Geology of the Lord Range, Central Southern Alps, New Zealand

Abstract

Gently dipping Torlesse rocks, in the Lord Range, occupy the core of a shallow post-metamorphic synform. Only two lithotypes are present; a dominant sequence of regularly bedded alternating sandstone and mudstone, and a relatively restricted sequence of maroon and green shale. The sandstones are poorly sorted litharenites, unusually deficient in feldspar, but otherwise containing the clastic assemblages typical of Torlesse sediments. Authigenic mineral assemblages grade from those of the prehnite-pumpellyite facies in the south-east, through pumpellyite-actinolite facies in a wide zone of semischists, into biotite zone assemblages of the greenschist facies. Texturally, the rocks grade from non schistose sandstones in the south-east into thoroughly laminated schists in the north-west. Large pre-metamorphic recumbent folds with associated tectonic slides are the earliest structures recognised. They are overprinted by a steeply dipping syn-metamorphic cleavage, with associated strongly asymmetric folds. Post-metamorphic deformation, probably related to massive uplift on the Alpine Fault, included formation of the broad, open Dan Synform and associated faulting. Alternative schemes of tectonic evolution are: 1. an early period of tectonic sliding in subhorizontal strata, passed into a phase of tight upright folding and subsequent overprinting of the metamorphic sequence. Large scale uplift on the Alpine Fault and a rather fortuitous rotation resulted in the present geometry of steeply dipping isograd and isotect surfaces cross-cutting recumbent folds; 2. (regarded as more probable) requires that the recumbent nature of the early folds is an original feature, but the dip of isograds and isotects differed initially and, with increasing strain to the north-west, the relative attitude of the isotects, isograds and bedding progressively changed. Subsequent folding, with uplift on the Alpine Fault to the west resulted in the present geometry.