Local shear zone network

minor pegmatites and fine-grained uranium- and thorium-rich granitic dykes Ho¨gdahl et al., 1998.

2. Regional deformation zones: the Storsjo¨n – Edsbyn deformation zone and the

Hassela shear zone On aeromagnetic anomaly maps, the SEDZ Bergman and Sjo¨stro¨m, 1994 appears as a 10 – 20-km wide and 300-km long zone between Eds- byn in the south and Lake Storsjo¨n in the north Fig. 2. There is independent evidence that the SEDZ continues to the north-northwest beneath the Caledonides Bergman and Sjo¨stro¨m, 1994. The SEDZ essentially separates \ 1.8 Ga rocks mainly Ljusdal and Revsund granitoids and older Svecofennian supracrustal rocks to the east from a younger, ca. 1.7 Ga TIB intrusion to the west. Structural data and the recognition of various kinds of mylonites show that the SEDZ has been active repeatedly Bergman and Sjo¨stro¨m, 1994. Ductile, retrograde and brittle – ductile mylonites are the most common types along this deforma- tion zone. Dextral, transpressive shearing result- ing in steep stretching lineations, has been suggested either to be connected to the emplace- ment of the Ra¨tan intrusion, or a post-emplace- ment phenomenon. In the Revsund granitoid affected by SEDZ-deformation, a coarse, dextral CS-fabric is the dominating structure. Sinistral shear zones occur as conjugate sets, or occasion- ally as later overprinting structures. The HSZ is localised along the boundary be- tween the Ljusdal Batholith and the older Sve- cofennian metasedimentary rocks greywacke-schist of the Bothnian Basin to the north Bergman and Sjo¨stro¨m, 1994; Sjo¨stro¨m and Bergman, 1996; Fig. 2. Previously, this boundary has been referred to as a primary fea- ture. It is a steep, WNW- to NW-striking domi- nantly dextral shear zone formed under wrench conditions. Narrow sinistral zones, conjugate to the dextral pattern, were probably formed during progressive bulk dextral deformation. Some of the former display a retrograde character reflecting shear during a late stage of the deformation. The dextral rotation of the HSZ into the SEDZ indicates either that the HSZ is older, or that they formed simultaneously Bergman and Sjo¨stro¨m, 1994. The timing of the main ductile deformation along HSZ is bracketed by its imprint on the 1.85 – 1.84 Ga Ljusdal Batholith Delin, 1993; Welin et al., 1993, and its syn-metamorphic rela- tion to the regional low-pressure metamorphism LPM at \ ca. 1.82 Ga Claesson and Lundqvist, 1995.

3. Local shear zone network

In the area where the HSZ joins the SEDZ, there are several local, ductile deformation zones dividing the Revsund granitoid into internally un- deformed, or only slightly deformed lenses Figs. 2 and 3. This pattern is apparent on various scales regional to outcrop, and the deformation zones are characterised by a coarse CS-fabric. The existence of anastomosing faults Lun- dega˚rdh et al., 1984 coinciding partly with ductile shear zones, indicates that brittle reactivation was favoured by ductile structures. The deformation zone along River Forsaa˚n at the southern end of Lake Locknesjo¨n is the most pervasively deformed of the local shear zones Fig. 3. It will be described in detail below, after a brief summary of the characteristic features of other zones in the area. East of the Forsaa˚n zone, there is a less persis- tent mylonite zone lacking CS-fabric Fig. 3, 2. To the west, there is a complex, ca. 5-km wide belt of anastomosing deformation zones, consist- ing of several localised shear zones that truncate older Svecofennian rocks as well as Revsund granitoids Fig. 3, 4. The foliation within the zones is steep and strike in a NNW – SSE direc- tion whereas the foliation between the zones is rotated around an axis subparallel to the stretch- ing lineation, which plunge gently to the SSE. The indicated fold structure is probably a result of rotation of pre-existing folds into the adjacent shear zones. The most prominent mylonites in this belt of deformation zones are characterised by a strong sub-horizontal lineation and a weak, steep foliation, i.e. an L \ S-fabric. In areas where dif- Fig. 3. Geological map of the western part of the Southern Massif in Fig. 2 showing the shear zones described in the paper modified after Lundega˚rdh et al., 1984; Lundqvist, 1996; Lundqvist and Korja, 1997; Sturkell et al., 1998, 1, dextral, west-side-up shear zone at the boundary between metasedimentary rocks and the Revsund granitoid; 2, discontinuous mylonite zone; 3, Forsaa˚n zone; and 4 a belt of anastomosing deformations zones dominated by L \ S-mylonites. ferent lithologies have been juxtaposed, the my- lonites are typically banded. Such a mylonite, affecting early Svecofennian rocks in the central part of the deformation zone, has been dated at 1802 9 2 Ma Ho¨gdahl et al., 1996. A prominent shear zone bounds the deformation zone to the east. In its northern part, this shear zone separates the Revsund granitoid to the east from metased- imentary rocks. Along with this study, recent mapping by the Geological Survey of Sweden shows that there has been intense deformation within both the grani- toid and the adjacent metasedimentary rocks to the east Lundqvist, 1996; L. Lundqvist Uppsala, personal communication, 1997. Within the coarse Revsund granitoid east of Lake Bo¨rjesjo¨n Fig. 31 steep, metre-wide ultramylonites occur, which strike ca. 330° and have a strong, oblique stretching lineation plunging ca. 45° to the south- east. Kinematic indicators shear bands, rotation of gneissosity verify oblique dextral- and south- west-side-up displacement. Such kinematic conditions have been recorded also in intensely deformed, very planar, steeply dipping metagreywackes ca. 200 m from the con- tact to the granitoid. In this case, the kinematics are verified by asymmetric boudinage and shear bands C, combined with a pronounced stretch- ing lineation plunging ca. 45° to the southeast. A weak, shallow plunging lineation is developed lo- cally on platy quartz. The differences in plunge between the lineations may either be the result of two distinct episodes of shearing or represent two stages of progressive shearing in a transpressive environment cf. Tikoff and Teyssier, 1994. 3 . 1 . The Forsaa˚n shear zone The deformation zone along Forsaa˚n Fig. 3, 3 was first recorded by Ho¨gbom 1894 and interpreted as a penetrative foliation in the Rev- sund granitoid. It was later re-interpreted as an enclave dominated by felsic metavolcanic rocks with a minor proportion of early-orogenic grani- toids and some lenses of Revsund granitoid Lun- dega˚rdh et al., 1984; Gorbatschev et al., 1997. The re-interpretation was based on the assump- tion that ductile structures should not exist in rocks classified as post-orogenic. Our study sup- ports Ho¨gboms interpretation, i.e. the protolith is a coarsely porphyritic granitoid cf. Fig. 4A and B. Although strain variations exist, the Forsaa˚n section is very homogeneous compositionally, lacking lithological variations. The deformation zone is ca. 1-km wide and can be traced for more than 10 km in a NNW – SSE to NW – SE direction. The deformational fabric is more or less continuous through the width of the zone and the boundaries to undeformed rocks are distinct Fig. 4B. Towards the northwest, the deformation zone follows Lake Locknesjo¨n where it affects early Svecofennian metavolcanic rocks Mansfeld et al., 1998. This part is recorded on Bouguer anomaly maps as a local gravity low Sturkell et al., 1998. Phanerozoic rocks and Caledonian thrust sheets cover the continuation farther northwest. The dominating structure in the granitoid along the deformation zone at Forsaa˚n is a coarse, penetrative, subvertical CS-fabric Fig. 4C, sometimes grading into pervasively deformed gneiss zones without CS-fabric Fig. 4D. With increasing strain, the CS-fabric is transformed into millimetre- to metre-wide mylonites. Locally, the CS-fabric is cut by mylonites indicating that the latter are slightly younger. Subordinate ultra- mylonites have been recorded, in which the folia- tion is defined mainly by platy quartz i.e. recrystallised ribbons and thin mica-rich bands. The bulk sense of shear is not obvious in the steep, NE-dipping Forsaa˚n shear zone and kine- matic indicators are contradictory. Dextral shear zones truncating a sinistral CS-fabric exist Fig. 4E, as well as sinistral shear zones truncating a pervasive gneissic foliation, or tensile quartz veins indicating sinistral rotation Fig. 4F, Fig. 5A and D. Altogether, these examples indicate a sequen- tial formation of sinistral and dextral kinematic patterns. In pervasively deformed gneissic parts, there is, at least locally, a faint asymmetric pattern indicat- ing dextral sense of shear in sub-horizontal sec- tions. Still, sinistral shear bands and minor shear zones dominate among the data collected Fig. 5A. However, more important is that dextral and sinistral shear bands C and minor shear zones are symmetrically arranged with respect to the pervasive, partly mylonitic C foliation Fig. 4E and Fig. 5A. Stretching lineations are generally weak and dominated by gentle to moderate plunges Fig. 5B. Constructed intersection lineations between minor shear zones or C and the mylonitic folia- tion or C tend to be perpendicular to the stretching lineations Fig. 5C. The former also show a variation in plunge from steep to moder- ate within the mylonitic foliation, comparable in amount to the variation in plunge of the stretch- ing lineations Fig. 5B and C. Fig. 4. A The undeformed, coarsely porphyritic granitoid sampled for U – Pb TIMS titanite analyses. The sample site, ca. 1 km west of the Forsaa˚n zone, is shown in Fig. 3. B Characteristic ductile mylonite truncating the porphyritic granitoid. The change from faintly gneissose granitoid to mylonite is abrupt. C Pervasive sinistral CS-fabric in the granitoid along the Forsaa˚n deformation zone. D High strain gneiss zone at Forsaa˚n. The light-coloured bands are made up by polygonised, former K-feldspar phenocrysts. Typical microstructures are shown in Fig. 6B. Sampling site for the U – Pb analyses cf. Fig. 3. E Intensely deformed granitoid dominated by a sinistral CS-fabric. Sinistral shear bands C running from upper right to lower left are distinct in the upper right part of the picture. These C show a dextral rotation into the mylonite in the central part of the picture. The mylonite contains asymmetrically folded quartz veins demonstrating dextral shear. Some dextral C running upper left to lower right are also visible above lens cap. F Tensile quartz veins displaced by a thin sinistral shear zone. Forsaa˚n deformation zone. The orientations of strain axes, indicated by two, conjugate dextral and sinistral shear zones, are NNW-trending close to horizontal X-axes stretching, WSW-trending close to horizontal Z-axes shortening and steep Y-axes Fig. 5C. X and Y thus plot in the fields of stretching- and constructed intersection lineations, respectively, and Z close to the field of poles to the mylonitic foliation, i.e. Z is more or less perpendicular to that foliation Fig. 5B and C. In terms of strain axes, the distribution of stretching and intersection lineations indicates that X and Y rotate within the shear plane, while Z is less variable and more or less orthogonal to that plane. Apparently, pure shear predominated during the development of the deformation zone and resulted in the development of the S \ L tectonites reflected by the poorly developed stretching lineations. Such conditions would also explain a sequential development of minor shear Fig. 5. Structural data from the Forsaa˚n deformation zone presented in an equal-area, lower hemisphere, stereographic projection. A Poles to steep, sinistral and dextral shear bands and minor shear zones define two groups that are symmetrically arranged with respect to the foliations shown in Fig. 5B. Predominantly pure shear is indicated. B The poles to foliations define a steep NE-dipping zone. Stretching lineations rotate in the shear zone but are dominated by shallow to intermediate plunges. C Constructed intersection lineations have steep to intermediate plunges in the shear zone. The intersection lineations plot in the field where stretching lineations are few in Fig. 5B, suggesting a perpendicular relationship between the lineations. Strain axes based on conjugate minor shear zones indicate subhorizontal stretching X, steep intermediate axis Y and subhorizontal shortening Z. Note that X plots among stretching lineations in Fig. 5B and that Y plots within the field of intersection lineations. Z is perpendicular to the shear zone, which is consistent with predominantly pure shear. D Schematic presentation of structures in the Forsaa˚n section. zones and the symmetric kinematic pattern shown in Fig. 5A – C. An important inference is that the kinematic data from the Forsaa˚n section within the granitoid deviate considerably from those recorded along the margin of the granitoid. This difference is significant for the interpretation that strain was partitioned. Some narrow, sinistral shear zones truncating the pervasive shear fabric, have a sub-horizontal lin- eation and tend to indicate lower magnetic suscep- tibility values than the gneissic zones. These shear zones may, therefore, represent a later phase of deformation under more oxidised conditions. Fur- ther diagnostic patterns will be described in the following section. Late brittle deformation is widespread in the area. It resulted in the formation of cataclasites in zones ranging from millimetres to several centime- tres in width, occasionally accompanied by pseudo- tachylite melts. Revsund granitoids that are deformed by brittle deformation have typically a deep red colour. Open cavities sporadically host small crystals of quartz, locally together with epidote and fluorite and in places calcite. 3 . 1 . 1 . Microstructures in the Forsaa˚n shear zone Microcline porphyroclasts in the pervasive CS- fabric at Forsaa˚n locally show rather coarse core- and-mantle structures Fig. 6A, formed by dynamic recrystallisation along the margins of the microcline megacrysts. Such fabrics are common in feldspars affected by deformation at temperatures of 400 – 500°C Passchier and Trouw, 1996, i.e. low- to medium-grade conditions. This tempera- ture range is also indicated by other feldspar microstructures, e.g. the porphyroclasts generally lack fracturing and micro-kink-bands typical of low-grade conditions 300 – 400°C. They also con- tain flame perthites typical of low- to medium- grade conditions 400 – 500°C, while myrmekites characteristic of medium- to high-grade conditions are absent Passchier and Trouw, 1996. These temperature estimates are, thus, comparable to the 500°C suggested for the development of CS-fabric in granites affected by shearing Gapais, 1989. In more deformed parts, the porphyroclasts are polygonised entirely with well-developed triple points between the crystals Fig. 6B. Bands of epidote with allanite cores wrap around some polygonised augen. Coexisting epidote and acces- sory amounts of chlorite and muscovite probably reflects saussuritisation of plagioclase; most chlor- ite appears to be a late replacement. The existence of titanites along C and C shows that they are part of the deformational fabric Fig. 6C. They occur preferably in biotite and in the saussuritised bands, minor amounts are found at quartz- and feldspar grain boundaries. Conse- quently, the titanites used for dating are part of a thoroughly recrystallised fabric showing some neomineralisation. Both the high degree of recrys- tallisation and the indicated temperature range of ca. 400 – 500°C are fundamental conditions when the relationship between the obtained age and the evolution of the shear zone is considered see below. The narrow sinistral shear zones that truncate the pervasive core-and-mantle fabrics and are char- acterised by intense grain-size reduction. Biotite partly chloritised, chlorite and dynamically re- crystallised quartz define a CS-fabric; kinked and bent muscovite occurs locally Fig. 6D. Large quartz grains show elongate subgrains and contain bands that are recrystallised dynamically. There is evidence of both grain-boundary migration recrys- tallisation most common by interlobate, highly irregular grain boundaries and subgrain rotation recrystallisation. The latter is displayed by the existence of small grains with slightly different optical orientation along grain boundaries. These recrystallisation mechanisms indicate temperatures probably exceeding ca. 400°C. The microstructures and the mineralogy indicate somewhat lower tem- peratures compared with that of the pervasive pattern. The microstructures of the sinistral zones also appear less evolved, and combined with the recognition in the field that they may be more oxidised lower magnetic susceptibility values than the gneissic zones, support the inference of a late development. 3 . 2 . Microstructures along the eastern margin of the granitoid The ultramylonites developed in the granitoid close to the eastern margin Lake Bo¨rjesjo¨n area, Fig. 6. Microstructures from the Forsaa˚n section and the sheared margin to the metagreywacke in the east. A In the CS-fabric, K-feldspar porphyroclasts one in the central part are surrounded partly by a mantle of dynamically recrystallised K-feldspar to the left of the large grain. Forsaa˚n shear zone. Length of photograph corresponds to 5.8 mm. X nicols. B With higher strain cf. Fig. 4D the K-feldspar porphyroclast are often completely polygonised. Locally well-developed triple points exist between newly formed crystals. Forsaa˚n zone. Length of photograph corresponds to 5.8 mm. X nicols. C Titanites occur along C- subhorizontal and C- planes SW – NE preferably in biotite and bands of saussurite that occasionally wrap around quartz and feldspar. Bt, biotite; Ti, titanite; Sau, saussuritised plagioclase. The sample is from a high strain gneiss zone within the Forsaa˚n zone. Length of photograph corresponds to 3 mm. D The fabric in thin sinistral shear zones along Forsaa˚n is defined by partly chloritised biotite, chlorite and dynamically recrystallised quartz. Length of photograph corresponds to 3 mm. X nicols. E In intensely mylonitised granitoid, s-porphyroclasts recrystallised to asymmetric ribbons in the upper central part of the picture and C at low angle to the mylonitic foliation indicate dextral sense of shear. These microstructures are typical for the mylonites east of Lake Bo¨rjesjo¨n Fig. 31, Length of photograph corresponds to 5.6 mm. F Muscovite fish large grains in the lower part of the picture with internal strain are found in the deformed metagreywackes east of Lake Bo¨rjesjo¨n. The crystals in the quartz plate in the upper part of the picture have boundaries indicating grain boundary migration recrystallisation. Length of photograph corresponds to 5.6 mm. X nicols. Fig. 3, 1 have a pronounced foliation made up by alternating platy quartz and bands containing fine-grained, brown biotite. The quartz plates have internal CS-fabrics defined by optically ori- ented, dynamically recrystallised quartz crystals, i.e. microstructures typical of dislocation creep. Kinematic indicators are fairly common, includ- ing composite asymmetric augen, s-porphyro- clasts and locally existing C at a low angle to the mylonitic foliation C, Fig. 6E. K-feldspar por- phyroclasts generally show intense static? sericitisation. An important condition is that the ultramylonites lack chlorite and epidote minerals, i.e. exhibit no records of retrograde, low tempera- ture deformation. In the metagreywackes ca. 200 m from the contact to the granitoid, a pseudoclastic texture is still visible in spite of strong shearing. The defor- mational fabric contains dynamically recrys- tallised quartz plates in which the crystals indicate grain boundary migration recrystallisation cf. above. Fine-grained, brown biotite dominates over white mica, but larger muscovite fish with internal strain undulose extinction and rare kink- bands exist Fig. 6F. The microstructures in the adjacent metasedi- mentary rocks are, thus, comparable with those in the mylonitised granitoid, suggesting that they formed simultaneously along a major shear zone. The comparatively low metamorphic grade of the greywacke close to the intrusion, including a pseu- doclastic texture, indicates that the greywacke has been juxtaposed by post-emplacement shearing.

4. U – Pb geochronology