magmatism in the Eastern Saghro area (Anti-Atlas, Morocco)

The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show T DM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption. Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580–560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr–Nd isotopic compositions and Nd T DM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts. Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas.

Seismic sequence stratigraphy, basin structuring, and hydrocarbon implications of Cretaceous deposits (Albian–Maastrichtian) in central Tunisia

Albian–Maastrichtian successions of the central and southern Atlas Mountains of Tunisia were investigated using seismic data, complemented by well and outcrop information. Basin-wide deformation can be related to tectonics and appears to have been the major control on variable distribution of Albian–Maastrichtian strata. The northeast–southwest Albian–Cenomanian extension was focussed on major faults and minor antithetic and synthetic faults creating structural depressions and grabens. Transtensional inversion was started during the Turonian. It was characterized by change of the field stress that changed from northeast–southwest to east–west then northwest–southeast during the Coniacian–Campanian. These transtensional events were followed by a northwest–southeast Campanian–Maastrichtian transpression as evidenced by the reactivation of preexisting faults and formation of new reverse faults and associated blind subtle anticlines. The structural rejuvenations were also influenced by holokinesis, which caused the block rotation. This diapirism coupled with sea-level fluctuations has controlled the sediment distributions. Seismic stratigraphic schemes developed for two adjacent sub-basins and two depocenters suggest coeval changes in deposition. Six second-order seismic-sequences are identified in the southern Gafsa sub-basin. These sequences are unequally distributed between depocenters. The Mèjel Bel Abbès sub-basin and Ali Ben Khalifa depocenter contain four second-order seismic-sequences, whereas only three seismic-sequences are identified in the Gamouda depocenter. Third-order sequences are defined by unconformities along the slopes and on the platforms where Upper Cretaceous sediment volume and sedimentation rates decrease. The major sequence boundaries recognized in the subsiding basins are correlated with outcrop unconformities and are considered to have regional significance, possibly controlled by eustatic sea-level changes. Each sequence constitutes the interaction and the dual signatures of the eustasy and the regional or local tectonics. The results of this integrated seismic stratigraphic and structural analysis have implications for evaluating the hydrocarbon potential. The distribution of Upper Cretaceous stratigraphic and structural traps and seals enable us to assess the petroleum potential of central Tunisia.

A Rheic cause for the Acadian deformation in Europe

The Acadian (mid-Devonian) deformation in NW Europe has typically been interpreted as the culminating event of the Silurian closure of the Iapetus Ocean. This view has been challenged by the recognition of an intervening early Devonian transtensional event across part of the assembled Laurussian continent. Instead, the Acadian shortening must be driven by a renewed ‘push from the south’, involving subduction of the Rheic Ocean, and either flat-slab subduction or impingement of another Gondwana-derived continental fragment. A problem with either hypothesis is the lack of Acadian deformation or even correlative unconformity in the segment of the Rhenohercynian Zone between the Acadian belt and the Rheic suture. The possibility is explored that this Rhenohercynian segment was juxtaposed with the Acadian belt and the Midland Microcraton only during latest Acadian and/or Variscan tectonics. If so, a major lithospheric suture lies buried just south of the Variscan Front, along the Bristol Channel Fault Zone, and the missing Acadian terranes must now lie elsewhere along the orogen. A case is made that they are related to the allochthonous terranes of NW Iberia. In any case, the Acadian event in Europe should properly be regarded as proto-Variscan rather than late Caledonian.

Sedimentary geology as a key to understanding the tectonic evolution of the Mesozoic–Early Tertiary Paikon Massif, Vardar suture zone, N Greece

This paper demonstrates how sedimentary evidence can provide a key to elucidating the regional tectonic and palaeoceanographic significance of a complex partly metamorphic terrane, in this case the Paikon Massif. The Paikon Massif is located centrally within the Vardar–Axios zone of N Greece and is critical to the reconstruction of Neotethys in the E Mediterranean region. The massif is here restored as a single tectono-stratigraphic unit of Jurassic to Early Tertiary age that originated along the margin of a continental unit known as the Serbo–Macedonian Zone. In this area, a Mesozoic oceanic basin, the Almopias Ocean, opened in the Triassic between the Serbo–Macedonian continent to the NE and another continental unit, the Pelagonian Zone, to the SW. Initial deep-water deposition along the northwesterly passive margin of the Almopias Ocean is represented within the Paikon Massif by metamorphosed redeposited carbonates and hemipelagic sediments (Gandatch Formation). Above, this the Jurassic Paikon Volcanic Group comprises two volcanogenic units of island arc-type (Livadia and Kastaneri Formations). The arc developed along the SW margin of the Serbo–Macedonian continent in response to NE subduction of the Almopias Ocean. The subduction also resulted in the opening of a related back-arc basin in Mid-Late Jurassic, represented by the Guevgueli ophiolite, now to the NE of the Paikon Massif. The massif underwent ductile deformation and inferred HP/LT metamorphism during Late Jurassic time, related to subduction beneath the Serbo–Macedonian active margin. Exhumation took place rapidly by Kimmeridgian time, followed by a shallow-water transgression (Khromni Limestones). Subsequent emergence and fluvial deposition (Ghrammos Formation) reflect a regional extensional or transtensional event. Following a marine transgression, influenced by eustatic sea-level rise and tectonic subsidence (Aptian–Albian), a carbonate platform developed throughout the Paikon Massif (Cretaceous Transgressive Limestones). Short-lived emergence (Turonian) was followed by deepening in the Campanian–Maastrichtian and pelagic carbonate deposition (Buff Pelagic Carbonates). Overlying terrigenous turbidites (Tchouka Flysch) form part of an Early Tertiary foredeep in a regional contractional setting. Late-stage shortening resulted in bivergent thrusting and folding to form the Paikon Massif as a large anticlinal pop-up structure near the axis of the suture zone. Localised transpression and Neotectonic extension complete the picture.

Tectonic paleostress fields and structural evolution of the NW-Caucasus fold-and-thrust belt from Late Cretaceous to Quaternary

The NW-Caucasus fold-and thrust belt essentially corresponds to the inverted western Flysch Zone of the Great Caucasus Mountains, a deep basin that developed from Late Jurassic to Eocene times between the Scythian Plate to the north and the Transcaucasian terranes to the south (the Shatsky Ridge, SW of the NW-Caucasus zone). The Flysch Basin was strongly affected by compression in Late Eocene times, when the characteristic WNW trending folds and thrusts of the NW-Caucasus belt developed (some authors regard the main compressive deformation as Miocene in age). By means of remote sensing analysis, we elucidate the geometry of major structures in the belt: WNW trending south-vergent thrusts and folds, and major vertical and transverse NNE–SSW to NE–SW deep fault zones. The later structures are interpreted as ancient faults that were active during the development of the Flysch Basin. Paleostress investigations reveal seven main tectonic episodes in the evolution of the NW-Caucasus since Late Cretaceous. Combining structural interpretation, remote sensing analysis and paleostress field reconstruction, we propose a model for the structural evolution of the belt. During the Late Cretaceous–Paleocene, the western Caucasus zone was under transtensional regime with an E–W to NE–SW trending σ 3 that generated oblique normal movements along NNE–SSW transverse faults and WNW–ESE margins of the Flysch Basin. This tectonism could correspond to rifting related to the formation of the Eastern Black Sea Basin. At the Paleocene–Eocene boundary, a transpressional event with an E–W to NW–SE trending σ 1 developed and the NNE–SSW to NE–SW trending faults could have been inverted. This event could correspond to an attenuation in the Eastern Black Sea Basin formation or to the incipient accretion of the Transcaucasian terranes. During the Eocene, another E–W to NW–SE oblique extension (-transtensional event) affected the Flysch Basin that could be related to a known rifting phase in the Eastern Black Sea Basin. Strong NNE–SSW to NE–SW compression characterises Late Eocene tectonism. The fold-and-thrust belt developed at this time as a result of the direct collision of the Shatsky Ridge with the Scythian Plate. A NE–SW extension followed the Late Eocene event, related to basin development around the newly formed fold belt. A WNW–ESE oblique contraction affected the belt during the early Miocene as the result of Arabian Plate convergence with the Caucasian system. The latest inferred event is a compressional regime, with NNW–SSE trending σ 1 that is affecting the NW-Caucasus belt from Sarmatian times until the present. Under this oblique compression, the belt has deformed as in a dextral shear zone and the thrust surfaces have acquired lenticular shapes. This study highlights the occurrence of oblique movements in the NW-Caucasus area prior to and after the dominant Late Eocene compression. From the Late Cretaceous until the Eocene, the structural development of the NW-Caucasus was closely related to the evolution of the Eastern Black Sea Basin. From the Late Eocene until Quaternary times, it was rather related to the Arabia–Eurasia plate convergence.

Paleozoic age of the Walden Creek Group, Ocoee Supergroup, in the western Blue Ridge, southern Appalachians: Implications for evolution of the Appalachian margin of Laurentia

In the foothills of the Great Smoky Mountains in Tennessee, low-grade metamorphic rocks of the Walden Creek Group, Ocoee Supergroup, contain a fossil assemblage composed of trilobites, bryozoans, ostracodes, pelmatozoans, algae, and agglutinated foraminifers, indicating a Silurian or younger age. The new paleontological data contradict older ideas on the Neoproterozoic age of the Walden Creek Group. The Walden Creek Group was not deposited in a continental rift basin preceding the opening of the Iapetus ocean but, possibly, in a pull-apart basin formed in a transtensional event during the Acadian orogeny. There is no field evidence indicating that the Walden Creek Group is separated from the lower units of the Ocoee Supergroup by a basin-wide unconformity and hiatus. A tentative geodynamic model is presented for the Ocoee Supergroup basin; the model is dependent on paleontological data indicating a Paleozoic age for the Walden Creek Group and concentrates on lithologic, petrographic, and basin characteristics. The model allows for the possibility that the entire Ocoee Supergroup was deposited in a Paleozoic transtensional basin.

1.88–1.87 Ga post-kinematic intrusions of the Central Finland Granitoid Complex: a shift from C-type to A-type magmatism during lithospheric convergence

The evolution of the Svecofennian Orogen spans from 1.95 Ga to 1.80 Ga with the main crust-forming activity having taken place 1.90–1.87 Ga ago. Subsequent deformation was largely concentrated to southern Finland where transpressional structures indicate lithospheric convergence until 1.80 Ga. Much of the Svecofennian Orogen in Finland is occupied by the Central Finland Granitoid Complex (CFGC) that mainly consists of I-type granodiorite and granite. These 1.89–1.88 Ga intrusive rocks are typically foliated and considered synkinematic. A distinct suite of post-kinematic, undeformed or slightly foliated plutons crosscut the synkinematic rocks. They are predominantly quartz monzonites and monzogranites, and have been divided into three types according to their petrographic, mineral, and chemical characteristics. Compared to the calc-alkaline synkinematic granitoids, the post-kinematic suite generally has an alkaline affinity, with higher Fe, Ti, K, Ba, Zr and Nb and lower Mg, Ca and Sr at a given SiO 2 content. The Type 1 post-kinematic plutons are peraluminous and contained a prominent sedimentary component in their source. The Types 2 and 3 plutons are marginally metaluminous to peraluminous. The Type 2 granites, especially those in the western CFGC, approach the classic 1.65–1.54 Ga rapakivi granites of southern Finland in their petrographic characteristics, magmatic association (tholeiitic mafic rocks) and elevated contents of incompatible elements, and hence have A-type characteristics. The Type 3 plutons contain a pyroxene-bearing margin (Type 3a) or contain pyroxene throughout (Type 3b). The Type 3b rocks have C-type (Charnockite magma type) affinities, and some plutons of this subgroup are alkaline. The Type 3a plutons appear to be transitional between the Type 2 and Type 3b plutons. The synkinematic magmatic episode in the CFGC area probably involved partial melting of intermediate high-K rocks in the lower crust, with a magmatic addition from a mafic underplate. This magmatic episode left a hornblende- and biotite-poor granulitic residue in the lower crust. Heat from the mafic underplate triggered partial melting of the granulite to produce the post-kinematic magmatism, and also introduced mafic material into the post-kinematic magmas. The post-kinematic magmatism took place over quite a short time period (ca. 15 Ma) during overall lithospheric convergence. It extended beyond the CFGC area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from C-type to A-type. The post-kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust. Partial melting of the mafic lower crust produced C-type magmatism soon after the first melting episode. The crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal A-type magmas were formed in the western CFGC.

Deformation history of a Mesozoic forearc basin sequence on Alexander Island, Antarctic Peninsula

The Mesozoic Fossil Bluff Group of Alexander Island contains trench-slope and forearc basin deposits that formed during the eastward subduction of oceanic crust beneath the Antarctic Peninsula region. The oldest rocks of the group were deposited in the Early to Middle Jurassic on the upper trench slope, with the forearc basin (sensu stricto) forming in Late Jurassic times. Deposition continued until at least Early Cretaceous (Albian) times. The Fossil Bluff Group was affected by three phases of deformation, as follows: 1. D1 — Movement on a major fault (the LeMay Range Fault) in the accretionary complex during the Middle Jurassic. Structural evidence suggests that the fault had a strike-slip component, and that the dip-slip component varied along strike from normal to reverse. 2. D2 — Basin inversion in the late Early Cretaceous, whilst deposition was still occurring. Fold patterns and fault movement directions indicate that inversion occurred in a dextral transpressional setting. 3. D3 — Late Cretaceous or Tertiary post-inversion extension. This caused the opening of a linear graben, George VI Sound, in a dextral transtensional setting. Phases (1) and (3) can be most easily explained by an oblique subduction model, with SE-directed subduction causing strike-slip motion on large-scale N-S trending structures in both arc and forearc. Dextral transtensional structures, such as those resulting from D3, formed when the forearc was in extension; whereas in a compressional forearc setting, sinistral transpressional structures would have resulted. As basin inversion (D2) occurred by dextral transpression, it cannot be explained by the above geometric model. It is likely, therefore, that the subduction direction was different during the D2 inversion event and that it corresponded to a Pacific-wide mid-Cretaceous compressional event. The D3 transtensional event corresponded with a dramatic decrease in spreading and subduction rates in early Tertiary times.

Transform migration and vertical tectonics at the Romanche fracture zone, equatorial Atlantic

The Romanche transform offsets the Mid‐Atlantic Ridge (MAR) axis by about 950 km in the equatorial Atlantic. Multibeam and high‐resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the R/V Akademik Strakhov as part of the Russian‐Italian Mid‐Atlantic Ridge Project (PRIMAR). Morphobathymetric data show the existence on the northern side of the transform of a major 800‐km‐long aseismic valley oriented 10° to 15° from the active valley; it disappears about 150 km from the western MAR segment. The aseismic valley marks probably the former location of the Romanche transform (“PaleoRomanche”) that was active up to roughly 8–10 Ma, when the transform boundary migrated to its present position. A temporary microplate developed during the migration and reorientation of the transform. This microplate changed its sense of motion as it was transferred from the South American to the African plate. A prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side, reaching its shallowest part (shallower by over 4 km than the predicted thermal contraction depth) in a zone opposite the eastern MAR axis/transform intersection (RTI). Flat‐top peaks on the summit of the transverse ridge are capped by acoustically transparent, weakly stratified, shallow water platfonn/lagunal/reef limestones. This limestone unit is a few hundred meters thick and overlies igneous basement. Evaluation of the seismic reflection data as well as study of samples of carbonates, ventifact basaltic pebbles and gabbroic, peridotitic and basaltic rocks recovered at different sites on the transverse ridge, suggest that (1) the summit of the transverse ridge was above sea level at and before about 5 Ma; (2) the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate; its summit was flattened by erosion at sea level during subsidence; (3) the transverse ridge is an uplifted sliver of lithosphere and not a volcanic constructional feature; and (4) transtensional and transpressional tectonics have affected the transverse ridge. Hypotheses on the origin of the Romanche transverse ridge include (1) lateral heat conduction across the RTI; (2) shear heating; (3) lithospheric flexure due to thermal stresses in the cooling lithosphere; (4) viscoelastic deformation of the lithosphere; (5) hydration/dehydration of mantle peridotites; and (6) longitudinal flow of melt and igneous activity across the RTI. These processes cannot by themselves explain the transverse ridge, although some of them could contribute to its formation to a small extent. Vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence. Uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former (PaleoRomanche) and the younger transform boundaries, before and during the transition to the present boundary. After migration of the transform boundary to its present position, transpression was replaced by transtension and by subsidence of the transverse ridge. An aseismic axial rift valley impacting against the transform valley about 80 km west of the present RTI suggests eastward ridge jumping that probably followed transform migration. Localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements, such as those that formed an ultradeep (&gt;7800 m) pull‐apart basin along the transform valley.

Geological interpretation of combined Seabeam, Gloria and seismic data from Anegada Passage (Virgin Islands, north Caribbean)

The Anegada Passage (sensu lato) includes several basins and ridges from Southeast of Puerto Rico to the corner of the Virgin Islands Platform. Seabeam (Seacarib I) and Gloria long-range sidescan sonar surveys were carried out in this area. These new data allow us to propose an interpretation of the Anegada Passage. Most of the features described are related to wrench faulting: A two-stage story is proposed for the creation of the basins. A first extensional event during Eocene(?)-Oligocene-lower Miocene time created Virgin Islands, St Croix Basins and the tilted blocks of St Croix Ridge. A second transtensional event from Pliocene to Recent gave the present day pattern to this area. However, the displacement along the strike-slip faults is no more than 15 km long. The proposed geodynamic model is based on the separation of the northeastern Caribbean boundary into two blocks. In the West, the indenter of Beata Ridge gives a northeastern motion to Hispaniola Block. In the East, as a result of Hispaniola Block's motion, the Puerto Rico—Virgin Islands Block could escape in an east-northeast direction.