Radical changes in bulk shortening directions during orogenesis: Significance for progressive development of regional folds and thrusts

Abstract

Polyphase folding and metamorphism of rocks in the eastern Mount Isa Inlier resulted from four distinct orogenic cycles. These consist of NE–SW shortening (O 00) from 1672 to ∼1648 Ma, involving a succession of five phases of deformation, which was responsible for the development of the first generation NW–SE striking macroscopic folds, plus cleavages that are mainly only preserved in porphyroblasts. N–S shortening (O 1) followed to 1600 Ma, involving at least three phases of deformation. This period of N–S directed orogenesis rotated large portions of the NW–SE trending regional folds into W–E trends and the dominance of locally well-preserved W–E trending sub-vertical matrix foliations as well as the intensification of foliation parallel to bedding. Subsequent W–E shortening (O 2) from 1600 to ∼1515 Ma was accompanied by the development of N–S trending folds. These dominate the Mount Isa Inlier and where best developed involved the formation of at least three near orthogonal foliations. Where these large-scale folds regionally are weakly developed or not visible, only one sub-vertical foliation has developed. Finally NW–SE shortening (O 3) produced locally developed SW–NE trending folds and weak crenulations until ∼1500 Ma. The asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during O 00 indicates thrusting to the SW. Similar inclusion trail asymmetries in garnet, staurolite and andalusite porphyroblasts that developed in O 1, indicate thrusting to the N confirming a change in thrust direction across the Wonga Duchess belt to the W. Formation of the Meerenurker roof thrust near Mount Isa by thrusting to the SW during O 00 with subsequent development of the Kokkalukkanurker duplex below by thrusting to the S during O 1 reduces the displacement required for the roof thrust from 234 to ∼65 kms. Foliations preserved in O 00 garnets are truncated by foliations associated with the development of O 1 in the matrix. O 1 garnet, staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during O 2. The inclusion trails entrapped within O 00 and O 1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case. Numerous phases of sub-vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during O 00 and O 1.