Ernesto Abbate

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.

A one-million-year-old Homo cranium from the Danakil (Afar) Depression of Eritrea

One of the most contentious topics in the study of human evolution is that of the time, place and mode of origin of Homo sapiens. The discovery in the Northern Danakil (Afar) Depression, Eritrea, of a well-preserved Homo cranium with a mixture of characters typical of H. erectus and H. sapiens contributes significantly to this debate. The cranium was found in a succession of fluvio-deltaic and lacustrine deposits and is associated with a rich mammalian fauna of early to early-middle Pleistocene age. A magnetostratigraphic survey indicates two reversed and two normal magnetozones. The layer in which the cranium was found is near the top of the lower normal magnetozone, which is identified as the Jaramillo subchron. Consequently, the human remains can be dated at ∼1 million years before present.

The eugeosynclinal sequences

The eugeosynclinal sequences of the Northern Apennines are characterized by the presence of ophiolites both as a primary stratigraphic substratum and as olistostromes and olistoliths interbedded in the sedimentary succession. A further characteristic is extensive turbidite sedimentation of Late Cretaceous-Eocene age, which preceded similar terrigenous deposition in the miogeosyncline. The eugeosynclinal sequences underwent intense horizontal displacements from a Tyrrhenian area, overriding the miogeosynclinal rocks. In spite of tectonic dismembering, several of the original stratigraphic successions have been reconstructed. The eugeosynclinal sequences have been grouped into: (1) Helminthoid Flysch sequences (Upper Triassic-Eocene); (2) Vara Supergroup (Jurassic?-Paleocene); (3) Calvana Supergroup (Upper Cretaceous-Eocene) and (4) Canetolo Complex (Paleocene-Miocene?). The Helminthoid Flysch sequences include most of the eugeosynclinal deposits, generally with remarkable thickness of calcareous and, subordinately, arenaceous turbidites of Late Cretaceous age. In this paper stratigraphic units of the Helminthoid Flysch and of the other major sequences are described, as well as their mutual tectonic relations. Schematically, the following upward geometric succession has been assumed, starting from the top of the miogeosynclinal rocks of the Tuscan and Umbrian sequences: the Canetolo Complex, the Calvana Supergroup, the Helminthoid Flysch sequences and the Vara Supergroup. Locally, this succession is present only in part, due to tectonic reductions. According to the structural positions and on the basis of various stratigraphic and sedimentological considerations, we have attempted to sketch the paleogeography of the eugeosyncline. The reconstruction is largely incomplete up to the Lower Cretaceous, whereas numerous paleogeographic data are available for the Upper Cretaceous and the Eocene. The areas of deposition of the Vara Supergroup and of those parts of the Helminthoid Flysch sequences with abundant Upper Cretaceous and Paleocene arenaceous turbidites, are believed to have been located along the internal (western) margin of the eugeosyncline. To the east of it, it is supposed, was located the basin of the Helminthoid Flysch sequences with calcareous turbidites. The external (eastern) margin of the eugeosyncline was occupied by the Calvana Supergroup and by the Canetolo Complex. The relations between the Alpine realms and the Apennines geosyncline are briefly discussed, especially in respect to the provenance of the clastics of the Apennines turbidites. The Corsica-Sardinia Massif is supposed to have been the main source area during the Cretaceous and Paleocene for the arenaceous clastics, whereas the coeval calcareous turbidites mainly derived from the South-Alpine zone and from the internal margin of the Brianconnais facies belt. A critical review of alternative paleogeographic reconstructions proposed by various authors, has also been attempted.

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.

Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis

[1] The Red Sea is one of the best exposed young rift basins in the world. Its flanks on both the African and Arabian sides are characterized by basement uplifts parallel to the margins and by active erosion. Through the integration of 37 new apatite fission track (FT) analyses and regional geology, we elucidate the uplift and denudational history of the Eritrean continental margin along the southern Red Sea and, in particular, the development, timing, and past and present morphostructural features of its onshore portion. FT data indicate that at around 20 Ma, the Eritrean margin was affected by a crustal cooling event due to a postrifting accelerated phase of denudation. This cooling has the same age as those already detected on the conjugate Arabian margin (Yemen and Saudi Arabia). FT ages increase from 10-20 to 300-400 Ma with increasing distance and elevation from the coastal areas toward the interior. This trend indicates a diminishing amount of eroded section in the same direction. We use FT and structural data to define the position of the main border fault along the margin where the original scarp was located. We estimate rates of vertical denudation of 190-200 m/Myr at the border fault and of <60-70 m/Myr on the plateau. On the basis of these new data, we argue against an Oligocene tectonic unroofing of the margin through a low-angle detachment assumed by previous authors, but we maintain the essential role of the erosional denudation in the development of present margin morphology.

GEOLOGY OF THE HOMO -BEARING PLEISTOCENE DANDIERO BASIN (BUIA REGION, ERITREAN DANAKIL DEPRESSION)

This paper deals with the geological context of the northernmost site in the East Africa Rift system which has yielded Homo erectus -like remains. They are dated ca. 1 Ma and have been found in the deltaic deposits of the Alat Formation belonging to the Dandiero group. This newly defined group crops out extensively in an elongated belt from the Gulf of Zula to the North to the Garsat area to the south. In the Buia-Dandiero area it ranges in age from the Early to the Middle Pleistocene, and incorporates six formations, from bottom up: the fluvial Bukra Sand and Gravel, the deltaic and lacustrine Alat Formation, fluvial Wara Sand and Gravel, the lacustrine Goreya Formation, the fluvio-deltaic Aro Sand and alluvial Addai Fanglomerate. This succession is bounded by two major unconformities, which separate it from the Neoproterozoic basement and from the overlaying Boulder Beds fanglomerate, and has been designated the Maebele Synthem. The latter is the result of two lacustrine transgression and regressions evidenced by two depositional sequences. The unconformities bounding the Maebele Synthem are related to the tectonic history of the basin fill and its substrate. The development of the two sequences was, instead, mainly controlled by lake level fluctuations and, hence, by climatic variations connected with the weakening and strengthening of the monsoons in the northwestern Indian ocean. The environment where the Buia Homo lived was a savannah with some scattered water pools. This environment probably extended farther north along the western coastal plain of the Red Sea, and was a preferential pathway for the dispersal of the hominids from East Africa toward Eurasia.

Insights on the thermal evolution of the Ligurian Apennines (Italy) through fission-track analysis

Apatite fission-track analysis has been applied on 24 samples from the geometrically upper units (Internal and External Ligurids) of the Northern Apennines nappe pile. They yield ages between c. 20 Ma and c. 6 Ma. Two samples were also dated with fission tracks in zircon (ages of c. 155 Ma and c. 199 Ma). Together the data reveal that during post-Late Cretaceous compression the Internal Ligurids reached temperatures to completely anneal fission tracks in apatite (T>c. 120 °C) but not in zircon (T<c. 300°C). After Eoccne continental collision these rocks were cooled (below c. 120°C) and started again to retain fission tracks in apatite. Since the Oligocene a further burial under episutural sediments ensued that caused a partial to total apatite fission-track annealing. During a final phase of cxhumation the sedimentary cover was removed. This exhumation can be linked to Late Miocenc extensional tectonics, that started at c. 8–9 Ma in the central and southern portions of the study area.

Geology of Ethiopia: A Review and Geomorphological Perspectives

The Ethiopian region records about one billion years of geological history. The first event was the closure of the Mozambique ocean between West and East Gondwana with the development of the Ethiopian basement ranging in age from 880 to 550 Ma. This folded and tilted Proterozoic basement underwent intense erosion, which lasted one hundred million years, and destroyed any relief of the Precambrian orogen. Ordovician to Silurian fluviatile sediments and Late Carboniferous to Early Permian glacial deposits were laid down above an Early Paleozoic planation surface. The beginning of the breakup of Gondwana gave rise to the Jurassic flooding of the Horn of Africa with a marine transgression from the Paleotethys and the Indian/Madagascar nascent ocean. After this Jurassic transgression and deposition of Cretaceous continental deposits, the Ethiopian region was an exposed land for a period of about seventy million years during which a new important peneplanation surface developed. Concomitant with the first phase of the rifting of the Afro/Arabian plate, a prolific outpouring of the trap flood basalts took place predominantly during the Oligocene over a peneplained land surface of modest elevation. In the northern Ethiopian plateau, huge Miocene shield volcanoes were superimposed on the flood basalts. Following the end of the Oligocene, the volcanism shifted toward the Afar depression, which was experiencing a progressive stretching, and successively moved between the southern Ethiopian plateau and the Somali plateau in correspondence with the formation of the Main Ethiopian Rift (MER). The detachment of the Danakil block and Arabian subcontinent from the Nubian plate resulted in steep marginal escarpments marked by flexure and elongated sedimentary basins. Additional basins developed in the Afar depression and MER in connection with new phases of stretching. Many of these basins have yielded human remains crucial for reconstructing the first stages of human evolution. A full triple junction was achieved in the Early Pliocene when the MER penetrated into the Afar region, where the Gulf of Aden and the Red Sea rifts were already moving toward a connection via the volcanic ranges of northern Afar. The present-day morphology of Ethiopia is linked to the formation of the Afar depression, MER, and Ethiopian plateaus. These events are linked to the impingement of one or more mantle plumes under the Afro-Arabian plate. The elevated topography of the Ethiopian plateaus is the result of profuse volcanic accumulation and successive uplift. This new highland structure brought about a reorganization of the East Africa river network and a drastic change in the atmospheric circulation.

An extensive apatite fission-track study throughout the Northern Apennines nappe belt

Abstract This paper takes into consideration more than 100 apatite fission-track analyses on samples coming from an approximately west-east cross-section throughout the Northern Apennines. This collisional chain is made of structural units and nappes (Ligurian and Tuscan Nappe) accreted to the Adriatic Foreland during the Neogene, which overthrust the Miocene turbiditic successions of the Cervarola and Marnoso-arenacea Formations. Different cooling ages and degrees of annealing delineate different evolution histories for these units. Exhumation of the western outcrops of the Ligurian Nappe can be placed at 8 Ma and follows a first denudation event occurred in Eocene times. Timing of exhumation decreases eastwards. A break in this general trend is shown by the Apuan Alps, that occupy an intermediate position and yielded the youngest cooling ages. In the external part of the Marnoso-arenacea foredeep deposits this tendency could not be tested because total annealing of the apatite system has not been reached. In this case, modeling of data allows evaluating maximum burial temperatures.