Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings

Abstract

Metamorphic core complexes (MCCs) are interpreted as domal structures exposing ductile deformed high-grade metamorphic rocks in the core underlying a ductile-to-brittle high-strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension. Extension is supposedly the driving force that has governed exhumation. However, numerous core complexes, notably Himalayan, Karakoram and Pamir domes, occur in wholly compressional environments and are not related to lithospheric extension. We suggest that many MCCs previously thought to form during extension are instead related to compressional tectonics. Pressures of kyanite-and sillimanite-grade rocks in the cores of many of these domes are c. 10–14 kbar, approximating to exhumation from depths of c. 35–45 km, too great to be accounted for solely by isostatic uplift. The evolution of high-grade metamorphic rocks is driven by crustal thickening, shortening, regional Barrovian metamorphism, isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension. Extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel (channel flow) following peak metamorphism beneath a passive roof stretching fault. In Naxos, low-angle normal faults associated with regional Aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening. The fact that low-angle normal faults exist in both extensional and compressional tectonic settings, and can actively slip at low angles (< 30°), suggests that a re-evaluation of the Andersonian mechanical theory that requires normal faults to form and slip only at high angles (c. 60°) is needed.

The Cycladic Blueschist Unit on Tinos, Greece: Cold NE Subduction and SW Directed Extrusion of the Cycladic Continental Margin Under the Tsiknias Ophiolite

“…The cause of this heating remains debated. While Avigad and Garfunkel (1991) suggest the heating is related to crustal extension, we argue Barrovian metamorphism is a result of crustal thickening as observed on Naxos, following continental collision between Eurasia and Adria/Cyclades (Lamont et al, 2019; Searle & Lamont, 2019).…”

“…Deformation on the TSZ was responsible for exhumation of rocks from midcrustal depths much later in the orogenic history. It is unclear whether the top‐to‐NE extensional fabrics on the TSZ represents overall crustal extension or relative extension along a passive roof fault in an overall compressional regime (Means, 1989; Searle & Lamont, 2019). The TSZ was responsible cutting out ~25 Myr of subduction history by juxtaposing the Upper Unit containing the Tsiknias Ophiolite, metamorphic sole, Mirsini Unit (pelagic metasediments) and Akrotiri Units in the hanging wall against the CBU in its footwall.…”

“…Tinos island is a part of the Attic Cycladic Massif (ACM; Durr et al, 1978; Papanikolaou, 2013) (Figure 1), which experienced a complete orogenic cycle including ophiolite obduction, HP metamorphism through crustal thickening, and regional metamorphism to crustal extension (Lamont et al, 2019; Lamont, Roberts, et al, 2020; Searle & Lamont, 2019). The island represents a NW‐SE trending dome (Figure 2) and comprises two distinct tectonometamorphic units (Upper Unit and Lower Unit) that are separated by a greenschist facies low‐angle normal‐sensed shear zone (the Tinos Shear Zone, TSZ), which displays top‐to‐NE kinematics (Avigad & Garfunkel, 1989; Brichau et al, 2007; Bröcker, 1990; Bröcker & Franz, 1994, 1998; Bröcker et al, 1993, 2004; Jolivet et al, 2010; Katzir et al, 1996; Melidonis, 1980; Ring & Layer, 2003).…”

The Anatomy and Origin of a Synconvergent Grenvillian-Age Metamorphic Core Complex, Chottanagpur Gneiss Complex, Eastern India

“…Despite the convergent nature of these tectonic settings, MCC development is associated with extension-related processes such as slab rollback, intrusion driven extension [13], or orogenic collapse under fixed boundary conditions or slow plate convergence [14]. Searle and Lamont [15] demonstrate that MCCs can form in entirely compression regimes unaffected by any extensional tectonism.…”

The Age, Origin, and Emplacement of the Tsiknias Ophiolite, Tinos, Greece

“…This represents the final closure of the Vardar Ocean, and during this stage, the Tsiknias Ophiolite, metamorphic sole, and Mirsini Unit would have been thrust to the SW onto the depressed and underthrusted Cycladic (Adriaic–Apulian) continental margin. Following extrusion from 80‐km depth and upon reaching mid‐crustal depths later in the orogenic history, these high‐pressure rocks were then juxtaposed against the Upper Unit by the greenschist‐facies top‐to‐NE shear zone (The Tinos Shear Zone) active during the Miocene (Figure —Stage 5), although it remains debated whether this structure was active during regional extension or during a phase synorogenic extrusion (Searle & Lamont, ). The onset of regional extension, exhumation, and the intrusion of the Tinos I‐type granite intruded at ~14.6 Ma (Brichau et al, ; Figure —Stage 6). A component of E‐W shortening caused upright folding during the Miocene that affected all units and folded the detachments.…”