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Mesopotamia — 1 Comment

  1. A comment on the interpretation of the top age of the Mesopotamia slab and thus also the base age of the Zagros slab. These ages are currently interpreted as relating to latest Cretaceous slab break-off, which is evidenced apparently by rapid exhumation of HP-LT rocks in the Zagros. This view was recently challenged in Deevsalar et al. 2018 (JGSL 175, 915-933). Their paper explored the cause of Eocene within-plate type magmatism in the far west of Iran which has now been recognised in a few studies quoted in that paper.

    The relevant text is quoted here:
    “Firstly, the onset of voluminous magmatic activity in the Urumieh-Dokhtar Magmatic Arc and further east during the Eocene has already been ascribed to rollback-induced slab steepening (e.g. Verdel et al. 2007; 2011; Mouthereau et al.
    2012). Verdel et al. (2011) argued that slab-rollback may have been preceded by a period of flat slab subduction lasting from the mid-Cretaceous, as evidenced by a regional unconformity separating Cretaceous sediments from overlying Paleogene volcanic rocks (Stöcklin 1968). In this model, there need not have been a slab break-off event around the Cretaceous-Palaeogene boundary [[This is what is referenced to in the above description]]. Late Cretaceous deformation in the Sanandaj-Sirjan Zone (Tillman et al. 1981) and a north-eastward shift in the locus of subduction-related magmatism from the SaSiZ to northern Iran and the UDMA also support the conclusion that a Tethyan slab was continuously attached to the surface during the mid-Cretaceous to Eocene (e.g., Guest et al. 2006; Omrani et al. 2008), albeit perhaps becoming shallower in angle at this time. Rapid exhumation in the Zagros mountains at ~65 Ma (Agard et al. 2011) could have several potential causes, including changes in collision rate or in the thickness and/or density of the subducted crust (Iannace et al. 2007; Brun & Faccenna 2008; Rosenbaum & Mo 2011). Furthermore, an upsurge in mantle-derived magmatism is commonly ascribed in the literature to slab break-off. However, this upsurge is not recorded until 10’s of Ma after the proposed break-off event of Agard et al. (2011), and as a corollary, nor did any extensive magmatism occur close to the Zagros Suture around ~65 Ma ago (Verdel et al. 2011).”

    This comment is therefore to state that it seems difficult to reconcile a slab break-off event around 65 Ma ago with the available data because it is not the only possible explanation for exhumation and because the age of relevant break-off-related magmatic activity does not fit. I feel that ascribing a top age to the Mesopotamia slab of early Eocene (~45-40 Ma) may fit a wider range of data.

    The knock-on implication is that the short burst of early Eocene within-plate magmatism is Eastern Iran, which is followed by the resumption of ‘normal’ subduction related magmatism of the Urumieh Dokhtar Magmatic Arc, marks the onset of what would be a very short-lived period of Zagros slab subduction, between the early-mid Eocene and the Oligocene collision of Arabia and Eurasia.

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