sgr ms thesis - University of Maine

Chapter 7
MECHANISMS FOR SECONDARY CLAST MODIFICATION: A DISCUSSION
The thermal influence from the intruded granitic matrix of the Shatter Zone
had a significant influence on clast size distribution (CSD) data trends,
specifically for Bar Harbor Formation Type 3 clasts. The marked decrease in
small clast populations and boundary shape values as well as the increase in
clast circularity implies a non-fractal mechanism was active in the modification of
clast size, shape, and abundance. Potential mechanisms are explored for post
brecciation modification and I determine that disaggregation through partial melt
of clasts is the dominant mechanism at play. Transient two-dimensional models
are produced in order to quantitatively characterize the migration of solidus
temperatures through a conductively heated clast, and to determine the evolution
of CSD as clasts begin to partially melt. I show that disaggregation of clasts can
lead to a bi-fractal and eventually non-fractal CSD. Magma flow, a constraint
ignored in the thermal-mechanical model, is required to physically disaggregate
the melted clasts. Thermal fracture is another possible secondary mechanism
and is treated in a transient two-dimensional thermal stress model. Results show
that late-stage thermal fracture occurred in diorite clasts, but less is known about
the thermal stress characteristics of the structurally anisotropic Bar Harbor
Formation.
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