The evolution of the Meliata-Hallstatt ocean and its significance for the early evolution of the Eastern Alps and Western Carpathians

Abstract

The evolution of the Meliata-Hallstatt ocean and of its southern and northern margin in the Inner Western Carpathians are described. The main rifting began during the Pelsonian and the sea-floor spreading ended at the beginning of the Middle Carnian contemporaneously with the Raibl event. The subduction began in the latest Triassic and the final closing of the ocean, accompanied by uplift of the adjacent marginal areas, was dated as basal Oxfordian. Remnants of the oceanic-suboceanic sequence occur in two tectonic positions: (1) Obducted nappes that contain, mostly as tectonic melanges, the whole Middle Triassic to Middle Jurassic sequence, including large bodies of dismembered ophiolites of Ladinian to Early Carnian age. (2) Evaporite melanges at the base of nappes that are derived from the northern margin of the ocean. These evaporite melanges consist of Late Permian evaporitic matrix and blocks of a dismembered Ladinian to Lower Carnian ophiolitic sequence. In the Inner Western Carpathians both the northern and southern marginal sequences of the Meliata-Hallstatt ocean and the transitions into the carbonate platforms are present in different nappes. The southern marginal development is characterized by Middle Carnian distal clastic Raibl Beds and by Middle Jurassic rhyolithic volcanism, both missing in the northern marginal development. Because the subduction-related Middle Jurassic volcanism (contemporaneous with the turbidites in the southern Meliaticum) is restricted to the marginal area south of the Meliata-Hallstatt ocean, southward-directed subduction is indicated. To the south, on the adjacent carbonate platform, a Jurassic basin with Aalenian to Bajocian ophiolites opened. This may be a back-arc basin. In the Eastern Alps only the northern marginal zone and the adjacent carbonate platform are preserved, but parts of the dismembered ophiolites in the Haselgebirge may be of Ladinian to Late Carnian age like in evaporitic melanges in the same tectonic position in the Inner Western Carpathians. The final closing of the Meliata-Hallstatt ocean is indicated by the abrupt end of the turbiditic sedimentntation in the oceanic-suboceanic domain and by uplift in the marginal areas, where Lower Oxfordian radiolarites are overlain by shallow-water Upper Oxfordian to Tithonian or Neocomian limestones (Silica Nappe, Hallstatt Nappes). With the evidence of the final closing of the Meliata-Hallstatt ocean near the Middle/Late Jurassic boundary, the Cimmerian orogenesis (in the genetic sense of Şengör, 1984, 1985) is now also proven in the Inner Western Carpathians and in the Eastern Alps. A continuation of the Meliata-Hallstatt ocean into the Pontide “Paleotethys” (=Cimmerian ocean sensu Kozur, 1990) through the TransylBANian oceanic domain and the Strandzha Unit is assumed. A connecction with the Vardar ocean is impossible, because in the Vardar Zone no Triassic ophiolites or basic volcanics are present and the ophiolites have Jurassic age.