Geology topics
Publications and source records attributed to N. M. Ratcliffe.
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40 Ar/ 39 Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mountain massifs, as well as 40 Ar/ 39 Ar and K-Ar mineral and whole-rock ages from Paleozoic metamorphic rocks, suggest that the thermal peak for the dominant metamorphic recrystallization in western New England occurred 465 ± 5 m.y. ago (Taconian). Although textural data indicate a complex metamorphic-tectonic history for Paleozoic rocks, no evidence in rocks at least as high as kyanite grade dictates an Acadian age for the Barrovian metamorphism. Available 40 Ar/ 39 Ar and K-Ar data suggest that the low-grade metamorphism and cleavage formation in Taconic allochthons and the higher-grade metamorphism and emplacement of the Berkshire massif allochthon are Taconian. 40 Ar/ 39 Ar age data from a poorly defined terrane beginning near the east margin of the Green Mountain massif and extending along the eastern one-third of the Berkshire massif as far south as Otis, Massachusetts, suggest that the area has been retrograded during a metamorphism that peaked at least 376 ± 5 m.y. ago (Acadian). Available age and petrologic data from western New England indicate the presence of at least three separate metamorphic-structural domains of Taconian age: (1) a small area of relict high-pressure and low-temperature metamorphism in northern Vermont (T-1 domain), (2) a broad area in Vermont and eastern New York of normal Barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient (T-2 domain), and (3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks extending from near the Cortlandt Complex northeastward through Dutchess County, New York, to the Berkshire massif in western Massachusetts (T-3 domain). Areas of maximum metamorphic intensity within the T-3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting (Berkshire massif) or from recumbent folding (Manhattan Prong) of remobilized North American continental crust in the later stages of the Taconic orogeny.
Since 1978, the U.S. Geological Survey (USGS) has cored cataclastic rocks at six localities along border faults of the Early Mesozoic Newark basin in New York and New Jersey. This drilling was done as part of fault definition studies for the USGS Earthquake and Reactor Hazard Programs. The purposes of these studies are to: (1) determine the attitude and location of major faults, (2) assess evidence for recent reactivation, and (3) identify the movement history and depth of formation of the faults. The results of these studies up to 1980 are summarized in Ratcliffe (1980) and a study of a site in Rockland County, N.Y. appears in Ratcliffe 1982). The attitude and movement history of these faults is particularly important because of the possible association of recent low-level seismicity in the New York-New Jersey area with the Ramapo-Flemington fault system (Aggarwal and Sykes, 1978). Previous studies have shown that the Ramapo fault dips to the southeast at angles varying from 70° to 45° and strikes N. 40° E. to N. 10° E. In all of the cored fault zones, the actual contact of hanging-wall and footwall blocks is expressed as a relatively narrow zone, 3 to 8 inches thick, of dark, finely comminuted, fluxionbanded gouge. The intensity of cataclasis decreases rapidly upwards so that rocks 60 ft or so above the fault are not strongly cataclastic. Gneiss, dolostone, and phyllonite in the footwall block are generally weakly deformed more than 30 ft below the fault. Movement sense on the Ramapo fault is largely dip-slip and right-oblique normal faulting. In the summer of 1983, two holes were drilled through the border fault of the Newark basin near Oldwick, New Jersey, in the Gladstone 7.5minute quadrangle. Figure 1A shows the location of the drill site in relation to regional geology and the major faults. The fault drilled in this study connects to the south with the Flemington fault, which trends southwestward across the Newark basin, as shown. To the north, the fault can be traced along the valley that extends towards Mendham, N. J., beyond the limits of exposed Mesozoic rocks, to connect with the Ramapo fault near Morristown N. J. (fig. 1A; Ratcliffe, 1980). For this reason, we use the name "Flemington" for the border fault in the region of the drill site. A detailed map (fig. 1B) shows the local geology along the border fault from Pottersville, N. J. southward to the axis of the Oldwick syncline.
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