Metamorphic Domes in Northern Tunisia: Exhuming the Roots of Nappe Belts by Widespread Post‐Subduction Delamination in the Western Mediterranean

Abstract

Cenozoic extension in the Western Mediterranean has been related to the dynamics of back‐arc domains. Although, in most of its orogenic belts extension propagated into the fore‐arc nappe domains. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell, where HP/LT rocks (350°C at 0.8 GPa), were exhumed by the sequential activity of extensional detachments after heating and decompression (410°C–440°C at 0.6–0.3 GPa) in a plate convergent setting. Normal faults thinning the Tunisian Tell detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving Tellian Triassic evaporites in the hanging‐wall forming halokinetic structures in the Mejerda basin late Miocene. The deeper‐detachment bounds metamorphic domes formed by marbles and metapsammites from the Atlas domain. Illite crystallinity on Triassic rocks shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell‐Atlas nappe belt, respectively. New U‐Pb 49.78 ± 1.28 Ma rutile ages from Tellian metabasites, together with existing phlogopite 23–17 Ma K‐Ar ages in Atlas marbles from the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell rocks underthrusted the Kabylian flysch in the early Eocene. Further, early Miocene shortening thrusted the metabasites over lower‐grade sediments, producing HP/LT metamorphism and ductile stretching at the base of the Atlas belt. The exhumation of midcrustal roots of Western Mediterranean nappe belts after tectonic shortening is a common feature related to tearing at the edges of the subduction systems and inboard delamination of their subcontinental lithospheric mantle.

Plain Language Summary

Mountain belts are formed by shortened sedimentary rocks. The Tell cordillera in Northern Tunisia is interpreted as a classic mountain belt developed through protracted shortening from the late Cretaceous until Present, formed by folded and overthrusted rocks, and intruded by salt bodies. However, we show here that conversely, some of the supposed salt bodies are formed by metamorphic rocks that were originally buried at depths of approximately 26 km. Moreover, the remaining salt structures in the Tunisian Tell formed in relation to the late‐stage thinning and collapse of the mountain belt, as they intrude through extensional faults into late Miocene sediments. We characterize the temperature and pressure conditions reached by the metamorphic rocks and obtain a 49 Ma age of an early metamorphic event by radiometric dating of rutile. Metamorphic rocks where also exhumed in other Western Mediterranean mountain belts like the Betics, Rif, Algerian Tell after the main shortening stage. We relate this process to delamination, a deep mantle tectonic mechanism, which strips the nappe belt crustal domain from its underlying mantle root.

Key Points

• The Tunisian Tell hosts High‐Pressure Low‐Temperature (HP/LT) domes underlying late Miocene extensional detachments, driven by slab delamination

• Metamorphic and rutile U‐Pb data shows that rocks in Northern Tunisia reached HP/LT conditions in the early Eocene and early Miocene

• Halokinetic structures in Northern Tunisia are rooted in the Mejerda detachment and related to late Miocene extension