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.

Exhumation of the ultrahigh‐pressure continental crust in east central China: Cretaceous and Cenozoic unroofing and the Tan‐Lu fault

“…D253b biotite, from the faulted western edge of the Dawu dome (Figure 13), yielded a disturbed spectrum for which we adopt the total fusion age of 84 Ma. The two thermal events are best recorded by potassium feldspar, which indicates initial cooling prior to 90 Ma and reheating and cooling at -75 Ma; these thermal events are accompanied by the two stages of deformation (dextral shear and sinistral faulting; see also Table 3 and Webbet al we obtained stress axes by the "pressure-tension (P-B-T) axes" method [Turner, 1953] and calculated stress tensors by the "numerical dynamic analysis" of Spang [1972] and the "grid search" technique of Hardcastle [1989]. In addition to stress orientation the computation of the reduced stress tensor determines the ratio R, which expresses the relationship between the magnitudes of the principal stresses.…”

Palaeostress Inversion in a Multiphase Deformed Area: Kinematic and Structural Evolution of the Çankırı Basin (Central Turkey), Part 1 – Northern Area

“…To avoid problems due to relative block and fault plane interactions [see Pollard et al (1993) and Twiss & Unruh (1998)] sampling sites were as small as possible and structurally homogeneous (Hancock 1985). In addition, the displacement should also be as small as possible (a few centimetres) so that it should not accommodate significant strain (Hardcastle 1989), therefore the principal strain and stress axes should remain parallel.…”

“…On the other hand, numerical methods are more robust and have been more widely used (e.g. see Angelier 1979Angelier , 1984Angelier , 1994Etchecopar et al 1981;Angelier et aL 1982;Armijo et al 1982;Gephart & Forsayth 1984;Michael 1984;CareyGailhardis & Mercier 1987;Reches 1987;Hardcastle 1989;Gephart 1990;Marret & Almandinger 1990;Fleischman & Nemcok 1991;Will & Powell 1991;Yin & Ranalli 1993;Nieto-Samaniego & Alaniz-Alvarez 1997).…”

“…Inhomogeneity and anisotropic material properties, fault interactions (especially in strike-slip settings), the presence of rotational deformation, and the non-coaxial stress and strains From: BOZKORT [monoclinic or triclinic symmetry of Twiss & Unruh (1998)] can either cause local variations in the stress field or a very high deviation between the maximum resolved shear stress and the slip direction (Pollard et al 1993). This decreases the reliability of the Stress-inversion procedure and makes the identifl~Rion of different deformation phases more difficult, but is helped if constrained by stratigraphic controls and overprinting and cross-cutting relationships (Nemcok & Lisle 1995;Hardcastle 1989;Angelier 1994). Fault reactivation is another source of difficulty as not all inversion procedures can cope with it.…”

Transcurrencia a lo largo de la Falla Sierra de Varas (Sistema de fallas de la Cordillera de Domeyko), norte de Chile.

“…Cabe señalar, que -en este caso-todos los datos intro ducidos al programa fueron, en realidad, compatibles con las direcciones de 'stress' establecidas para cada estación de medidas. Segundo, clasifi ca los datos sobre mesofallas en cuatro grados de confi abilidad establecidos por Hardcastle (1989), que van desde grado 1 (muy confi able) hasta grado 4 (poco confi able), otorgando mayor peso relativo a aquellas medidas con mayor grado de confi abilidad y restando importancia a aquellas con menor grado de confi abilidad. A la izquierda, se muestra el análisis dinámico con el estereograma (proyección en red de Schmidt, hemisferio inferior) que incluye el datum de cada falla.…”