Model of successive granite sheet emplacement in transtensional setting: Integrated microstructural and anisotropy of magnetic susceptibility study

Abstract

[1] This study presents a model of successive emplacement of three granite plutons in transtensional deformation regime controlled by the preextensional synconvergent history of the lower and middle crust in the central Vosges Mountains (France). The complex compressional orogenic structure recorded a vertical exhumation-related fabric in midcrustal levels which was overprinted by the regional extension. Three successively emplaced granite sheets exploited the inherited vertical anisotropy obliquely oriented with respect to the applied tensile stress. This progressive opening of original steep fabric leads to the sequential generation of free spaces toward the south, and emplacement and deformation of granite sheets. Repeated granite intrusions are responsible for reheating of southern margins of the already partially solidified granites, which became reactivated under lower ambient thermal conditions related to overall $6 Ma cooling of rapidly exhumed crust. The results of anisotropy of magnetic susceptibility (AMS) fabric modeling suggest a highly partitioned oblique extension divided into pure shear-dominated deformation close to the central and northern margins of intrusions and wrenchdominated shear along the southern margins. The AMS modeling suggests the importance of overprinting of the intrusive fabrics by the transtensional deformation.

Some remarks on fabric overprints and constrictional AMS fabrics in igneous rocks

“…We show that the variations of fabric symmetries and degrees of AMS anisotropy may result from complex deformation overprints. Such a possibility was already supported by independent structural studies (Kratinová et al 2007(Kratinová et al , 2010Parry et al 1997;Schulmann et al 2009). …”

“…Several modern structural studies show that granitoids often exhibit multiple foliations (Ž ák et al 2007, 2008), but the associated variations in AMS fabrics were never discussed. Kratinová et al (2007), Schulmann et al (2009) and Kratinová et al (2010) combined both structural and AMS fabric studies to unravel polyphase deformation of migmatitic terrains and intrusions of granite sheets. These studies showed the presence of multiple fabrics and development of prolate shapes of AMS ellipsoids in areas of fabric transpositions developed at a high angle between two planar fabrics.…”

“…Structural geology of magmatic bodies provides a number of transitional examples from gravity-driven ascent leading to development of flat fabrics and to highly partitioned transtensional deformation (e.g., Kratinová et al 2007Kratinová et al , 2010Kratinova et al this volume). The development of prolate fabrics at the expense of a horizontal oblate foliation is another example of complex interplay between orientation of initial anisotropy and superimposed deformation tensor.…”

The significance of Late Devonian ophiolites in the Variscan orogen: a record from the Vosges Klippen Belt

“…335 Ma in the northern Vosges (Altherr et al 2000) most likely reflects the magmatic arc related to this event. The age of arc magmatism coincides with a major extension associated with partial melting and emplacement of sheeted intrusions in the southern highgrade core zone of the Vosges (Rey et al 1992;Kratinová et al 2007;Schulmann et al 2009). It is therefore the central part of the Vosges Mountains that probably recorded the extensional and high temperature event associated with the southward subduction of the Rhenohercyian basin.…”

Relationships between magmatism and extension along the Autun–La Serre fault system in the Variscan Belt of the eastern French Massif Central

“…Although well documented (Burg et al 1994;Faure 1995), the Stephanian-Permian NE-SW stretching must be distinguished from the NW-SE Namurian-Westphalian extension reported from the western part of the Massif Central (Faure 1995 Laurussia has resulted in thrust tectonics and metamorphism in the Vosges Mountains (Fluck et al 1987;Rey et al 1989;Schulmann et al 2002), and in the Black Forest Eisbacher et al 1988;Echtler and Chauvet 1992). The thickened crust experienced synorogenic to postorogenic extension represented by detachment faults and syntectonic plutons, documented in the Vosges Mountains (Rey et al 1992;Kratinova et al 2007), and in the Black Forest (Krohe and Eisbacher 1988;Eisbacher et al 1988;Echtler and Chauvet 1992).…”