The geology and gravimetric studies of the Tophouse district, South Nelson

Abstract

The Tophouse District encloses three important structural blocks - the Upper Paleozoic Brook Street Volcanics, the Maitai-Te Anau Series and the Triassic Alpine greywacke. Each is separated from the other two by faults. The active, transcurrent, dextral Wairau Fault strikes at 065° and separates the two Upper Paleozoic blocks to the north from the Triassic block to the south. The Brook Street Volcanics are assumed to lie on the western flank of a marginal syncline and the Maitai-Te Anau Series on the eastern flank. The Waimea Fault has faulted the axis of the marginal syncline and has thus brought the Brook Street Volcanics and the Maitai-Te Anau Series together. The Te Anau Series contain schistose sediments at the base and the Red Hill Ultramafics and haematised volcanics at the top. All formations strike north - south and exept the Red Hill Ultramafics, dip steeply east. The Red Hill Ultramafics have an apparent horizontal attitude. The ultramafic rocks consist of massive serpentinite, harzburgite, dunite and pyroxene peridotite. These rocks are surrounded by steeply dipping crushed serpentinite. Dykes of grossulerite and amphibolite intrude peridotite in the south-east corner of the ultramafic mass. All formations of the Maitai Series are present and occur in correct stratigraphic order, from limestone at the base, through grey laminated argillite and red and green laminated argillite to green tuffaceous sandstone at the top. The sandstone is faulted against the saussuritized spilite and agglomerate of the Brook Street Volcanics. The Wairau Fault has moved dextrally at the rate of 0.36 inches per year since the last glaciation. Active movement along the Wairau Fault and correlation between the Maitai - Te Anau formations of the Tophouae District and similar formations of West Otago points to a 300 mile trans-current displacement between the two areas along the Wairau-Alpine Fault system.