sgr ms thesis - University of Maine

This thermal fracture model explains the late stage fracture for diorite
clasts, but it does not take into account the anisotropic nature of the Bar Harbor
Formation. Rock heterogeneity creates a far more complex thermal stress
problem because crack formation is not solely dependent on the thermal
gradient. Clarke et al. (1998) discuss the three different possibilities for fracture
development in anisotropic xenoliths. The first is determined by the thermal
gradient and this is used in the above model (thermal gradient cracking). The
second mechanism for crack formation in a layered material becomes operative
when one layer has a differing thermal expansion coefficient than its counterpart
(thermal expansion mismatch cracking). Third, a material may have an
anisotropic fabric that leads to preferred crack propagation in one direction
(thermal anisotropy cracking). Bar Harbor Formation rocks can exhibit all three of
these fracture types due to their compositional layering, and they may be more
susceptible to thermal fracture than suggested by this model. The thermal
gradient cracking model therefore provides a low end-member possibility for the
proliferation of thermal cracks in Bar Harbor clasts.
If thermal fracture was prevalent, each clast would fracture according to a
new size distribution: for thermal fracture, D s = 2.156 (Glazner and Bartley,
2006). The new value of D s is the combination of the explosive fracture event and
the secondary thermal fracture event, and repeated fracture events always
increase D s by a fractional amount (Jebrak, 1997). Also, field evidence suggests
that not all diorite clasts experienced thermal fracture, therefore late-stage
thermal fracture did not have a significant effect on diorite dike CSD results.
114