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Horizontal principal stress orientations inferred from well bore elongations ("breakouts") in petroleum exploration wells on the Atlantic outer continental shelf yield the following results by area (from north to south): Georges Bank (9 wells, total depths between 4.30 and 6.66 km)-maximum horizontal compressive stress (Snmax ) orientation approximately E-W; Baltimore Canyon (14 wells, total depths between 3.74 and 5.58 km)-SHmax orientation between N 50° E and N 70 E; and the southeast Georgia Embayment (3 wells, total depths between 1.83 and 2.13 km)-$Hmax orientation between N 8°E and N 23°E. All wells lie within 70 km of the continental slope and the inferred least horizontal principal stress orientations are generally aligned perpendicular to the local trend of the slope. Focal mechanisms of earthquakes directly adjacent onshore indicate congressional deformation (thrust and strike-slip faulting) resulting from NE to ENE Snmax orientation. In addition, available unambiguous data on young faulting on the continental shelf suggest thrust and/or strike-slip faulting and in-situ stress magnitudes measured on the Scotian shelf, offshore Canada, indicate a strike-slip faulting stress regime with Snmax oriented about NE (Ervine and Bell, 1987). It appears that the state of stress on the continental margin is broadly consistent with the NE to ENE maximum horizontal compression characterizing most of mid-plate North America, however superimposed continental slope effects, including flexure due to sediment loading, may be locally large enough to rotate the principal stresses by as much as 40°. INTRODUCTION Knowledge of the in-situ stress field is invaluable for assessing seismic hazards in areas such as the eastern United States with a relatively low level of seismicity and where major active faults have not been identified. Results of previous studies (Zoback and Zoback, 1980; Yang and Aggarwal, 1981; and Wentworth and Mergner-Keefer, 1983) suggested that a zone of NW-oriented compression exists along the Atlantic seaboard of the U.S. in contrast to the NEto ENEoriented maximum horizontal compressive stresses found throughout much of the midcontinent region of North America (Sbar and Sykes, 1973; Zoback and Zoback, 1980; M. L. Zoback and others, 1986). The evidence for a distinct Atlantic coast stress province includes poorly constrained earthquake focal mechanisms (Yang and Aggarwal, 1981) and small (1-2 m) late Tertiary reverse fault offsets (Prowell, 1983). However, new stress data suggests that the NE to ENE compressive stress state characterizing the midcontinent region may be continous into the mid-plate region of the Western Atlantic basin making the existence of a separate coastal stress province doubtful. These new stress data include: 1) better-constrained earthquake focal mechanisms in the Charleston, South Carolina area (Talwani, 1982) and in the Western Atlantic basin (Nishenko and Kafka, 1982), as well as new mechanisms and a re-analysis and error assessment (Quittmeyer and others, 1985, C.T. Statton, 1984, written communication) of earlier focal mechanisms in the New York-New Jersey area that suggested NW-compression (Yang and Aggarwal, 1981), 2) Analysis of well bore elongations in southeastern Canada and the northeastern U.S. (Plumb and Cox, 1987) and on the Scotian shelf (Prodrouzek and Bell, 1985), 3) in-situ stress (hydraulic fracturing) measurements in a 1 km-deep well in the New York-New Jersey area (Zoback and others, 1985) and in several holes near the Georgia-South Carolina border (M. D. Zoback and others, 1986). To further evaluate the state of stress along the Atlantic coast area we analyzed high resolution four-arm dipmeter logs for stress-induced borehole elongations ("breakouts") in Continental Offshore Test wells (COST) and other petroleum industry exploration wells drilled on the outer continental shelf. Numerous studies using commercially available dipmeter data indicate that the average azimuth of these elongations is generally very consistent within a given well or oil field (Cox, 1970; Babcock, 1978; and Brown and others, 1980). Comparison with other types of data (Gough and Bell, 1981 and 1982; Hickman and others, 1985; Plumb and Hickman, 1985; Teufel, 1985) and theoretical analyses (Bell and Gough, 1979 and 1982; and M. D. Zoback and others, 1985) suggest that the consistent azimuth of the elongation is parallel to the minimum horizontal compressive stress orientation. We determined borehole elongation azimuths for wells from three fields on the eastern U.S. continental margin (Georges Bank, Baltimore Canyon, and Southeast Georgia Embayment) in order to evaluate stress orientations in this region. ANALYSIS OF LOG DATA TO DETERMINE ELONGATION DIRECTIONS The elongation directions reported here were measured from field dipmeter logs obtained from the U.S. Minerals Management Service and also directly from the private companies who drilled the holes (see the Acknowledgments for a complete list of companies who provided logs). Dipmeter logs utilize an oriented-four arm caliper tool which records hole geometry in two orthogonal directions (for a complete description of this tool see Schlumberger, Limited, 1981). Torque in standard logging cable results in a clockwise rotation of the tool under normal operating conditions when the well bore is approximately circular. In zones of well bore elongation this rotation may temporarily be interupted when one set of caliper arms expands and locks into the elongated axis of the hole. Babcock (1978), Bell and Gough (1982), Cox (1983), and Dart (1985) give a detailed discussion of identification and discrimination of elongation ("breakout") zones from dipmeter logs and also provide numerous examples. The primary criteria used in this study to distinguish elongation or breakout zones from other forms of well bore enlargment and non-symmetrical caving are essentially identical to those reported in Dart (1985) and are repeated below: 1. The logging tool must exhibit normal rotation in circular parts of the hole. 2. Normal tool rotation is interrupted in elongation zones. 3. One of the caliper pairs must exceed the borehole diameter relative to the bit size. 4. The direction of elongation and azimuth of hole drift in cases where there is a vertical deviation of the hole must not coincide. Such non-verticality of the well bore may induce drill-pipe wear in the form of asymmetric borehole elongation (Plumb and Hickman, 1985). GEOLOGY OF THE STUDY AREAS The United States Atlantic continental margin extends from the Georges Bank southward to southern Florida, between the coastal plain and the outer continental rise (Figure 1). The physiography of the continental margin consists of a gently sloping shelf to depths of approximately 200 m, a continental slope on which bathymetric depths increase rapidly to about 2,000 m, and the broad continental rise that extends seaward to the abyssal plain (Emery and Uchupi, 1972). The continental margin is underlain by a series of structural platforms and basement depressions that trend parallel to the coastal plain (Maher, 1971; Klitgord and Behrendt, 1979). These platforms and basins are syn-rift and post-rift, fault-controlled structural features associated with the separation of the North American and African continental landmasses (Klitgord and Behrendt, 1979; Ziegler, 1983). Basin subsidence occurred in conjunction with the extensional thinning and cooling of the rifted crust and the thick accumulation of terrigenous and marine sediments (Hallam, 1971; Falvey, 1974; Bott, 1979; Steckler and Watts, 1985). On the basis of the available geologic and geophysical data it appears that the entire Atlantic continental margin, from the Scotian shelf off the Canadian Maritime Provinces to the Southeast Georgia Embayment-Blake Plateau Basin, has undergone a similar tectonic and depositional evolution. References to apparent similarites in structure and lithology among the three areas studied and the Scotian shelf are made by Adinolfi and Jacobson (1979), Judkins and others (1980); Grow (1980); Scholle and Wenkam (1982); Poag and Valentine (1985); Libby-French (1983); Maher (1971); Emery and Uchaupi (1972); and Ziegler (1983). COST wells and petroleum exploration wells used in this study were drilled in three of the major structural basins in the Atlantic continental margin: The Georges Bank basin offshore from Massachusetts, the Baltimore Canyon Trough offshore from New Jersey and the Southeast Georgia Embayment offshore from Georgia and northern Florida. These wells were generally located on the outer continental shelf, within 70 km of the shelf-slope break, but in a few cases the wells were drilled on the continental slope itself (see Figure 4). Wells in all three basins penetrated sedimentary strata consisting of unconsolidated to poorly consolidated Tertiary coarse sands, gravels and soft clay overlying poorly consolidated to well indurated Cretaceous and Jurassic interbedded siltstone, sandstone, and shale, limestone, dolomite and evaporite deposits (Scholle and Wenkam, 1982; Scholle, 1977, 1979 and 1980). Table A-2 correlates lithology with type of well-bore elongation for selected wells in each of the three basins. Basement rock was drilled in two of the basins, Paleozoic rocks in the Georges Bank Basin and igneous intrusives and metamorphic rock in the southeast Georgian Embayment. Paleozoic basement was not reached in the Baltimore Canyon Trough (Scholle, 1979, 1977, 1980; and Scholle and Wenkam, 1982). Depositional environments of the rocks drilled in these three basins varied from nonmarine and restricted marine inner shelf to outer shelf open marine, slope and deep water pelagic (Poag and Valentine, 1985). The identification and correlation of certain strati graphic units and Figure 1. Eastern U.S. Atlantic continental margin. Study areas are hachured. Location of figures 2, 1, and 6 shown by boxes. Base modif

Principal stress directions on the Atlantic continental shelf inferred from the orientations of borehole elongations