Background

The Lockne Area, Jamtland, Sweden, provides unique opportunities to study the interior structure of a relatively large impact crater. Relatively recent erosion has exposed many outcrops that can be studied geologically and geophysically. The purpose of this investigation is to investigate the correlation between electric resistivity and fracture frequency of crystalline rocks using the electro-magnetic, Very Low Frequency-Resistivity (VLF-

R), method. I also present the result of a modified window mapping method to estimate the fracture frequency of impact-affected crystalline rocks and the unaffected surrounding crystalline rocks. Window mapping of glacially eroded crystalline outcrops is considered to give the most reliable estimate of the fracture frequency in this area compared to scan line mapping (Priest 1993), which has an orientation bias.

Impact Fracturing

During a meteorite impact, the fractures are mainly caused by the tensile stresses in the target induced by the rarefaction wave. The rarefaction wave is the response to the compressional high-pressure shock wave generated by the impact as the shock wave meets the free surface, which is a surface of zero stress. The rarefaction wave is equal in strength to the shock wave but of opposite sign, starting at the free surface and moving downwards when the shock wave arrives. The shock wave and the rarefaction wave occur simultaneous at the free surface. Close to the free surface, the rarefaction wave reduces the stress caused by the shock wave but, at a distance further from the free surface, the difference in speed between the shock wave and the tensile rarefaction wave causes tensile stress. This stress causes fractures to develop in the target when it reaches the tensile strength of the target material. The rarefaction wave moves away from the crater center into the target, causing fracturing to a distance of several times the crater depth or diameter (Melosh 1989). Fractures are also formed during the subsequent excavation flow and the crater modification process.

The fracturing in impact crater structures has been studied from the deep drilling at the Puchezh-Katunki impact structure (Russia), where a 5374 m borehole was drilled and 3082 m of drill core were retrieved (Masaitis et al. 1999). The authigenic breccia took up the interval between 546-5374 m. In this impact crater, with a diameter of 80 km, strong brecciation occurs down to 3.5- 4 km depth. The size of the undisturbed rock-blocks does not exceed 3-5 m. Below 4 km, the core recovery was poor but, according to the acoustic properties, the diameters of the blocks increase to many tens of meters at 5 km (Masaitis et al. 1999).

The porosity of the crystalline rocks down to 5 km was measured and it was found that the decrease of porosity follows a power law. A correlation between fracture frequency, electric resistivity and porosity has not been established yet. Drilling performed at the Popigai impact crater (Russia) shows that fractured rocks in the ring structure can be allochtonous to a very large depth (Masaitis et al. 1975). In the annular ring structure, large blocks of sedimentary rocks have been found immersed in the fractured crystalline rocks. This indicates that considerable movements have also occurred within the brecciated crystalline crater basement.

The impact-generated fractures overprint and may reactivate existing fractures. The more recent fractures cannot generally be separated from older ones. The spatial variation of the fracture frequency values can, however, reveal the impact-generated overprint as being several orders higher than the fracture frequency of the unaffected surroundings.

Fig 1. Location of the study area in Sweden where two major deformation zones are seen, the Storsjö Edsbyn Deformation Zone (SEDZ) and the Hassela Shear Zone (HSZ). The HSZ can be seen as thin lines in the area map (after Högdahl 2000). The light grey areas in the area map are the extent of the Tandsby Breccia (after Lindström et al. 1996). The sites for fracture frequency and electric resistivity measurements are marked with dots. The triangle marks the location of the outcrop shown in figure 2. The large box (broken lines) at section B shows the transition zone. The step in fracture frequency / electric resistivity is marked by the narrow box. The direction to the GBR radio transmitter in southern England is SV. Coordinate numbers refer to the Swedish National grid. Thick lines indicate the sections A and B on which the measurements are projected.

Geological Setting of the Lockne Area

The study area in and around the Lockne structure is situated in Jamtland, Sweden (N 63°00'20", E 14° 49'30", Fig. 1). It is situated just east of the Caledonian thrust front in an area with autochonous Lower Palaeozoic sedimentary sequences on Proterozoic crystalline basement. The deposits resulting from the Lockne structure were suggested by Wickman et al. (1988) to be the result of a meteorite impact. The Lockne crater is an impact crater formed in a sea environment, which is now exposed on land allowing detailed studies of the impact-induced effects to be made.

The Lockne impact structure was formed in the Ordovician Sea 455 Ma ago (Grahn and Nolvak 1993). An up-to-date description of the crater structure and relevant references is given in Lindstrom et al. (this volume).

The structure has been covered by Caledonian thrust nappes of at least 3000 m thickness (Karis and Stromberg 1998). Thus, the structure has been preserved from erosion for a long time. Presently, the cover rocks have been eroded so that the front is less than 1 km NW of the suggested margin of the structure. A small outlier of the lowermost Caledonian over-thrust nappe called the Tramsta nappe has been preserved from erosion due to its low topographic level in the crater depression (Lindstrom and Sturkell 1992).

Radial erosional gullies cut the "brim" of the structure (von Dalwigk and Ormo 2001; Lindstrom et al. this volume). They eroded through the allochtonous to the autochthonous breccia and, at some locations, to the less fractured basement.

Pre-Impact Geology

Proterozoic. In the Lockne Area, the oldest rocks are represented by a sequence of metavolcanites, the Boron volcanic suite (Mansfeld et al. 1998), occurring on the east side of the southern part of Lake Lockne. After the peak metamorphism, there was an extensive magmatism of Revsund granite. The Revsund granite occupies vast areas in the western part of the Bothnian Basin (Gorbatschev 1997).

The major part of the Lockne Area consists of Revsund granite with ages from 1.82 Ga to 1.65 Ga (Gorbatschev 1980). A prominent ductile deformation zone is located to the southwest of Lake Lockne, the Hassela Shear Zone, striking NNW. Further to the southwest, another deformation zone, the Storsjon Edsbyn Deformation Zone (SEDZ), has affected the basement. The Storsjon Edsbyn Deformation Zone extends from the Bothnian Sea to Lake Storsjon. The two deformation zones experienced ductile deformation during the late stage of the Svecokarelian orogeny between 1.85 and 1.7 Ga ago (Högdahl 2000). The youngest crystalline rocks in the area are represented by Äsby dolerite sills, which are 1.2 to 1.3 Ga old (Patchett et al. 1987; Souminen 1991). After the intrusion of these dolerites and before the transgression of the Cambrian Sea, the area was exposed to weathering (Karis and Strömberg 1998).

Cambrian to Ordovician pre-impact autochthonous cover rocks. The autochthonous Lower Palaeozoic is of standard Baltoscandian type (Lindström and Sturkell 1992). Prior to the meteorite impact, the deposition of the Dalby Limestone began.

The Impact Event

The Lockne Impact Crater formed in a marine environment with a water depth of less than 500 m (Lindström et al. this volume), 455 million years ago (Grahn and Nölvak 1993). The resulting structure has an inner crater diameter of 7.5 km that is surrounded by a zone, about 2.5 km wide, with a brecciated crystalline basement and a few traces of pre-impact sedimentary rock called the "brim" (Lindström et al. this volume).

Authigenic breccia. Authigenic breccia (autochtonous or parautochtonous) is an impact generated monomict breccia of the locally occurring rocks, remaining relatively coherent during the crater formation. It shows no signs of significant transport and contains no exotic fragments. The fragments are angular and resemble breccias formed by other geological mechanisms, such as volcanic explosions or tectonic movements (French 1993). Authigenic breccias are often given local names. In the Lockne impact structure, it has been named Tandsbny Breccia (Lindström and Sturkell 1992).

Tandsbyn Breccia consists of intensely fractured but otherwise little altered fragments of local Proterozoic basement rocks. The lithology varies depending on the source rocks, the most common being Revsund Granite but dolerites or metavolcanites as protoliths have also been found (Lindström and Sturkell 1992). On several locations, the Tandsbyn Breccia grades into more intact basement granite. Cataclastic zones and displacements occur as well as spaces filled with bituminous materials. The Tandsbyn Breccia has a bituminous matrix containing angular fragments of quartz and feldspar, lithic fragments and fragments of fossils and shale. The fragments of Alum shale are slightly deformed along the margins but have an intact interior. The fractures in the Tandsbyn Breccia have a random distribution (see Fig. 7 in Simon 1987). The fracture fill is similar to that of Loftarsten (see below) down to very small dimensions and cemented with chlorite, calcite, and fluorite (Simon 1987). A study to investigate the occurrence of shocked quartz has been made in thin sections of Tandsbyn Breccia and it was found to contain quartz with a lamellar structure reminiscent of shocked quartz, but it was not as distinct as examples of Planar Deformation Features (PDF) in quartz from other established impact craters (Lindström and Sturkell 1992). The PDFs found in connection with this structure were found in the allogenic breccia (Therriault and Lindström 1994).

Allogenic breccias. Breccias with rock fragments that have been incorporated from different sources form allogenic breccia deposits, usually deposited in and around the structure (French 1993). In the Lockne impact structure, the Lockne Breccia (Simon 1987) and the Loftarsten (Thorslund 1940) are resurge sediments containing ejecta formed during the impact. Orientations of sets of PDFs corresponding to those found in known impact craters have been measured by Therriault and Lindström (1994) in 25 quartz grains from the rock type Loftarstone from the Lockne impact structure.

Post-Impact Geology

After the impact, the sedimentation continued, with the Dalby Limestone as the lowermost unit. The youngest autochthonous deposit, still existing in the Lockne Area, is the Slandrom limestone from the upper Caradoc (458-448 Ma) identified in this area by Thorslund (1940) and Lindström et al. (1983).

Allochtonous units of the Caledonian thrust nappes. The base of the Caledonian thrust front, the lower allochton, is composed of thrusted rocks of late Precambrian to early Palaeozoic age with a very low-grade metamorphism. The main orogenic displacement occurred in the Silurian, extending over the already existing crater. A small nappe, called the Tramsta nappe, still covers an area in the center of the topographic depression. Large parts of brecciated basement of sizes from 10 to 10 000 m3 (Lindström et al. this volume) have been found lying on top of transported sediments. These large clasts are interpreted as having been ejected from the crater to a distance of several kilometers and then transported back towards the structure with the Caledonian nappes in the general direction of the Caledonian thrusting. The thrusting direction was E-SE, with an estimated 10-40 km range of transportation distance of the units of impact age (Lindström et al. 1996).

Post Caledonian tectonic events. Shearing and fracturing of the crystalline basement in the Lockne Area unrelated to the meteorite impact occurred mainly before the Caledonian orogeny (described earlier) (e.g., Gorbatschev 1997; Hogdahl 2000). However, the displacement of the thrust front and the impact structure indicate some post-Caledonian shearing and faulting, which also displaced the Âsby dolerite sills. At present, the top of the basement surface dips about 1° to the northwest (Thorslund 1940).

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