Ductile and brittle structural evolution of the Laxemar-Simpevarp area: an independent analysis based on local and regional constraints

This report discusses the main aspects of the ductile and brittle deformational evolution of the Laxemar-Simpevarp area. Based on the interpretation of existing potential field geophysical data, it is suggested that the structural ductile grain of the region is controlled by large, c. EW trending shear zones with an overall sinistral strike-slip kinematics. The Oskarshamn Shear Zone (OSZ) and the Mederhult lineament are two examples of these shear zones and it is proposed that the ductile lineaments mapped in Laxemar-Simpevarp are genetically linked to shearing accommodated by these shear zones. The structural interpretation of the geophysical imagery of the Laxemar-Simpevarp regional model area and the available meso-scale structural information indicate that the Laxemar-Simpevarp study area can be interpreted as the analogue of a large-scale S/C' structural pattern. In detail, the Aespoe shear zone and other similarly oriented ductile shears represent C' shear bands that deform sinistrally the intervening EW lineaments (the S surfaces), which locally are significantly crenulated/folded in response to their asymptotic bending into the C' shears. This geometric and kinematic interpretation implies that, in contrast to existing reconstructions and models, EW- and not NE-trending shear zones become the main structural ductile feature of the region. Shear forces acting parallel to these main zones can successfully explain all the ductile structures described and reported from the area. The greatest compressive stress at the time of ductile shearing would trend NE-SW. The brittle deformation history of the region is complex and results from the multiple reactivation of fracture- and fault sets caused by the many orogenic episodes that affected the area during 1.5 Gyr of geological brittle evolution. Fault-slip data from outcrops and oriented drill cores were used to compute paleo-stress states. In the general absence of time markers that help constrain the relative timing of the multiple faulting events, the metamorphic grade of mineralizations coating the striated planes was of help in distinguishing earlier and 'higher-grade' from later and 'colder' structures. Faulted Ordovician limestones on the island of Oeland, however, provided a useful marker to distinguish pre- from post-Ordovician paleo-stress tensors. Two compressional, c. NNW-SSE and NNE-SSW oriented shortening episodes generated sets of conjugate, steep strike-slip fractures coated by epidote and quartz. These sets (referred to as 'set I' and 'set II', respectively) formed during the late stages of the Svecokarelian and possibly also of the Gothian Orogeny, soon after the region entered the brittle deformation domain c. 1.5 Ga ago. Although it is natural to expect extensional periods affecting the region at the end of these major orogenic episodes, no systematic set of brittle extensional fractures could be identified with confidence. The Mesoproterozoic Sveconorwegian orogeny, which in the area is believed to have caused an overall c. EW shortening, generated fractures and faults that are assigned to 'set III', a set of conjugate strike-slip faults, which constrain a c. EW σl. Late Sveconorwegian extension/transtension is known to have caused intrusion of 978-946 Ma old and c. NS trending doleritic dykes. The Caledonian shortening, oriented from c. NW-SE to EW, reactivated set III but also formed a new, similarly oriented set of sub-vertical strike-slip fractures in the Ordovician limestones of Oland. Permian transtension was oriented NE-SW and caused a prominent set of moderately-dipping NW-SE trending normal faults in the pre-Cambrian basement of the study area. A second set of normal faults accommodating NW-SE extension is also documented. NE-SW trending clastic dykes filled by remobilised Cambrian sandstones may be the result of this extensional phase. Two other, distinct NE-SW and NNWSSE oriented shortening events (referred to as 'set b' and 'set c', respectively) are recorded in the Ordovician limestones and can be tentatively linked to the far-field effects of the Laramide and Alpine orogenies. Each of these deformation processes caused the reactivation of suitably oriented pre-existing faults and fractures, thus significantly complicating the unravelling of the brittle deformation history.