Possible paleostress tensor configurations derived from fault‐slip data in eastern Vermont and western New Hampshire

Abstract

The configuration of six possible paleostress tensors have been derived from 152 faults measured in eastern Vermont and western New Hampshire. Populations of potentially genetically related faults were separated using two techniques. Tensor configurations for each population were derived using a linear least squares inversion method based in part on the work of Reches [1987] and a grid search inversion method which tests over 100,000 possible tensors for compatibility with all or portions of the data. Faults belonging to the oldest population (set R; n=24) occur primarily in high‐grade rocks. This set is composed of semiductile, northeast trending reverse and west–northwest trending left‐lateral faults. Fault fabrics of quartz rods and thin mylonite layers suggest that the host rocks were at crustal levels of 8–10 km during faulting. The derived tensor indicates a roughly east–west σ1 and near vertical σ3. Set R faults are offset by normal faults (sets N1 and N2) and are interpreted to be pre‐Mesozoic in age, perhaps related to late Paleozoic Alleghanian compression. Normal and normal oblique faults mineralized with chlorite, calcite, and strained quartz (n=73) have been separated into two populations (sets N1, n=49; and N2, n=24) even though these faults are likely of similar age. Host rocks were probably at moderate crustal depths of perhaps 5 km during faulting. Faults of both sets are most likely related to Mesozoic rifting. Tensor configurations indicate that σ1 plunges steeply northwest (N1) and southeast (N2), and σ3 plunges gently, roughly east–west. These faults cut presumably Mesozoic age dikes and are themselves offset by normal oblique faults (set T) and right‐lateral faults (set RL). Strongly deviatoric, near vertical σ1 suggests a thermally driven tectonic regime during the development of normal faulting in New England. Set T (n=25) is composed of normal oblique slip faults mineralized similarly as sets N1 and N2. The plunge of σ1 is 50°N and σ3 plunges gently east. Field relations indicate set T faults document the transition from normal (sets N1 and N2) to strike‐slip faulting regimes (set RL). A compatible sequence of Mesozoic stress fields is suggested de Boer et al. [1988]. North–northeast trending, right‐lateral faults of set RL (n=7) are mineralized with quartz‐limonite‐calcite‐chlorite. Calcite twins are bent and fractured, and quartz grains show minor internal strain, suggesting that these faults developed while host rocks were at relatively shallow crustal levels. The plunge of σ1 is gently northeast and, σ3 plunges gently east–southeast. Seven distinctly mineralized (feldspar‐siderite‐quartzoalcite‐1imonite‐chlorite) gently northwest dipping, thrust faults of set F define the sixth tensor. These faults are cut by essentially vertical, east‐side‐up faults and are assumed to be Mesozoic in age by analogy with similar faults in western Massachusetts.