Ahmed, Ahmed Sayed Ali (2010) Tectono-metamorphic evolution of the Eastern Fold Belt, Mount Isa, NW Queensland, Australia. PhD thesis, James Cook University.

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Abstract

Section A

Polyphase folding and metamorphism of rocks in the eastern Mount Isa Inlier resulted from four distinct orogenic cycles. These consist of NE–SW shortening (Ooo) from 1672 to ~1648 Ma, involving a succession of five phases of deformation, which was responsible for the development of first generation NW–SE- striking macroscopic folds, plus cleavages that are mainly only preserved in porphyroblasts. N–S shortening (O1) 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 (O2) 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 (O3) 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 Ooo indicates thrusting to the SW. Similar inclusion trail asymmetries in garnet, staurolite and andalusite porphyroblasts that developed in O1, 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 Ooo with subsequent development of the Kokkalukkernurker duplex below by thrusting to the S during O1 reduces the displacement required for the roof thrust from 234 to ~ 45 kms. Foliations preserved in Ooo garnets are truncated by foliations associated with the development of O1 in the matrix. O1 garnet, staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during O2. The inclusion trails entrapped within Ooo and O1 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 Ooo and O1.

Section B

The relative timing of the sequence of events is problematic where deformation partitioning strongly separates zones dominated by folds that formed in different periods of orogeny. This is particularly the case where they form orthogonal to one another in plan view. A succession of foliation inflection/intersection axes preserved within porphyroblasts (FIAs) readily resolves the sequence. The succession of FIAs, as well as foliations in the matrix in the Soldiers Cap Group of the eastern Mount Isa Inlier, reveals W–E-directed bulk horizontal shortening followed N–S bulk horizontal shortening in spite of the apparent regional lack of curvature of the W–E-trending folds around those trending N–S. Furthermore, N–S-and W–E-directed bulk shortening both resulted in at least 2 cycles of sub-vertical and sub-horizontal foliation development that define the W–E-and N–S-trending FIAs.

Section C

Foliation Inflexion/Intersection Axes preserved in porphyroblasts (FIAs) that grew throughout Isan orogenesis reveal four significant changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from SW–NE, N–S, W–E to NW–SE. This anticlockwise change in the direction of bulk horizontal shortening implies an anticlockwise shift in relative plate motion with time during the long lasted 1670–1500 Ma Isan orogeny. Dating monazite grains amongst the foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined. Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for SW–NE shortening, 1645±7 Ma for N–S shortening, and 1591±10 Ma for that directed W–E. Inclusion trail asymmetries indicating shear senses of top to the SW for NW–SE FIAs and dominantly top to the N for W–E FIAs reflect thrusting towards the SW and N. No evidence for tectonism related to early SW–NE bulk horizontal shortening has previously been detected in the Mount Isa Inlier. Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE–SW-and N–S-directed thrusting as the ages of tectonism are similar. The possibility of an AUSWUS linkage as part of Rodinia is increased in that 1670–1500 Ma period of tectonism in the Mt Isa province exactly spans the gap between two periods of extended tectonism within the USA.

Section D

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (1650–1500 Ma). Low-P/high-T metamorphism prevailed during NE–SW bulk horizontal shortening. This was followed by medium-P/high-T and high-P/high-T tectonism that accompanied the N–S and the W–E shortening periods, respectively. The sediments of the Soldiers Cap Group were deposited between 1675–1650 Ma in a tectonic setting that has the characteristics of an intra-continental rift basin. Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was a major source of heat.

Item ID:	19044
Item Type:	Thesis (PhD)
Keywords:	deformation partitioning, inclusion trails, porphyroblasts, thrusting, shear sense, Mount Isa Inlier, foliation inflection/intersection axes, bulk horizontal shortening, Soldiers Cap Group, tectono-metamorphic, Isan Orogeny, volcano-sedimentary, intra-continental, Mesoproterozoic, orogeny, geomorphology, North West Queensland
Date Deposited:	28 Nov 2011 23:01
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 100%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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