Tectono-sedimentary Evolution Research Articles

Évolution tectono-sédimentaire des bassins flexuraux profonds et à taux de sédimentation réduit : exemple du bassin de flysch de Saint-Jean-de-Luz (Pyrénées-Atlantiques, France) au Crétacé supérieur

The integrated analysis of thrust structures and fades allows us to interpret the late Cretaceous flysch basin of Saint-Jean-de-Luz as a North-Pyrenean foreland basin active since Turanian times. This example illustrates the tectono-sedimentary evolution of fold-and-thrust belts in the early stage of shortening in deep marine and starved environments. The formation and the migration of the depocenters are related to the propagation of a blind frontal thrust. The sedimentary infill of the basin is subdivided into numerous depositional sequences, tectonic in origin. They consist of lowstand system tracts related to an increase in tectonic activity, and highstand system tracts related to a decrease in this activity.

Along-strike structural variations in the foreland thrust and fold belt of the southern Urals

The foreland thrust and fold belt of the southern Urals has a number of along-strike structural changes in which regional-scale structures terminate abruptly. One important along-strike structural change is the southern termination of the Precambrian-cored Bashkirian Anticlinorium, which plunges southward beneath the synformal Zilair Nappe. Balanced and restored dip cross-sections, a N-S-oriented strike section, together with a geological interpretation of a CDP reflection seismic profile have been used to determine the 3D structure and the amount of shortening in the foreland thrust and fold belt in the area of the Bashkirian Anticlinorium and the Zilair Nappe. The geometry of the foreland thrust and fold belt is that of a west-vergent, basement-involved thrust stack with shortening on the order of 13% to 17% (approximately 17 km). The small amount of shortening allows a correlation to be made between the along-strike structural changes and pre-existing basement structures. In particular, the southern termination of the Bashkirian Anticlinorium coincides with a Precambrian-age aulacogen, the Sernovodsk-Abdulino Aulacogen, developed in the basement. An E-W-oriented tear fault, the Belaya tear fault, appears to have developed in the basement, along the northern margin of the Sernovodsk-Abdulino Aulacogen. The Belaya tear fault was active during much of the history of the foreland thrust and fold belt, and exerted strong control on its structural and tectonosedimentary development. Because of the small amount of shortening involved, the foreland thrust and fold belt of the southern Urals provides an important example for understanding the effects of pre-existing basement structures on the development of foreland thrust and fold belt's.

Late Cenozoic tectono-sedimentary development of the Selendi and Uşak-Güre basins: a contribution to the discussion on the development of east–west and north trending basins in western Turkey

This paper expands the K–Ar dating and palynologically controlled stratigraphical data base reported in earlier papers to the north trending Selendi and Uşak-Güre basins located to the north of east–west trending Alaşehir graben in western Turkey. These north trending basins began to form during Early Miocene times and most of their basin fills accumulated before 14 Ma, except for the youngest Asartepe formation. Recent studies of both east–west grabens and north trending basins show that they started to develop simultaneously during Early Miocene times under the north–south extensional regime, and the classification of the structures as ‘replacement’ and ‘revolutionary’ has no meaning for the Alaşehir graben and the basins located to its north.

Abstract: Tectono-Sedimentary Evolution of the Middle Pontides from Jurassic to Lower Cretaceous, Turkey 

Abstract: Tectono-Sedimentary Evolution of the Middle Pontides from Jurassic to Lower Cretaceous, Turkey 

Tectono-sedimentary evolution of the Upper Prealpine nappe (Switzerland and France): nappe formation by Late Cretaceous-Paleogene accretion

The Upper Prealpine nappe of the Swiss and French Prealps consists of a composite stack of various tectonic slivers (Gets, Simme, Dranse and Sarine sub-nappes, from top to bottom). The structural superposition and stratigraphic content of the individual sub-nappes suggests a successive stacking at the South Penninic/Adriatic transition zone during the Late Cretaceous and Early Paleogene. The present paper deals with two aspects. (1) new data obtained from the Complexe de base Series of the Dranse sub-nappe which underlies the Helminthoid Sandstone Formation, and (2) the development of a geodynamic accretionary model for the Upper Prealpine nappe stacking.The Complexe de base Series reveals a succession of black shales at the base, grading upward into variegated red/green and red shales which were deposited in an abyssal plain environment starved of clastic input. It is overlain by the Helminthoid Sandstone Formation. The combined analysis of planktic and agglutinated benthic foraminifera and comparisons with other Tethyan series suggest an Albian to Campanian age of the Complexe de base succession. Tectonic transport of the abyssal plain segment into a trench environment allowed for the stratigraphic superposition by the Helminthoid sandstone sequence. The present findings combine well with the general scheme of the Upper Prealpine nappe stack and several single results on parts of the nappe stack. We take that opportunity to present a comprehensive model for the tectono-sedimentary evolution of the Upper Prealpine nappe.We suggest that Late Jurassic-Early Cretaceous asymmetric (?) extension at the South Penninic-Adriatic margin created an extensional alloehthon. Later during the mid-Cretaceous, the start of convergence drove the obduction of oceanic crust on the northern margin of the extensional allochthon. The resulting ophiolitic/continental source supplied clasts to the trench basin in front (Manche turbidite series), and the backarc basin (Mocausa Formation) and abyssal plain (Perrières turbidite series) to the South. During Middle to Late Coniacian the main Adriatic margin was thrusted over the obductionrelated mixed belt and established an incipient accretionary prism containing the former trench, backarc and abyssal plain basin fill series. During this stage the Gueyraz (melange) Complex formed, which separates the trench series from the retroarc and abyssal plain formations. On top of the incipient accretionary prism a forearc basin developed hosting the Hundsrück Formation. The frontal abyssal plain formation (Complexe de base) still received few turbiditic intercalations. From Campanian time on, the forearc basin was bypassed and deposition of the Helminthoid Sandstone Formation occurred on the Complexe de base succession. During the Maastrichtian the abyssal plain and trench fill succession (Dranse nappe) was accreted to the incipient wedge, and in front of a newly active buttress, the Gurnigel trench basin was established. Another accretionary event during latest Paleocene/earliest Eocene added parts of that trench series to the base of the wedge (Sarine nappe). During the Late Eocene the accretionary wedge and remaining trench fill series (Gurnigel nappe) were thrusted en-bloc over the Middle Penninic limestone nappes and partly overtook the latter. Continued shortening of the resulting nappe pile and out-of-sequence thrusting accomplished the overriding of the Middle Penninic units over the former South Penninic Gurnigel trench series (inversion of palaeogeographic domains).

Late Cretaceous subsidence history of the Betic Continental Margin (Jumilla-Yecla region, SE Spain)

A combined approach of subsidence and tectono-stratigraphic analyses is presented to document the vertical tectonic motions of the crust in the Betic margin (SE part of the Iberian Plate) during the late Cretaceous (late Albian-late Maastrichtian), when the basin was evoling from a passive margin into a convergent one. The overall patterns of tectonic subsidence and paleogeography are characterized by the following phases: (1) middle late Albian: regional tectonic pulse (98 Ma), major basin reorganization; (2) latest Albian-early middle Cenomanian: relatively rapid subsidence, wide carbonate platform development; (3) late middle Cenomanian-early Coniacian: regional uplift and related tectonic pulses at 94.5 and 93.5 Ma followed by a prolonged low-subsidence phase; (4) intra-Coniacian extensional tectonic event (88.5 Ma); (5) late Coniacian-late Santonian interval: relative tectonic quiescence and moderate subsidence rates, wide carbonate platform development; (6) latest Santonian-earliest Campanian: compressional tectonic event (83.5 Ma)-restructuring of the Betic margin; (7) early Campanian-early Maastrichtian: low-subsidence, narrow mixed carbonate-siliciclastic platforms; (8) “mid”-Maastrichtian: tectonic event (71 Ma)-abrupt paleographic changes; (9) late Maastrichtian: overall, moderate subsidence, high terrigenous influx; and (10) latest Maastrichtian: regional tectonic event, major basinal reorganization. Characterization of these periods, which include the first compressional (or transpressional) tectonic phases of the Alpine Orogeny in the Betic Margin, permits the integration of the late Cretaceous tectono-sedimentary evolution of the Betic Margin into the geodynamic evolution of Iberia. On the basis of this integration, tectonic pulses and related changes in subsidence are interpreted as the result of variations on intraplate stresses, which are superimposed on the general, longer term subsidence of the passive margin. These intraplate stresses were governed by (1) the North Atlantic spreading and the consequent decrease in the sinistral relative motion between Iberia and Africa; (2) the progressive opening of the Bay of Biscay between the late Aptian and the early Campanian; and (3) the onset of the oblique convergence between Africa and Europe in the late Cretaceous.

Petroleum Systems of the Burgua Depression, Tachira and Apure States, Venezuelan Andes: ABSTRACT

The Burgua Depression (southeast extension of the Tachira Depression) underwent a severe subsidence during the Andean orogeny. Its tectonosedimentary evolution was controlled by Paleozoic discontinuities. The sedimentary sequence contains Jurassic/Lower Cretaceous syn-rifting transgressive rocks, synDeformational Paleocene/Eocene and Neogene rocks. Deformation is diachronic from SE (Oligocene) to NW (Pleistocene). From oil-rock correlation data, Navay and Los Cuervos samples render excellent potential source rocks. The analyses indicate a marine algal-bacterial organic matter source for Navay and higher plants for Los Cuervos, which plot as moderate yield type II kerogen and low yield type II-III, respectively. Although, the samples are immature (0.50-0.55% R6), ID modelling (BasinMod) suggests that they reached maturation during the Andean orogeny. Oil seeps correlate with Navay and Los Cuervos and were expelled at the peak of oil generation (0.85-0.90% R{sub o}). These source rocks define the Navay-K/T(!) and Los Cuervos-K/T(!) petroleum systems. Potential clastic and carbonatic reservoirs were identified in surface and wells. The Guafita Formation is the regional seal, never extending northward of the Brujas High. Modelling indicates that migration started during late Pliocene and took place northward and south/southeastward of the Andean foredeep.

Different foreland basins in Italy: examples from the central and southern Adriatic Sea

The Central Adriatic Basin (CAB) and the Southern Adriatic Basin (SAB) are foreland (or foredeep) basins relative to the non-coeval Apenninic and Dinaric-Hellenic orogenic systems and currently characterized by different physiographies; these are separated by a major forebulge of the Adriatic lithosphere running throughout the Adriatic region from Istria to the Gargano promontory. The clastic basin filling of the CAB is mostly Plio-Pleistocene whereas it is Upper Oligocene-Pleistocene in the SAB. Stratigraphy reveals that the CAB has experienced an overfilling regime throughout the Plio-Pleistocene whereas the SAB has been probably continuously underfilled during the same time span. In the CAB the foreland plate dips westward at an average angle of 5°–6°, whereas in the SAB the basement dips to the east at an angle of 1°–2°. This discrepancy is congruent with the already noticed distinction between east- and west-dipping subduction systems and possibly has geodynamical implications. In the CAB the non-homogeneous distribution of thrust sheets suggests strike-slip tectonic movements along roughly NE-trending lineaments that occurred up to the Middle-Late Pliocene. On the western part of the SAB, the overall structural assemblage recalls the fabric typical to strike-slip regimes and includesboth extensional and contractional structures. It is proposed that the Messinian salinity crisis and relative eustatic lowstand affected the subsequent tectono-sedimentary evolution of the CAB in terms of subsidence rate and depositional style. One, still tentative, hypothesis is that water and sediment removal in the basin lasted for a sufficient time span to promote viscoelastic relaxation, temporary migration of the forebulge towards the thrust belt and downwarping of the foreland plate.

The Lower Miocene siliceous zone: a marker in the palaeogeographic evolution of the northern Apennines

The occurrence of coeval silica-rich and volcaniclastic layers within diverse tectonic domains of the northern Apennines provides the opportunity to discuss and redefine the old term “lithozone” in a modern light, showing its interest when used for stratigraphic correlation of genetically related deposits. Silica-rich horizons (bedded cherts and nodules) have been reported in the Lower Miocene strata of circum-Mediterranean countries as occurring at distinct stratigraphic levels. In the northern Apennines at least, they can be included in a single siliceous zone, representing the upper part of an unconformity-bounded depositional sequence. The siliceous zone is characterized by a reduced rate of sedimentation, with the local aspect of a condensed sequence, and the almost ubiquitous presence of lithologic markers: cherts, ash layers, bentonites, and locally glaucony. None of them can be considered distinctive of the zone, but their assemblage is. The local intercalation of turbidite siliciclastics complicates the picture, especially the location of the lower boundary of the zone. These terrigenous units form the lower part of the sequence and dilute the overlying siliceous and volcanogenic horizons, but do not mask them completely. Turbidites cause a thickening of the sequence and of the intervals between the markers, up to two orders of magnitude in the main foredeep area where the Cervarola and Falterona Formations were deposited. Smaller turbidite bodies occur in the semi-allochthonous basins located onto the thrust belt (Antognola Formation), whereas the ramp connecting the foredeep and foreland to the east is virtually turbidite-free (Bisciaro Formation). In semi-allochthonous and foredeep depocentres, both sand input and frequence of resedimentation events decrease upwards within the siliceous zone, concurrently with a concentration of chert and volcaniclastic layers. Carbonate or mixed carbonate-siliciclastic megaturbidites are interbedded in the more shaly, upper sections of the Cervarola and Falterona Formations and are probably correlatable with calcareous tempestites (or relatively shallow-water turbidites) of the Bisciaro area. The siliceous zone has a geochemical signature, too, showing a silica enrichment, which is not accompanied by major or minor elements concentrations. This supports the hypothesis that silica is related more directly to biological activity than to volcanic events. Abrupt facies variations from deep-water turbidite/hemipelagite to shelf deposits, concurrently with a change in the terrigenous supply, occur at the top of the zone, allowing its interpretation as a sequence boundary. This boundary is marked by obvious unconformities both in the semi-allochthonous basins and in the foredeep, laterally passing into correlative conformable surfaces in the foreland ramp. The regional significance of the siliceous zone and its possible use as a stratigraphic marker over a great areal extent suggest using a single name for this unit instead of the chaotic terminology which has grown up in the past. The evidence of a pronounced tectonic (Burdigalian) phase that caused a substantial rearrangement of all orogenic basins at the upper zone boundary, supports the idea that the siliceous zone marks a divide in the tectono-sedimentary evolution of the Apenninic orogenic system.

Sedimentary evolution of basins in mobile belts: examples from the Tertiary terrigenous sequences of the Peloritani Mountains (NE Sicily)

ABSTRACTThe Tertiary covers of the Peloritani Mountain Belt (NE Sicily) provide a complete stratigraphical record of tectonic events related to collision in the Central Mediterranean region. The tectonosedimentary evolution is inferred from interpretation of new field data and indicates various stages of polyphase deformation. The Peloritani Mountain Belt is composed mostly of crystalline units representing the active margin of the European Plate that was thrust over the descending African Plate during the Tertiary. Late Eocene‐early Oligocene syn‐orogenic deposition took place within a fore‐arc basin located along the leading edge of the Peloritani Mountain Belt. From the late Oligocene to late Langhian, terrigenous deposition occurred throughout the mountain belt and extended into perched basins, located in southern areas. The basin was fed from the north, from source areas located in the hinterland of the orogenic belt. Deposition was controlled by a combination of active thrusting, regional subsidence and sea‐level change. During the early Serravallian sudden tectonic inversion took place, associated with collapse of hinterland areas and uplift of former low‐lying southern areas of the mountain belt. These processes were related to onset of opening of the Tyrrhenian Sea that was completed during the Serravallian‐Tortonian, and resulted in the deposition of a northwestward prograding clastic fan, fed by source areas located in the southern area of the mountain belt. This setting characterized Messinian and Plio‐Pleistocene deposition, and was controlled by both active tectonics and eustasy. The Recent evolution of the Peloritani Mountain Belt is characterized by major progressive uplift of the southern margins of the Tyrrhenian Basin, and local active subsidence related to downfaulting. Such processes resulted in the uplift of mid‐Pleistocene fan‐delta deposits and late Pleistocene marine terraces deposits to various altitudes above present sea‐level.

L'écaille des Gaux (Ardennes beiges): un exemple d'évolution tectono-sédimentaire complexe lors du développement d'un pli de propagation

The kinematics of internal structuration in the ≪Gaux thrust sheet≫ (Charleroi Coal basin; Belgium) is explained with the help of a geometric model. This model is controlled by the distribution of ≪ Landelies red breccia ≫ outcrops that was syntectonically deposited, probably during the Namurian. We suggest that the tectono-sedimentary evolution was controlled by (1) the growth of a Fault-propagation fold, (2) the formation of collapse structures and (3) localized interbed extension. © 1994 Springer-Verlag.

Tectono-sedimentary development of backarc continental basin in Yunnan, southern China

Sedimentological analyses of the strata occurring in the Cretaceous-Paleogene continental basin of western and central Yunnan enabled the writers to reconstruct the sedimentary history of the basin within the framework of the regional tectonics of the Yunnan-Burma region. The sedimentary record in the basin is characterized by diachroneity within the various sedimentary facies. This diachroneity resulted from increasing uplift in the west that produced a steadily increasing drainage gradient toward the east. The record of uplift is especially reflected in the changes within the sandstones. Granitic batholiths extend north-south through the full length of the uplifting hinterland along the Yunnan-Burma border region. Sedimentation in the continental basin exhibits a close relationship with a multiphase granitic emplacement history revealed through radiometric dating. A close relationship is also evident with an ophiolite-bearing flysch basin exposed today in the Indo-Burma Ranges of western Burma. The writers interpret the Cretaceous-Paleogene sequences as follows. The Yunnan-Burma region was the site of a westward-facing continental arc-trench system with a forearc basin and a backarc basin developed above continental crust. Tectonic and sedimentologic data suggest that the development of the backarc continental basin was controlled by asymmetric arching of the continental arc, in which structural vergence was toward the west and granitic magmatism took place along its axis.

Stratigraphy and structure of the Maltese graben system

The Maltese graben system is a series of Miocene–Quaternary extensional basins located in the foreland of the Sicilian Apennine–Maghrebian fold and thrust belt. The tectono-sedimentary development of the graben was determined from the first fully integrated analysis of the area to include onshore field and hydrological borehole data, together with offshore reflection seismic data and exploration wells. The NW–SE- and ENE–WSW-trending rifts have a coeval, four phase tectono-sedimentary evolution revealed by a succession of platform and pelagic carbonates. The pre-rift phase (>21 Ma) is followed by a early syn-rift phase (21–6 Ma) characterized by evidence for relatively minor extensional faulting including the development of neptunian dykes. Major fault activity occurred during the succeeding late syn-rift phase (<5 Ma) which is characterized by growth faults and fault control on facies and the areas of deposition/non-deposition. This was followed by passive graben infilling during the post-rift phase (probably < 1.5 Ma). Despite low measured extension factors ( β = 1.03–1.17) across the rift structures, major basin bounding extensional faults have throws of up to 2.2 km. Fault slip data indicate that both NW–SE and ENE–WSW rift trends were generated in response to N–S stretching. These results are in accord with Argnanis (1990) description of a major N–S orientated transfer fault located 50 km west of the present study area.

Petroleum Geology of Solimoes Basin, Brazil

Solimoes basin is located at the northwestern portion of the South American continent, adjacent to a subandean pericratonic belt. It constitutes a Paleozoic intracratonic basin covering an area of about 600,000 km[sup 2] and displaying a maximum sedimentary thickness of 3400 m. The tectono-sedimentary evolution can be largely related with important events that occurred at the western border of the continent. The regional flexural subsidence and tectonic deformation observed are correlated to two major phases: Pre-Andean (Paleozoic) and Andean (Mesozoic and Cenozoic). In the Andean phase, compression with shear components, of Triassic age, known as the Jurua Tectonism, became very important for oil exploration of the basin. The stratigraphic conditions considered of greatest interest for petroleum exploration comprise Devonian gas and oil sourcing shales and Carboniferous sandstones reservoirs with an evaporitic seal. The main traps are anticlines related to reverse faults. In this context, Petrobras discovered Jurua gas field in 1978 and the Rio Urucu gas and oil field in 1986. These were the first fields discovered in Brazilian Paleozoic basins. Total petroleum fields discovered in Brazilian Paleozoic basins. Total petroleum reserves discovered represent 400 million bbl of equivalent oil. Present migration and accumulation call for previous accumulation in intrabasinalmore » Paleozoic highs and related stratigraphic traps. Late tectonism seems to have resulted in redistribution and/or escape of fluids into new traps. Less disturbed areas are expected to contain original accumulations.« less

Upper Palaeozoic tectono-sedimentary evolution of the Arabian and adjoining plates

From the Devonian to possibly the early Carboniferous, the Arabian and adjacent plates were structurally affected by a regional Hercynian (Variscan) tectonic event. This tectonic event is interpreted to be the crustal response to regional compression caused by the initiation of subduction of the Palaeo-Tethyan oceanic crust near the margin of Gondwana along Turkey and Iran. The uplifted areas associated with this tectonic event were largely eroded during the late Carboniferous and early Permian when glacio-fluviatile and marginal marine conditions prevailed. The Palaeozoic era closed in the late Permian with a major transgression which deposited massive carbonates over most of a peneplaned Arabian plate. Also in late Permian, as well as Triassic, active rifting along the Zagros suture projected extensional stresses across the Arabian plate causing substantial extensional movements along most major basement-controlled, fault systems.

Oligocene to Early Miocene sedimentation and tectonics in the southern part of the Calabrian‐Peloritan Arc (Aspromonte, southern Italy): a record of mixed‐mode piggy‐back basin evolution

AbstractThe Calabrian‐Peloritan Arc (southern Italy) represents a fragment of the European margin, thrusted onto the Apennines and Maghrebides during the Europe‐Apulia collision in the late Early Miocene. A reconstruction of the pre‐Middle Miocene tectono‐sedimentary evolution of the southern part of the Calabrian‐Peloritan Arc (CPA) is presented, based on a detailed analysis of the Stilo‐Capo ďOrlando Formation (SCO Fm). Deposition of the SCO Fm occurred in a series of mixed‐mode piggy‐back basins. Basin evolution was controlled by two intersecting fault systems. A NW‐SE oriented system delimited a series of sub‐basins and fixed the position of feeder channels and submarine canyons, whereas a NE‐SW oriented system controlled the axial dispersal of coarse‐grained sediments within each of the sub‐basins. From base to top, sedimentary environments change from terrestrial and lagoonal to upper bathyal over a timespan of approximately 12 Myr (late Early Oligocene‐late Early Miocene). During this interval, extensional tectonic activity alternated with oblique backthrusting events, related to dextral transpression along the NW‐SE oriented faults. This produced a characteristic pulsating pattern of basin evolution.Oligocene‐Early Miocene evolution of the W. Mediterranean basin was dominated by ‘roll back’ of the Neotethyan oceanic lithosphere. Considerable extension in the overriding European Plate gave rise to the formation of a back arc‐thrust system. The initial stages of Calabrian Basin evolution are remarkably similar to the evolution of rift basins in the back arc (Sardinia). The Calabrian basins, which are inferred to have originated as thin‐skinned pull‐apart basins, were subsequently incorporated into the Apennines‐Maghrebides accretionary wedge by out‐of‐sequence thrusting, and became decoupled from the back arc. Periodic restabilization of the accretionary wedge, resulting in an alternation of backthrusting and listric normal faulting, provides an explanation for the structural evolution of these mixed‐mode basins. The basins of the southern part of the CPA may be termed ‘spanner’ or ‘looper’ basins, in view of their characteristic pulsating structural evolution, superimposed upon their migration toward the foreland. This new term adequately accounts for the occurrence of tectonic inversions in long‐lived piggy‐back basins, as expected in the light of the dynamics of accretionary wedges.

A new controlled source tensor AMT system

The Lunigiana basin is a NW trending tectonic depression located in the Northern Apennines of Italy. The basin is bounded by active normal faults, characterized by moderate to strong seismicity, with historical earthquakes reaching up to magnitude 6 (February 14, 1834) and instrumental events with magnitude up to 5.4 (June 21, 2013).Using the classical methodologies of active tectonic studies (i.e. remote sensing and digital topography analysis, field mapping, structural geology, and tectonic geomorphology), the main active faults have been mapped for their geometries, kinematics and evidence of activity. Both fault systems bounding the basin to the northeast and southwest have revealed clear evidence of fault capability, i.e., tectonic displacement of the latest Pleistocene-Holocene sediments and geomorphic markers. Then, the study focused on the southwestern Arzelato-Mulazzo-Tresana fault system (AMT), depicting its Late Quaternary tectono-sedimentary evolution and linkage relationship between important fault segments. The throw measurement of key geomorphic markers on the central fault of the AMT allowed to derive medium- (last 350–250 ka) and short-term (18–13 ka) vertical slip rates: 0.44–0.62 and 0.45–0.68 mm/yr, respectively.Lastly, an exploratory trench for paleoseismological analysis was dug across the fault scarp where geomorphic indicators of active surface faulting were most pronounced, just south of the village of Mulazzo.Its paleoseismological analysis revealed at least two events of surface faulting, with a cumulative throw in excess of 70 cm, occurred in historical times. Radiocarbon dating of the exposed alluvial and slope deposits allowed to bracket the surface rupture events to 14th to 19th century CE and after the 18th century CE. Typically, in the active extensional belt of the Apennines, the observed offsets correspond to earthquakes of magnitude >6.3, also confirmed by the empirical fault-scaling laws. This magnitude is, therefore, higher than that currently inferred for this seismic source (Mw 5.7÷6.0 in the DISS catalog) and is comparable to that experienced in the adjoining Garfagnana basin (Mw 6.5 on September 7, 1920). The findings of this research suggests a need to revise the seismic hazard of the Lunigiana basin, including greater consideration of the surface faulting hazard in the seismic microzonation studies.More generally, our findings provide new insight into the comprehension of a) the still unclear deep geometry (listric or not?) and the hierarchy of bounding faults, and b) provide new elements to decipher the puzzling seismotectonic relationships of Lunigiana with the adjoining basins on both its ends, characterized by quite different structural settings.

Tectono-sedimentary development of an extensional basin: the Neogene Megara Basin, Greece

The ?Miocene/Pliocene to Pleistocene succession exposed in the Megara Basin comprises a grossly coarsening-upwards sequence at least 1 km thick. The oldest strata, exposed in the NW, record the establishment of alluvial fans and axial streams. These drained into a lacustrine environment subject to minor wave and, possibly, tidal action. The lower part of the central basin fill is comprised of marls, siltstones, sandstones and conglomerates. They record the pulsed infill of the predominantly freshwater ‘Lake Megara’ by progradation of a network of shallow multichannel streams, which drained a western hinterland containing ultrabasic rocks, cherts and limestones. Frequent examples of large-scale soft sediment deformation structures and stratigraphic thickening into hangingwall rolloven indicate active fault control of sedimentation. The lake/river interface featured Gilbert deltas, which were subject to periodic transgression by ?climatically-controlled lake expansions. The upper part of the ?Pliocene basin fill largely comprises fluviatile gravels, but with two important marine transgressional horizons and renewed calcareous mar1 deposition, including a basinwide tufa horizon, towards the top. The overlying ?Pleistocene alluvial clastics were deposited by a NW-flowing system and thicken markedly into a major basin-bounding fault. Deposition of these fluvial deposits was interrupted towards the top of the sequence by the rapid deposition of monomict limestone breccias of debris flow origins, which are believed to mark a catastrophic event. At some time in the late Pleistocene (c. 1 Ma), the basin bounding faults became inactive and the entire basin fill was inverted in response to uplift in the footwall of the active major fault strand that bounds the SE margin of the Alkyonides Gulf.

Plio-Quaternary geodynamic evolution of a segment of the Peruvian Andean Cordillera located above the change in the subduction geometry: the Cuzco region

The Cuzco region, which is located above a change in subduction geometry, appears to be characterized by a variable Plio-Quaternary tectono-sedimentary evolution essentially located along the major fault system that separates the High Plateaux from the Eastern Cordillera. After the higher surface formation of the High Plateaux, a set of Neogene basins were filled by Miocene “ fluvio-torrential” series and by Plio-Pleistocene fluvio-lacustrine deposits. The Neogene series have been affected by compressional tectonic forces attributed to the Late Miocene. This compression is followed by roughly E-W trending syn-sedimentary extensional tectonics attributed to the Pliocene; it is related to reactivation of the pre-existing major faults, basin evolution, and volcanic activity concentrated along the faults. In the Early Pleistocene, fluvio-lacustrine deposits are affected by syn- and post-sedimentary compressional tectonism it is characterized by shortening that trends both N-S and E-W and produces folding and faulting of the sedimentary cover. Extensional tectonism trending roughly N-S has been taking place from the Middle Pleistocene to the Present; it is coeval with shoshonitic volcanic activity, and with sedimentation of fluvio-lacustrine terraces, torrential fans and moraines. Quaternary and active normal faults due to this tectonism, are located in a narrow zone more than 100 km-long between the High Plateaux and the Eastern Cordillera, and two 15 km-long fault sectors in the Eastern Cordillera. Characteristic Pleistocene scarps, 400 m or more high, are due to the cumulative normal offset, and there are also little scarps, with heights ranging between 2 and 20 m, which are related to Holocene fault reactivations. Recent fault reactivation on the Cuzco fault system, during the April 5, 1986 earthquake ( m b = 5.3), is due to the N-S trending extension. This state of stress, located at a mean elevation of roughly 3730 m, is generally homogeneous to different scales. The active Cuzco normal faults may be a consequence of adjustment between the compensated Western Cordillera and the undercompensated Eastern Cordillera, this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge. The variation of the state of stress, during the Plio-Quaternary is in agreement with the variations of the compressional boundary forces. It may be explained by variation of the convergence rate or by the variation of pull-slab forces.

Evolution sédimentaire et tectonique du bassin protérozoïque supérieur de Comba (Congo) : Stratigraphie séquentielle du Supergroupe Ouest-Congolien et modèle d'amortissement sur décrochements dans le contexte de la tectogénèse panafricaine

The Comba trough infilled by the Upper Proterozoïc West Congolian Supergroup trends N60°E and cuts across the Congo-Zaïre Archean craton. The trough is perpendicular to the Nyanga-Niari basin which itself represents the foreland of the Pan-African Mayombe belt. Moreover, the Comba trough was connected to other troughs extending further East beneath the Mesozoïc-Cenozoïc of the Congolian basin. The tectono-sedimentary evolution of the Comba basin was initiated between 950 and 700 Ma. The trough was formed by a NW-SE extension. It was filled up by periglacial fluvio-lacustrine sediments which correspond to the deltaïc formations of the “Bouenza” Group and the Upper Diamictite (“Tillite” supérieurerd), respectively. Then, a general climatic warm up produced a rise of the eustatic sea level which flooded the lake of Comba as well as the lakes and shelves of the Congo-Zaïre craton. This marine sedimentation is represented by the Schisto-calcaire Group which in terms of sequence stratigraphy we interpret as: a transgressive system tract (red marls of the lower part of the SCIa facies); a condensed section (pink dolomite of the upper part of the SCIa facies); a highstand system tract deposited along a carbonate ramp. The latter corresponds to a lateral facies variation and not a vertical facies succession as it was classically assumed. This highstand system tract was composed of: an external shelf (SCIb facies); a barrier (SCIc facies); a lagoon (SCII facies); a beach (“Kisantu Oolithe” of the SCIIIa facies and SCIIIb facies); a tidal flat (SCIIIc facies) with fluvial (SCIV facies) and tidal (Niari Breccia facies) channels. Between 700 and 600 Ma, a first compressive phase was responsible further west for the eastward Pan-African Mayombian thrusting. This phase was accompanied by the deposition of a thick molassic formation (Mpioka Formation) on the foreland belt and extending into the Comba basin. A late shortening phase of the Mayombian belt propagated into the Comba basin which was affected by strike-slip deformation along its southern edge. This strike-slip deformation induced important disruptions of the regional Mayombian tensor and a particular structural pattern of the Comba basin, characterized by strike-slip damping structures. Sections of these structures are interpreted as positive, negative, and dual “flowers”. This particular structural pattern provides an explanation of the Combian structures as they reflect local deviation of the Pan-African shortening axes. As a conclusion, the evolutionary stages of the Comba basin are discussed in the wider context of the Upper Proterozoïc of the Congo-Zaïre craton.