Main Extensional Events Research Articles

Late Devonian paleogeography in the framework of global plate tectonics

This paper presents Late Devonian (Frasnian–Famennian) global and regional paleogeographic maps displaying present day coastlines, tectonic elements' boundaries, subductions, rifts, spreading centers, transform faults, paleogeographic configuration and volcanism 370 million years ago. The regional maps illustrate the paleoenvironment and paleolithofacies distribution. The Late Devonian was a time of the onset and development of a major collisional event, the Variscan orogeny. The trench-pulling (or slab-pull) effect of the north dipping subduction, which developed along the Laurussia margin, caused the creation of the back-arc Rheno-Hercynian basin, as well as the transfer of tectonic elements. These tectonic elements included Saxothuringian, Southern Proto-Carpathian and Balkan terranes. The Antler and Ellesmerian orogenies constituted major collisional events in North America. The spreading of the Paleotethys Ocean constituted the main extensional event. This spreading is associated with the movement of tectonic elements towards Laurussia, Siberia and Northern Kazakhstan. In addition, a branch of the Paleotethys Ocean was opened between South China and Gondwana, during the Late Devonian times. The spreading was displayed along the proto-Andean margin of western Gondwana and is thought to have opened the newly proposed Chilean Ocean. The development of major rift systems took place throughout Laurussia and Siberia. Late Devonian rifting was associated with volcanic activity, especially prominent in the Viluy rift in Siberia. The deposition, during the Late Devonian time, is characterized by the existence of large carbonate platforms with reefs on large continents and synorogenic flysch in collisional areas. The sea-level dropped towards the Devonian- Carboniferous boundary. The climate was undergoing change from greenhouse to icehouse. The following plate tectonic events could have influenced the extinction of biotas at the Frasnian-Famennian boundary: 1) The very extensive basaltic volcanism in Laurussia and Siberian and along the arcs. 2) The closure of the Rheic Ocean and the development of the Variscan orogeny. 3) Intensive spreading in the Paleotethys and the development of numerous subductions and volcanic arcs. These events resembled a transition from a rift to a drift phase, during the Permian–Mesozoic break-up of Pangaea. This break-up was associated with other biota extinctions. • The paper presents 10 paleogeographic Late Devonian maps. • Late Devonian was a time of the onset of Variscan orogeny. Spreading of the Paleotethys Ocean was the main extensional event. • Late Devonian rifting was associated with volcanic activity, especially prominent in the Viluy rift in Siberia. • The plate tectonic events could have influenced the extinction of biotas.

Late Oligocene to Pliocene Extension in the Maltese Islands and Implications for Geodynamics of the Pantelleria Rift and Pelagian Platform

AbstractThe tectonic and geodynamic evolution of the Pelagian platform and of the Pantelleria Rift System, located in the foreland of the Apennine‐Sicilian‐Maghrebian Belt, from the Late Oligocene onward, is still debated. Here we present a new interpretation based on structural data collected in the Malta and Gozo islands. With a tectonic back‐stripping approach, we recognized two main extensional events separated by a period of tectonic quiescence: (D1) Early Miocene WNW‐ESE extension and (D2) Late Miocene to Pliocene N‐S extension that led to the opening of the Pantelleria Rift System. During both extensional events the Maltese Islands and the Pelagian Platform were located in the foreland of the Apennine‐Sicilian‐Maghrebian Belt (Western Mediterranean Arc), in a period of slab rollback. We suggest that both tectonic events are an evidence of the extensional regime imposed on the foreland area by the rollback of the subducting slab. The switch in the main extension axis between D1 and D2 (WNW‐ESE vs. N‐S) can be interpreted as a result of the varying orientation and distance of the trench due to the progressive development of the Western Mediterranean Arc and provides an independent tool to “monitor” the slab rollback. Results obtained in this study allowed, for the first time, to bind the tectonic evolution of the area with the geodynamic evolution of the Western and Central Mediterranean.

The tectono-magmatic setting of the Hercynian upper continental crust exposed in Calabria (Italy) as revealed by the 1:10,000 structural-geological map of the Levadio stream area

This study presents a structural-geological map illustrating the tectonic contact between the Stilo-Pazzano Phyllite Unit and the Mammola Paragneiss Unit. Their juxaposition is exposed in the Paleozoic upper continental crust cropping out in the southern Serre Massif. Since the geometry of the contact has never been defined before, a detailed geological survey has been carried out in the Levadio stream area, i.e. one of the most promising areas to constrain the tectono-magmatic setting of the Hercynian upper continental crust exposed in Calabria. A structural-geological map, at a scale 1:10,000, has been constructed, including also Upper Carboniferousgranodiorites, belonging to the Serre batholith, and Lower Permian felsic dykes, both useful time markers to constrain the minimum age for the juxtaposition of the Stilo-Pazzano Phyllite and Mammola Paragneiss units wall rocks. According to petrographic and structural observations, the attempt to discriminate on the geological map the main deformation events has been made also by the use of tracing distinct fold-axial-planes, each referring to the related contractional deformation event. This approach makes the geological map intuitive for the relative timing of the deformational events affecting the Stilo-Pazzano Phyllite and Mammola Paragneiss units. The geological mapping of the Levadio stream area shows that the post-Visean – pre-Upper Carboniferous tectonics was characterized by a main extensional event, that was responsible for the juxtaposition of the Mammola Paragneiss Unit (below) and the Stilo-Pazzano Phyllite Unit (above) under greenschist facies conditions. Three subsequent, progressively weaker contractional deformation events occurred in response to the late Hercynian emplacement and incremental growth of the Serre batholith.

DEFORMATION HISTORY AND CORRELATION OF PAIKON AND TZENA TERRANES (AXIOS ZONE, CENTRAL MACEDONIA, GREECE)

Paikon and Tzena terranes are situated in the centre part of Axios zone, between Almopia and Paionia ophiolitic belts. Tectonostratigraphic data reveal that both have been affected by the same polyphase deformation and metamorphism, as well that they have the same lithostratigraphic column. The first deformation phase took place during the Middle to Late Jurassic and is associated with ophiolite obduction, nappe- stacking, terrane accretion and crustal thickening (D1). Metamorphism does not exceed greenschist facies (M1). Relict HP-LT metamorphic assemblages predating M1 metamorphism are possibly developed during subduction processes and overloading of the obducted ophiolites on the continental margin, characterized the initial stages of deformation. Compressional tectonics and intense thrusting with the same kinematics continued in Lower Cretaceous time, affected all pre-Upper Cretaceous units and the obducted ophiolites (D2). This phase is associated with low-greenschist metamorphism (M2). The first main extensional event occurs in the Late Cretaceous, related to basin formation and sedimentation (D3). During Paleocene to Eocene, D4 intense imbrication of all tectonic units towards mainly SW takes place again. Nappes collapse and finally crustal exhumation taken place during Oligocene to Miocene, associated with low - angle normal faults, with a main top to the SW sense of movement (D5). In Miocene to recent times, high - angle normal and strike-slip faults are formed in an extensional to transtensional strain regime (D6), associated with Neogene to Quaternary basin formation and terrane dispersion. The basement rocks of both terranes are of Pelagonian origin, exhumed as a multiple tectonic window.

Tectonic and climatic controls on the deposition of the Permo-Carboniferous Grant Group and Reeves Formation in the Fitzroy Trough, Canning Basin, Western Australia

Interpretation of seismic and well data from the Fitzroy Trough, Barbwire Terrace and Lennard Shelf provides constraint for the tectonic and climatic control on the deposition and architecture of the Permo-Carboniferous Reeves Formation and Grant Group in the Fitzroy Trough.An angular unconformity is recognised at the base of the Reeves Formation, recording the middle Carboniferous Meda Transpressional Movement. This separates two major extensional phases that affected the basin: the Devonian–Early Carboniferous Pillara Extension and the Late Carboniferous Point Moody Extension. The Reeves Formation is interpreted to have been deposited during the Late Carboniferous Point Moody Extension phase, and as such is a syn-rift sequence. Its distribution is restricted to the Fitzroy Trough, with a thickness in-excess of 1875 m deposited in the half-graben, clearly observed to thicken toward the actively moving Fenton Fault. Compressional anticlines underlying the Reeves Formation, and located at left-lateral bends in the Fenton Fault, indicate associated dextral strike-slip movement.Extensional faulting ceased prior to deposition of the overlying Grant Group. Thickening of the Grant Group into the Fitzroy Trough is interpreted to be the result of thermal sag following the main extensional event, and associated compaction of the underlying thick syn-rift sequence and minor displacement along the Fenton Fault.The well-established subdivision of the Grant Group into the Betty, Winifred and Carolyn formations is interpreted to correlate directly with the subdivision of the group proposed by Redfern (1990) on the Barbwire Terrace into Hoya, Calytrix and Clianthus formations. Evidence for climatic control on the depositional style and architecture includes the prominent glacially eroded Base Grant Group unconformity and prominent erosional surfaces associated with facies (diamictites, varved mudrocks) that all suggest a glacial depositional environment. Deglaciation and relative rise in base level gave rise the mudstone dominated middle unit. The absence of glacial signature within the uppermost Clianthus Formation on the Barbwire Terrace (laterally equivalent to the Carolyn Formation on the Lennard Shelf) is attributed to restriction of the remnant ice sheet or glaciers to the margins of the basin, at the final stage of glaciation of the basin.

Reply to the comments of Jean Philip on the paper entitled

AbstractWe reply to the comments of J. Philip regarding the structure of La Nerthe range (southern Provence, France) and the timing of the deformation. We first agree with J. Philip on the structural independence of La Nerthe and L’Etoile ranges. We then discuss the allochthonous and autochthonous models. The allochthonous model mainly relies on a reactivation of a N-verging thrust during the Oligocene. There are no evidences for a Middle Rupelian thrusting event and the interpretation of the Oligocene series in southern Provence area was entirely revised. J. Philip’s argumentation is solely based on the existence of steep dipping Rupelian limestones. However we demonstrate that they could be tilted along normal faults as it is the case in the Marseille basin. Recent works clearly show that the Oligocene Marseille and Saint-Pierre basins have a similar tectonic history resulting from two main extensional events. The last point debated by J. Philip is the age of the strike-slip faults. As it is pointed in our contribution the strike-slip fault planes cut folded strata and were reactivated during an extensional event. This strike-slip faulting event occurred between the latest stages of the main Bartonian compressional event and the beginning of the Early Rupelian extensional tectonics. As pointed by J. Philip the E-trending faults of Saint-Pierre basin acted as normal faults during the Oligocene. We however suggest that these faults were inherited from the Late Eocene strike-slip tectonics and reactivated during the Oligocene.

Thermo-mechanical evolution and rheology of the northern sector of the Tyrrhenian–Apennines system

The connection between the thermal field and the mechanical properties in the northern sector of the Tyrrhenian–Apennines system is analysed. The thermal setting is the result of three main extensional events, which have taken place within relatively small space and time intervals. The oldest tectono-thermal event took place in eastern Corsica 14 Myr ago and the second between Capraia and Elba islands 7 Myr ago. The third and youngest rift episode, with an average age of 3 Myr, affected a wider area extending east of Elba Island and including most of the peri-Tyrrhenian zone. By assuming lithosphere extension, the expected Moho temperature increases eastward from 550 to 750 °C. Uniform stretching cannot account for the high heat flux in the zone affected by the youngest tectono-thermal event. For this zone, we propose a thermo-mechanical model, which incorporates the removal of 15 km of mantle lithosphere, as a result of eastward asthenosphere flow induced by the Apennines subduction. This yields a predicted heat flux in agreement with that observed. Lithosphere mechanical strength analysis indicates that the thickness of the uppermost crustal–brittle layer decreases eastward from 15 to 10 km, while the temperature discriminating the rheological behaviour is rather uniform, being on average, 320 °C.

Tectonics and crustal structure of the Campania continental margin: relationships with volcanism

¶The crustal structure of the Campania continental margin is synthesized from outcrop, seismic reflection and gravimetric data. Outcrop and subsurface geological data reveal the presence of NE–SW faults, E–W faults and NW–SE faults. An older extensional event occurred along NW–SE faults and was followed by the main extensional event linked to the activity of NE–SW normal faults. The latter were active between 700 and 400 ka producing half-grabens filled by more than 5 km of Quaternary deposits. The stratigraphic signature of these tectonic events corresponds to a Lower Pleistocene marine unconformity-bounded unit overlain by Middle Pleistocene rocks belonging to a transgressive-regressive cycle. A crustal section of the Campania margin displays an asymmetric linked fault system characterized by a 10–12 km-deep main detachment level, listric normal faults and rollover anticlines. Structural and stratigraphic data document that the inception of volcanic activity at Vesuvius occurred at 400 ka, just after the main extensional event, and the volcano is located at the margin of a rollover anticline.

Jurassic to Holocene tectonics, magmatism, and metallogeny of northwestern Mexico

The present metallic distribution in northwestern Mexico is the culmination of superposed magmatism, tectonism, erosion, and burial over more than 150 m.y. Detailed palinspastic reconstructions of preextensional configurations—the first study of its kind for this region—clarify the interplay among these features on the present distribution and character of mineralized geologic systems. This new synthesis goes beyond previous metallogenic investigations of northwestern Mexico by separating events into specific timing and structural relationships, and by restoring the geology to its preextensional configuration. Metallogenic factors such as enrichment, preservation, and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern Mexico. The analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history, rather than simply metallic belts or interpreted angle of a subducting slab. Three main extensional events in the Oligocene‐Holocene have been restored, and the palinspastic distributions have been analyzed. Reconstructions reveal the following: (1) Mineralization events, igneous centers, and sedimentary sequences are continuous across the Gulf of California and other areas with large amounts of extension. (2) Middle Tertiary gold-silver mineralization in Baja California may be the western part of the Sierra Madre Oc*Present address: BHP Billiton., Avenida America Vespucio Sur 100, 8th floor, Las Condes, Santiago, Chile; e-mail: John-Mark.G.Staude@ BHPBilliton.com. cidental metallogenic province, thus expanding the previously recognized extent of this province’s mineralization. (3) Late Cretaceous‐early Tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km, with parts of the belt buried beneath the younger Sierra Madre Occidental volcanic fields. (4) Interpreted alignments of older geologic features, including lineaments of ore deposits, are displaced in the reconstructions. (5) Sedimentary-rock‐hosted gold deposits and low-angle-detachment gold systems are closely related and occur around core complexes. By using structurally restored time slices, it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions, whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic-rock hosted. These characteristics express themselves in the regional distribution of deposit types. Second, mineralization of widely differing ages is spatially superposed, commonly associated with coeval magmatic and tectonic events. The structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern Mexico.

Le filtrage des anomalies gravimetriques; une cle pour la comprehension des structures tectoniques du Boulonnais et de l'Artois (France)

Abstract The geology of the Boulonnais has been well studied since the early part of the last century [Gosselet and Bertaut, 1873; Olry, 1904; Pruvost and Delepine, 1921]. Extensive coal exploration added substantially to the general understanding of the geology of the region but as outcrop is poor, many questions remain. Gravity methods used in the analysis of geological structures have had a long and successful history in helping to study the earth's crust for scientific and applied objectives. Regional gravity data are particularly useful in mapping geographic distribution and configuration of density contrast of rocks. Previous gravity research shows the main trends of the structure. In most cases the regional Bouguer gravity hides the relationship between the geology and the shape of the anomaly caused by the perturbing body. New information can be obtained by filtering the maps. The purpose of filtering a map is to remove unwanted characteristics and enhance desirable characteristics that are diagnostic for the geology. Because of their simple mathematical forms, most potential field filters are in the spectral domain. It is advisable to transform the original unfiltered field to the spectral domain, apply the filter, then transform the filtered map back to the spatial domain for use in the interpretation. Several spectrally filtered versions of the original gravity map are used in this regional interpretation. In the case of the Boulonnais the most useful filters have been the horizontal component and the first vertical derivative. In the first instance computing the horizontal gradients of the gravity field permits us to localise the limit of the blocks and then the fault positions. The gravimetric field above a vertical contact of rock with different density shows a low on the side of the low density rocks and a high on the side of the high density rocks. The inflection point is located just on the contact of the two types of rocks. This contact can be outlined by locating the maxima of the horizontal gradient. In the case of a low dipping contact maxima stay close to the contact, but are displaced down dip. In the second instance the first vertical derivative acts as a booster for the short wavelength; this attenuates or destroys the effect of the regional field. The resulting map shows a better structure because in complex areas they give a better definition of the different bodies by separating their effects. In the case of the Boulonnais the first vertical derivative allows us to distinguish the depressed region from the uplifted one. The structural evolution of the Boulonnais-Artois area includes two main extensional events in the late Palaeozoic-early Cretaceous interval and an inversion in mid-late Palaeocene time. The new gravity data in combination with recent field and published data have provided a new insight into the structure of the Boulonnais-Artois area and a new interpretation is proposed. -- Fault patterns are oriented 110N and 040N in the Boulonnais and 140N in Artois areas. -- The linkage between the faults shows a relay geometry with transfer zones [cf. Morley et al., 1990 and Pea-cock and Sanderson, 1994]. The best example is located between Sangatte (near the tunnel) and Landrethun faults where overlapping synthetic faults with a relay ramp are imaged. -- There is no major continuous fault zone but a complex en echelon fault system. -- Linkage between Boulonnais and Artois fault is not well constrained. An important discontinuity between the two regions is apparent. This model underlines the importance of overlapping fault tips with the generation of transfer zones. These structures are also known in the Wessex and Weald basins [Stoneley, 1982; Chadwick, 1993] where heritage and inversion are significant.

Structure and evolution of the Galicia Interior Basin (Atlantic western Iberian continental margin)

An industrial multichannel seismic reflection data set is presented on the Galicia Interior Basin (G.I.B.), a sedimentary basin that has been poorly studied until now. Seismic stratigraphic units defined in other areas of the western Iberian continental margin as well as some deeper reflections can also be identified within this basin, although they show some particular characteristics. The G.I.B. is bounded to the east by a major fault identified beneath the continental shelf and to the west by an alignment of highs (“Western Banks”). The structural framework of the area consists of NNW-SSE-trending normal faults cross-cut by NE-SW transfer faults. The inception of the G.I.B. probably ocurred during Triassic times. Within the Late Jurassic-Early Cretaceous rifting phase it probably experienced several extensional episodes, the most intense one being during Valanginian times. Evidence for compressional tectonic activity in this basin during Cenozoic times has also been found. When comparing the G.I.B. with the adjacent “deep Galicia margin” a gap in the timing of the main extensional events as well as some differences in the structural styles have been found. In a wider regional context, an integration of data about the timing of the processes of continental rifting and oceanic spreading along the whole western Iberian margin is consistent with a northward propagating rift for the Late Jurassic-Early Cretaceous phase.