Structure and tectonic synthesis of the Marlborough block, Northern New England fold belt, Australia

Harbort, Terrence Anthony (2002). Structure and tectonic synthesis of the Marlborough block, Northern New England fold belt, Australia PhD Thesis, School of Physical Sciences, The University of Queensland.

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Author Harbort, Terrence Anthony
Thesis Title Structure and tectonic synthesis of the Marlborough block, Northern New England fold belt, Australia
School, Centre or Institute School of Physical Sciences
Institution The University of Queensland
Publication date 2002
Thesis type PhD Thesis
Supervisor Rod Holcombe
Darcy Milburn
Total pages 127
Language eng
Subjects 0403 Geology
Formatted abstract
The Marlborough Block consists of seven thrust-bounded sheets consisting of a serpentinised ophiolite, dismembered into four sheets, intercalated with at least three other fault sheets. These other sheets contain deformed metavolcanic and metasedimentary units. Two of the latter sheets consist of an upper greenschist to lower amphibolite facies assemblage and a lower greenschist facies assemblage, respectively. The third contains strongly contact metamorphosed rocks of a probable upper-greenschist facies assemblage. Variably foliated, to mylonitic metagranite is a major constituent of one of the high-grade sheets.

Three of the ophiolitic sheets are dominantly ultramafic consisting of harzburgite, with minor lherzolite, pyroxenite and dunite, plus pods of chromite and pyroxene gabbro intruded by dolerite dykes (Murray 1974, Bruce 2000). These rocks are interpreted to represent the lower region of a mid-oceanic ridge magma chamber. On the boundary of the central ophiolite sheet is a thin band of serpentinite melange composed of metamorphosed gabbro, actinolite schist, quartzite and serpentinite blocks, encased in a serpentinite matrix. This band is interpreted to represent a wide thrust zone that collected components from different levels during thrusting. The fourth sheet is composed of an ultramafic unit similar to the other three sheets and a metasomatised gabbro unit.

The westernmost non-ophiolitic sheet is composed of weakly foliated to mylonitic granite intruding pelites, psammites and amphibolites. These rocks have been regionally metamorphosed under pressure temperature conditions of ~450oC to 550oC and 3.5-4.5 kpa. The metasedimentary rocks are metamorphosed clastic sediments derived from a siliceous continental source. Metagranitic rocks are derived from the partial melting of the adjacent metasedimentary rocks Amphibolites are metamorphosed sub-alkaline tholeiitic ocean floor basalts. These rocks are regionally correlated with the Broome Head Metamorphics and are derived either from deeper crustal level of the Shoalwater Terrane or rocks that are basement to the northern New England Fold Belt.

The central non-ophiolitic sheet is composed of two fault-intercalated groups of rocks, one group composed of pelitic schist, greenstone and manganiferous banded quartzite and a second, subordinate group consisting of quartzofeldspathic schist. The first group is composed of rocks derived from pelagic sediment, with ocean floor midoceanic ridge basalt and may be derived from the adjacent Wandilla Terrane. The second group consists of metamorphosed clastic sedimentary rocks and may be derived from the Shoalwater Terrane. Thrust intercalated marble blocks are most likely derived from the Mt. Holly Beds of the Calliope Volcanic Assemblage. This sheet has been regionally metamorphosed in the temperature range 420oC-460oC and pressure 3.5 kbar - 4.5 kbar.

The easternmost tectonic sheet of the Marlborough Block is composed of two units, contact metamorphosed by the Kunwarara Igneous Complex. The first unit consists of homfelsed mafic rocks possibly equivalent to the mafic components of the mafic/ultramafic complexes. The second unit consists of cordierite andalusite schist, possibly thermal metamorphosed equivalents of the westernmost non-ophiolitic sheet. These rocks have been thermally metamorphosed under temperature conditions of 550-650oC and pressure conditions of 1-3 kbar.

A shallowly dipping to subhorizontal brittle detachment defines the base of the block, separating it from an underlying Permian thrust belt and exposing the block as a relatively thin (<2km) out of sequence nappe sheet. Sheets within the nappe sheet are separated by steeply dipping (~45-65o) thrust faults with wide (up to 1000m) zones of strong ductile deformation, including mylonite zones. Associated with these faults is an intense pervasive fabric that deforms an earlier transpositional layering (formed during subduction accretion) in the non-ophiolitic rocks. Ductile deformation associated with this thrusting reoriented early fabrics and developed shallow stretching lineations. Later steep normal and strike-slip faults offset these earlier structures

The Marlborough Block is a thin (< 3 kilometers) out-of-sequence nappe sheet. The Marlborough Detachment, an undulating, low angle (<15o) to flat lying dominantly brittle structure, defines the base of this nappe sheet. The nappe sheet is dissected by at least four steeply dipping thrust faults that do not pierce the basal detachment. These thrusts separate alternating ophiolitic and non-ophiolitic units. Associated with these internal boundaries are wide prominent zones of ductile deformation. A pervasive fabric S2, (Ss in ophiolitic units) is related to these faults and in metasedimentary rocks deforms an earlier intense fabric probably related to subduction-accretion (Si). A mineral elongation lineation L2 (Ls in ophiolitic units) is developed on the S2 surface.

Three structural domains are identified in each tectonostratigraphic sheet, each defined by the amount of ductile response caused by the bounding faults. A static domain, generally greater than one kilometre from the fault boundary, is only weakly deformed by the thrusting event and is dominated by fabrics formed prior to thmsting. A transition domain is situated between the static domain and the fault domain. Fault related deformation in this zone is moderate, with S2 weakly developed adjacent to the static zone and gradually increasing in intensity approaching the thrust domain. The fault domain includes the fault boundary and an intensely deformed zone about 200 metres wide surrounding the boundary. S2, the dominant fabric in this domain, transposes S1 and intensifies closer to the boundary to a mylonitic fabric.

The static domain in ophiolitic units is characterised by large coherent blocks of mafic/ultramafic rocks surrounded by highly sheared serpentinite fabric Ss. This fabric is regionally correlated with S2. These blocks decrease in size across the transition domain and become more elongate as the serpentinite bands increase in width and intensity. Within the fault domain, sheared serpentinite dominates over small elongate clasts of mafic/ultramafic. Shear intensity in this domain increases towards the contact to form a well-defined mylonitic fabric commonly with sheath folding and two or three shear foliations.                   ................................ 
Keyword Geology, Structural -- Queensland -- Marlborough Region

Document type: Thesis
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