Palinspastic Reconstruction Research Articles

The Curinga-Girifalco fault zone (northern Serre, Calabria) and its significance within the Alpine tectonic evolution of the western Mediterranean

Abstract Hercynian basement rocks and Mesozoic ophiolites of the Calabria-Peloritani terrane drifted in the present position during the opening of western Mediterranean basins (namely Liguro-Provencal and Tyrrhenian basins) since the Oligocene. Basement rocks were partly involved by Alpine (late Cretaceous—Eocene) deformation and metamorphism before the onset of the drifting process. Even though the kinematics of the Alpine deformation in Calabria has been already defined, restoration of structural and kinematic data to the original position and orientation before the opening of the western Mediterranean has never been performed. In this work we present new structural and petrological data on a major tectonic contact of Alpine age exposed in central Calabria (Serre Massif). Structural and kinematic data are then restored at the original orientation in the early Oligocene time, to allow a correct tectonic interpretation. In the Serre Massif the Hercynian basement is sliced into three nappes emplaced during the Alpine orogeny. The upper nappe is formed by a nearly continuous section of the Hercynian crust, consisting of medium- to high-grade metamorphic rocks in the lower portion. The intermediate nappe mainly consists of orthogneisses, whereas the lower nappe is chiefly composed of phyllites. The contacts between the Alpine nappes are outlined by well developed mylonitic and cataclastic rocks. The Curinga-Girifalco Line is a well exposed shear zone that overprints mainly metapelitic rocks of the upper nappe and granitoid orthogneisses of the intermediate nappe. Mylonites of the intermediate nappe typically show overgrowths on garnet and hornblende with grossular-rich and tschermakitic composition, respectively. The Alpine mineral assemblage indicates that deformation took place in epidote-amphibolite facies at pressures ranging from 0.75 to 0.9 GPa. In the investigated area mylonites strike roughly WNW–ESE, with shallow dips towards SSW. Kinematic indicators in mylonites are mostly consistent with a top-to-the-SE shear sense in the present geographic coordinates. The mylonitic belt is affected by later extensional faults outlined by South-dipping cataclasite horizons. Published geochronological data indicate that mylonites and cataclasites developed in Eocene and early Miocene times, respectively. Considering rotational parameters coming from paleomagnetic studies and large-scale palinspastic reconstructions, the shear sense of the Curinga-Girifalco Line has been restored to the early Oligocene position and orientation. Through restoration a top-to-the-S shear sense is obtained. This result is in striking agreement with the convergence direction between Africa and W-Europe/Iberia during Eocene, computed from the North Atlantic magnetic anomalies. Our geodynamic reconstruction, combined with structural and petrological evidence, allows to relate the Curinga-Girifalco mylonites to a thrust related to the southeastern front of the double-verging Alpine chain. The adopted method could be used also for other exotic terranes, such as the Kabylie or the Corsica-Sardinia, to better constrain geometry and evolution of the southern Alpine belt.

Assessment of factors of periodicity in Phanerozoic carbonate accumulation

Periodicity in the evolution of carbonate accumulation (CA) depends on settings and processes governed by three (vertical, latitudinal, and circum-continental) types of zonality. It is also controlled by oceanic hydrodynamics and variations in global paleorelief of the bottom [1]. The history of origin and evolution of carbonates is intricate and diverse. The role of predominant factors in many aspects of this process remains debatable so far [2]. The emergence of the plate tectonics theory and lithology of lithospheric plates [3] reduced the list of debatable points in this field. Nevertheless, quantitative assessments of global sedimentation processes in the ocean are insufficient. The present work is dedicated to the study of the Phanerozoic time series of global CA based on analysis of the available quantitative data on carbonates [4] by methods of the spectral analysis. The purpose of the work is to study the latent periodicity in the formation of limestone and dolomite sequences. The first paleoclimatic studies carried out by paleomagnetologists in the initial 1960s showed that Phanerozoic CAs are mainly concentrated in the tropical belts between 30 ° N and 30 ° S of the Northern and Southern hemispheres [5]. Based on new palinspastic reconstructions of the Phanerozoic drift of continents, the authors of [6] calculated dynamics in the latitudinal distribution of CA areas within these belts and assessed variations in sedimentation patterns. The results obtained demonstrated once again opposite patterns in accumulation of terrigenous and carbonate sequences in the Phanerozoic. For example, despite processes of erosion and redeposition with time, recent carbonate bodies gradually contracted relative to ancient accumulations. In contrast, terrigenous sedimentary bodies expanded. Correspondingly, authors of [6] proposed two scenarios for the distribution of CA areas. The initial distribution pattern is given after [4], whereas the reconstructed version with the consideration of erosion is based on original data [6]. We analyze both of these scenarios. The spectral analysis of carbonate time series was performed using two methods: the regression method (mainly, the method of maximal entropy combined with the method of least squares) and the wavelet analysis (wavelet of Morlet). We analyzed distribution areas and accumulation volumes of various carbonate sediments in the Phanerozoic (after [4]), temporal dependences of CA sources on carbon and calcium cycles, as well as some processes providing the influx of these elements to the ocean. Correspondingly, we estimated dynamics of CA in terms of the above-mentioned latitudinal zonality based on [6]. In total, we analyzed 29 time series. Together with previous results of the spectral analysis of sea level fluctuations and marine biota diversification [7, 8], the new correlations provide insights into variations in the CA environments related to dislocations of lithospheric plates.

Permian geodynamic setting of Northeast China and adjacent regions: closure of the Paleo-Asian Ocean and subduction of the Paleo-Pacific Plate

Northeast China and adjacent regions are located in the central East Asian continent and consist tectonically of both the Paleo-Asian and Paleo-Pacific orogens between the Siberian platform and Sino-Korean (North China) block. This paper discusses some hotly-debated issues concerning the Permian geodynamic setting of these regions, based on a comprehensive analysis of available geological, geochemical, paleobiogeographical and paleomagnetic data. Spatial and temporal distribution of ophiolites and associated continental marginal sequences, Permian sedimentary sequences, spatial distribution and geochemistry of Permian magmatic rocks, and the evolution of paleobiogeographical realms imply: (1) that the Permian marine basins in northeast China and adjacent regions include remnants of the Paleo-Asian Ocean in southeastern Inner Mongolia and central Jilin Province, and active continental margins of the Paleo-Pacific Ocean; (2) that the suture between the Siberian and Sino-Korean paleoplates was finally emplaced in the Permian and is located in areas from Suolunshan (Solonker) eastwards through regions north to the Xar Moron river in southeastern Inner Mongolia, and then central Jilin province to the Yanji area; and (3) that the Permian crustal evolution of northeast China and adjacent regions, as well as parts of the Siberian paleoplate, was influenced by subduction of the Paleo-Pacific oceanic plate. Finally, the Permian tectonic framework and paleogeography of northeast China and adjacent regions in central East Asia are discussed briefly, and Early and Late Permian palinspastic reconstruction maps are provided.

Palinspastic reconstruction of Proterozoic basement-related aeromagnetic features in north-central New Mexico: Implications for Mesoproterozoic to late Cenozoic tectonism

Aeromagnetic maps from north-central New Mexico show east-northeast–trending linear features that are offset dextrally across the north-striking Picuris-Pecos, Tusas-Picuris, and (possibly) Nacimiento fault systems. Geologic structures that correspond to these regional aeromagnetic lineaments, where exposed in Phanerozoic uplifts, are major ductile shear zones of Proterozoic age that juxtapose folded metasedimentary rocks associated with areas of low aeromagnetic value with metavolcanic and metaplutonic rocks associated with areas of high aeromagnetic value. Regional aeromagnetic lineaments serve as piercing lines that suggest at least ∼55 km and perhaps as much as ∼90 km of net dextral separation since 1.4 Ga. The restored aeromagnetic lineaments are interpreted to represent a series of east-northeast–striking, dominantly north-vergent, thrust-sense shear zones that formed initially during the ca. 1.65 Ga Mazatzal orogeny and were variably reactivated during the ca. 1.4 Ga magmatic and deformational event.

Heterogeneous mantle complex, crustal processes, and obduction kinematics in a unified Pindos-Vourinos ophiolitic slab (northern Greece)

Abstract The Eocene-Miocene Mesohellenic Trough is an elongate sediment-filled tectonic basin trending NW across central Greece and into Albania. Neotethyan oceanic rocks, including Triassic-Jurassic rift-related volcanic rocks and deep-sea sediments, accretionary mélange and ophiolitic complexes, crop out along its margins. These units were tectonically emplaced onto the Pelagonian microcontinent to the east and the Apulian-African continental margin to the west. In northern Greece, the mid-Jurassic Vourinos ophiolite on the eastern margin of the trough is geographically separated from the synchronous Pindos ophiolite along the western margin by a minimum c. 20 km distance. The sedimentary fill of the trough obscures their presumed subsurface continuation, although magnetic surveys identify thick ‘ophiolitic’ rocks beneath the basin. We interpret these ophiolites as parts of the same oceanic slab, two parts of a single larger oceanic complex we now term the Mesohellenic ophiolite. Comparable ophiolitic complexes to the south (the Koziakas and Othris) and the ophiolites of the Mirdita complex to the north in Albania are considered as members of this same complex. Geological and petrological data from the Vourinos and Pindos ophiolites define intra-slab heterogeneity. Vourinos essentially is a ‘Penrose-style’ ophiolite with ‘supra-subduction’ compositions; the less continuous Pindos ophiolite shows coexisting mid-ocean ridge basalt and island arc characteristics. Ophiolitic rocks that seem to represent geographical overlap between these characteristic associations crop out along their northern (Dotsikos strip) and southern (Mesovouni) margins. Variations in mantle strain conditions across the ophiolitic slab have been mapped, and demonstrate a single orientation of deformation; this is explained by variable strain kinematics that persisted across the ductile-brittle boundary. A continuity from ductile to brittle emplacement structures spans the Mesohellenic Trough, independent of petrological association, and indicates the original relative positions of these ophiolites within the oceanic slab. These structures illustrate tectonic ‘steps’ of obduction from the ridge crest onto the Pelagonian margin to the east, and can be relatively timed by the overlap of magmatism with ductile deformation in different parts of the slab. Hence, rotations of original horizontality are dated to the period preceding cessation of ductile field deformation, while still in the oceanic environment. The morphology defined by these structures and the horizontal rotation of stratigraphy are analogous to a spoon- or scoop-shaped nappe originating in the ductile field at its base, and crossing into the brittle field rapidly at its leading edge (Vourinos), whereas the mylonitic deformation characterizes the ‘trailing’ end (Pindos). Age relations require that geochemical variation between the two complexes must be explained within a model of synchronous generation, possibly with apparent ‘supra-subduction zone’ rifting of originally heterogeneous mantle, or an overlapping series of diverse processes of magma generation in an initially homogeneous mantle. Indications of the original ridge crest directions suggest the operation of several simultaneous spreading centres, separated by transform faults or ‘pseudofaults’. A palinspastic reconstruction of the slab constrains applicable oceanic models and provides the basis of future research.

Application of trishear fault-propagation folding to active reverse faults: examples from the Dalong Fault, Gansu Province, NW China

Determining accurate fault slip rates at 1 ka to 1 Ma timescales requires well-constrained palinspastic reconstructions of dateable geomorphic and/or geologic markers. Although general kinematic models have been developed to simultaneously reconstruct both bedrock (e.g. bedding and fault attitudes) and neotectonic markers (e.g. strath terraces) along active strike-slip and thrust faults, it is not clear if these models can also account for deformation along steeply dipping (>45°) reverse faults. To address this problem, we have investigated the active, ∼50° dipping, Dalong reverse fault system. This ∼40-km-long fault system forms part of the Aksai restraining stepover along the active, left-slip Altyn Tagh Fault in northwestern China. Our geometric and kinematic analyses show that conventional fault-bend fold models cannot satisfy the steeply-dipping fault geometry we observe in the bedrock record. Likewise, standard fault-propagation fold models fail to match our measurements of a set of fluvial terraces. However, by expanding the trishear model of fault-propagation folding to track both bedrock and neotectonic markers, we are able to match both sets of records. In particular, we have developed trishear kinematic models for two sites (Liuchenzi and Qingyazi) using the numerical modeling program, Fault/Fold v.5.0. This work indicates that an important implication of active trishear fault-propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace. Thus, to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall, terrace profiles across active reverse faults must extend 1–2 km on either side of this zone of deformation.

Crustal constraints on the origin of mantle seismicity in the Vrancea Zone, Romania: The case for active continental lithospheric delamination

The Vrancea zone of Romania constitutes one of the most active seismic zones in Europe, where intermediate-depth (70–200 km) earthquakes of magnitude in excess of M w = 7.0 occur with relative frequency in a geographically restricted area within the 110° bend region of the southeastern Carpathian orogen. Geologically, the Vrancea zone is characterized by (a) a laterally restricted, steeply NW-dipping seismogenic volume (30 × 70 × 200 km), situated beneath (b) thickened continental crust within the highly arcuate bend region of the Carpathian orocline, and (c) miscorrelation of hypocenters with the position of known or inferred suture zones in the Carpathian orogenic system. Geologic data from petroleum exploration in the Eastern Carpathians, published palinspastic reconstructions, and reprocessing of industry seismic data from the Carpathian foreland indicate that (1) crust of continental affinity extends significantly westward beneath the external thrust nappes (Sub-Carpathian, Marginal Folds, and Tarcau) of the Eastern Carpathians, (2) Cretaceous to Miocene strata of continental affinity can be reconstructed westward to a position now occupied by the Transylvanian basin, and (3) geologic structure in the Carpathian foreland (including the Moho) is sub-horizontal directly to the east and above the Vrancea seismogenic zone. Taken together, these geologic relationships imply that the Vrancea zone occupies a region overlain by continental crust and upper mantle, and does not appear to originate from a subducted oceanic slab along the length of the Carpathian orogen. Accordingly, the Vrancea zone appears to potentially be an important place to establish evidence for active lithospheric delamination.

An investigation of lower crustal deformation: Evidence for channel flow and its implications for tectonics and structural studies

High-grade metamorphic terranes generally have similar deformation fabrics characterised by a shallowly dipping transposition foliation and recumbent folds. Deformation paths are typically non-coaxial and strain magnitudes extreme. We refer to this association of structures and metamorphism as the high-grade nappe association (HGNA), and argue that it is common, and represents crustal-scale (kilometres thick) shear zones. The structure of such rocks is generally interpreted as comprising stacked thrust sheets separated by thrusts. We concluded, however, that the ‘thrusts’, which are ubiquitously parallel to the transposition foliation, are not thrusts in the normal sense, but are various discontinuities, rotated by large pervasive shear strains. The thrust-like appearance may be further enhanced by late localised shear strain. The sense of shear may be constant across an HGNA shear zone, giving it the geometry and kinematics of a crustal-scale (kilometres thick) detachment zone. Alternatively it may reverse across the body, consistent with channel flow. Structural evidence therefore supports current ideas on the behaviour of the middle to lower crust during orogeny, and illustrates the deformation mechanisms involved. We describe the association with special reference to the Monashee complex, and discuss the implications of our interpretation for the kinematics of high-strain zones, palinspastic reconstruction, and interpretation of deformation fabrics at various scales.

Geometry and kinematics of compressional growth structures and diapirs in the La Popa basin of northeast Mexico: Insights from sequential restoration of a regional cross section and three‐dimensional analysis

Diapirism in the La Popa basin NE Mexico was triggered and sustained by compressional décollement tectonics. Coeval development of contractional structures and diapirs was recorded by growth strata of Late Cretaceous‐Tertiary age. A palinspastic reconstruction of a regional cross section, based on the restoration of growth strata related to both diapirs and detachment folds, was carried out. On the basis of this reconstruction and field data, a three‐dimensional geometrical study was done to quantify the movement of evaporites within the detachment horizon and two diapirs. The latter allowed determining the tectonodiapiric potential of a structure or set of structures (i.e., the amount of ductile rocks forced to flow in association with the growth of a tectonic structure, which is not accommodated by thickening of the ductile horizon nor by the tectonic structure itself, and is capable of producing diapiric phenomena). This study demonstrates that there is a direct coupling between tectonodiapiric potential and diapiric activity. Geometrical results show that the main factors that controlled the amount of ductile material moved as a consequence of compressional tectonics were shortening, the depth to detachment, the folding style, and the style of deformation process (i.e., pure shear/simple shear). In response to folding, the overburden experienced a downward movement that was the major cause of diapiric phenomena. In addition to field and geometrical data, mechanical data indicate that forceful intrusion was the prevailing emplacement mechanism. Temperatures (>150°–197°C) and strain rates (in the order of 10−16–10−17 s−1) calculated for top of the evaporite layer were adequate for the flow of these rocks from the initiation of compression onward. However, an inferred overpressure in the ductile layer should be considered the key factor for the emplacement mechanism.

Sedimentary record of Triassic intraplate extension in North China: evidence from the nonmarine NW Ordos Basin, Helan Shan and Zhuozi Shan

The Helan Shan and Zhuozi Shan of the NW Ordos basin, China, contain thick (up to 4 km) sequences of nonmarine Triassic strata. These rocks represent a major intraplate sedimentary basin, the paleogeography, tectonic setting and provenance of which are poorly understood and controversial. Studies of the sedimentary geology of the basin, supported by new palinspastic reconstruction of younger deformation, demonstrate that the basin filled from three sides by fluvial, lacustrine-deltaic and alluvial fan depositional systems. The basin forms a westward-thickening wedge that reaches its maximum thickness along the western margin of the Helan Shan and thins to a relatively constant 600–800 m east of the Zhuozi Shan. The stratigraphy of the basin is strongly asymmetric; alluvial fan strata are restricted to the extreme western margin of the basin and interfinger with axial fluvial deposits low in the section and deep lacustrine facies high in the section. Much of the eastern part of the basin is dominated by west-flowing meandering river and deltaic systems. Large structures of Triassic age have not been identified in the Helan Shan or Zhuozi Shan, but small Triassic normal faults have been documented in the western and central Helan Shan. These characteristics most strongly support an extensional origin for the Triassic basin in NW Ordos. The basin is interpreted to have been a north-trending half graben, bound along its western margin by an east-dipping normal fault, presently concealed beneath Quaternary cover west of the Helan Shan. The eastern margin, now found in the Zhuozi Shan, has simple ramp-margin geometry. Driving mechanisms for this extension are not obvious due to limited documentation of Triassic structure throughout the region, but probably relate to far-field stresses from the Qinling or Jinsha active margins interacting with the stable Ordos block.

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin

According to palinspastic reconstructions, the Neo-Tethys opening took place during the Permian between the Cimmerian fragments in the north and the Indo-Arabian margin in the south. Igneous remnants of this opening are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed in the Saih Hatat tectonic window. They consist predominantly of pillowed basaltic flows among which three groups have been distinguished. Group 1 is tholeiitic and characterized by low TiO2 and incompatible trace element contents, and a large range of εNdi values. Group 1 basalts are associated with distal sediments and plot near the boundary of or within the MORB field in the Pb–Pb correlation diagrams and between the MORB and Bulk Silica Earth (BSE) fields in εNdi–(206Pb/204Pb)i diagram. Group 2 basalts are alkaline and differ from Group 1 ones by their higher TiO2, La and Nb contents, and lower and more homogeneous εNdi values (+3 to +5). Group 2 volcanics are similar to alkali basalts from oceanic islands and share with Group 1 similar initial Pb ratios. Group 3 consists of tholeiitic and alkali basalts which are interbedded either with carbonate-platform sediments from the Saih Hatat window or with distal sediments from the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative εNdi (+1.6 to −2). Group 1 likely derived from the mixing of depleted and enriched sources while Group 2 derived exclusively from an enriched source. There is no indication that continental crust was involved in the genesis of both Groups 1 and 2. In contrast, the low to negative εNdi values of Group 3 suggest that the magmas of this group were contaminated by the Arabian continental crust during their ascent. The geochemical features of the Middle Permian plume-related basalts suggest thus that the basement of the Hawasina basin was not genuine oceanic crust but either the thinned Arabian rifted continental margin or the continent–ocean transition zone of the Neo-Tethys.

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin

According to palinspastic reconstructions, the Neo-Tethys opening took place during the Permian between the Cimmerian fragments in the north and the Indo-Arabian margin in the south. Igneous remnants of this opening are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed in the Saih Hatat tectonic window. They consist predominantly of pillowed basaltic flows among which three groups have been distinguished. Group 1 is tholeiitic and characterized by low TiO 2 and incompatible trace element contents, and a large range of εNd i values. Group 1 basalts are associated with distal sediments and plot near the boundary of or within the MORB field in the Pb–Pb correlation diagrams and between the MORB and Bulk Silica Earth (BSE) fields in εNd i–( 206Pb/ 204Pb) i diagram. Group 2 basalts are alkaline and differ from Group 1 ones by their higher TiO 2, La and Nb contents, and lower and more homogeneous εNd i values (+3 to +5). Group 2 volcanics are similar to alkali basalts from oceanic islands and share with Group 1 similar initial Pb ratios. Group 3 consists of tholeiitic and alkali basalts which are interbedded either with carbonate-platform sediments from the Saih Hatat window or with distal sediments from the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative εNd i (+1.6 to −2). Group 1 likely derived from the mixing of depleted and enriched sources while Group 2 derived exclusively from an enriched source. There is no indication that continental crust was involved in the genesis of both Groups 1 and 2. In contrast, the low to negative εNd i values of Group 3 suggest that the magmas of this group were contaminated by the Arabian continental crust during their ascent. The geochemical features of the Middle Permian plume-related basalts suggest thus that the basement of the Hawasina basin was not genuine oceanic crust but either the thinned Arabian rifted continental margin or the continent–ocean transition zone of the Neo-Tethys.

Palinspastic reconstructions of southeastern Marlborough, New Zealand, for mid‐Cretaceous‐Eocene times

Southeastern Marlborough, New Zealand, preserves many complete sections through the Cretaceous/ Tertiary (K/T) boundary. Attempts to understand the paleogeography of these sections are hampered by the pervasive, Neogene deformation of the area associated with the propagation of the modern Pacific/Australian plate boundary through New Zealand. In this paper, we produce palinspastic maps of southeastern Marlborough for five intervals of Cretaceous and Paleogene time, based on a retro‐deformed, pre‐Neogene geographic model. Retro‐deformation takes account of: displacements on five major faults; distributed, between‐fault shortening; and a uniform, vertical axis, clockwise rotation of 100°. The mapped intervals are: (1) part of the Urutawan‐Motuan (middle‐late Albian, c. 105–102 Ma); (2) the Piripauan (latest Coniacian to late Santonian, 86.5–84.5 Ma); (3) the Early to early Late Haumurian (late Santonian‐Campanian, 84.5–72 Ma); (4) the late Late Haumurian to late Teurian (late Maastrichtian to late Paleocene, 68–58 Ma); and (5) the Waipawan‐Mangaorapan (early Eocene, 55–51 Ma). During the Cretaceous and Paleogene, southeastern Marlborough lay on the generally north‐facing, Pacific margin of proto‐New Zealand. The palinspastic maps record the progressive drowning of what we infer to be a faulted platform, the “Marlborough paleo‐platform”, that formed the eastern boundary of a large embayment, the “Marlborough paleo‐embayment”. In the late Early and early Late Cretaceous, terrigenous clastic sediments were deposited on the platform at mostly shelf to upper bathyal depths. Ngaterian (late Albian‐Cenomanian) and Piripauan (latest Coniacian to late Santonian) paleoshorelines lay within the study area and were oriented northeast‐southwest. Subsequently, regional, passive subsidence of the continental margin resulted in transgression towards the south and southeast and a switch from terrigenous clastic to biogenic sedimentation. By the end of the Cretaceous, much of the Marlborough paleo‐platform was at outer shelf to bathyal depths; by the early Eocene, it lay entirely at bathyal depths. During the latest Cretaceous and Paleogene, the position of the Marlborough paleo‐embayment coincided approximately with a significant boundary in sedimentary regime, separating dominantly biogenic sediments in the east from mixed biogenic‐siliciclastic sediments to the west. The palinspastic maps show internal consistencies that give us some confidence in the new analysis. Differences from previous maps are attributed both to the retro‐deformation and also to variations in the locations, values, and number of data points used to construct isopachs. Locally restoring paleogeography by retro‐deforming structures is likely to be of most use where the amount of deformation is high (e.g., >20% shortening and/or some tens of kilometres of fault displacements), where the isopachs are well constrained by robust data points, and where regional or global controls on sedimentary and biological patterns are significant and of interest.

Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia

A >2.5-km-thick succession of Tertiary strata in the Eastern Cordillera of southern central Bolivia consists of predominantly fluvial and lacustrine deposits. The age of the base of the succession is middle Paleocene, and its upper part (which is erosionally truncated) is probably late Oligocene-early Miocene. The lower 50-130 m of the succession consists of interbedded fluvial sandstone and mudstone (including minor paleosols) of the Santa Lucia Formation. These strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the Santa Lucia Formation and lower part of the Impora Formation that may represent much of late Paleocene-Eocene time. Above the paleosol zone is a thin zone (∼10 m) of lacustrine carbonate and clastic rocks in the uppermost Impora Formation, and this in turn is overlain by a >2000-m-thick, upward-coarsening succession of clastic fluvial deposits in the Cayara, Camargo, and Suticollo Formations. Paleocurrent data indicate that fluvial channels that deposited the Santa Lucia, Impora, and Cayara Formations flowed mainly westward, whereas channels responsible for the Camargo and Suticollo Formations flowed generally eastward. Modal sandstone petrographic data show a long-term evolution from subarkosic (during Santa Lucia deposition) to quartz-arenitic (during Impora and Cayara deposition) to sublitharenitic (during Camargo and Suticollo deposition) compositions. We argue that the lithofacies and sediment-accumulation history of this succession are most consistent with deposition in an eastward-migrating foreland basin system. E -mail: decelles@geo.arizona.edu. Critical to this interpretation is the zone of extreme stratigraphic condensation in the lower Impora Formation, the inferred upper Paleocene-Eocene part of the succession. The abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous, postrift thermal subsidence resulted in a gradual, continuous decay of early Tertiary sediment-accumulation rates. Stratigraphic condensation is, however, consistent with passage of the forebulge through the region, an interpretation that also can be reconciled with the record of foreland basin development in the Altiplano to the west and gross sediment accumulation patterns in the Subandean zone and modern foreland basin to the east. Simple flexural modeling and palinspastic reconstruction of the complete Cenozoic migration history of the foreland basin system suggest that ∼1000 km of foreland lithosphere has been underthrust westward beneath the Andean orogenic belt at the latitude of southern Bolivia. The amount of middle and lower crust that would have been added to the Andean infrastructure is sufficient to explain the present crustal thicknesses in Bolivia.

Palinspastic reconstruction of structure maps: an automated finite element approach with heterogeneous strain

Existing methods for the palinspastic reconstruction of structure maps do not adequately account for heterogeneous rock strain and hence cannot accurately treat features such as fault terminations and non-cylindrical folds. We propose a new finite element formulation of the map reconstruction problem that treats such features explicitly. In this approach, a model of the map surface, with internal openings that honor the topology of the fault-gap network, is constructed of triangular finite elements. Both model building and reconstruction algorithms are guided by rules relating fault-gap topology to the kinematics of fault motion and are fully automated. We represent the total strain as the sum of a prescribed component of locally homogeneous simple shear and a minimum amount of heterogeneous residual strain. The region within which a particular orientation of simple shear is treated as homogenous can be as small as an individual element or as large as the entire map. For residual strain calculations, we treat the map surface as a hyperelastic membrane. A globally optimum reconstruction is found that unfolds the map while faithfully honoring assigned strain mechanisms, closes fault gaps without overlap or gap and imparts the least possible residual strain in the restored surface. The amount and distribution of the residual strain serves as a diagnostic tool for identifying mapping errors. The method can be used to reconstruct maps offset by any number of faults that terminate, branch and offset each other in arbitrarily complex ways.

Pre-Mesozoic geodynamics of the Precaspian Basin (Kazakhstan)

This paper concentrates on the Pre-Mesozoic history of the Precaspian Basin representing more than half of the 20 km of its sedimentary thickness. We present the evolution of sediments deposited in the basin through time by several regional sections of the basin. These image, from the Devonian, the creation and evolution of a deep intercontinental basin which existed from Carboniferous to Permian. Palinspastic reconstructions show also the evolution of the basin's margins, their relations with the southern pre-Uralian foredeep in the east and carbonate build-ups in the southeast. We present a reconstruction for the early history of the Precaspian Basin, which became an individual basin only at the end of the Permian as a result of the Uralian orogeny. The basement of the Central Precaspian depression is characterised by a thin crust where a low-velocity layer is absent. A layer of abnormally high velocities exists below the crust, which on the basis of structural and gravitational field data is interpreted as eclogite. We put forward a scenario for the age and emplacement of eclogite lenses at the base of the crust. They are possibly implemented in the hangingwall, from a westward subduction zone existing during the Vendian, before the collision giving birth to the Proto-Urals. Most of the basin subsidence is due to the thinning (mainly during Devonian) and increase of the average density of the crust. Present crustal density has characteristics of intermediate to lower crust interpreted by some authors as oceanic. We propose that the loading effect of the eclogites emplacement may explain also a part of the Precaspian Basin subsidence.

Middle Paleozoic subduction belts: The leading factor in the formation of the Central Asian fold-and-thrust belt

The Paleozoic fold-and-thrust belt, confined between the European, Siberian, Tarim, and North China Precambrian continents, results from a complex evolution of the Paleo-Asian Ocean. At the end of the Ordovician, the Kazakhstan-Kyrghyz continent, originating from the accretion of island arcs and Gondwanan continental fragments, divided the Paleo-Asian Ocean into four oceanic basins, Uralian, Turkestan, Junggar- Balkhash, and Ob-Zaisan. The Middle to Late Paleozoic history of these oceanic basins, which closed completely in the terminal Carboniferous to Permian, is portrayed in eight detailed, 1:10,000,000 scale, palinspastic reconstructions for the Early Silurian (430 Ma), Early Devonian (Emsian, 390 Ma), Middle Devonian (Givetian, 380 Ma), Late Devonian (Famennian, 360 Ma), Early Carboniferous (late Visean to Serpukhovian, 330 Ma), early Late Carboniferous (305 Ma), Early Permian (280 Ma), and Late Permian (255 Ma) time slices. These reconstructions draw on 1:2,500,000 scale sedimentologic-paleogeographic maps and paleomagnetic measurements from ancient continents and Variscan orogenic zones of the Urals, Kazakhstan, Tien Shan, Junggaria, and Altay. The shrinking and collision-induced closure of the oceans were ensured by the three large and long-lived (100- 130 m.y.) Urals-Tien Shan, Junggar, and Siberian subduction belts, spanning thousands of kilometers, whose polarities remained stable. The belts were represented by systems of roughly parallel and branching subduction zones, each with a 10-30 m.y. lifespan, plunging beneath the Kazakhstan-Kyrghyz and Siberian continents. Taken together, they constituted a system that diverged in a southwesterly direction and ensured dierential rotations of the European, Siberian, and Kazakhstan-Kyrghyz continents. The Urals-Turkestan belt began to form at the beginning of the Silurian, and the Siberian and Junggar belts, at the beginning of the Devonian. The subduction belts ceased to exist as they were crushed between continents during a general collision that set on in the second half of the Devonian and in which the Junggar belt became involved prior to the beginning of the Permian. Geologic and paleomagnetic evidence points to oblique motions of oceanic plates being consumed in the subduction belts and, accordingly, to an oblique collision in the Urals and South Tien Shan foldbelts that propagated through time and space to finally give rise to large-scale post-collisional lengthwise strike slips. We believe the subduction belts to be surface manifestations of descending mantle convection flows that drove the long-lasting sinking of oceanic plates into the mantle.

Palinspastic restoration of the Anniston transverse zone in the Appalachian thrust belt, Alabama

Transverse zones are cross-strike alignments of cross-strike linking structures (lateral ramps, transverse faults, and displacement-transfer zones) in foreland thrust belts. Cross-strike links constitute one component of a three-dimensional system of fault surfaces, connecting strike-parallel structures (frontal ramps) and detachment flats both geometrically and kinematically. Three-dimensional palinspastic restoration provides the basis to consider causes of cross-strike alignments of cross-strike links into transverse zones, as well as the role of transverse zones in kinematic history. The Anniston transverse zone in the Appalachian thrust belt in Alabama (southeastern United States) exemplifies the types and distribution of cross-strike links within a transverse zone. Palinspastic reconstruction of the Anniston transverse zone relies on matching hanging-wall/footwall pairs of frontal and lateral ramps for geometric balance in three dimensions in palinspastic-map and cross-section views. Cross-strike links within the Anniston transverse zone are systematically distributed across strike, from hinterland to foreland, as domains of lateral ramps, transverse faults, and displacement-transfer zones, in response to variations in depth to basement beneath the thrust belt and variations in thickness of décollement-host weak rocks. Contrasting structural profiles characterize the thrust belt on opposite sides of the Anniston transverse zone as a result of abrupt along-strike changes at the cross-strike alignment of cross-strike links. The Anniston transverse zone is aligned with a northwest-striking basement fault that offsets the boundary faults of the Birmingham basement graben, suggesting kinematic partitioning within the advancing thrust sheets at a stress concentrator. The northwest-striking basement fault separates domains of contrasting structural profiles of basement fault systems, differing elevations of top of basement, and differing thicknesses of the regional décollement-host weak layer in the lower part of the sedimentary succession above basement rocks.

Archaean tectonics: a review, with illustrations from the Slave craton

Abstract The tectonic evolution of Archaean granite-greenstone terranes remains controversial. Here this subject is reviewed and illustrated with new data from the Slave craton. These data show that a thick, c. 2.7Ga, pillow basalt sequences extruded across extended sialic basement of the Slave craton at a scale comparable with that of modern large igneous provinces. The pillow basalts do not represent obducted oceanic allochthons. Basement-cover relationships argue for autochthonous to parautochthonous development of the basaltic greenstone belts of the west-central Slave craton, an interpretation that is further supported by geochemical and geochronological data. Similar data exist for several other cratons and granite-greenstone terrains, including the Abitibi greenstone belt of the Superior craton, where stratigraphic and subtle zircon inheritance data are equally incompatible with accretion of oceanic allochthons. Many classical granite-greenstone terrains, including most well-documented komatiite occurrences, thus appear to have formed in extensional environments within or on the margins of older continental crust. Closest modern analogues for such basalt-komatiite-rhyolite-dominated greenstone successions are rifts, marginal basins and volcanic rifted margins. Indeed, these environments have high preservation potential compared with fully oceanic settings. Collapse and structural telescoping of these highly extended volcano-sedimentary basins would allow for the complex structural development seen in granite-greenstone terrains while maintaining broadly autochthonous to parautochthonous tectonostratigraphic relationships. Seismic reflection profiles cannot discriminate between these telescoped autochthonous to parautochthonous settings and truly allochthonous accretionary complexes. Only carefully constructed structural-stratigraphic cross-sections, allowing some degree of palinspastic reconstruction, and underpinned by sufficient U-Pb zircon dating, can address the degree of allochthoneity of greenstone packages. Furthermore, seismic reflection profiles are essentially blind for the steep structures produced by multiple phases of upright folding and buoyant rise of mid- to lower-crustal, composite, granitoid and gneiss domes. Such structures are ubiquitous in granite-greenstone terrains and, indeed, most of these terrains appear to have experienced at least one phase of convective overturn to re-establish a stable density configuration, irrespective of the complexities of the pre-doming structural history. Buoyant rise of mid- to lower-crustal granitoid and gneiss domes can explain the typical size and spacing characteristics of such domes in granite-greenstone terranes, and the coeval deposition of late-kinematic, ‘Timiskaming-type’ conglomerate-sandstone successions in flanking basins. The extensional and subsequent contractional evolution of granite-greenstone terrains may have occurred in the overall context of a plate tectonic regime (e.g. volcanic rifted margins, back-arc basins) but highly extended, intraplate, rift-like settings seem equally plausible. Explaining the evolution of the latter in terms of Wilson cycles is misguided. Periods of intense rifting and flood volcanism (e.g. 2.73–2.70 Ga) may have been related to increased mantle plume activity or perhaps catastrophic mantle overturn events. Although there is evidence for plate-like lateral movement in late Archaean time (e.g. lateral heterogeneity of cratons, arc-like volcanism, cratonscale deformation patterns, strike-slip faults, etc.), the details of how these plate-like crustal blocks interacted and how they responded to rifting and collision appear to have differed significantly from those in Phanerozoic time. The most productive approach for Archaean research is probably to more fully understand and quantify these differences rather than the common emphasis on the superficial similarities with modern plate tectonics.

Regional electrical resistivity structure of the southern Canadian Cordillera and its physical interpretation

The regional geoelectric crustal structure of the southern and central Canadian Cordillera of western Canada is interpreted from the inversion of magnetotelluric data along five profiles crossing the physiographic morphogeological belts, with emphasis on the Intermontane and Omineca Belts. Decomposition of the tensor impedance response estimates demonstrates that large‐scale regional structures can be reasonably approximated along each profile as two‐dimensional, with dominant geoelectric strikes of either −30° or +15°, depending on profile location. These profile‐specific strike directions are consistent with a local clockwise rotation of crustal structures in the southern Intermontane and Omineca Belts suggested by others based on paleomagnetic data and palinspastic reconstructions. Comparing the resistivity models derived from two‐dimensional inversions of the distortion‐corrected data along each profile allows us to construct an orogen‐scale three‐dimensional resistivity model for southern British Columbia. Generally, the model shows a resistive upper crust overlying a conductive lower crust. The resistivity of the lower crust beneath the Intermontane Belt is independent of latitude and is similar for all profiles. In stark contrast, a 2 orders of magnitude variation in lower crustal resistivity is observed along strike in the Omineca Belt, with higher conductivities to the south in the region of Eocene extension and lower conductivities to the north in the unextended part of the belt. Such spatial association has noteworthy implications for the cause of lower crustal resistivity in active, or recently active, young regions. Our preferred interpretation of the observed lower crustal resistivities with other geophysical data is in terms of fluids, with brines dominating for the most part but partial melt possible at the base of the crust in specific localities. We attribute the along‐strike variation in the Omineca Belt mostly to variation in fluid content and interconnectivity, with the lowermost crust of the southern Omineca Belt being partially molten. This physical state difference is a consequence of degree of extension and implies that mantle‐derived fluids are important for lower crustal resistivity.