The development and 3D geometry of porphyroblast inclusion trails: significance for the tectonic evolution of the Lebanon Antiformal Syncline, New Hampshire

Bruce, M.D. (2007) The development and 3D geometry of porphyroblast inclusion trails: significance for the tectonic evolution of the Lebanon Antiformal Syncline, New Hampshire. PhD thesis, James Cook University.

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Using mineral elongation lineations for determining movement directions in porphyroblastic rocks is problematic because they are routinely subjected to reorientation due to reactivation of compositional layering during subsequent deformation events. Foliation inflexion or intersection axes preserved within porphyroblasts (FIAs) have been shown by numerous studies to be a consistent and reliable indicator of movement direction in metamorphic terrains that have experienced multiple episodes of bulk shortening. Furthermore, they appear to form orthogonal to the direction of relative plate motion. FIAs have most commonly been measured using multiple vertical thin sections with different strikes to determine the switch in inclusion trail curvature asymmetry that takes place across the FIA trend. An alternative method uses the “FitPitch” computer program to determine FIAs via the measurement of apparent dips of foliations defined by inclusion trails from a variety of thin section orientations. The results obtained from both methods are contrasted while the strengths and weaknesses of each approach are highlighted and discussed. “FitPitch” works best where the inclusion trails are predominately straight, whereas the “asymmetry” method requires that they display some degree of curvature. The two techniques compliment each other by permitting FIAs to be obtained from a larger proportion of samples and they can also be used as independent tests in samples that are suitable for both methods. The sense of shear operating during progressive bulk shortening is indicated by inclusion trail curvature recorded in thin sections cut at a high angle to the FIA. Unlike the “asymmetry” method, “FitPitch” does not give the shear sense directly but can be used to establish the most suitably oriented thin section in which to look for inclusion trail curvature.

The inclusion trail geometry within a first phase of porphyroblast growth can differ significantly from that preserved by further enlargement because the porphyroblast forms a rigid mass up against which the rock preferentially strains during ensuing events. The geometry of the first overgrown inclusion trails is affected by their primary orientation, including any pre-existing curvature, combined with any heterogeneous rotation of this foliation about the developing stretching lineation. This can impact the apparent timing of foliation intersection/inflection axes preserved within porphyroblasts (FIAs) that nucleated during the development of a sub-horizontal foliation, but is readily resolved. 3-D computer analysis of sigmoidal inclusion trails reveals that the asymmetry method for FIA determination is unaffected by the cut location relative to the porphyroblast core. Significantly, perfect spiral inclusion trail geometries can be produced from a sigmoidal shape in cuts up 30° away from the FIA. Therefore, since FIAs in most porphyroblasts bear no relation to matrix structures, there is a 17% chance that thin-sections cut relative to the foliation lie within 30° of a FIA and could contain such an apparent spiral. FIAs maintain consistent trends for the first phase of porphyroblast growth accompanying horizontal bulk shortening but may vary in plunge. FIAs have sub-horizontal plunges for porphyroblasts nucleating during gravitational collapse, but may vary in trend. For all periods of porphyroblast regrowth the data available indicates that FIAs remain consistently trending and sub-horizontal until the relative direction of plate motion causing orogenesis changes.

The progressive development and migration of patterns of deformation partitioning at all scales through the rock matrix commonly destroys any record of the ductile history associated with previous events making the problem of similar structures developing through multiple pathways generally intractable. However, records of the small-scale geometries that form as deformation commences and begins to partition through a rock are routinely trapped and protected by porphyroblasts because these large crystals nucleate and/or grow at this time. This allows examination of the geometry of microstructures formed at the start of deformation partitioning that were destroyed by the same event in the matrix, or which formed during an event prior to any preserved in the matrix. Porphyroblasts locally preserve oppositely concave microfolds (“millipedes”), which, in all examples that we have found, exclusively indicate a deformation history of bulk inhomogeneous shortening. Very similar structures have been formed experimentally during inhomogeneous simple shear but can readily be distinguished from those trapped in porphyroblasts that form during progressive bulk inhomogeneous shortening. Oppositely concave microfolds in some porphyroblasts reveal that deformation near orthogonal to a previously developed foliation occurred by axial plane shear driven rotation that led to rapid reactivational “card-deck-like” collapse of the pre-existing foliation. Differentiated crenulation cleavages may result from the same process providing yet another reason for the cessation of porphyroblast growth at the start of differentiation.

The succession of foliation intersection/inflection axes preserved as inclusion trails in porphyroblasts (FIAs) from the Lebanon Antiformal Syncline of SE New Hampshire, USA, document changes in bulk shortening geometry associated with emplacement and refolding of a large-scale recumbent fold (the Blue Hills Nappe). A sequence of four FIAs trending NW-SE, NE-SW, E-W and NNW-SSE has been distinguished based upon overprinting criteria plus inclusion trail composition, texture and orientation. This succession indicates a general clockwise rotation in the direction of horizontal bulk shortening during Acadian orogenesis. Metamorphic phase relations and pseudosections used to examine the P-T conditions during deformation and FIA development indicate an increase in both pressure and temperature associated with inversion of the stratigraphic sequence. Analysis of asymmetries recorded by the inclusion trails reveals a spectacular dominance of top to the south-east directed, non-coaxial horizontal shear during nappe emplacement and subsequent refolding that is attributed to repeated episodes of gravitational collapse in the orogen core to the west that progressively rotated, translated and amplified originally sub-vertical folds and foliations into sub-horizontal orientations.

Item ID: 7874
Item Type: Thesis (PhD)
Keywords: Lebanon Antiformal Syncline, South East New Hampshire, Blue Hills Nappe, porphyroblasts, inclusion trails, bulk inhomogeneous shortening, porphyroblast nucleation, porphyroblast growth, crenulation cleavage development, bulk shortening, foliation intersection/inflection axes
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Appendix E has not been uploaded. They are Quicktime files of rock sections.

Date Deposited: 11 Jan 2010 06:05
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040311 Stratigraphy (incl Biostratigraphy and Sequence Stratigraphy) @ 50%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 50%
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