Modeling, describing, measuring and interpreting porphyroblast inclusion trails to understand inter-relationships between deformation, metamorphism and tectonism

Fay, C. (2014) Modeling, describing, measuring and interpreting porphyroblast inclusion trails to understand inter-relationships between deformation, metamorphism and tectonism. PhD thesis, James Cook University.

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The main objectives of the research presented in this PHD thesis are:

• to demonstrate numerically the relevance of using microstructures encapsulated within porphyroblasts as a quantitative proxy for unravelling lengthy orogenic deformation history;

• applying this quantitative measurements of foliation intersection/inflexion axis (FIAs) method to deciphering the sequence of main shortening directions responsible for most of the deformation features observed in fold interference patterns and the arcuate shape of orogens using the example of the Adelaide Geosyncline (South Australia);

• to describe with great precision the sequence of deformation and metamorphic events and their inter-relationships during one single FIA event affecting the study area;

• to understand the long-lasting stable metamorphic conditions of mid-crustal rocks displaying multiple generations of synchronous staurolite and andalusite using a combination of thermo-dynamic modelling coupled with microstructural analysis.

The first part is based on the data on foliation intersection/inflection axes preserved in porphyroblasts (FIAs) and show that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates. This contrasts with 99% of investigations of "rigid" objects in non-coaxially deforming media where the objects rotate. When anastomosing shear zone formation around relatively strong objects in a weaker matrix is modelled, no "porphyroblast" rotation occurs. Formation of these anastomosing zones controls the development of this phenomenon, labelled "gyrostasis". If such zones are absent, porphyroblasts rotate. In weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress. Since shear zones are controlled by the orientations of principal axes of stress, initial anastomosing zones retain their orientations and positions during subsequent noncoaxial deformation. The porphyroblast is isolated from the embedding non-coaxially deforming material and material close to the porphyroblast continues to deform coaxially; no local rotation occurs. This has major significance since porphyroblasts can be routinely used to access lengthy tectonic histories destroyed in the matrix by reactivation. In particular, changes in relative directions of bulk shortening, associated with orogenesis, can now be determined within ancient orogens.

In the second part of the thesis, porphyroblast microstructures in rocks from a narrow portion of the sigmoidal-shaped Adelaide geosyncline indicate that the Delamerian Orogeny was a product of five changes in the direction of bulk shortening. The progression from NNW-SSE to WSW-ENE to SSW-NNE to WNW-ESE to NNESSW directed bulk shortening, resulted from shifts in the direction of relative plate motion as orogenesis progressed between 522 and 478 Ma. These directions were determined from a succession of 5 FIAs (Foliation Intersection/Inflection Axes preserved within porphyroblasts). The overprinting relationships between the multiple generations of regional folds that developed were resolved using the timing criteria provided by this FIA succession. The S-shape of the orocline has been interpreted as a fold and thrust belt product of either oblique convergence or the development of asymmetric syntaxis zones but this is not the case. Rather the orocline resulted the overprinting of a succession of near orthogonal plan view changes in the direction of convergence. This reflects much of the geodynamic evolution of the Eastern Australian portion of the Gondwana margin during its early Paleozoic history.

In the third part of the thesis, excellent inclusion trails in a staurolite and andalusite-bearing sample preserve 3 main phases of growth of both phases during the early stages of 3 deformation events. Subtle extra periods of growth of both phases occur, being most obvious for andalusite porphyroblasts, which commonly occur as clusters of large crystals that vary from several to tens of degrees in orientation and can encapsulate staurolite grown in an earlier or the same deformation event. All foliations defined by all inclusion trails intersect in a FIA (a foliation intersection axis preserved within porphyroblasts) trending at 25° indicating no change in the direction of horizontal components of bulk shortening while the porphyroblasts grew. Wellpreserved microstructural relationships between successive foliations within porphyroblasts allow a detailed analysis of the approaches to inclusion trail description and interpretation that have resulted from 25 years of quantitative FIA based studies. Spiral-shaped inclusion trails in most porphyroblast clusters contain portions of millipede geometries. The latter clinch the dominant role of bulk shortening in porphyroblast growth even in an environment that is overall non coaxial and which results in the same asymmetry later on in each deformation event. Any role for porphyroblast rotation is strongly refuted by differing stages in the development of these bulk-shortening geometries preserved within staurolite and the andalusite that immediately enclosed them as does such variation in adjacent clusters. They strongly suggest that discrepancy in the orientation of inclusion trails in porphyroblast cores is a function of the early effects of bulk shortening driving porphyroblast growth and cannot be used to imply later porphyroblast rotation. Staurolite and andalusite have grown slightly before, after and synchronously, without reacting with each other, during the early stages of 3 separate deformations. This strongly supports microstructural and more recent metamorphic data that the early stages of bulk shortening start porphyroblast growth; it also indicates that the commencement of the development of a differentiated foliation in the vicinity of a porphyroblast will always stop growth.

In the four[th] part of this thesis, it is revealed that the southern portion of the Adelaide fold belt contains a large region where synchronous to interleaved growth of staurolite and andalusite porphyroblasts has occurred. The truncation and continuity of inclusion trails versus matrix foliations reveal multiple periods of growth of staurolite and andalusite in many samples. The measurement of FIAs (foliation intersection axes preserved within porphyroblasts) revealed a succession of 5 changes in the bulk shortening direction during orogenesis from initially NNW-SSE when the first FIA (I), trending at 75°, formed in garnet. FIAs II through V are present in garnet, staurolite and andalusite. Indeed, staurolite and andalusite grew in the same sample during the development of at least one of FIAs II, III, IV and V. Pseudosections show a remarkably narrow range of PT conditions where this is possible and define a very confined location in PT space where these rocks developed at least 10 foliations over the ~ 30 million years that FIAs II through V developed. Garnet growth occurred early in some samples, but not in others with very similar bulk chemistry, where staurolite and andalusite grew in the same FIA event. This behaviour resulted from slight changes in Mn content and allowed a very tightly constrained PT path to be defined on pseudosections in combination with the minerals that formed early and late in some multi FIA samples. These low-pressure high-temperature rocks remained at the same orogenic level throughout most of the very lengthy deformation history of the Delamerian orogenic cycle once staurolite and andalusite began to grow. They ceased to grow at the commencement of exhumation when retrogressive chlorite growth began in many samples when shortening was directed once again NNW-SSE.

Item ID: 39976
Item Type: Thesis (PhD)
Keywords: Adelaide geo-syncline; andalusite growth; FIAs; geodynamics; Gondwana; Kanmantoo group; metamorphic rocks; metamorphism; millipede microstructures; mineralogical chemistry; Porphyroblast continuum modelling; porphyroblast growth; PT stability during orogenesis; rock deformation; spiral trails from millipeding; s-shape orocline; staurolite growth; structural geology; tectonics
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 1: Fay, C., Bell, T.H., and Hobbs, B.E. (2008) Porphyroblast rotation versus nonrotation: conflict resolution. Geology (Boulder), 36 (4). pp. 307-310.

Date Deposited: 29 Oct 2015 02:16
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 33%
09 ENGINEERING > 0912 Materials Engineering > 091299 Materials Engineering not elsewhere classified @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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