UVM Theses and Dissertations
Format:
Print
Author:
Marcotte, Stephen B.
Dept./Program:
Geology
Year:
2004
Degree:
MS
Abstract:
Studies of convergent and obliquely convergent margins have suggested that crustal thickening and mafic magmatism leads to the partial melting of the lower crust and that pressure gradients accompanying deformation facilitate t1le migration of partial melts to shallower levels. However most natural examples of orogenic belts do not expose true lower crustal levels where melt formation,. extraction and transfer mechanism interact with deformation.
I present data from Fiordland, New Zealand where >5000 km2 of Early Cretaceous lower crust records feedback between melt transfer mechanisms and the development of a crustal scale transpressional shear zones at ~111 Ma. The Indecision Creek Shear Zone (ICSZ) formed during oblique convergence following the intrusion of a mafic batholith into the lower crust and the partial melting of crust below the batholith. We have established a 5 km wide subhorizontal strain gradient using variations in fold geometry and in the orientations of linear and planar fabric elements that define a 15 km wide subvertical lower crustal shear zone. Changes in the orientations of stretching lineations and foliations across the strain gradient show that this shear zone records triclinic transpression characterized by subvertical thickening and subhorizontal shortening. Forward modeling and finite strain measurements support this interpretation.
Along the margins of the ICSZ, zones of melt accumulation focused deformation. We observed leucosome that were injected parallel to foliation planes and to the axial planes of folds that define the shear zone fabric. These data and textural changes across the strain gradient show that the foliations were used as pathways for melt transport. These relationships suggest that feedback relationships among melt segregation and transfer through the lower crust was controlled by transpressional shear zones. The shear zones formed vertical conduits that channeled melts from the lower crust to middle and upper crustal levels.
The heterogeneous localization of strain in the ICSZ is pronounced at regional macroscopic and microscopic scales of observation. Analyses of microstructures in the ISCZ indicate fabric formation occurred at T = 650-700ʻ C and the contrasting strength of coexisting mineral facilitated strain partitioning. Shear zone strain were heterogeneously partitioned within the weakest mineral phases, which commonly resulted in the semi-brittle failure of relatively strong mineral phases. Using a scanning electron microscope I investigated replacement textures developed along plagioclase grain boundaries after the ICSZ. The growth of the hydrous mineral phase clinozoisite and diffusional mass transfer along grain boundaries suggest that metamorphic fluids were present during metamorphism. These replacement textures likely formed during the decompression and exhumation of the Fiordland Granulite belt.
Three-dimensional maps of the Earth surface are an innovative medium to present engaging and informative topographical and geological maps to all audiences. Three-dimensional geology maps can depict true geometrical relationships between different geological structures. Three-dimensional maps are useful for present information to other researchers and teaching the basic elements of topographical map interpretation and geological mapping. A detailed algorithm for creating images and movies of three-dimensional maps using Bryce 5TM is presented in this thesis.
I present data from Fiordland, New Zealand where >5000 km2 of Early Cretaceous lower crust records feedback between melt transfer mechanisms and the development of a crustal scale transpressional shear zones at ~111 Ma. The Indecision Creek Shear Zone (ICSZ) formed during oblique convergence following the intrusion of a mafic batholith into the lower crust and the partial melting of crust below the batholith. We have established a 5 km wide subhorizontal strain gradient using variations in fold geometry and in the orientations of linear and planar fabric elements that define a 15 km wide subvertical lower crustal shear zone. Changes in the orientations of stretching lineations and foliations across the strain gradient show that this shear zone records triclinic transpression characterized by subvertical thickening and subhorizontal shortening. Forward modeling and finite strain measurements support this interpretation.
Along the margins of the ICSZ, zones of melt accumulation focused deformation. We observed leucosome that were injected parallel to foliation planes and to the axial planes of folds that define the shear zone fabric. These data and textural changes across the strain gradient show that the foliations were used as pathways for melt transport. These relationships suggest that feedback relationships among melt segregation and transfer through the lower crust was controlled by transpressional shear zones. The shear zones formed vertical conduits that channeled melts from the lower crust to middle and upper crustal levels.
The heterogeneous localization of strain in the ICSZ is pronounced at regional macroscopic and microscopic scales of observation. Analyses of microstructures in the ISCZ indicate fabric formation occurred at T = 650-700ʻ C and the contrasting strength of coexisting mineral facilitated strain partitioning. Shear zone strain were heterogeneously partitioned within the weakest mineral phases, which commonly resulted in the semi-brittle failure of relatively strong mineral phases. Using a scanning electron microscope I investigated replacement textures developed along plagioclase grain boundaries after the ICSZ. The growth of the hydrous mineral phase clinozoisite and diffusional mass transfer along grain boundaries suggest that metamorphic fluids were present during metamorphism. These replacement textures likely formed during the decompression and exhumation of the Fiordland Granulite belt.
Three-dimensional maps of the Earth surface are an innovative medium to present engaging and informative topographical and geological maps to all audiences. Three-dimensional geology maps can depict true geometrical relationships between different geological structures. Three-dimensional maps are useful for present information to other researchers and teaching the basic elements of topographical map interpretation and geological mapping. A detailed algorithm for creating images and movies of three-dimensional maps using Bryce 5TM is presented in this thesis.