Geophysical investigations of the Stuoragurra postglacial fault, Finnmark, northern Norway

Abstract

Processed images of aeromagnetic, gravimetric and topographical data and geological maps combined with VLF ground measurements have been interpreted in mapping the main fault structures along the Mierujav'ri-Sværholt Fault Zone (MSFZ) in Finnmark, northern Norway. The 230 km long MSFZ is situated in the extensive Proterozoic terrain of Finnmark. Proterozoic albite diabases, which cause characteristic magnetic anomalies in the Masi area, have intruded along the MSFZ. A system of duplexes can be delineated along the MSFZ from the geophysical images. These interpretations have been followed up in the till-covered area with electromagnetic measurements and confirm the existence of the faults interpreted from the geophysical images. The postglacial Stuoragurra Fault (SF) lies within the MSFZ. It is a southeasterly dipping reverse fault and can be traced fairly continuously for 80 km in the Masi-Iešjav'ri area. Detailed geophysical investigations and drilling have been carried out in the Fidnajåkka area 10 km to the south of Masi. A ca. 1 m thick layer of fault gouge detected in the drillholes is thought to represent the actual fault surface. Resistivity measurements reveal low-resistivity zones in the hanging-wall block as well as in the foot-wall block of the SF. These low-resistivity zones lie within a several hundred metre wide belt and are interpreted to be due to fracturing of the quartzites along the regional MSFZ. Within the Fidnajåkka area, however, the resistivity of the hanging-wall block of the SF is typically lower than in the foot-wall, indicating more intense fracturing in the hanging-wall. Vertical electrical soundings show a low-resistivity layer at depth in the eastern hanging-wall block, which corroborates other evidence that the fault dips to the southeast. The refraction seismic data reveal low seismic velocities along the SF which are interpreted to be caused by faulted and fractured bedrock. Detailed topographical data proved very useful for estimating the dip of the fault zone in the upper part of the subsurface. Additional data from ground-penetrating radar measurements could be used to map the thickness of the overburden and, when combined with the digital topography, these measurements could be used to map the topography of the bedrock surface. Highly reflective bedrock on the ground-penetrating radar records is interpreted to be fractured and weathered quartzite and single reflectors can be interpreted as fault zones. Within the survey area, there exist two possibilities for the course of the postglacial reverse fault at depth: a—the fault is listric with a dip of 50° in the uppermost 10 m of the subsurface and a dip of 30° at a depth between 25 and 40 m, or b—the fault continues at depth from the surface with a dip of 50–60°. If model a is correct, the SF will be represented by a 1 m thick gouge which occurs in two drillholes and the fault will be parallel with the foliation at depth. It is concluded that the SF is controlled by older faulting, on a local as well as regional scale. The earliest detectable displacement along the MSFZ is of Proterozoic age and the latest movements occurred less than 9000 yr ago. The postglacial faulting occurs most commonly either along the margins of the duplex structures or within them. Northeast of Iešjav'ri, however, the young faulting occurs within the main fault zone.