Stratigraphic evolution of Triassic arc-backarc system in northwestern Croatia

Abstract

Middle Triassic arc-related extensional tectonics in the western Tethys generated a complex pattern of intraand backarc basins. We studied volcano-sedimentary successions of subsided continental-margin blocks (Mts. <umberak and Ivan71ica) and of dismembered incomplete ophiolite sequences interpreted as remnants of a backarc basin (Mts. Medvednica and Kalnik) in northwestern Croatia. We dated the successions with radiolarians, conodonts, foraminifers, algae, and sponges.The continental margin experienced a phase of accelerated subsidence in the late Anisian that was approximately coincident with the onset of intermediate and acidic volcanism; pelagic sediments with volcaniclastics accumulated atop subsided carbonate platforms. These relatively shallow basins were later infilled completely by prograding platforms in the late Ladinian-Carnian. In the backarc basin, sea-floor spreading initiated near the Anisian-Ladinian boundary and continued into the late Carnian. Pillow basalts were erupted and interlayered with radiolarian cherts and shales.The studied area was a part of a larger Triassic arc-backarc system preserved in the southern Alps, Alpine-Carpathian Belt, Dinarides, and Hellenides. Volcano-sedimentary successions of Mts. Medvednica and Kalnik are relics of the Meliata-Maliak backarc basin. In comparison to other previously dated oceanic remnants of this system, the longest continuous sea-floor spreading is now documented in one restricted tectonic unit. Évolution stratigraphique d'un système arc-arrière-arc triasique au nord-ouest de la CroatieMots clés. -Système arc-arrière-arc, Bassin de Meliata-Maliak, Trias moyen, Trias supérieur, Biostratigraphie, Radiolaires, Conodontes, Volcanisme, Croatie.Résumé. -Au Trias moyen, la tectonique d'extension liée aux arcs volcaniques de la Téthys occidentale engendre un système complexe de bassins intra-et arrière-arcs. Dans le NW de la Croatie, nous avons étudié les séquences volcanosédimentaires des blocs subsidents de la marge continentale (monts <umberak et Ivan71ica) et du bassin d'arrière-arc (monts Medvednica et Kalnik). Les successions ont été datées grâce aux radiolaires, conodontes, foraminifères, algues et éponges.La marge continentale a subi une phase de subsidence accélérée à l'Anisien supérieur. Cette phase a plus ou moins coincidé avec l'installation d'un volcanisme intermédiaire et siliceux. Les sédiments pélagiques et volcanoclastiques se sont accumulés au sommet des plates-formes carbonatées subsidentes. Plus tard, au Ladinien supérieur-Carnien, ces bassins relativement peu profonds ont été complètement remplis par des plates-formes progradantes. Dans le bassin d'arrière-arc, l'expansion du fond océanique a débuté au voisinage de la limite Anisien-Ladinien et a duré jusqu'au Carnien supérieur. Des pillow-lavas basaltiques y sont intercalées dans des cherts à radiolaires et des shales.La région étudiée faisait partie d'un système d'arrière-arc plus large qui a été préservé dans les Alpes du sud, la chaîne des Carpathes, les Dinarides et les Hellenides...

Triassic metasediments in the internal Dinarides (Kopaonik area, southern Serbia): stratigraphy, paleogeographic and tectonic significance

“…Furthermore Middle Jurassic formation ages of ophiolitic mélanges in the Di- narides-Albanides (e.g. Goričan et al 1999Goričan et al , 2005Halamić et al 1999;Babić et al 2002;Chiari et al 2002;Gawlick et al 2008Gawlick et al , 2009b suggest that early Late Jurassic obduction of the ophiolites was presumably preceded by the emplacement of olistostromes that were tectonically overprinted forming the mélanges below the ophiolites. In the closer Kopaonik region, however, sediments younger than the Kopaonik Formation were not found below the ophiolitic mélange, probably because obduction-related thrusts cut across older formations in this area.…”

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)

“…On account of similar textural features all these mélange sectors are assumed to constitute a single unit (Babić et al 2002) referred to as the Kalnik Unit (Hass et al 2000), which brings strong evidence for a discrete oceanic domain (Repno oceanic domain = ROD) within the Meliata-Maliac ocean system (Babić et al 2002;Goričan et al 2005;Slovenec & Lugović 2008). However, excluding the Mt Medvednica ophiolite mélange where magmatic megaclasts or olistoliths have been geochemically studied in detail and stages of terminal spreading and initial convergence were dated by isotopes (Lugović et al 2007;Slovenec & Lugović 2008) other sectors still wait for such complementary data that would confirm other sectors as true integral parts of the Kalnik Unit.…”

“…Its exact tectonic position in the framework of Alpine-Dinaric-Pannonian triple junction zone sensu Hass & Kovács (2001) is not clearly explained yet (e.g. Pamić & Tomljenović 1998;Tari & Pamić 1998;Hass et al 2000;Pamić 2002;Babić et al 2002;Goričan et al 2005;Schmid et al 2008;Robertson et al 2009). However, a position within the transitional zone between the External and Internal Dinarides seems to be most acceptable (Placer 1999).…”

Displaced South Alpine and Dinaridic elements in the mid-Hungarian zone

“…In the Medvednica and Kalnik Mts blocks of rift-type peperitic basaltoid rocks also occur . Ladinian and Carnian to Norian radiolarian assemblages were found in radiolarian chert, in direct stratigraphic contact with pillow basalt (Halamić and Gorićan 1995;Gorićan et al 2005) constraining a Middle to Late Triassic age of such blocks in the mélange. In the Medvednica Mts Bathonian to Callovian radiolarians were encountered in radiolarite beds intercalated into a polymict olistostrome (debrite) succession containing pebble-sized sandstone, siltstone, chert and basalt clasts and olistoliths in a shale matrix (Halamić et al 1999).…”