The Timing of Strike-Slip Deformation Along the Storstrømmen Shear Zone, Greenland Caledonides: U–Pb Zircon and Titanite Geochronology

Abstract

The Storstrømmen shear zone (SSZ) in the Greenland Caledonides is widely interpreted to have formed in a transpressional regime during sinistral, oblique collision between Baltica and Laurentia in the Silurian to Devonian. New mapping of the SSZ at Sanddal documents a 100 m thick, greenschistfacies mylonite zone cutting the eclogite to amphibolite-facies gneiss complex. We present U–Pb ion probe geochronology on zircon and titanite from a variety of lithologies that shows the SSZ was active from late Devonian to the Carboniferous (at least until 350 Ma). The age of thrusting in the foreland is not well known, but must be younger than the age of eclogite-facies metamorphism at ~400 Ma. It is, therefore, possible that contraction is the same age as strike-slip motion, and that transpression is a viable model. The timing of the SSZ is synchronous with dextral strike-slip displacement on the Germania Land deformation zone. Simultaneous displacement on sinistral and dextral, conjugate shear zones suggests that the SSZ is part of a strikeslip fault system that led to lateral escape of material northward (present day coordinates) during the waning stages of plate convergence between Laurentia and Baltica.SOMMAIRELa zone de cisaillement de Storstrømmen (SSZ) dans les Calédonides du Groenland est généralement comprise comme ayant été formée durant un régime de transpression sénestre lors de la collision oblique entre Baltica et Laurentie, du Silurien au Dévonien. Une nouvelle cartographie de la SSZ à Sanddal décrit une zone de 100 m d’épaisseur de mylonite au faciès des schistes verts qui recoupe un complexe de gneiss au faciès éclogite à amphibolite. Notre analyse géochronologique par sonde ionique U-Pb sur zircon et titanite sur diverses lithologies, montre que la SSZ a été active de la fin du Dévonien jusqu’au Carbonifère (au moins jusqu’à 350 Ma). L’âge du chevauchement dans l’avant-pays n’est pas bien connue, mais il doit être plus jeune que le métamorphisme au faciès d’éclogite à ~400 Ma. Il est donc possible que la contraction soit du même âge que le mouvement de coulissage, et que la transpression soit un modèle viable. La chronologie de la SSZ est synchrone au mouvement de coulissage dextre de la zone de déformation de Germania Land. Les déplacements simultanés, sénestre et dextre, sur des zones de cisaillement conjuguées permettent de penser que la SSZ fait partie d’un système de décrochement qui a engendré une éjection latérale de matériau vers le nord (selon les coordonnées actuelles) durant les stades de convergence des plaques Laurentie et Baltica.

Middle to Late Devonian–Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

“…The timing of final core complex exhumation can be constrained to Middle to Late Devonian-early Carboniferous and possibly linked to the regional Serpukhovian unconformity on the Finnmark Platform (see Figs. 5a, b, e, f, and g and 6a-c; Bugge et al, 1995), in accordance with Sartini-Rideout et al (2006) and Hallett et al (2014) in northeastern Greenland.…”

“…By comparison, in northeastern Greenland, SartiniRideout et al (2006) and Hallett et al (2014) proposed that ultra-high-pressure basement rocks were exhumed along large, mylonitic, Caledonian shear zones in Late Devonianearly Carboniferous times . The study of Sartini-Rideout et al (2006) also shows that the last stages of exhumation were accommodated by steep, brittle normal faults that strike parallel to major Caledonian shear zones, i.e., similar to the main segment of the MFC striking parallel to the SISZ along the SW Barents Sea margin (Fig.…”

“…10a-c). Additionally, Late Devonian-early Carboniferous timing of exhumation for basement rocks in northeastern Greenland and formation of a regional mid-Carboniferous (Serpukhovian) unconformity (Sartini-Rideout et al, 2006;Hallett et al, 2014;McClelland et al, 2016) corresponds to a realistic approximation for the final stages of late-to post Caledonian extension, ending with the formation of DevonianCarboniferous collapse basins along exhumed NE-SW-to ENE-WSW-trending basement ridges (Fig. 10d-f).…”

“…We propose that the SISZ and potential other Caledonian shear zones along the SW Barents Sea margin were inverted as low-angle extensional shear zones during late-to post-Caledonian orogenic extension and subsequent collapse. This is based on analog examples in northeastern Greenland (Sartini-Rideout et al, 2006;Hallett et al, 2014;McClelland et al, 2016), western Norway (Séranne and Seguret, 1987;Séranne et al, 1989;Wilks and Cuthbert, 1994;Osmundsen and Andersen, 2001), mid-Norway (Braathen et al, 2000), and Lofoten-Vesterålen Steltenpohl et al, 2004Steltenpohl et al, , 2011Osmundsen et al, 2005). Extensional reactivation of such ductile shear zones along the Barents Sea margin may have initiated in the Early Devonian, as in LofotenVesterålen (Steltenpohl et al, 2011), through orogenic collapse dominated by top-NW movement along the SISZ.…”

Mid/Late Devonian-Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

“…We propose that the SISZ and potential other Caledonian shear zones along the SW Barents Sea margin were inverted as low-angle extensional shear zones during late/post-Caledonian orogenic extension and subsequent collapse. This is based on analog examples in northeast Greenland (Sartini-Rideout et al, 2006;Hallett et al, 2014;McClelland et al, 2016), western Norway (Séranne & Seguret, 1987;1185Séranne et al, 1989Wilks & Cuthbert, 1994;Osmundsen & Andersen, 2001), mid-Norway (Braathen et al, 2000) and Lofoten-Vesterålen Steltenpohl et al, 2004;Osmundsen et al, 2005). Extensional reactivaton of such ductile shear zones along the Barents Sea margin might have initiated in the Early Devonian as in LofotenVesterålen (Steltenpohl et al, 2011), by crustal thinning and orogenic collapse through dominant 1190 top-to-the-NW displacement along the SISZ and later exhumation of the SISZ and underlying basement as a metamorphic core complex.…”