sgr ms thesis - University of Maine

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7.1. Potential Mechanisms for CSD and CBS modification A secondary mechanism is required to produce the observed results for CSD, CBS, and CCA. Potential mechanisms include 1) thermal disaggregation, or the disintegration of clasts, by partial melt and assimilation into the surrounding magma and 2) thermal fracture of clasts during magma intrusion. These two options are considered because they are are relatively easily treated numerically, and because field observations suggest that they played important roles in the modification process. Although abrasion would play a part during the active stages of wall rock readjustment, it cannot explain the nonfractal distribution of sizes for Type 3 Bar Harbor Formation clasts. Clasts that are host to late stage fracture show no evidence for kinetic impact or abrasion between other clasts, therefore the only input that could produce fracture at this stage is heat transfer from the enveloping matrix. 7.1.1. Thermal Attrition Thermal disaggregation requires supersolidus temperatures to be reached and presence of magma flow to physically disaggregate the clast (e.g. Braun and Kriegsman, 2001). Thermal fracture is dependent on a heated material’s thermal expansivity and requires the elastic response to sharp thermal gradients in rock. I assume these two components to be agents of thermal attrition that had a marked effect on Bar Harbor clasts and less of an effect on diorite clasts. Thermal attrition is defined as any process or mechanism that directly enhances the potential for removal or assimilation of clast material into the intruding 91
magma. To prove the viability of thermal attrition, thermal-mechanical equations are solved using the finite element method (COMSOL Multiphysics). Partial melt can play a large part in the volumetric removal of clasts and has been linked to alteration of clast size distribution trends and reduced D s values (Farris and Paterson, 2007). I assume that clast volume is disaggregated once the solidus temperature is achieved, altering clast sizes with respect to time from the start of intrusion (Marko et al., 2005; Clarke, 2007). Because of this, clast size distribution evolves with the progression of heat conduction. However, this assumption requires flow of the matrix magma in order to disperse the products of partial melting, in addition to diffusional processes that would facilitate mixing. Field observations do suggest local flow around clasts, but I did not quantitatively evaluate the problem. 7.2. Methods: Partial Melting of Clasts 7.2.1. Model Setup and Important Parameters In order to address thermal-mechanical coupling, I use two geometries (figure 7.1): one is an ideal spherical clast and the other is an outcrop image from Type 3 Shatter Zone. These two geometries are used to compare ideal conditions with the complex clast geometries observed in the field. The twodimensional models are closed systems that examine the instantaneous immersion of Bar Harbor Formation metasedimentary clasts with D s of 2.5 in a hot magma matrix. The model’s outer boundaries are periodic. Nodes are defined by a triangular mesh. All clast-matrix boundaries have a fine boundary 92
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FRACTAL ANALYSIS AND THERMAL-ELASTI
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LIBRARY RIGHTS STATEMENT In present
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emplacement of mafic magma, resulti
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© 2011 Samuel Geoffrey Roy All Rig
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TABLE OF CONTENTS ACKNOWLEDGEMENTS
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5.3.5. Clast Circularity Analysis .
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LIST OF FIGURES Figure 2.1. Figure
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LIST OF TABLES Table 6.1. Table 7.1
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subvolcanic magmatic systems, the l
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Chapter 2 GEOLOGIC SETTING 2.1. Reg
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Somesville granite suite to the wes
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Biotite is metamorphic in origin, a
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Chapter 3 PHYSICAL DESCRIPTION OF S
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thinned lithosphere allows for part
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A B C Figure 3.1. Formation of a la
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1997; Branney and Kokelaar, 1998; J
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Effusive-Type Eruption max 10 0 - 1
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magma quickly intrudes the develope
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Piston Downsag Piecemeal Funnel Tra
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fragmentation, and thermal energy h
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Chapter 4 THERMAL FRAMEWORK FOR CON
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contrast, conduction is heat transf
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A B Grt C Grt D Crd Crd E 1 mm F Cr
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! ! ! ! Biotite Zone Garnet Zone !
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4.3.1. Model Setup The model is use
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to make a “hockey puck” style g
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t = 0s t = 1E11 t = 1E12s t = 1E13s
Page 55 and 56: 850 750 650 550 450 400m Into Chamb
Page 57 and 58: approximately 140m thinner. The Sha
Page 59 and 60: Chapter 5 ROCK MECHANICS AND THE FR
Page 61 and 62: A β δ σ 1 σ 3 σ t c a σ 1 σ
Page 63 and 64: friction along the closed crack sur
Page 65 and 66: h h/2 h/4 Figure 5.2. A cube displa
Page 67 and 68: where N (≥r) is the count of clas
Page 69 and 70: to place a minimum size limit when
Page 71 and 72: moment of sudden fracture tip failu
Page 73 and 74: produced results in a self-similar
Page 75 and 76: 13 ln(area) (pixels) 12 11 10 9 ln(
Page 77 and 78: eaction processes were involved in
Page 79 and 80: Chapter 6 ANALYSIS AND RESULTS The
Page 81 and 82: Type 1 Type 2 500 m N Type 1 Type 2
Page 83 and 84: A B Figure 6.3. Type 2 Shatter Zone
Page 85 and 86: A B Figure 6.5. Centimeter-scale bo
Page 87 and 88: A B Figure 6.6. Brecciation texture
Page 89 and 90: The final breccia classified in thi
Page 91 and 92: esolution images of each box were s
Page 93 and 94: Clast circularity data were produce
Page 95 and 96: A 1000 Clast Size Distribu on: Type
Page 97 and 98: diorite dike with D s = 2.62 and R
Page 99 and 100: 6.3.3. Clast Circularity Analysis (
Page 101 and 102: with decreasing clast size in all T
Page 103 and 104: possibilities as postulated previou
Page 105: Chapter 7 MECHANISMS FOR SECONDARY
Page 109 and 110: mesh to better evaluate large therm
Page 111 and 112: consistent with Type 3 breccias. Th
Page 113 and 114: A temperature versus time plot was
Page 115 and 116: Dimensionless time 0 0.05 0.1 0.15
Page 117 and 118: Figure 7.4 displays model results f
Page 119 and 120: thermal model ignores advection of
Page 121 and 122: Zone experienced a relatively weake
Page 123 and 124: 7.4. Thermal-Induced Fracture Field
Page 125 and 126: Metapelite/hornfels Diorite dike E
Page 127 and 128: 7.4.2. Results and Discussion COMSO
Page 129 and 130: This thermal fracture model explain
Page 131 and 132: disaggregation of Bar Harbor clasts
Page 133 and 134: REFERENCES Acocella, V. (2007). Und
Page 135 and 136: Billen, M.I. and Gurnis, M. (2001).
Page 137 and 138: Clarke, D. B. (2007). Assimilation
Page 139 and 140: Fisher, R.V. (1960). Classification
Page 141 and 142: Hartmann, W.K. (1969). Terrestrial,
Page 143 and 144: Johnson, S.E.; Paterson, S.R.; Tate
Page 145 and 146: Mandelbrot, B.B. (1967). How long i
Page 147 and 148: Oliver, N.H.S.; Rubenach, M.J.; Fu,
Page 149 and 150: Schoutens, J.E. (1979). Empirical A
Page 151 and 152: Wiebe, R.; Holden, J.; Coombs, M.;
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