3 NEtectonics: The Implications of Crustal Heterogeneity for Structure, Rheology, and Tectonics I

Thursday, 27 March 2008
8:00 AM - 11:30 AM

QUANTIFYING THE DEVELOPMENT OF AN EARLY ACADIAN LINEATION PRESIDENTIAL RANGE, NH

Paper number: 3-3
Program: THEME SESSIONS
Time: 8:30 AM – 8:50 AM

Authors

Abstract

This study quantifies the development of a variably aligned, coarse-grained pseudoandalusite lineation in the schists of the Devonian Littleton Formation, Presidential Range, NH. The lineation lies within the S1 axial plane, and is hinge-parallel to early Acadian D1 nappes. For each site, trend and plunge measurements were plotted in a rectangular coordinate system as vectors of unit length. The vectors were averaged, and the resulting mean vector had a length between 0 and 1 that reflected the degree of alignment. For example, a mean vector of length 1 would mean that all lineations at that site had the exact same trend and plunge. This methodology was applied to 42 sites on Mt. Jefferson, 42 sites on Mt. Madison and 26 sites on Mt. Washington. At each site, approximately 50 crystals were measured in an approximately 50x50 cm2 area. Vector lengths for Mt. Jefferson ranged from 0.48 to 0.97, for Mt. Madison from 0.36 to 0.96, and for Mt. Washington from 0.23 to 0.99. Areas with high average vector lengths were assumed to have experienced high strain. The crystals' long-axis orientations are assumed to lie within the λ1- λ2 plane. From these data, a map of early Acadian strain distribution across the Presidential Range is derived.

The differences in length between the mean vectors were proportionally exaggerated, and the mean vectors were spatially plotted on a map of the mountain range. There is an overall southwesterly trend and shallow to moderate plunge of the well-aligned sites. The trends are oblique to D1 macroscopic fold axial trace strikes by 30° to 90° degrees, indicating a reclined fold geometry. Upright and inverted limb position does not seem to affect lineation alignment. Thinly bedded (cm-scale) bedrock units appear to contain a higher amount of porphyroblast growth, but there is no clear correlation between lineation alignment and rheologic variations between Littleton Formation members. In D1 hinges, the lineation is generally poorly aligned. Despite intersecting planar fabrics (S1, S0), these data suggest that D1 hinge zones did not act as nucleation sites for aligned porphyroblasts. Instead, this method allows identification of localized, high-strain zones that may have been formed by non-homogenous early Acadian strain.