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(Ramapo Fault) and Phyllonitic Ductile Shear

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(Ramapo Fault) and Phyllonitic Ductile Shear BRITTLE FAULTS (RAMAPO FAULT) AND PHYLLONITIC DUCTILE SHEAR ZONES IN THE BASEMENT ROCKS OF THE RAMAPO SEISMIC ZONES NEW YORK AND NEW JERSEY, AND THEIR RELATIONSHIP TO CURRENT SEISMICITY NICHOLAS M. RATCLIFFE U.S. Geological Survey Reston, Virginia 22190 Introduction and rationale for investigations Field investigations at the northern end of the Newark grain of this zone is accompanied by faults of Pro­ basin begun more than 10 years ago led to the publi. terozoic, Paleozoic, Mesozoic and possibly Tertiary and cation of several articles (Ratcliffe, 1971; Ratcliffe and recent age. others, 1972) in which a case was made for long con~ tinued fault activity in the vicinity of the border fault Despite the abundant evidence for reactivation of (Ramapo fault) in New York and New Jerse~. These ar­ older faults in the Mesozoic, (Ratcliffe J 1971) the ticles also called attention to the observation that a Mesozoic and younger faults are not uniformly series of low-level earthquakes appears to be spatially distributed and do not everywhere coincide with the associated with these ancient faults (Fig. 1). traces of older faults. Unfortunately, it is not presently possible to produce a tectonic map of this area that ac­ Since that time, seismic-detention capabilities of the curately shows the distribution of Mesozoic as opposed Lamont-Doherty network have been greatly improved to older faults. In general, previous studies have not at­ and further supplemented by a dense array of stations tempted to differentiate Mesozoic faults except where located on the northern extension of the Ramapo fault Mesozoic rocks are actually offset. The geologic­ in the vicinity of Consolidated Edison's nuclear­ tectonic template, therefore, is quite incomplete at pre­ generating plants at Indian Point (Fig. 2)1. sent, and any plan to test the hypothesis that geologic structures of Mesozoic age are responsible for current Aggarwal and Sykes (1978) document a pattern of seismicity is severly limited by the lack of appropriate seismicity (Figs. 1 and 2) that illustrates that numerous geologic data. Figure 1 shows a new preliminary com­ low-level events (Nuttli magnitude 1.5 to approximately pilation of fault features. 3) are occurring"throughout a 30 km-wide zone roughly centered on the trace of the Ramapo fault at depths of 0 Tectonic framework and fault distribution in northern to about 10 km. Fault-plane solutions (Fig. 2) obtained New Jersey and southeastern New York by them and by Yang and Aggarwal suggest that motion is currently taking place on northeast-striking steeply Figure 1 shows the distribution of major tectonic southeast-dipping surfaces. First-motion studies in~ features in the Raplapo seismic zone. Two classes of dicate southeast-to-northwest compression and faults are distinguished: therefore a reverse-faulting mechanism. They conclude a ) faults with associated semiductile fabric in ad­ that curreet seismicity in the Ramapo zone is being con­ jacent rockS and phyllonite or blastomylonite trolled by reactivation of the northeast-striking steeply t at fault co~tacts; southeast-dipping faults which make up the predomi­ b ) more brittle faults characterized by discrete nant structural grain in this area. They further suggest breaks and cataclastic fabrics. that the Mesozoic border fault, the Ramapo fault, and its northern splays (Fig. 1) are the actual surfaces being This grouping of faults by and large also serves to reactivated. distinguish thrust faults, largely class (a), from normal or oblique dips lip faults, class (b). The geologic framework of the Ramapo seismic zone, however, is extremely complex. A multitude of faults of 1 Figure 2 is reproduced with permission of the authors from a paper by Jin·Ping Yang and Y,P, Aggarwal, Seismotectonics of north­ different ages have different physical characteristics and eastern United States and adjacent Canada, submitted for publication contrasting histories. The northeast-trending structural . to the' Journal of Geophysical Research. 278 Hg. 1. Preliminary tectonic map of the Reading Prong, northern 'i1''PII'; ;~.·.,;t LOC;1ti':l;L, Vi U.S. Geoiogicai ~urvey Newark basin, and Manhattan Prong. showing distribution drill holes along the Ramapo fault are identified. This map of "brittle" versus more ductile style fault zones and is preliminary and is likply 1(' be :::!!t::::-;:;;,! ~i.i~"ldl1liil;l} as earthquake epicenters. CompilatioH uses data of Drake, dating of faults and examination of fault-zone materials "T1 Lyttle, and Owens (1979), New York· State geologic map continue. Line A-A identifies the loc~tion of cross section mr (Fisher, Isachsen and Rickard, 1971) New York State brittle (Fig. 4). Epicenter locations from data of Lamont-Doherty ti fracture map (Isachsen and McKendree, (977), New Jersey Geological Observatory published in Regional Seismicity (J) -I Geological Survey maps at 1:62.500. and new data Bulletin of the Lamont-Doherty Network. Palisades. N. Y. C o (Ratcliffe, unpub. data) principally along the Ramapo- Horizontal errors are less than 2 km (diameter of circles). m til o 'Tj Z tr1 ~ '-m :;0 mtil >-<! Clm EXPLANATION o ol' o Jb - Jurassic basatt and diabase >-<! J - jurassic sedimentary rocks above ~ first lava flow Z J 1 - JlJrasslC and Triassic s.edrmentary tJ roc!o;s 1 o SEDIMENTARY AND METASEDIMENTARY ROCKS c: E DEVONiAN AND 8 } UPPER SILURIAN trl Devonian and Sliurian sedimentary rocks ~....r....r- Unconformity ,~ UPPER AND d 'Tj ~ MIDDLE OR DOV1C IAN til Martinsburg formation EZJ r WaUoomsac Formation Manhattan Schist, including ~ t'l BaJmville 1 and Jacksonburg probable allochthonous umts Taconic allochthons Limestones of Cambrian age -l ~ ~ Unconformity ~ ORDOVICIAN ;; AND CAMBRIAN til ~ Carbonate ~ocks ~ Unconformity '""'VV"'V............... CJ and outliers lOt New York State usage 20 KILOMETERS ! /0;~; lGNEOUS ROCKS HHust ~ault: teeth on upthrOWfl SI02"; [ill DEVONIAN iocatly mat~~d by zones of p~'fnonlte 1 OR OLDER Peeksvllie Granite J ~ Steeply dlPPI ng phytlonlte 20rte SILURIAN OR ORDOVICIAN 1Q~1 UPPER Nepheline syenite a! ORDOVICIAN o 8eemerville and lamprophyre dike j nstrumentalfy located earthquake epicenter Normal: Qf obti Que normal fault of pmbable jurassIC age barb vertical fault o N o00 281 FIELD STUDIES OF NEW JERSEY GEOLOGY AND GUIDE TO FIELD TRIPS On this map, numerous faults of both kinds are pre­ assume that classes of faults showing these features at sent within the Ramapo seismic zone. Definition of the surface in. a seismic area are the faults most likely faults in New York State is somewhat better than in associated with the current activity. The obverse argu­ New Jersey, especially within the Reading Prong, ment is that healed, more ductile faults that do not show because of recent mapping in a tier of quadrangles ex­ evidence of movement in an earthquake-generating tending from the Hudson River west to Greenwood mode are probably not responsible for producing Lake. Recent geologic mapping is somewhat limited in present-day earthquakes. the New Jersey Highlands, and investigations specifical­ lyon faults have not been conducted except locally. Acceptance of this premise has profound significance because it suggests that present-day seismicity in A series of previously unrecognized, interconnected crystalline terranes such as the northeastern United Mesozoic faults are now recognized as a result of recon­ States should be occurring in tectonic areas only where '1aissance studies over the past several years. The pro­ brittle fractures have in the proper attitude for reactiva­ bable interconnection of the Flemington and Hopewell tion by present stress fields. faults with the Ramapo fault system is worthy of note. Investigations in the Morristown-Bloomingdale to My present feeling is that we have not yet Chester area suggest that individual splays of the main demonstrated either from experiment, theory, or em­ Ramapo fault extend southwestward from a point just pirical observation that the premise restricting seismic north of Mohistown to interconnect with the Flem­ activity to surface brittle fracture zones is at all valid. ington fault south of Mendham. This zone of faulting The Ramapo seismic zone is the best locality, but coincides with a zone of concentrated earthquake ac­ perhaps the most complex area in the eastern U.S. in tivity in the Bloomingdale-Denville area (Fig. 0, which to empirically test this premise. Faults bounding the Green Pond syncline are inter­ Semiductile shear zones, expressed by phyllonite or preted to be Triassic and Jurassic, although reactivation diapthoritic phyHonite-blastomylonite are common of older faults is likely, especially on the Reservoir fault. throughout the Proterozoic rocks of the Hudson The Tuxedo Lake valley, however, is bordered by abun­ Highlands. A zone of especially intense concentration dantly block-faulted rocks that have zeolite-chlorite" of these faults is located in the Canopus zone north of· mineralization and irregular wear grooves typical of Peekskill. Detailed mapping has allowed tracing of Mesozoic faults. These rocks almost certainly mark a these faults northwestward toward Carmel, N. Y., and zone of Jurassic faulting. A similar zone extends north­ southwestward to near Boonton, where ductile shear eastward to Sebago Lake from the Sloatsburg area. zones are truncated by the Ramapo fault. Throughout this belt, metamorphic mineral fabrics found in fault The Ramapo fault itself extends from the Peapack­ zones indicate progressive increase in grade north-· Gladstone area northeastward
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