TSK 11 Göttingen 2006 Angerer & Greiling Rock fabrics in palaeo- weathering profiles below basement-cover interfaces (AMS-study on drill cores from the Caledonian margin, Central Sweden) Vortrag Thomas Angerer1 Reinhard O. Greiling1 Basement-cover-interfaces are impor- tant crustal boundaries. In many cases they act as detachment hori- zons. Criteria like pre-erosional base- ment characteristics, intensity of palaeo- weathering and post-erosional processes during burial stage lead to a huge vari- ety of observable alteration and fabric features of basement-cover-interfaces, which may influence the shear-strength. Unconformity-parallel planar fabrics in the weathering profile were facilitated by palaeo-alteration and later processes (Angerer 2005 unpubl. data). Such fabrics may be a factor for lowering the shear-strength (e.g. Wintsch et al. 1995). The probably ubiquitous exis- tence of those fabrics at basement-cover- interfaces is investigated in case studies by means of AMS-fabric analysis, which is a sensitive indicator of rock fabric changes. The present case study is based on sections from two drill cores across the erosional unconformity between Fennoscandian Granite (Revsund) and Cambrian Gärdsjön Fm. (Långviken SGU 73007 and Hara 79002, see Fig. 1) (petrographic descriptions in Gee, 1978 and Gee et al. 1982). 1 Geologisch-Paläontologisches Institut, Uni- versität Heidelberg, Im Neuenheimer Feld 234 Figure 1: Geological overview of the Cen- tral Caledonides. Marked are the two drill hole localities. Hara SGU 79002 drill core In the Hara drill core, the basement- cover-interface is reached at 180m drill depth. The upper part of the base- ment is a 5.5m thick tectonic slice with weathered and brecciated granite. The footwall of this slice is a shear surface with a foliated cataclasite and a thin graphitic black shale horizon. Below the slice there is a gradual change downward from strongly weathered to fresher gran- ite. This can be considered as a primary palaeo-weathering profile below an ero- sional unconformity. The samples are paramagnetic with very low bulk sus- ceptibility (κbulk) values. Dark granitic samples are graphite bearing and clay rich due to weathering and/or brittle 1 Angerer & Greiling TSK 11 Göttingen 2006 Figure 2: Compilation of the AMS-parameters bulk susceptibility (κbulk), eccentricity (P ′) and inclination of magnetic foliation (κmin = surface pole) and magnetic lineation (κmax) in dependance of distance from unconformity. 2 TSK 11 Göttingen 2006 Angerer & Greiling brittle deformation. The feldspar-to- clay alteration during weathering is a process that increased the bulk suscep- tibility. Both magnetic foliation and lin- eation are steep in the granite. Whereas the foliation inclination remains rela- tively stable, the lineation inclination decreases considerably together with ec- centricity P ′ and shape factor T (latter not visualized here) towards the black shale horizon. These changes of param- eters, especially the systematic lowering of the lineation inclination, can be best explained with a multi-phase superposi- tion of unconformity-related fabrics (see Fig. 2, upper part). Långviken SGU 73007 drill core The autochthonous granite in the Långviken drill core below 437m drill depth is divided into a more strongly weathered and brecciated zone down to 6m below the unconformity (low-κ- granite) and a relatively fresher zone further below (high-κ-granite). The transition is rather discrete. However, there are parts of low-κ- inside the high-κ-granite. The high-κ-granite is magnetite bearing (large crystals of up to 2mm), whereas the low-κ-granite is free of magnetite, but bearing Fe- clay and hematite as the susceptibility- dominating phases. With the decrease of κbulk, P ′ decreases, as well. The magnetic lineation is steep in the high-κ-granite and changes to shallow in the low-κ-granite (see Fig. 2, lower part.) The magnetic foliation re- mains sub-parallel to the steep primary gneissic foliation. The change of the magnetic fabric towards the unconfor- mity reveals characteristics like in the Hara drill core, therefore similar super- position processes can be assumed. In the Långviken drill core, 12m above the basement-cover-unconformity, the sequence is cut by a thrust, which trans- ported a slice of granitic basement (7m thick) with overlying cover sediments. It is a detached uppermost part of the autochthonous granite. Pyrrhotite ap- pears dispersed in the deformed ma- trix of the central part and the cata- clastic part in form of small irregular crystals (0.02mm). This has an im- pact on κbulk and P ′, because of the magnetic field dependence of pyrrhotite. However, the AMS-ellipsoid shaping ef- fect of pyrrhotite is not important com- pared to the rock fabric defining shear- deformation. Through the entire slice the granite is sheared in a ductile way and altered with a green colour. Both footwall and hanging-wall of the granite are cata- clastically deformed. The distinct rock foliation is pronounced and flat-lying towards the top and bottom. Mag- netic foliation parallels the main petro- graphic foliations. In the centre part of the sheared granite, fabrics are slightly steeper and partly composed by two sets of surfaces. The deformation gradient through the slice can be traced by κbulk, by P ′, which reaches 70% in the lower cataclasite, and by the inclination of the magnetic lineation. Towards both rims of the sheared granite, the magnetic lin- eation becomes shallow, which is clearly caused by simple shear deformation (see Fig. 2, middle part). Systematic unconformity-related magnetic fabrics The results show how useful AMS is to trace even cryptic flat-lying fab- rics in weathered granite, which appar- ently can be developed below basement- cover-interfaces. In both profiles unconformity-related fabric change is 3 Angerer & Greiling TSK 11 Göttingen 2006 Figure 3: This sketch explains the change of AMS trajectories when a secondary fab- ric superposes a primary one. Here, the pri- mary fabric is a flat-lying diagenetic feature and the secondary is the steep Caledonian gneissic foliation. In case of the Långviken allochthonous slice, the secondary fabric is a flat-lying fabric, which was induced by simple shear during thrust tectonics. traceable: towards the unconformity the magnetic lineation decreases its inclina- tion, whereas magnetic foliation stays stable and AMS-ellipsoid shapes be- come less pronounced (general decrease of eccentricity P ′). The AMS fab- rics resulted from a superposition of a prominent steep fabric (gneissic folia- tion) and a cryptic flat-lying one (see Fig. 3). The flat-lying fabric is a dia- genetic pure shear feature (flattening), produced during the burial stage. It was facilitated by palaeo-weathering, which produced mainly clay minerals. The AMS-trends are independent of κbulk, which behaves differently in the two cores, due to the different alteration properties of the paramagnetic (Hara) and ferromagnetic s.latu (Långviken) phases. The allochthonous basement slices are examples for the detachment of the uppermost basement parts, fea- turing unconformity parallel slip. The flat-lying fabric may have facilitated propagation of Caledonian detachments sub-parallel with this fabric during oro- genic deformation. Apparently, burial compaction (pure shear) and lateral slip (simple shear) can produce quite similar AMS-fabrics. References Gee DG, Kumpulainen R & Thelander T (1978) The Tasjön Décollement, central Swedish Caledonides. SGU Serie C Nr. 742: p 35 Gee DG, Snäll S & Stejskal V, (1982) SGU Alunskifferprojektet - Prospecteringsrapport BRAP 82502. Wintsch RP, Christofferson R & Kronenberg AK (1995) Fluid-rock reaction weakening of fault zones. J Geophys Res 100(B7):13021– 13032 4