Pietro Passerini

Olistostromes and olistoliths

Abstract The Northern Apennines are a typical area of slide deposits. Sliding phenomena gave rise to various products ranging from gravity nappes to olistostromes and olistoliths. The latter differ from gravity nappes with regard to size and internal structure. Current research in the Northern Apennines suggests using the terms “olistostrome” and “olistolith” with a somewhat different meaning from that originally proposed by Flores (1955). The main differences concern the size limits and the relative position in the sedimentary sequences. Olistostromes occur in Jurassic to Pliocene deposits pertaining to eu-, mio-, late and postgeosynclinal sequences. They are particularly common in Upper Cretaceous to Miocene formations. The material of the Cretaceous and Eocene olistostromes generally comes from the ophiolites and the rocks overlying the ophiolitic suite. The olistostromes occur as thick layers or breccias or paraconglomerates, and, like the olistoliths, are intercalated in the eugeosynclinal flysch formations. The Oligo-Miocene olistostromes are also made of material from the eugeosynclinal sequences, but they are interbedded in the miogeosynclinal flysch or in the late geosynclinal formations. They generally appear as argillaceous bodies with scattered rock fragments (mostly limestones). The genesis of olistostromes and olistoliths is strictly related to the migration of the flysch basins from west to east. Slumping was caused by the eastward orogenic wave: olistostromes were discharged from the uplifted areas and/or from the front of the advancing nappes.

The miogeosynclinal sequences

Abstract The Tuscan and Umbrian sequences represent miogeosynclinal deposition in the Northern Apennines. They are composed of five major rock groups, which are here described in detail and interpreted. The sequences start with a terrigenous basal section, Upper Carboniferous to Carnian, composed of clastic rocks with shallow-marine, lagoonal and continental facies, and of acid volcanics. This group probably represents the Hercynian postgeosynclinal stage. A carbonate-evaporitic section follows, Noric to Rhaetic, overlain by Hettangian neritic limestones. These facies are of wide extent; a geosynclinal basin in the Northern Apennines had probably not yet differentiated. A progressive deepening of the sea began in the Sinemurian; the carbonate-siliceous section (Lower Jurassic-Lower Cretaceous) is made of frequently siliceous micritic limestones and marlstones, and contains bedded cherts, but locally there are biostromal and oolitic limestones. In parts of the geosyncline, submarine dissolution on top of tectonic rises caused the development of paraconformities. The pre-flysch section includes pelitic and marly formations, with some bedded chert, and graded limestones (prototurbidites) in the Tuscan sequence (Lower Cretaceous to Lower Oligocene); limestones and marlstones in the Umbrian sequence (Upper Cretaceous to Middle Miocene). The pre-flysch stage indicates a further deepening of the sea and a considerable slowing of the rate of deposition, but the bottom physiography was in parts irregular and paraconformities were formed. The topmost flysch section consists of thick turbidite formations, migrating in age from west to east, Early-Middle Oligocene to Early Miocene in the Tuscan sequence, Early to Late Miocene in the Umbrian sequence. Each flysch unit starts abruptly and is typically arenaceous in the lower part (ortotubidites), but it becomes progressively marly-argillaceous in the upper part (kataturbidites).

Introduction to the geology of the Northern Apennines

According to Aubouin (1965) the sedimentary evolution of the Northern Apennines geosyncline is divided into a geosynclinal stage proper, represented by eu- and miogeosynclinal sequences, a late geosynclinal and a postgeosynclinal stage. In the Apennines the eugeosynclinal rocks are almost entirely allochthonous. Their interpretation as autochthonous is held to be unrealistic on structural and paleogeographic grounds. The late geosynclinal stage is defined here mainly on the base of tectonic criteria: sediments deposited over folded eugeosynclinal rocks, later subjected to lateral tectonic transport in the same manner as their allochthonous substratum. Owing to the eastward progression of tectonic movements in the Northern Apennines, the tecto-sedimentary stages tend to overlap and coexist (e.g., Oligocene-Miocene miogeosynclinal flysch and late geosynclinal sediments). The eugeosynclinal stage is characterized by the presence of ophiolites and the early development of flysch. Four main groups of sequences are distinguished: (1) Upper Cretaceous to Eocene Helmynthoid Flysch sequences; (2) Jurassic to Eocene Vara Supergroup; (3) Upper Cretaceous to Middle Eocene Calvana Supergroup; and (4) Paleogene Canetolo Complex. The largely autochthonous miogeosynclinal rocks are represented by the Tuscan (Lower Triassic to Lower Miocene) and Umbrian (Carnian to Upper Miocene) sequences. The late geosynclinal sequences are Middle Eocene to Messinian in the Emilian Apennines, Lower and Middle Miocene in Tuscany and Romagna. The postgeosynclinal sediments are Upper Miocene to Pleistocene in the southwest (Tuscany-Latium), Pliocene and Pleistocene in the northeast and east (Emilia and Marche). Four major structural areas are distinguished: 1. (1) The Tyrrhenian area, a zone of intense crustal shortening, containing the axis of symmetry between the structure of the Apennines and that of the Western Alps and “Alpine Corsica”. 2. (2) Southwestern Tuscany, characterized by a fault block structure and an incomplete Tuscan sequence. 3. (3) The main fold range, with reverse faults, overturned folds and overthrusts, all directed eastward and northeastward. There are two major northwest-southeast lines of thrusting and overturned folding, and two major tectonic windows (Alpi Apuane and Monte Pisano), in which the Tuscan sequence is doubled. 4. (4) The outer foothills and Po Valley, where the asymmetric folding and reverse faulting become gradually attenuated. Metamorphism is generally of low grade (greenschist facies), and affects the lower part of the Tuscan sequence in small areas of western Tuscany (Alpi apuane, Monte Pisano, Montagnola Senese, Elba Island). Metamorphism tends to be associated with the tectonic doubling of the sequence. Theories on the tectonic interpretation of the Northern Apennines are summarized. The authors are inclined to accept features of both the “decollement” nappe model of Trevisan et al. (1965) and the orogenetic landslide model of Migliorini (1948) and Merla (1951). The emplacement of the allochthon was essentially through gravitational gliding; detachment and gliding affected also, in parts, rocks of the Tuscan sequence (Tuscan nappe of the Alpi Apuane). Uplift and differential movements (block faulting?) in the miogeosyncline from the Triassic to the Cretaceous are indicated by slumping and unconformities. Much slumping occurred during the Cretaceous in the eugeosyncline. The first recorded eastward gliding movements of nappes are Upper Cretaceous to Paleocene in the southern part of the eugeosyncline, Eocene in the northern part of it. Parts of the eugeosynclinal sequences were folded in the Lower-Middle Eocene and at the Eocene-Oligocene transition. The advancement of nappes onto the miogeosynclinal rocks, accompanied by folding, began at the Oligocene-Miocene transition. It continued, gradually moving eastward, until the Lower Pliocene. The detachment of rocks of the Tuscan sequence in the Alpi Apuane and southern Tuscany (Tuscan nappe) seemingly occurred in the Lower and Middle Miocene.

Strike-slip faults in a rift area: a transect in the Afar Triangle, East Africa

Abstract The Afar Triangle, a diffuse triple junction where the Red Sea, Ethiopian and Gulf of Aden rifts converge, is examined along an E-W cross section in order to recognize traces of strike-slip faulting summarily known from earlier studies. Both field evidences from slickensides and airphotograph or satellite image data indicate that strike-slip faults, although less numerous than normal ones, occur throughout this area. These faults mainly strike parallel or at small angles relative to rifting axes, rather than transversal to them as would be expected if they were transforms. Strike slip subparallel to rifts is explained through lateral displacement between the major lithospheric plates around the junction or, subordinately, by a domino fault mechanism in zones of diffuse transform deformation. Faults at small angles with the rift axes often constitute conjugate systems suggesting along-axis compression, which is considered to be frequently induced by the lateral intraplate shift mentioned above. In other cases, this compression may develop in intervals between mantle plumes wedging up along a rift, or at the head of a propagating rift. The main lateral displacements among the boundary lithospheric plates during the last million of years are supposed to have been sinistral. This does not challenge the notion of mainly divergent plate movements, but adds to this divergence an anticlockwise shift of the plates around the junction.

Geothermal exploration in the Republic of Djibouti : thermal and geological data of the Hanlé and Asal areas

Abstract During geothermal exploration in the Republic of Djibouti in the period 1987–1989, two deep wells were drilled in the Hanle area and four in the Asal area. The two wells at Hanle encountered low temperatures (maximum recorded temperature was 124°C at 2020 m depth). Volcanic rocks of Miocene age were found at the bottom of one Hanle well. Minor recent crustal stretching may account for the absence of any significant thermal anomaly. The four wells at Asal met high temperatures (up to 358°C at 2095 m depth), and high-enthalpy fluid was produced from two of them. Drilling data and new structural studies contribute to the development of a model for the Asal Rift, which is characterized by very steep faults with no evidence of flattening at depth and by the migration of the spreading axis from southwest to northeast. The authigenic mineral assemblage studied in all the wells is in good agreement with the measured temperatures in five of the six wells.

Patterns of faulting in the Mont Blanc granite

Abstract Faults at the meso scale in the Mont Blanc granite have been analyzed considering their strike, dip and pitch of the striae. The resulting pattern is characterized by mainly SW-NE striking, strongly dipping faults with a pronounced dip-slip character, associated with oblique or transversal strike-slip faults. The dominant dip-slip, NE-trending faults constitute a downward converging fan more or less concordant with the wedge shape of the granite mass. Reverse faults largely prevail over normal ones. In some sections of the massif (especially the NW-section) the fault pattern can be analyzed in terms of a plane strain model under compression with the NW azimuth. Towards the southeast, two-dimensional models of deformation become increasingly inadequate, and the pattern needs being analyzed in three dimensions: it can be simulated by a half-cylindrical envelope of fault planes whose axis strongly dips towards NW. Dip-slip movement characterizes the fault planes lying at the bottom of this half-cylindrical “gutter”, while the movement becomes strike-slip in the faults whose planes lie at the gutters flanks. Although this is only a geometrical simulation, it may again be consistent with the hypothesis of a NW-directed overall compression. In all the examined sections of the massif the angle of dip of fault planes is much higher than expected by the Mohr-Coulomb theory with horizontal maximum compression. We suggest that this may be due to an association of faulting with bulk deformation, which may have resulted in both a pattern of shearing stresses along very inclined planes, different from that of perfectly elastic materials, and some rotation of already formed fault planes. The different patterns of NW- and SE-dipping faults, the bimodal distribution of the dip angles around the dominant NE-strike, and other features suggest that faulting occurred in more than one phase.

LONGITUDINAL STRIKE-SLIP FAULTS IN OCEANIC RIFTING : A MESOSTRUCTURAL STUDY FROM WESTERN TO SOUTHEASTERN ICELAND

Abstract Mesostructural analysis carried out in several localities of western, southern-central and southeastern Iceland shows an unexpectedly frequent occurrence of strike-slip faults parallel to, or lying at relatively small angles with, the axis of rifting. Some faults can be interpreted in terms of shear parallel to the rifts, whereas others form conjugate systems referable to rift-parallel compression. Axial shear can be explained by the accommodation of differences in the directions of tectonic extension between different segments of the axial rift system and subordinately by lateral (rift-parallel) displacements between the major lithospheric plates. Axial compression may be attributable to rift-oblique transform shear or to mantle transport parallel to the spreading axis compensating local deficiencies in mantle upwelling. The data regard 1974 fault surfaces, mainly small-scale.

The geosyncline concept and the northern apennines

Abstract The area of the Northern Appennines is considered a geosyncline on account of strong subsidence followed by an Alpine-type deformation. The Northern Apennines are now regarded mainly as a structural unit, characterized by an arched system of folds with overthrusts and decollement nappes and an eastward polarity. Depositionally the Northern Apennines cannot be entirely separated from the Central Apennines and the Alps. The first definite appearance of a geosynclinal basin is considered Lower Cretaceous. A separation between eu- and miogeosyncline is based on the early appearance of flysch and the allochthonous position of the rocks of the former on those of the latter. The presence of ophiolites is not considered as a distinctive criterion in the case of the Apennines, because they occur in a primary position in only one of the allochthonous sequences. There is also not much evidence for a physical separation between eu- and miogeosyncline, in the form of a geanticlinal ridge, nor is it certain whether the various flysch units represent separate troughs, or simply reflect areas of separate sediment supply. A very characteristic feature in the history of the Northern Apennines is the eastward migration of sedimentary facies, tectonics and magmatism. The flysch troughs migrated eastward, with an often repeated vertical succession of proturbidites (initial thin turbidites in pelitic formations), orthoturbidites (thick arenaceous flysch) and cataturbidites (a gradual declining of turbidite deposits). Each turbidite cycle was followed by horizontal tectonic movements and in some cases, by emergence. Olistostromes accompany many turbidite units, thus showing the close relations between uplift in the source areas at the western margins of the troughs, and subsidence in the latter.

The northern apennines geosyncline and continental drift

Abstract The history of the Northern Apennines is difficult to be accounted for without recurring to relative horizontal displacement of crust masses. The ophiolitic stage of the eugeosyncline might be considered as the product of divergent drift, with disruption of the sialic crust, and formation of an “oceanic” crack. The subsequent closure of the latter can be ascribed to a reversal of drifting from divergent to convergent. The convergent movement was presumably the cause of the orogenesis, and also continued after complete closure of the ophiolitic scar. The eugeosynclinal area was probably located in the Tyrrhenian Sea between Corsica and the present Tuscan coast; basin reconstruction indicates that the area has been sensibly reduced by orogenic squeezing. In the present state of knowledge on the Northern Apennines, it is not necessary to assume horizontal displacements exceeding some hundreds of kilometres. Larger displacements, as usually put forward in continental drift theories, are not proved in the Northern Apennines area.

Mesoscopic faults in the Bregaglia (Bergell) massif, Central Alps☆

Abstract The strike, direction of dip and pitch of the striae along mesoscopic faults in the Oligocene granodiorite-tonalite of Val Masino-Val Bregaglia (Bergell) are analysed. Most fault planes are steeply dipping, and show strike-slip or oblique-slip motion. Dominant strikes are NNW or NNE. A relative chronology of fault sets is suggested based on the presence of different minerals (chlorite and epidote) on fault planes. The pattern of mesoscopic faults in the Val Masino-Val Bregaglia massif does not follow the earlier tectonic trends of the Pennidic nappe edifice, nor even the trend of the nearby section of the Insubric Line considered at both regional and mesoscopic scales. The mesoscopic analysis of the Val Masino-Val Bregaglia massif thus reveals a fault system largely oblique to the major Alpine lineaments. The observed fault pattern does not reveal traces of thrusting referable to late Alpine orogenic phases, and can be related to subsequent deformation, dominated by strike-slip movements; this pattern does not match the traditional schemes of extensional dip-slip faulting following orogenesis. It records a stage of tectonic evolution which follows nappe emplacement, yet it precedes vertical or extensional post-orogenic tectonics.