Jacky Ferrière

Primary or secondary distal volcaniclastic turbidites: how to make the distinction? An example from the Miocene of New Zealand (Mahia Peninsula, North Island)

Abstract Miocene marine volcaniclastic deposits occur in Mahia Peninsula (North Island, New Zealand) and were sedimented in a forearc setting related to the Hikurangi trench subduction system. These deposits are interbedded with hemipelagic marls, and correspond to simple or amalgamated centimetric- to metric-thick turbiditic sequences. Volcaniclastic material is mainly composed of vitric particles, with crystals (quartz, plagioclase with minor biotite, amphibole, pyroxene and oxides), which are well represented in the coarse-grained fraction. The glass shards are mainly rhyolitic in composition. Three types of volcaniclastic turbidites were distinguished with geochemical data, because distinction is impossible on sedimentary characteristics. (1) Primary monomagmatic turbidites contain both magmatic (bubble wall pumice and shards) and phreatomagmatic (blocky shards with few vesicles and hydroclastically fragmented pyroclasts) vitric particles. The chemical compositions of the vitric particles and the crystals are very homogeneous suggesting a cogenetic origin. These turbidites directly result from unique eruptive events and are probably related to the entrance of hot subaerial pyroclastic flows into the sea, which also led to their transformation into subaqueous gravity flows. (2) Secondary monomagmatic turbidites never contain phreatomagmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites. There is a strong linear correlation in the compositions that suggest that the glass particles are derived from the same magma. Crystals also show a compositional homogeneity. These deposits reflect the succession of several eruptions related to a unique magmatic system and result of the reworking of volcaniclastic material after relatively short storage on the shelf. (3) Secondary multimagmatic turbidites do not display compositional homogeneity of their vitric and mineral components. This implies that the volcaniclastic material has been stored during a relative long period on the shelf before remobilization, and that this sedimentation records the volcanic activity of multiple magmatic sources. Consequently, it has been possible to distinguish primary volcaniclastic turbidites that are directly related to the volcanic activity, from secondary turbidites that result from reworking of previously deposited material on the shelf. Monomagmatic turbidites can be used as stratigraphic and magmatic markers whereas multimagmatic cannot. Multimagmatic turbidites, however, record the evolution of the volcanic arc during longer periods. This leads to the conclusion that the Mahia Peninsula volcaniclastic turbidites distally record the evolution of the source volcanic arc.

Extensional deformation across an active margin, relations with subsidence, uplift, and rotations: The Hikurangi subduction, New Zealand

On the basis of field studies in the forearc domain of the Hikurangi subduction margin, in the eastern North Island of New Zealand, we analyze the evolution of the subduction since its onset 25 Myr ago. Analyses of brittle deformation within this forearc domain have revealed evidence of extensional deformation, contemporaneous with the subduction of the Pacific plate beneath the North Island. The type and origin of extensional deformation during the development of the active margin are discussed. The problem is crucial because this forearc domain was previously considered as having undergone almost continuous compression during the late Cenozoic. Two distinct events of late Cenozoic extensional deformation are identified. The youngest one is Quaternary in age; it affects limited areas where important uplift prevails. The orientation of extensional paleostress axes is perpendicular to that of uplift axes, hence consistent with the classical hypothesis of gravitational collapse affecting the upper part of the margin. Of particular interest is the older extensional event, middle-late Miocene in age. It affected most of the forearc domain during a long time span that was also characterized by widespread margin subsidence. Some structures attributed to this event are syndepositional, including rollover structures, and there are mainly high-angle normal faults that affect not only the Miocene sediments but also the pre-Miocene basement. The orientations of paleostress axes obtained by inversion of fault data sets are multiple; after corrections taking into account the rotational history of the margin during the late Cenozoic, they are found to be generally consistent with two major trends. Because these extensional structures developed throughout a long period of the margin subsidence, it is proposed that they reflect the process of tectonic erosion that affected the Hikurangi active margin during the middle and late Miocene, between about 15 and 5 Myr ago.

The Hellenic ophiolites: eastward or westward obduction of the Maliac Ocean, a discussion

Ophiolitic bodies in the Dinaro-Hellenic mountain belt are among the most important ones in the Peri-Mediterranean Alpine chains. The characteristic feature of this ophiolitic belt is its Middle to Late Jurassic age of obduction. The ophiolitic bodies form two major belts on each side of the Pelagonian zone: an east Pelagonian belt in the Vardarian domain and a Supra-Pelagonian ophiolitic belt (SPO) to the west. The different hypotheses relative to the origin of the SPO present geodynamic evolution models accounting for most of the available data: a mid-Triassic episode of rifting; a Ladinian–Jurassic episode of sea-floor spreading forming notably the Maliac Ocean; a Middle to Late Jurassic convergent period with subduction and obduction episodes, and finally, a late episode of Tertiary compressional deformation responsible for the westward thrusting of the Jurassic ophiolitic nappes over the external zones. Despite many studies dating from the early 1970s, the eastern or western Pelagonian origin of these ophiolites, especially the SPO, is still under dispute. Whatever the adopted hypothesis, we consider that the main SPO bodies (N-Pindos, Vourinos, Othris, Evia, Argolis) have the same origin because of their geographic continuity and of the similarities in their geological characteristics. We propose that this ocean corresponds everywhere to the Maliac Ocean, defined in Othris from the well-preserved sedimentary (oceanic margin) and ophiolitic nappes thrust during the Late Jurassic obduction onto the Pelagonian platform. There is strong evidence for the existence of two deep basins on both sides of the Pelagonian continental ridge during Triassic–Jurassic times. They correspond, respectively, to the Vardar area to the east and the Pindos domain to the west, one of these domains being at the origin of the SPO. The hypothesis of an eastward emplacement of the SPO from the Pindos domain is based mainly on sedimentological data from the margin series and on structural analyses of ophiolitic bodies. However, we conclude the westward obduction of the Maliac Ocean, originating from the Vardar area, to be the best fitting model. This westward model is supported by paleogeographic and structural constraints on regional scale. Notably, the absence of obducted ophiolites in the Jurassic series of the Koziakas units (units attributed to the western Pelagonian margin) and of the Parnassus domain (on the eastern side of the Pindos basin) is difficult to reconcile with an eastward obduction from the Pindos domain. Other observations, such as the distribution of ophiolitic detritus in the internal and external zones, also promote the westward Late Jurassic obduction of the Maliac Ocean. Our preferred model offers a consistent explanation for the mechanism and timing of the emplacement of the SPO, as well as providing insight on the origin and emplacement of the Vardarian ophiolites. Following this hypothesis, there is no need for a clear boundary between the SPO and the west Vardarian ophiolitic bodies as they were obducted from the same oceanic basin and during the same Jurassic tectonic event. In this paper, we develop evidence in favor of the eastern Pelagonian origin for the SPO (our adopted model) and provide discussion on the data supporting the main alternative hypothesis (western origin for the SPO).

Initiation d'un bassin transporté: l'exemple du ≪ sillon méso-hellénique ≫ au Tertiaire (Grèce)*

Abstract The Mesohellenic trough (MHT), northern Greece, is a Tertiary molassic piggy-back basin that formed in several stages. This evolution is related to the underthrusting of the external Hellenides below the internal Hellenides. Sedimentologic and tectonic studies of its western border allow us to reconstruct the first stages of the basin differentiation and to propose a new interpretation of the mechanisms at the origin of the MHT. The MHT is made of several overlapping individual basins that evolved successively through time, from west to east, starting in the Upper Eocene. These basins develop along the border of dissymetrical faulted flexures, which are controlled by the characteristics of the major tectonic units.