THE RELATIONS BETWEEN PERI-GONDWANA BLOCKS AND THE INTERIOR OF GONDWANA IN THE LATE NEOPROTEROZOIC AND EARLY PALEOZOIC TIMES

It is widely accepted that many blocks within the orogenic edifices along the southern margins of Laurussia were located in Late Neoproterozoic and Early Paleozoic times along the periphery of Gondwana. However, the relations between their history and that of the adjacent more internal areas of Gondwana was little studied. Here are examined the relations between the Cadomian-type blocks of Europe and the Middle East and between West and North Africa and Arabia (WNAA) next to which they were most likely located.

The WNAA part of Gondwana was shaped by the assembly and amalgamation of many units and elimination of intervening oceanic areas (with a combined width of perhaps a few thousand kilometers) that was completed 650-620 Ma ago. It is only then that the Late Neoproterozoic configuration could have been established. The earlier positions of the peri-Gondwana blocks are not well constrained, but several lines of evidence suggest that many of them were accreted to WNAA in the late stages of its assembly. When these blocks formed the fringe of WNAA they evolved in an active margin setting, being affected by the Cadomian orogenic phase until the early Cambrian. In contrast, in this time interval the more internal WNAA region was stabilized and was deeply eroded in the aftermath of the Pan African orogeny, and became a stable platform. The tectonic relations between these two domains are not known, so several plate tectonic models of the accretion and development of the Cadomian orogeny can be envisaged. To better constrain them, more needs to be known about the arrangement of magmatic arcs and back-arc basins, and about the transport of detrital zircons in this zone.

Since the Early Cambrian an extensive veneer of northward transported mature silici-clastic sediments accumulated on the WNAA platform. The extension of this cover to the peri-Gondwana domain can be recognized in Anatolia where platformal conditions were established. Sediments accumulated also in more western peri-Gondwana blocks, but in the known ones igneous and tectonic activity of various types continued in the Cambrian and Ordovician times, indicating a different tectonic regime. This activity varied considerably laterally and also in time and was variously interpreted as recording an active margin setting in which rifts and back-arcs formed, and also transtensional deformation, rifting and breakup of the Gondwana margin after subduction along it ended. These events are not obviously related to the evolution and subsidence pattern of the more internal WNAA platform. It is also not clear where the large volume of sediments transported northward from the platform accumulated. The transition from the platform to the more active peri-Gondwana area is not exposed, but it follows that the WNAA platform was not delimited by a passive margin that passed northward into an ocean but by a few hundred kilometers wide belt where tectonic and igneous activity continued at least 80-100 Ma after the platformal sediment cover began to accumulate. A passive margin formed along the edge of this part of Gondwana only after some of the peripheral blocks rifted away and drifted northward.

Thus, the relation between the peri-Gondwana blocks and WNAA provide new insights, and also raise new questions, regarding the history of these areas.