THE IMPORTANCE OF SCALE AND HETEROGENEITY WHEN USING STRETCHING LINEATIONS TO CONSTRAIN REGIONAL STRAIN AND INFERRED DISPLACEMENT

Statistical averages of temporally related mineral and/or stretching lineations are often used to infer paleo-displacement directions in orogens (i.e., paleogeodesy). In the Cascades core, Washington, statistical maxima of these lineations are subhorizontal and parallel to this Cretaceous arc and often used to infer arc parallel strain and thus displacement. In a linked abstract (Miller et al., this volume) we discuss the long-lived development and multiple processes by which these lineations form. In this abstract we explore the importance of scale and heterogeneity of mineral lineations when interpreting regional strain and displacement. Mapping from 1 to 103 m2 indicates that spatial and temporal heterogeneity occur at all scales, with particularly spatial heterogeneity increasing dramatically with decreasing scales (increasing area). The intensity of lineation development relative to foliation is also variable in most domains implying heterogeneous strain and the potential for overestimating the importance of these lineations. Strain modeling indicates that increasing heterogeneity in lineation orientation or intensity dramatically decreases the required orogen-scale strains and/or displacements. For example, statistical averages of stretching lineations and associated strains in the Cascades have been used to infer 150% to 300% orogen parallel stretch. Whereas our strain modeling using directed tensor averages suggest that only 50% to 100% orogen-parallel stretch is required. Our studies suggest that these smaller values could be compensated for at the orogen scale (i.e., regional folding, doming, or flow in pluton aureoles), thus requiring little regional orogen-parallel stretch and no orogen-parallel displacement. We also speculate that the structural heterogeneity observed in the Cascades core may be a fundamental character of geologic displacement fields as has been suggested for flow in other non-linear dynamic systems.