Large-scale distributed deformation controlled topography along the western Africa–Eurasia limit: Tectonic constraints

In the interior of the Iberian Peninsula, the main geomorphic features, mountain ranges and basins, seems to be arranged in several directions whose origin can be related to the N–S plate convergence which occurred along the Cantabro–Pyrenean border during the Eocene–Lower Miocene time span. The Iberian Variscan basement accommodated part of this plate convergence in three E–W trending crustal folds as well as in the reactivation of two left-lateral NNE–SSW strike-slip belts. The rest of the convergence was assumed through the inversion of the Iberian Mesozoic Rift to form the Iberian Chain. This inversion gave rise to a process of oblique crustal shortening involving the development of two right lateral NW–SE shear zones. Crustal folds, strike-slip corridors and one inverted rift compose a tectonic mechanism of pure shear in which the shortening is solved vertically by the development of mountain ranges and related sedimentary basins. This model can be expanded to NW Africa, up to the Atlasic System, where N–S plate convergence seems also to be accommodated in several basement uplifts, Anti-Atlas and Meseta, and through the inversion of two Mesozoic rifts, High and Middle Atlas. In this tectonic situation, the microcontinent Iberia used to be firmly attached to Africa during most part of the Tertiary, in such a way that N–S compressive stresses could be transmitted from the collision of the Pyrenean boundary. This tectonic scenario implies that most part of the Tertiary Eurasia–Africa convergence was not accommodated along the Iberia–Africa interface, but in the Pyrenean plateboundary. A broad zone of distributed deformation resulted from the transmission of compressive stresses from the collision at the Pyrenean border. This distributed, intraplate deformation, can be easily related to the topographic pattern of the Africa–Eurasia interface at the longitude of the Iberian Peninsula.Shortening in the Rif–Betics external zones – and their related topographic features – must be conversely related to more “local” driven mechanisms, the westward displacement of the “exotic” Alboran domain, other than N–S convergence. The remaining NNW–SSE to NW–SE, latest Miocene up to Present convergence is also being accommodated in this zone straddling Iberia and Morocco, at the same time as a new ill-defined plate boundary that is being developed between Europe and Africa.     

Variation of palaeostress patterns along the Oriente transform wrench corridor, Cuba: significance for Neogene–Quaternary tectonics of the Caribbean realm

In this study, we address the late Miocene to Recent tectonic evolution of the North Caribbean (Oriente) Transform Wrench Corridor in the southern Sierra Maestra mountain range, SE Cuba. The region has been affected by historical earthquakes and shows many features of brittle deformation in late Miocene to Pleistocene reef and other shallow water deposits as well as in pre-Neogene, late Cretaceous to Eocene basement rocks. These late Miocene to Quaternary rocks are faulted, fractured, and contain calcite- and karst-filled extension gashes. Type and orientation of the principal normal palaeostress vary along strike in accordance with observations of large-scale submarine structures at the south-eastern Cuban margin. Initial N–S extension is correlated with a transtensional regime associated with the fault, later reactivated by sinistral and/or dextral shear, mainly along E–W-oriented strike-slip faults. Sinistral shear predominated and recorded similar kinematics as historical earthquakes in the Santiago region. We correlate palaeostress changes with the kinematic evolution along the boundary between the North American and Caribbean plates. Three different tectonic regimes were distinguished for the Oriente transform wrench corridor (OTWC): compression from late Eocene–Oligocene, transtension from late Oligocene to Miocene (?) (D₁), and transpression from Pliocene to Present (D₂–D₄), when this fault became a transform system. Furthermore, present-day structures vary along strike of the Oriente transform wrench corridor (OTWC) on the south-eastern Cuban coast, with dominantly transpressional/compressional and strike-slip structures in the east and transtension in the west. The focal mechanisms of historical earthquakes are in agreement with the dominant ENE–WSW transpressional structures found on land.     

Coastal neotectonics in Southern Central Andes: uplift and deformation of marine terraces in Northern Chile (27°S)

Neotectonic observations allow a new interpretation of the recent tectonic behaviour of the outer fore arc in the Caldera area, northern Chile (27°S). Two periods of deformation are distinguished, based on large-scale Neogene to Quaternary features of the westernmost part of the Coastal Cordillera: Late Miocene to Early Pliocene deformations, characterized by a weak NE–SW to E–W extension is followed by uppermost Pliocene NW–SE to E–W compression. The Middle Pleistocene to Recent time is characterized by vertical uplift and NW–SE extension. These deformations provide clear indications of the occurrence of moderate to large earthquakes. Microseismic observations, however, indicate a lack of shallow crustal seismicity in coastal zone. We propose that both long-term brittle deformation and uplift are linked to the subduction seismic cycle.     

Terrane assembly and geodynamic evolution of central–western Hoggar: a synthesis

After a review of the rock sequences and evolution of the eastern and central terranes of Hoggar, this paper focusses on the Neoproterozoic subduction-related evolution and collision stages in the central–western part of the Tuareg shield. Rock sequences are described and compared with their counterparts identified in the western and the eastern terranes exposed in Hoggar and northern Mali. The Pharusian terrane that is described in detail, is floored in the east by the Iskel basement, a Mesoproterozoic arc-type terrane cratonized around 840 Ma and in the southeast by Late Paleoproterozoic rock sequences (1.85–1.75 Ga) similar to those from northwestern Hoggar. Unconformable Late Neoproterozoic volcanosedimentary formations that mainly encompass volcanic greywackes were deposited in troughs adjacent to subduction-related andesitic volcanic ridges during the c. 690–650 Ma period. Abundant arc-related pre-collisional calc-alkaline batholiths (650–635 Ma) intruded the volcanic and volcaniclastic units at rather shallow crustal levels prior to collisional processes. The main E–W shortening in the Pharusian arc-type crust occurred through several stages of transpression and produced overall greenschist facies regional metamorphism and upright folding, thus precluding significant crustal thickening. It was accompanied by the shallow emplacement of calc-alkaline batholiths and plutons. Ages of syn-collisional granitoids range from 620 Ma in the western terranes, to 580 Ma in the Pharusian terrane, thus indicating a severe diachronism. After infill of molassic basins unconformable above the Pan-African greenschists, renewed dextral transpression took place in longitudinal domains such as the Adrar fault. The lithology, volcanic and plutonic suites, deep greenschist facies metamorphism, structures and kinematics from the Adrar fault molassic belt previously considered as Neoproterozoic are described in detail. The younger late-kinematic plutons emplaced in the Pharusian terrane at 523 Ma [Lithos 45 (1998) 245] relate to a Cambrian tectonic pulse that post-dates molasse deposition. The new geodynamic scenario presented considers several paleosubductions. The major east-dipping subduction, corresponding to the closure of a large Pan-African oceanic domain in the west (680–620 Ma) post-dates an older west-dipping “Pharusian” subduction (690–650 Ma?) to the east of the eastern Pharusian terrane. Such a diachronism is suggested by the 690 Ma old eclogites of the western part of the LATEA terrane of central Hoggar [J. African Earth Sci. this volume (2003)] that are nearly synchronous with the building up of the Pharusian terrane, thus suggesting that the 4°50^′ lithospheric fault represents a reactivated cryptic suture.     

Age of the Corsica–Sardinia rotation and Liguro–Provençal Basin spreading: new paleomagnetic and Ar/Ar evidence

The age of spreading of the Liguro–Provençal Basin is still poorly constrained due to the lack of boreholes penetrating the whole sedimentary sequence above the oceanic crust and the lack of a clear magnetic anomaly pattern. In the past, a consensus developed over a fast (20.5–19 Ma) spreading event, relying on old paleomagnetic data from Oligo–Miocene Sardinian volcanics showing a drift-related 30° counterclockwise (CCW) rotation. Here we report new paleomagnetic data from a 10-m-thick lower–middle Miocene marine sedimentary sequence from southwestern Sardinia. Ar/Ar dating of two volcanoclastic levels in the lower part of the sequence yields ages of 18.94±0.13 and 19.20±0.12 Ma (lower–mid Burdigalian). Sedimentary strata below the upper volcanic level document a 23.3±4.6° CCW rotation with respect to Europe, while younger strata rapidly evolve to null rotation values. A recent magnetic overprint can be excluded by several lines of evidence, particularly by the significant difference between the in situ paleomagnetic and geocentric axial dipole (GAD) field directions. In both the rotated and unrotated part of the section, only normal polarity directions were obtained. As the global magnetic polarity time scale (MPTS) documents several geomagnetic reversals in the Burdigalian, a continuous sedimentary record would imply that (unrealistically) the whole documented rotation occurred in few thousands years only. We conclude that the section contains one (or more) hiatus(es), and that the minimum age of the unrotated sediments above the volcanic levels is unconstrained. Typical back-arc basin spreading rates translate to a duration ≥3 Ma for the opening of the Liguro–Provençal Basin. Thus, spreading and rotation of Corsica–Sardinia ended no earlier than 16 Ma (early Langhian). A 16–19 Ma, spreading is corroborated by other evidences, such as the age of the breakup unconformity in Sardinia, the age of igneous rocks dredged west of Corsica, the heat flow in the Liguro–Provençal Basin, and recent paleomagnetic data from Sardinian sediments and volcanics. Since Corsica was still rotating/drifting eastward at 16 Ma, it presumably induced significant shortening to the east, in the Apennine belt. Therefore, the lower Miocene extensional basins in the northern Tyrrhenian Sea and margins can be interpreted as synorogenic “intra-wedge” basins due to the thickening and collapse of the northern Apennine wedge.     

Structure and late cenozoic evolution of the upper lithosphere in Southwest Tuscany (Italy)

Geological and geophysical data on southwest Tuscany are reviewed in order to define the structure and evolution of the upper lithosphere from the Miocene to the Quaternary. Petrologic studies reveal the existence, below all of Tuscany, of Hercynian and older polyphased metamorphic rocks and of Hercynian granite, whose top is an important seismic reflecting horizon. The basement is characterized by NE-SW trending structures, in contrast with the main NW-SE “Alpine” structures of the uppermost levels. The heat flow map shows two broad areas with values higher than 80 mW/m², reaching maximum values of 10.5 and 15 H.F.U. in the geothermal areas, which are also characterized by negative Bouguer anomalies. A Landsat study revealed a NE-SW band of subcircular structures passing through Larderello and coinciding with a regional fault system and a steep rise in the Moho. Petrologic, geochemical and radiometric data on the Tuscan igneous rocks show that partial melting took place in the Tuscan crust at different levels and to varying degrees from the Miocene to Quaternary, producing a continuous “Alpine” granitic layer. The known Tuscan intrusive bodies and two batholiths below the Larderello and Mt. Amiata geothermal fields represent culminations of the “Alpine” granite. The rise of the Tuscan magmas was closely correlated to a post-Tortonian tensional tectonics and followed its N-E migration. Tensional tectonics started after the last compressional phase (10–11 Ma B.P.) as a consequence of the anticlockwise rotation of Italy, the opening of the Tyrrhenian Sea and the swelling of the mantle below southwest Tuscany.     

Post-plutonic unroofing and morphogenesis of the Attic–Cycladic complex (Aegea, Greece)

The Attic–Cycladic complex of Greece comprises an Eocene high-P unit with blueschist occurrences. Unroofing of this unit took place in Oligocene–Miocene times and was accompanied by a regional low-P medium-T overprint and Miocene granitic plutonism.Apatite fission-track ages of 14 “crystalline” samples from the islands of Tinos, Mikonos and Serifos range between 13.1 and 5.3 Ma, corresponding to the middle and late Miocene. The frequency distributions of confined track lengths are characterised by high arithmetic means of 14.2–15.1 μm and by standard deviations from 0.9 to 1.6 μm. Thermochronological modelling of the data indicates rapid cooling between 10 and 6 Ma and subsequent deceleration of the cooling rates. For a short time, the Miocene plutons of Tinos, Mikonos and Serifos experienced maximum cooling rates above 50 °C/Ma.These exceptionally high cooling rates cannot be explained by strong vertical uplift and fast regional erosion. Such a process is disproved by preserved remnants of a former peneplain, including inselbergs and kaolinized tropical subsoil. On Mikonos, fast post-plutonic cooling of the lower plate was apparently accompanied by simultaneous sedimentation on the upper plate. We propose a post-plutonic cooling model which assumes strong periplutonic heat flow into much cooler host rocks and fast extensional unroofing.     

Tethyan, Mediterranean, and Pacific analogues for the Neoproterozoic–Paleozoic birth and development of peri-Gondwanan terranes and their transfer to Laurentia and Laurussia

Modern Tethyan, Mediterranean, and Pacific analogues are considered for several Appalachian, Caledonian, and Variscan terranes (Carolina, West and East Avalonia, Oaxaquia, Chortis, Maya, Suwannee, and Cadomia) that originated along the northern margin of Neoproterozoic Gondwana. These terranes record a protracted geological history that includes: (1) 1 Ga (Carolina, Avalonia, Oaxaquia, Chortis, and Suwannee) or 2 Ga (Cadomia) basement; (2) 750–600 Ma arc magmatism that diachronously switched to rift magmatism between 590 and 540 Ma, accompanied by development of rift basins and core complexes, in the absence of collisional orogenesis; (3) latest Neoproterozoic–Cambrian separation of Avalonia and Carolina from Gondwana leading to faunal endemism and the development of bordering passive margins; (4) Ordovician transport of Avalonia and Carolina across Iapetus terminating in Late Ordovician–Early Silurian accretion to the eastern Laurentian margin followed by dispersion along this margin; (5) Siluro-Devonian transfer of Cadomia across the Rheic Ocean; and (6) Permo-Carboniferous transfer of Oaxaquia, Chortis, Maya, and Suwannee during the amalgamation of Pangea. Three potential models are provided by more recent tectonic analogues: (1) an “accordion” model based on the orthogonal opening and closing of Alpine Tethys and the Mediterranean; (2) a “bulldozer” model based on forward-modelling of Australia during which oceanic plateaus are dispersed along the Australian plate margin; and (3) a “Baja” model based on the Pacific margin of North America where the diachronous replacement of subduction by transform faulting as a result of ridge–trench collision has been followed by rifting and the transfer of Baja California to the Pacific Plate. Future transport and accretion along the western Laurentian margin may mimic that of Baja British Columbia. Present geological data for Avalonia and Carolina favour a transition from a “Baja” model to a “bulldozer” model. By analogy with the eastern Pacific, we name the oceanic plates off northern Gondwana: Merlin (≡Farallon), Morgana (≡Pacific), and Mordred (≡Kula). If Neoproterozoic subduction was towards Gondwana, application of this combined model requires a total rotation of East Avalonia and Carolina through 180° either during separation (using a western Transverse Ranges model), during accretion (using a Baja British Columbia “train wreck” model), or during dispersion (using an Australia “bulldozer” model). On the other hand, Siluro-Devonian orthogonal transfer (“accordion” model) from northern Africa to southern Laurussia followed by a Carboniferous “Baja” model appears to best fit the existing data for Cadomia. Finally, Oaxaquia, Chortis, Maya, and Suwannee appear to have been transported along the margin of Gondwana until it collided with southern Laurentia on whose margin they were stranded following the breakup of Pangea. Forward modeling of a closing Mediterranean followed by breakup on the African margin may provide a modern analogue. These actualistic models differ in their dictates on the initial distribution of the peri-Gondwanan terranes and can be tested by comparing features of the modern analogues with their ancient tectonic counterparts.     

Aeromagnetic signatures over western Marie Byrd Land provide insight into magmatic arc basement, mafic magmatism and structure of the Eastern Ross Sea Rift flank

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed.     

Pliocene and Quaternary regional uplift in western Turkey: the Gediz River terrace staircase and the volcanism at Kula

Along the upper reaches of the Gediz River in western Turkey, in the eastern part of the Aegean extensional province, the land surface has uplifted by 400 m since the Middle Pliocene. This uplift is revealed by progressive gorge incision, and its rate can be established because river terraces are capped by basalt flows that have been K–Ar and Ar–Ar dated. At present, the local uplift rate is 0.2 mm a⁻¹. Uplift at this rate began around the start of the Middle Pleistocene, following a span of time when the uplift was much slower. This was itself preceded by an earlier uplift phase, apparently in the late Late Pliocene and early Early Pleistocene, when the uplift rate was comparable to the present. The resulting regional uplift history resembles what is observed in other regions and is analogously interpreted as the isostatic response to changing rates of surface processes linked to global environmental change. We suggest that this present phase of surface uplift, amounting so far to 150 m, is being caused by the nonsteady-state thermal and isostatic response of the crust to erosion, following an increase in erosion rates in the late Early Pleistocene, most likely as a result of the first large northern-hemisphere glaciation during oxygen isotope stage 22 at 870 ka. We suggest that the earlier uplift phase, responsible for the initial 250 m of uplift, resulted from a similar increase in erosion rates caused by the deterioration in local climate at 3.1 Ma. This uplift thus has no direct relationship to the crustal extension occurring in western Turkey, the rate and sense of which are thought not to have changed significantly on this time scale. Our results thus suggest that the present, often deeply incised, landscape of western Turkey has largely developed from the Middle Pleistocene onwards, for reasons not directly related to the active normal faulting that is also occurring. The local isostatic consequences of this active faulting are instead superimposed onto this “background” of regional surface uplift. Modelling of this surface uplift indicates that the effective viscosity of the lower continental crust beneath this part of Turkey is of the order of 10¹⁹ Pa s, similar to a recent estimate for beneath central Greece. The lower uplift rates observed in western Turkey, compared with central Greece, result from the longer typical distances of fluvial sediment transport, which cause weaker coupling by lower-crustal flow between offshore depocentres and eroding onshore regions that provide the sediment source.     

Late Tertiary–Quaternary tectonics of the Southern Apennines (Italy): New evidences from the Tyrrhenian slope

This paper summarises the results of combined structural and geomorphological investigations we carried out in two key areas, in order to obtain new data on the structure and evolution of the Tyrrhenian slope of the southern Apennines. Analysis by a stress inversion method [Angelier, J., 1994. Fault slip analysis and paleostress reconstruction. In: Continental Deformation. P.L. Hancock Ed., Pergamon Press, Oxford, 53–100] of fault slip data from Mesozoic to Quaternary formations allowed the reconstruction of states of stress at different time intervals. By integrating these data with those deriving from the stratigraphic and morphotectonic records, chronology and timing of the sequence of the deformation events was obtained.The tectonic history of the region can be related to four deformation events. Structures related to the first event, that was dominated by a strike-slip regime with a NW–SE oriented σ1 and was active since Mid–Late Miocene, do not significantly affect the present day landscape, as they were strongly displaced and overprinted by subsequent deformation events and/or deleted by erosion. The second and third events, that may be considered as the main responsible for the morphostructural signature of the region, are comparable with the stretching phases recognised offshore and considered to be responsible for the opening and widening of the Tyrrhenian basin. In particular, the second event (with an E–W oriented σ3), took place in the Late Miocene/earliest Pliocene and was first dominated by a strike-slip regime, that was also responsible for thrusting and folding. Since Late Pliocene, it was dominated by an extensional regime that created large vertical offsets along N–S to NW–SE trending faults. The third event, that was dominated by extension with a NW–SE oriented σ3, started in the Early Pleistocene and was responsible for formation of the horst-and-graben structure with NE–SW trend that characterises the Tyrrhenian margin of the southern Apennines. The fourth deformation event, which is characterised by an extensional regime with a NE–SW trending σ3, started in the late Middle Pleistocene and is currently active.     

Magnetic overprints in granitoids and metamorphic rocks from Limousin (France): evidence for Late Variscan rotations, crustal folding and tilting

Generally, the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics. Nevertheless, this work demonstrates that successive remagnetizations recorded in Early Carboniferous metamorphic and plutonic units, without clear bedding criteria, can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation, followed by a folding phase, a tilting phase and a second regional clockwise rotation.Metamorphic, ultrabasic, tonalitic and granitic rocks from different parts of Limousin (western French Massif central; 45.5°N/1.25°E), which underwent metamorphism during Devonian–Early Carboniferous or were intruded in the Early–Middle Carboniferous, were sampled in order (a) to identify the magnetic overprinting phases and the related tectono-magmatic events and (b) to constrain the regional and plate tectonic evolution of Limousin. Paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90% of the magnetization isolated in rocks older than 330 Ma are overprints. In agreement with results from adjacent areas of the Variscan belt, the major overprinting phases occurred: (a) in the last stages of the major exhumation phase [332–328 Ma; mean Virtual Geomagnetic Pole (VGP) “Cp”: 37°N/70.5°E], (b) during the post-collisional syn-orogenic extension (325–315 Ma; VGP “B”: 11°N/114°E), (c) in the Latest Carboniferous and Early Permian (VGP “A1”: 27°N/149°E) and (d) in the Late Permian (VGP “A”: 48°N/146°E). The Middle–Late Carboniferous overprints “Cp” and “B” are contemporaneous with emplacement of leucogranitic, crustal derived plutons, and probably result from the hydro-thermal activity related to the magmatism. The drift from “Cp” directions to “B” directions implies that after 330 Ma, Limousin underwent a clockwise rotation by 65°, together with the Central Europe Variscides. The “Bt” components, the VGPs of which deviate from the mean apparent polar wander path (APWP) of the belt, are interpreted as “B” overprints tilted during Late Variscan tectonics, that is, in the time range 325–315 Ma. The first and most important generation of “Bt” overprints was tilted during NW–SE folding associated with NE–SW shortening, updoming and emplacement of leucogranitic plutons. The second generation reveals southeastward tilting due to NE-striking normal faulting. The drift from “B” to “A1” directions implies that Limousin has participated to the second clockwise rotation by 40° of the whole belt in Westphalian times.     

Nomenclature and resolution in Holocene glacial chronologies

Most Quaternary research in Canada during the first half of the twentieth century focused on Pleistocene glaciation. Given the dramatic shifts in climate during the Pleistocene, it is not surprising that the Holocene was viewed as a time of benign climate. Holocene climate variability was first recognized around the middle of the century when paleoecologists found evidence that the early part of the epoch was warmer and drier than the later part. In 1970s and 1980s, another generation of geologists, geographers, and botanists began to recognize more complexity in Holocene climate and vegetation in western Canada. Several millennial-scale glacier “advances” postdating the early Holocene warm interval were defined, including the Garibaldi Phase (6.9–5.6 ka), the Tiedemann–Peyto Advance (3.5–1.9 ka), and the Little Ice Age (AD 1200–1900). Subsequently, application of dendrochronological techniques and stratigraphic studies in glacier forefields showed that the Little Ice Age was itself more complex than previously thought. During that 700-year period, glaciers repeatedly advanced and retreated in response to climatic variability on time scales ranging from centuries to decades. Recent work shows that the glacier record of the Garibaldi Phase and the Tiedemann and Peyto advances are similar in complexity to the Little Ice Age, with multiple advances of glaciers separated by intervals of more restricted ice cover. Researchers have also identified other times in the Holocene when glaciers expanded from restricted positions – 8.20, 4.90–3.80, and 1.70–1.40 ka. Continued research undoubtedly will reveal additional complexities, but with what is currently known the appropriateness of terms such as “Tiedemann Advance,” “Peyto Advance,” and “Little Ice Age” can be questioned. Only short periods of time separate these episodes as currently defined, and it seems likely that intervals of restricted glacier cover within each of these millennial-length intervals are just as long as the intervals separating them.     

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

Integrated studies and revisions of sedimentary basins and associated magmatism in Peru and Bolivia (8–22°S) show that this part of western Gondwana underwent rifting during the Late Permian–Middle Jurassic interval. Rifting started in central Peru in the Late Permian and propagated southwards into Bolivia until the Liassic/Dogger, along an axis that coincides with the present Eastern Cordillera. Southwest of this region, lithospheric thinning developed in the Early Jurassic and culminated in the Middle Jurassic, producing considerable subsidence in the Arequipa basin of southern Peru. This 110-Ma-long interval of lithospheric thinning ended 160 Ma with the onset of Malm–earliest Cretaceous partial rift inversion in the Eastern Cordillera area.The lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and/or transpressional deformations. In particular, the Altiplano plateau corresponds to a paleotectonic domain of “normal” lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere. The high Eastern Cordillera of Peru and Bolivia results from Late Oligocene–Neogene intense inversion of the easternmost thinned area.     

Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments

The Hidaka Collision Zone (HCZ), central Hokkaido, Japan, is a good target for studies of crustal evolution and deformation processes associated with an arc–arc collision. The collision of the Kuril Arc (KA) with the Northeast Japan Arc (NJA), which started in the middle Miocene, is considered to be a controlling factor for the formation of the Hidaka Mountains, the westward obduction of middle/lower crustal rocks of the KA (the Hidaka Metamorphic Belt (HMB)) and the development of the foreland fold-and-thrust belt on the NJA side. The “Hokkaido Transect” project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical/geological research including seismic refraction/reflection surveys and earthquake observations. An E–W trending 227 km-long refraction/wide-angle reflection profile found a complicated structural variation from the KA to the NJA across the HCZ. In the east of the HCZ, the hinterland region is covered with 4–4.5 km thick highly undulated Neogene sedimentary layers, beneath which two eastward dipping reflectors were imaged in a depth range of 10–25 km, probably representing the layer boundaries of the obducting middle/lower crust of the KA. The HMB crops out on the westward extension of these reflectors with relatively high Vp (>6.0 km/s) and Vp/Vs (>1.80) consistent with middle/lower crustal rocks. Beneath these reflectors, more flat and westward dipping reflector sequences are situated at the 25–27 km depth, forming a wedge-like geometry. This distribution pattern indicates that the KA crust has been delaminated into more than two segments under our profile. In the western part of the transect, the structure of the fold-and-thrust belt is characterized by a very thick (5–8 km) sedimentary package with a velocity of 2.5–4.8 km/s. This package exhibits one or two velocity reversals in Paleogene sedimentary layers, probably formed by imbrication associated with the collision process. From the horizontal distribution of these velocity reversals and other geophysical/geological data, the rate of crustal shortening in this area is estimated to be greater than 3–4 mm/year, which corresponds to 40–50% of the total convergence rate between the NJA and the Eurasian Plate. This means that the fold-and-thrust belt west of the HCZ is absorbing a large amount of crustal deformation associated with plate interaction across Hokkaido Island.     

Compressional- and transpressional-stress pattern for Pliocene and Quaternary brittle deformation in fore arc and intra-arc zones (Andes of Central and Southern Chile)

Kinematic analysis of fault slip data for stress determination was carried out on Late Miocene to Quaternary rocks from the fore arc and intra-arc regions of the Chilean Andes, between 33° and 46° south latitudes. Studies of Neogene and Quaternary infilling (the Central Depression), as well as plutonic rocks of the North Patagonian Batholith along the Liquiñe–Ofqui Fault Zone, have revealed various compressional and/or transpressional states of stress. In the Pliocene, the maximum compressional stress (σ₁) was generally oriented east–west. During the Quaternary, the deformation was partitioned into two coeval distinctive states of stress. In the fore arc zone, the state of stress was compressional, with σ₁ oriented in a N–S to NNE–SSW direction. In the intra-arc zone the state of stress was transpressional with σ₁ striking NE–SW. Along the coast, in one site (37°30′S) the Quaternary strain deformation is extensional, with an E–W direction, which can be explained by a co-seismic crustal bending readjustment.     

A tectonic interpretation of “Eburnean terrane” outliers in the Reguibat Shield, Mauritania

The Reguibat Shield comprises a western “Archaean terrane” and eastern “Eburnean terrane” juxtaposed during the early Palaeoproterozoic Eburnean Orogeny. Metasedimentary rocks of probable Palaeoproterozoic age are preserved as flat-lying klippen (Kediat Ijil and Guelb Zednes) and steep imbricate zones (El Mahaoudat range and Sfariat Belt). These are interpreted to record a phase of thrust tectonics that emplaced a continental margin succession onto a composite Archaean foreland prior to ca. 2.06 Ga sinistral transcurrent deformation. Together, these events reflect partitioned Eburnean transpression.     

A cryo-SEM study of aggregate and floc structure changes during clay settling and raking processes

The destabilization of kaolinite suspension by anionic flocculant addition occurs in three zones; free settling, hindered settling and compression which usually includes a final bed raking process in mineral processing practice. This paper reports changes in the kaolinite aggregate and floc structures in the different settling and raking zones by cryo-vitrification/cryo-SEM techniques with image analysis combining micro- and macro-flocs. Cryo-SEM images indicate that, even during free settling, fine clay particles are bridged predominantly in edge–edge (E–E) with some face–face (F–F) configurations forming single, small flocs and some chain structures. When these small flocs and chains settle into the hindered settling zone, the collision between flocs and chains results in “honeycomb” network structures formed with lateral chain-like extension. The settled bed consists of these honeycomb structures with both inter-aggregate and intra-aggregate trapped water and has relatively low bed density (e.g. < 12 wt.% for a 2 wt.% slurry). The effect of the raking process in dramatically improving thickener underflow solids has been extensively studied but the structural changes in flocs and aggregates in this process are less well defined. Raking the compression zone for 1 h at 3 rpm can release some of the trapped water in the “honeycomb” structure and the bed density for 2 wt.% slurry improves dramatically to more than 36 wt.%. Cryo-SEM illustrates the extensive restructuring of flocs from predominantly E–E to predominantly F–F in many areas. The STructural IMage ANalysis (STIMAN) software is used to combine a series of images at magnifications from 1000× to 8000×, including both macro- and micro-flocs. This structural analysis comparing the un-raked and raked bed samples gives increases in total particle area of 30% and in relative particle area of 6%. The relatively low energy rake action of the shear stress results in the disruption of the E–E chains and the honeycomb structure, partly releasing the trapped water and inducing some E–E to F–F aggregate restructuring are clearly illustrated in these results.     

Induced seismicity in the Castanhão reservoir, NE Brazil — Preliminary results

The Castanhão reservoir was built in the state of Ceará, a dry region in Northeastern Brazil, to regulate the flow of the Jaguaribe River, for irrigation, and for power generation. It is an earth-filled dam, 60 m high, with a water capacity of 4.5 × 10⁹ m³. The seismicity in the area has been monitored since 1998, with a few interruptions, using one analog or one digital station and, during a few periods, a three-station network. The first earthquakes likely to be induced events were detected in 2003, when the water level was about 20 m high. In early 2004 a very heavy rainfall season quickly filled the reservoir. Shortly after, an increase in the seismic activity occurred and many micro-earthquakes were recorded. We suggest that this activity resulted from an increase in pore pressure due to undrained response. Therefore, we may classify this cluster of micro-earthquakes as “initial seismicity.” We deployed a network with four analog stations in the area, following this activity, to determine the epicentral zone. At least three epicentral areas under the reservoir were detected. The spatio-temporal analysis of the available data revealed that the seismicity occurs in clusters and that these were activated at different periods. We identified four sets of faults (N–S-, E–W-, NW–SE-, and NE–SW-oriented), some of which moved in shallow crustal levels and as recently as the Quaternary (1.8 Ma). Under the present-day stress regime, the last two sets moved as strike-slip structures. We suggest a possible correlation between dormant faults and the observed induced seismicity.