Precambrian Research, 50 (1991) 269-282 269 Elsevier Science Publishers B.V., Amsterdam A cleavage triple point and its meso-scopic structures: the Mustio Sink (Svecofennides of SW Finland) Reinoud P. Veenhof' and Harm Stel Institute of Earth Sciences, Free University, de Boelelaan 1085, 1081 HVAmsterdam, The Netherlands (Received March 29, 1990; accepted after revision November 13, 1990) ABSTRACT Veenhof, R.P. and Stel, H., 1991. A cleavage triple point and its meso-scopic structures: the Mustio Sink (Svecofennides of SW Finland) Precambrian Res., 50: 269-282. A cleavage-triple-point (CTP) structure is analyzed, located at the west side of the Mustio gneiss dome in the Svecofen- nides of southwest Finland. The presence of the CTP and the pattern of mesoscopic fold structures exclude the origin of the Mustio dome by successive interference of fold phases. The highly variable deformation structures are explained in a single-phase deformation model by using the theoretical specific strain environments of a CTP. These environments are (1) horizontal oblation on top oi;a dome, (2) transition from horizontal to vertical oblation on the flanks of a dome, and (3) vertical constriction in the center of the CTP. It is shown that each strain environment is associated with specific development of foliation, folds, mesoscopic fold interference and strain intensity. The theoretical strain environments are confirmed by strain analysis. Introduction tion pit which steepens towards the center (Fig. 2). The strain fields range from vertical con- striction in the center through foliation paral- In this paper, the evolution of the Mustio lel oblation at the flanks, towards pure hori- mantled gneiss dome in the Svecofennides of zontal oblation at the top of a dome. At least southwestern Finland, is analyzed with refer- three interfering rising bodies are required to ence to two models: (1) interference of succes- create a triple point. At the onset of doming, sive cylindrical folds, or (2) single phase non- the cover will passively rotate or actively slide cylindrical doming. Mapping of the triangular into the forming sink. At this stage, a horizon- synform west of the dome reveals a cleavage tal flattening field (with concurrent foliation triple point (CTP; Brun et al., 1981) structure fabric development) will develop on top of the of S) foliation (to be referred to as Mustio sink, culminating domes or antiforms and will radi- Figs. 1 and 2). The occurrence of CTP's was ally spread (Dixon, 1975). As the sink pro- predicted by Gorman et al. (1978) and Ram- gressively develops, a horizontal compression berg (1981), modelled by Dixon and Sum- field will develop in the center of the sink. It mers (1983) and first mapped by Brun et al. originates as bi-axial compression in the center (1981) in the Finnish Karelides. Its main and migrates progressively towards uniaxial characteristic is a tri- or multi-angular folia- horizontal compression at the flanks of the CTP. This horizontal compression will over- 'Present address: International Tectostrat Geoconsul- tants BV, De Boelelaan 7. 1083 HJ Amsterdam, The print (Dixon and Summers, 1983) the previ- Netherlands. ously developed horizontal flattening field, 0301-9268/90/S03.50 © 1990—Elsevier Science Publishers B.V. 270 R.P. VEENHOF AND H. STEL [zr-ZT] met opelites mafic to intermediate metavolcanics massive amphibolites marbles:-?*? marble quarry \:-\-A quartzofeldspathic schists and gneisses ^-l tonalitic-granodioritic gneisses microcline granites _—tiy'~ reaction isograde (1):sillimanite*biotite* quartz * p cordiente. Kfeldspar. H О L__ fault 2 Fig. 1. Simplified geological map of the Mustio dome. Note the lithological symmetry' on the limbs of the dome. Reaction isograd (1) represents a wide reaction zone rather than a uni-variant boundary. See Fig. 3 for location of the map. (After Bleeker and Westra, 1987). more or less halfway between the center of the difficult distinction concerns simultaneous domes and CTP [i.e., the short axis (Z) of the cross-buckling or buoyancy of low density and strain ellipsoid changes from vertical to hori- low viscosity layers at depth, i.e., the distinc- zontal]. Hence, during the development of tion between a compressional, tensional or domes and sink, three different strain domains non-deviatoric stress field. Virtually, no crite- occur: ria exist (Van den Eeckhout et al., 1986), ex- - progressive horizontal oblation at the top of cept on the distribution of the total strain pat- the domes (zone 1); tern. Obviously, the Mustio dome formation is - transition from horizontal to vertical obla- related to the occurrence of a migmatized core, tion at the flanks of the CTP (zone 2); but whether this dome is a simultaneously - progressive vertical constriction in the center "cross-buckled" migmatized rock-column or a of the CTP (zone3). buoyancy-driven structure, is difficult to as- The presence of the CTP with its specific pat- certain. The minor deformational structures associated with the various processes will es- tern of foldaxes in the Mustio area, excludes sentially be the same. the origin of the Mustio dome by successive fold interference. Instead, its zonal strain dis- tribution allows to explain the various genera- Geological setting tions and types of folding, by one-phase inter- domal progressive deformation. The Mustio dome is situated in the Kemio- Doming is used here as a descriptive term. Orijarvi-Lohja-Jarvenpaa (KOLJ) supra- Several processes can lead to doming, e.g. bal- crustal gneiss and schist belt (Fig. 3). It is part looning (Bateman, 1984, 1986), buoyancy of the Svecofennian belt (1900-1880 Ma; Gaal (Griffin, 1979; Dixon, 1975) or crossfolding, and Gorbatschev, 1987) in which these late either simultaneous or successive (Park and orogenic granitic domes occur abundantly (Si- Bowes, 1983). Criteria to distinguish the var- monen, 1980). The structure consists of inter- ious processes have been raised and rejected mediate to mafic volcano-sedimentary basal (Coward, 1981; Bateman, 1984). The most series, overlain by argillaceous sediments (Si- CTP STRUCTURE IN THE SVECOFENNIDES OF SW FINLAND 271 6670 MAIN FOLIATION TREND LINE 90' DIP BEDDING (S.) T-Vl II LOCATIONS OF STRAIN ANALYSIS X X MAIN FOLIATION (SI) X S2' \ 1-0, t к. L 1,2 \ L MINERAL Fig. 2. Form surface and structural map of the Mustio sink. Note the triangular pattern of the main foliation and the steep dip of S, in the center of the synform. The locations of strain analysis (I—VIII) are indicated. See text for further discussion. monen, 1980); the core of the dome is mig- Petrography and metamorphism matized and intruded by microcline granites (Fig. 4). The KOLJ belt has been interpreted to represent an ancient island-arc setting (Gaal, According to the general stratigraphy of the 1982). However, its basement has to be dis- Svecofennides (Simonen, 1980 and the map- covered yet.. ping of Harme (1953) and Bleeker and Westra 272 R.P. VEENHOF AND H. STEL | JSUPRACRUSTAL ROCKS •:f0\ Ml CROC LINE GRANITE МЕТАТОМАЫТЕ/ ШGABBRO 10 20 km Fig. 3. Simplified geological map of the Kemio-Orijarvi-Lohja-Jarvenpaa metamorphic belt of SW Finland. Note the position of the Mustio dome southwest of the town Lohja and outside the orthopyroxene-in isograd which limits the granulite facies area known as the West Uusimaa complex. Note the extensive microcline occurrence south of the Mustio dome. (After Bleeker and Westra, 1987). (1987), the rocks of the Mustio area (Fig. 1) This entire sequence is intruded by microc- can be divided (from old to young) as follows: line granites and pegmatites, both as sills and - Quartzo-feldspathic schists and gneisses, dikes. Microscopic relations between porphy- consisting of fine to medium grained felsic roblasts and foliation development demon- rocks with low biotite content. They are inter- strate that peak metamorphism and migmatis- ation occurred post-F, to early-F (Bleeker and preted as meta-arkoses. Intercalations occur of 2 calc-silicate gneisses and marbles. Westra, 1987; Bleeker, 1984). - Pre-kinematic tonalitic to granodioritic in- trusives. They occur as conformable lenses of coarse grained biotite bearing orthogneisses. Relations and pattern of structural elements - Mafic to intermediate metavolcanics. They are associated with the tonalites and comprise The investigation of the Mustio sink was fo- biotite-hornblende gneisses, layered amphibo- cussed on the pattern of the following struc- lites and massive amphibolites (which can tural elements: bedding (So), main foliation reach ultramafic composition). They are in- (Si) and F, fold axis or lineation (LOxi), F2 terpreted as subaqueous volcanic sediments axial plane or crenulation cleavage (S2) and F2 mixed with basaltic flows and associated pil- fold axis or lineation (L,x2), mineral linea- low lavas. tion (Lmin). The labelling S, -L, x 2 accounts for - Metagreywackes, which consist of a quartz, to the succession of structures established in plagioclase and biotite. each separate exposure and does not refer to - Metapelites, consisting of quartz, plagio- regional events. The interpretation on the suc- clase, biotite, K-feldspar and sillimanite. A cessive generation of structures on the scale of gradually transition exists between the meta- the entire area, will be discussed later. greywackes and pelites. Although altered by metamorphism, lithol- т CTP STRUCTURE NI THE SVECOFENNIDES OF SW FINLAND form surface compilation map N Musiio 4 Lohja area, Finland (SW) /C Him filiititn iniU I ins I 1nigmititi nicnclism irmltes 4 Fig. 4. Compilation from maps (of Bleekerand Westra, 1987 and Van den Kerkhof, 1980) of the Mustioand Lohja area. Indicated are the major outcrops of granites and migmatites. Below the southern edge of the map (not compiled), mig- matitic and granitic bodies occur, striking E-W (see also Fig. 3). A complex cleavage-multiple-point occurs at the NE and E side of the Mustio dome. See text for further discussion. ogical boundaries are still clearly visible in thestructures (up to 30 percent melt products; pelitic and leptitic parts of the supracrustal se-Bleeker and Westra, 1987).