......................................................................................................................................2 Laccoliths and magmatism in strike slip settings .......................................................................3 Geotectonic setting of Central Europe at the Carboniferous-Permian transition .......................5 Variscan intramontane strike slip basins with prominent laccolith complexes..........................5 Types of laccolith complexes ...................................................................................................14 Discussion: Strike slip control on the evolution of laccolith complexes..................................17 Conclusions ..............................................................................................................................21 Acknowledgement ....................................................................................................................22 References ................................................................................................................................22 Laccoliths in transtensional basin systems 2 Abstract Comparing felsic laccolith complexes prominent in the Late Palaeozoic Ilfeld-, Saar-Nahe-, and Saale basins in Germany, a characteristic pattern related to transtensional tectonics is revealed. In contrast to central magma feeding systems recognized so far for laccolith complexes, individual units of the Late Palaeozoic Central European complexes apparently were fed synchronously by numerous feeder systems arranged laterally in a systematic pattern. The Ilfeld basin is a small strike slip pull apart basin in the SE of the Hartz Mountains cogenetical with a neighbouring rhomb horst – the Kyffhäuser. The Ilfeld basin represents a “frozen-in” early stage of laccolith complex evolution with small isolated intrusions and domes emplaced within a common level at the intersections of intra-basinal Riedel shears. In the Saar-Nahe basin, numerous medium-sized felsic subvolcanic to subaerial complexes emplaced at a common level have been recognized (Donnersberg type laccolith complex). The magmatic evolution of the Halle Volcanic Complex in the Saale basin culminated in the ± synchronous emplacement of voluminous porphyritic laccoliths within different levels of a thick pile of Late Carboniferous sediments (Halle type laccolith complex). Laccoliths in the Halle area might consist of several laccoliths typical for the Saar-Nahe Basin according to outcrop pattern and host sediment distribution. These three post-Variscan Central European basins are characterized by a dextral transtensional tectonic regime leading to a model for laccolith complex evolution: i) Initial lithosphere-wide faulting forms pathways for magma ascent. ii) Supracrustal pull-apart leads to the formation of a transtensional basin. iii) Continued transtension gives way to decompressional melting of the mantle lithosphere, especially if fertilized by previous magmatic activity as in the Variscan orogen. The mantle melts rise into the lower crust to differentiate, mingle or cause anatexis. iv) They homogenize and start crystallizing in a midto upper crustal magma chamber tapped during tectonic episodes. v) Resulting SiO2-rich magmas ascend along major transtensional faults into thick sedimentary basin fill. The amount of transtension and the amount of melt rising from the lithospheric mantle have major influence on type and size of the laccolith complex to be formed. Additionally, the presence of a midto upper crustal magma chamber is a prerequisite for the formation of the Donnerberg and Halle type laccolith complexes. End of abstract Laccoliths in transtensional basin systems 3 Based on a detailed study of one laccolith complex (i.e. the Halle laccolith complex, HLC) and a literature review on other laccolith complexes in Permocarboniferous Central Europe, we attempt to explain the tectonic controls on the formation of laccolith complexes in continental strike slip systems. This contribution should serve as a base for discussion about the definition of new types of laccoliths beyond the classic mechanical spectrum defined by Corry (1988) with a punch to Christmas tree geometry. Laccoliths and magmatism in strike slip settings Laccoliths are intrusive bodies with a flat lower and a curved upper contact. They are common features of intrusive mafic and felsic intracontinental magmatic provinces (Corry 1988; Friedman & Huffman 1998). Several aspects of their emplacement and geometry are not well understood. The classic approach of Gilbert (1877) in the Tertiary Henry Mountains in Utah has been developed further by Corry (1988) with finite element modelling and thorough field investigations (Johnson & Pollard 1973; see also: Jackson & Pollard 1988; and Kerr & Pollard 1998 for recent numerical modelling approaches). Corry postulated that the level of neutral buoyancy of magma and country rock is the major controlling factor for the initiation of laccolith formation. However, other parameters are important such as the presence of fluids, the stress field in the host rock, and dynamic features of the rising magma (crystallization, viscosity, magma driving pressure, transport rate). According to analogue modelling, laccolith emplacement also requires the presence of a weak layer, e.g. a sole thrust or a less competent lithology, near the level of emplacement (Roman Berdiel et al. 1995). There, the orientation of magma flow changes from vertical to horizontal. A sill of ~ 30 m thickness forms first and inflates upon reaching a critical expanse determined by the effective thickness of the overburden (Johnson & Pollard 1973; Pollard & Johnson 1973), given that the supply of magma is sufficient. Recent studies suggest that laccolith-like mechanisms play a major part in the emplacement even of large plutonic bodies (e.g. Vigneresse et al. 1999). Furthermore, the magma chamber below many caldera complexes has a laccolithic geometry ("lacco-caldera" according to Henry et al. 1997). Depending on the surrounding geology and tectonic setting, laccolith geometries might become much more complex than the simple mechanical models suggested (Morgan et al. 1998). Before turning to the laccoliths in the strike slip pull-apart basins of Permocarboniferous Central Europe, as a means of comparison, we would like to introduce some examples of Laccoliths in transtensional basin systems 4 similar tectonic environments showing different styles of magmatism. They represent different styles of deformation and magmatic activity ranging from ancient plutonic emplacement to recent volcanic features. In the north-western corner of the Arabian plate, near the triple junction of Anatolia, Arabia and Africa, Late Cenozoic elongate volcanoes, volcanic ridges and linear clusters of adjacent volcanic vents are rooted on tension fractures, which are a kilometre or several kilometres in length and show similar development in depth. Non-volcanic tension fractures are also common (Adiyaman & Chorowicz 2002). Late Jurassic strike slip intra-arc basins formed along the axis of earlier Early to Middle Jurassic extensional intra-arc basins in western North America. Volcanism occurred only in releasing bends in the Late Jurassic arc, producing more episodic and localized eruptions than in the extensional arc, where volcanism was voluminous and widespread (Busby 2002). The Ollo de Sapo domain of the northern part of the Variscan belt of Spain contains Precambrian and Ordovician metamorphic rocks intruded by the Guitiriz granite. The domain is bounded by two N-S transcurrent shear zones. Plutonism occurred in three steps: (1) development of N-S trending structural and magnetic fabrics; (2) concordant structures in granites and country rocks; and (3) development of shear zones along the eastern and western granite margins. The proposed emplacement model involves the northwards tectonic escape of a crustal wedge – the Ollo de Sapo Domain – bounded by two shear zones acting as conjugate strike slip zones (Aranguren et al. 1996). The island of Vulcano is composed of four main volcanoes which date from about 120 ka to historical times. The time-space evolution of the volcanism indicates a shifting of the activity from the southeastern sectors towards the northwest. Two main systems of NW-SE-trending right-lateral strike slip faults affect the island. NE-SWand N-S-trending normal faults are also present. This system of discontinuity is related to the stress field acting in the southern sector of the Aeolian Archipelago. Volcanological and geochronological data are also consistent with the opening of a pull-apart basin (Ventura 1994). These examples show that a strike slip tectonic environment can produce very different styles of magmatism. In the next section, the focus will turn on some specific examples from the Carboniferous-Permian transition in Central Europe. They will be compared to the well investigated classic examples of the Tertiary Colorado Plateau in the Western US. Laccoliths in transtensional basin systems 5 Geotectonic setting of Central Europe at the Carboniferous-Permian transition A number of rift related basin systems with pronounced magmatic activity developed in the wane of the Variscan orogenesis in the foreland and on the cratonic blocks of former Baltica (Arthaud & Matte 1977). Among these are the Oslo graben, Norway, the Whin Sill region, Northern England, the North Sea graben systems Central and Horngraben – the former dominated by tholeiitic flood basalts, the latter with chemically varied magmatism – and the central NE German Basin. The latter contains about 48,000 km3 of volcanic rocks with subordinate SiO2-poor lavas, but dominantly (~70%) SiO2-rich, calc-alkaline, subaerial ignimbrites and lava domes (Fig. 1, Benek et al. 1996; Breitkreuz & Kennedy 1999). The decaying Variscan Orogen itself, on the contrary, was characterized by