Preliminary bedrock geologic map of the Lahore 7.5-minute quadrangle, Orange, Spotsylvania, and Louisa Counties, Virginia

Burton, William C.- bburton@usgs.gov · 2019 · Report

Introduction Bedrock geologic mapping of the Lahore, Va., 7.5-minute quadrangle was completed as part of a broader project

This 2019 document, Preliminary bedrock geologic map of the Lahore 7.5-minute quadrangle, Orange, Spotsylvania, and Louisa Counties, Virginia, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

Pvq SOeg Oc Olg Old _Zb _Zh SOa PRELIMINARY BEDROCK GEOLOGIC MAP OF THE LAHORE 7.5-MINUTE QUADRANGLE, ORANGE, SPOTSYLVANIA, AND LOUISA COUNTIES, VIRGINIA

By William C. Burton Lake Anna Lake Anna Lake Anna North Anna River Run Berry Berry Run Pamunkey Creek Pamunkey Creek Creek Clear Rockey Little Creek Beverly Riga Run Riga Riga Run Run Terrys Terrys Run Run Run Terrys Run Horsepen Branch Pleasant Run Pleasant Run Run CARTER LN TATU M RD TATUM RD SHIRLEY RD MARQU IS RD MARQUIS RD MARQUIS RD BE RRY RUN R D DAYS BRIDGE RD ORANGE SPRIN G S RD ORANGE SPRINGS RD ARROW POINT DR COUNTY RD 654 SAINT JUST RD MON ROVIA RD MONRO IA RD INDEPE NDENCE RD WI N TE R LN LA KE POINTE DR KENDALL RD K END A L R D T OW ER R D THO RN H RD COUNTY RD 612 BELMO NT RD LAHORE RD LAHORE RD TE R RYS R U N RD BE N N E T F A R M RD PINEY WOODS RD P A M UNK E Y LN GOVE R NO RS POINT L N O A KW O OD D R MATTHEWS MILL RD MONTVUE LN DON RD OL D LAWYERS R D HETH DR GRASTY LN BUR R E LL DR E S LE R D A S TAWA Y BEAC H RD HENDE R SON LN CARTERS LN COOPER LN W OODL AWN FAR M DR APPL E LN DANIEL RD SU N N Y NO O K LN STAR VIEW LN BOSTON RD D AN EL S P O INT DR LANDS END DR SUNNYS IDE RD MILLVIEW R D SPOTSYLVANIA CO ORANGE CO ORANGE CO LOUISA CO SPOTSYLVANIA CO ORANGE CO Groomes Point Conway Point Bakers Point Frazers Point Dickersons Point Earls Point Bennetts Point Daniels Point Henrys Point Buzzard Woods Point Freemans Point Moores Point Simms Point Woodville Point Towsey Point Thornhill Danton Tatum Vulcan Dan Monrovia Lahore Sam Goodwins Cove Hickory Cut Graftons Bridge Pamunkey Bridge Frazers Bridge Days Bridge SOeg SOeg SOeg Old Old Old Oc Oc Oc _Zh _Zh _Zh _Zb _Zb _Zb Olg _Zb Oc Olg Gb Gt Gt Pvq _Zb SOa Pvq Pvq SOa SOa Gb Uf Uf Ts Ts Gt Gt 238000mE 238000mE 4224000mN 247000mE 77°52'30" 77°52'30" 77°55' 77°55' 77°57'30" 77°57'30" 78°00' 78°00' 38°15' 38°15' 38°12'30" 38°12'30" 38°10' 38°10' 38°07'30" 38°07'30" 4237000mN Garnet Garnet Garnet Biotite Biotite Biotite ROUTE 522 ROUTE 624 ROUTE 651 ROUTE 669 U.S. Department of the Interior U.S. Geological Survey FAULT BYRD MILL FAULT HOPAWAMSIC HARRIS CREEK FAULT Open-File Report 2019-1110 Terry's Run Berry Run Little Creek Byrd Mill fault (Taconic orogeny) Chopawamsic fault (Taconic orogeny) Intrusive and vein rocks Metasedimentary, metavolcanic, mafic, and ultramafic rocks Ellisville pluton Lahore pluton Chopawamsic Formation (volcanic arc) Aplite dikes CAMBRIAN TO NEOPROTEROZOIC(?) ORDOVICIAN LATE ORDOVICIAN SILURIAN TO ORDOVICIAN PERMIAN(?) CORRELATION OF MAP UNITS Pvq SOeg SOa SOa Olg Olg Old Olg Old Oc Oc _Zb _Zb _Zb _Zh _Zh SURFACE SURFACE A A FEET SEA LEVEL 2,000 3,000 3,000 1,000 1,000 2,000 A' A' FEET SEA LEVEL 2,000 3,000 3,000 1,000 1,000 2,000 BYRD MILL CHOPAWAMSIC FAULT FAULT Surficial deposits not shown NO VERTICAL EXAGGERATION 1000 feet 305 meters Accretionary wedge (Mine Run Complex melange zone III of Pavlides, 1989) Laurentian slope-rise metasediments (Mine Run Complex melange zone IV of Pavlides, 1989) Shores complex (Brown, 1986) ´ ´ Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government For sale by U.S. Geological Survey, Box 25286, Denver Federal Center, Denver, CO 80225; ://store.usgs.gov; 1-888-ASK-USGS (1-888-275-8747) Suggested citation: Burton, W.C., 2019, Preliminary bedrock geologic map of the Lahore 7.5-minute quadrangle, Orange, Spotsylvania, and Louisa Counties, Virginia: U.S. Geological Survey Open-File Report 2019-1110, 1 sheet, scale 1:24,000, ://doi.org/10.3133/ofr20191110. ISSN 2331-1258 (online) ://doi.org/10.3133/ofr20191110 Figure 1.—Stereonets (A-E ) and rose diagrams (F-G) showing results of the orientation of measured outcrop-scale planar and linear structures from mapped units in the Lahore quadrangle, Virginia, and from mapped units from the Ferncliff and Louisa quadrangles, Virginia. A, stereonet results of early Paleozoic (S1) schistosity in the Lahore quadrangle (contoured poles to planes); B, stereonet results of late Paleozoic (S2) schistosity in the Lahore quadrangle (contoured poles to planes); C, stereonet results of late Paleozoic (F2) fold axial planes in the Lahore quadrangle (poles to axial planes); D, stereonet results of mineral lineations, micro-crenulations, or mullion fabric associated with late Paleozoic (F2) fold axes in the Lahore quadrangle; E, contoured stereonet results of late Paleozoic (F2) fold hinges in the Ferncliff and Louisa quadrangles (from Burton and others, 2019); F-G, rose diagram results of joints in the Lahore quadrangle (F ) and joints located in the Ferncliff quadrangle (G) from Burton and others (2019); joints in G are located southeast of the "Ellisville neck" of the Ellisville pluton in the Ferncliff quadrangle. The numbers next to each principle peak (for example, 59°±8°) for joints in G corresponds to the average azimuth and standard deviation of the joint trend. The number of data measurements for each stereonet and rose diagram is indicated by "n." Stereonets and rose diagrams were plotted using the Structural Data Integrated System Analyzer (DAISY, version 5.05-8) software by Salvini (2015). MAGNETIC NORTH APPROXIMATE MEAN DECLINATION, 2019 TRUE NORTH VIRGINIA MAP LOCATION SCALE 1:24 000 CONTOUR INTERVAL 10 FEET NORTH AMERICAN VERTICAL DATUM OF 1988 Base from U.S. Geological Survey, Lahore, Virginia, 2016, scale 1:24,000 North American Datum of 1983 World Geodetic System of 1984 10,000-foot grid ticks based on Virginia coordinate system of 1983, north and south zones Projection and 1,000-meter Universal Transverse Mercator grid, zone 18 south Bedrock geology mapped by William C. Burton (2012-2015) GIS database and cartography by E. Allen Crider Jr. (2019) Cartography by Linda M. Masonic (2019) Edited by David A. Shields (2019) ° REFERENCES CITED Brown, W.R., 1986, Shores complex and mélange in the central Virginia Piedmont, in Neathery, T.L., ed., Southeastern Section of the Geological Society of America—Centennial field guide volume 6: Boulder, Colo., Geological Society of America, p. 209-214. Burton, W.C., Harrison, R.W., Malenda, H.F., Pazzaglia, F.J., and Crider, E.A., 2019, Geologic map of the Ferncliff and Louisa quadrangles, Louisa, Fluvanna, and Goochland Counties, Virginia: U.S. Geological Survey Scientific Investigations Map 2019-3429, 1 sheet, scale 1:24,000. [Also available at ://doi.org/10.3133/sim3429.] Burton, W.C., and Southworth, S., 2017, Discontinuities and connections in Piedmont and Blue Ridge terranes from Maryland to Virginia: Geological Society of America Abstracts with Programs, v. 49, no. 3. [Paper no. 4-7.] [Also available at ://doi.org/10.1130/abs/2017SE-290224.] Burton, W.C., Harrison, R.W., Spears, D.B., Evans, N.H., and Mahan, S., 2015, Geologic framework and evidence for neotectonism in the epicentral area of the 2011 Mineral, Virginia, earthquake, in Horton, J.W., Jr., Chapman, M.C., and Green, R.A., eds., The 2011 Mineral, Virginia, earthquake, and its significance for seismic hazards in eastern North America: Geological Society of America Special Paper 509, p. 391-406. [Also available at Burton, W.C., Spears, D.B., Harrison, R.W., Evans, N.H., Schindler, J.S., and Counts, R., 2014, Geology and neotectonism in the epicentral area of the 2011 M5.8 Mineral, Virginia, earthquake, in Bailey, C.M., and Coiner, L.V., eds., Elevating geoscience in the southeastern United States; New ideas about old terranes-Field guides for the GSA Southeastern Section Meeting, Blacksburg, Virginia, 2014: Geological Society of America Field Guide 35, p. 103-127, accessed July 30, 2019, at ://doi.org/10.

Carter, M.W., Burton, W.C., McAleer, R.J., DiGiacomo-Cohen, M.L., and Sauer, R.T., 2019, Geology of the Mineral and Lake Anna West quadrangles, Virginia: Virginia Division of Geology and Mineral Resources Publication 189, 41 p., 2 pls., scale 1:24,000. [Also available at ://www.dmme.virginia.gov/commerce/ProductDetails.aspx?product ID=3026.] Hughes, K.S., Hibbard, J.P., and Miller, B.V., 2013, Relationship between the Ellisville pluton and Chopawamsic fault—Establishment of significant late Ordovician faulting in the Appalachian Piedmont of Virginia: American Journal of Science, v. 313, no. 6, p. 584-612. [Also available at ://doi.org/10.2475/06.2013.03.] McAleer, R.J., Burton, W.C., Carter, M.W., Kunk, M.J., and Spears, D.B., 2017, Tectonothermal signatures of Taconian and Alleghanian orogenesis in the central Virginia seismic zone: Geological Society of America Abstracts with Programs, v. 49, no. 3. [Paper no. 10-7.] [Also available at ://doi.org/10.1130/abs/2017SE-290009.] Mixon, R.B., Pavlides, L., Powars, D.S., Froelich, A.J., Weems, R.E., Schindler, J.S., Newell, W.L., Edwards, L.E., and Ward, L.W., 2000, Geologic map of the Fredericksburg 30' x 60' quadrangle, Virginia and Maryland: U.S. Geological Survey Geologic Investigations Series Map I-2607, 2 pls., scale 1:100,000, 34-p. pamphlet. [Also available at ://pubs.er.usgs.gov/publication/i2607.] Pavlides, L., 1989, Early Paleozoic composite mélange terrane, central Appalachian Piedmont, Virginia and Maryland—Its origin and tectonic history, in Horton, J.W., Jr., and Rast, N., eds., Mélanges and olistostromes of the U.S. Appalachians: Geological Society of America Special Paper 228, p. 135-194. [Also available at ://doi.org/10.11 30/SPE228-p135.] Salvini, F., 2015, DAISY 3; The Structural Data Integrated System Analyser (version 5.05-8): Rome, Italy, Roma Tre University, Department of Geological Sciences, software available at ://host.uniroma3.it/pro getty/fralab/. Sinha, A.K., Thomas, W.A., Hatcher, R.D., Jr., and Harrison, T.M., 2012, Geodynamic evolution of the central Appalachian orogeny— Geochronology and compositional diversity of magmatism from Ordovician through Devonian: American Journal of Science, v. 312, no. 3, p. 907-966. [Also available at ://www.ajsonline. org/content/312/8/907.full.] Tarr, A.C., and Wheeler, R.L., 2006, Earthquakes in Virginia and vicinity 1774-2004: U.S. Geological Survey Open-File Report 2006-1017, 1 sheet, scale 1:1,000,000. [Also available at ://pubs.er. usgs.gov/publication/ofr20061017.] Tuttle, M., Carter, M.W., and Dunahue, J., 2015, Paleoliquefaction study of the earthquake potential of the central Virginia seismic zone (CVSZ): Geological Society of America Abstracts with Programs, v. 47, no. 7, p. 466. [Also available at ://gsa.confex.com/gsa/2015AM/web program/Paper265553..] Early Paleozoic (S1) schistosity Late Paleozoic (S2) schistosity Joints Joints (from Burton and others, 2019) Late Paleozoic (F2) fold axial planes Mineral lineations, micro-crenulations, or mullion fabric associated with late Paleozoic (F2) fold axes A B D F G 0% 5% 10% 15% 19% Contours 0% 2% 4% 6% 8% 10% 12% 14% Contours N 90° 180° 270° N 180° 270° 0% 2% 4% 6% 8% 10% 12% E Contours Late Paleozoic (F2) fold hinges (from Burton and others, 2019) N 180° 270° N 90° 180° 270° 331°±14° 291°±17° 43°±4° 16°±6° 59°±8° n=405 n=8 n=68 n=89 n=77 n=15 N 90° 180° 270° 320°±10° 286°±5° 226°±5° 48°±4° 14°±17° 66°±8° n=156 DESCRIPTION OF MAP UNITS [Minerals were identified in hand sample only and are listed in increasing order of abundance] INTRUSIVE AND VEIN ROCKS Vein quartz (Permian?)—White, massive, coarse-grained vein quartz Granodiorite of Ellisville pluton (Silurian to Ordovician)— Pale-yellow to white, massive to locally foliated, fine-to medium-grained, biotite-microcline-quartz-plagioclase granodiorite. Uranium-lead (U-Pb) zircon crystallization ages of 444±3 mega-annum (Ma, million years before present) and 437±4 Ma are reported by Hughes and others (2013) Aplite dikes (Silurian to Ordovician)—Pink to white, medium-finegrained to locally coarse-grained, quartz-plagioclase-potassium feldspar aplite dikes. The dikes crosscut hornblende diorite (Old) and granodiorite (Olg) of the Lahore pluton, indicating they are possibly contemporaneous with granodiorite of Ellisville pluton (SOeg). The dikes are usually less than 1 meter wide, but two larger outcrops are mapped in the northwest part of the map area Biotite diorite and granodiorite of Lahore pluton (Late Ordovician)—Light-to medium-gray weathering, greenish-gray when fresh, massive to locally well foliated, medium-grained biotite diorite and granodiorite; represents a felsic to intermediate phase of Lahore pluton. A U-Pb zircon crystallization age on monzonite of 446±5 Ma is reported by Sinha and others (2012) Hornblende diorite of Lahore pluton (Late Ordovician)—Light-to medium-gray weathering, greenish-gray when fresh, massive to locally well foliated, medium-grained hornblende diorite (±pyroxene) and minor gabbro METASEDIMENTARY, METAVOLCANIC, MAFIC, AND ULTRAMAFIC ROCKS Chopawamsic Formation (Ordovician)—Medium-to dark-gray, fine-grained, ±muscovite-biotite-quartz-plagioclase granofels and schist; and lesser pale-yellow, fine-grained, muscovite-potassium feldspar(?)-quartz-plagioclase metafelsite. Garnet was identified in hand sample at locations identified as "Gt" on map Metagraywacke, schist, and metasiltstone of Shores complex (Brown, 1986) (Cambrian to Neoproterozoic?)—Medium-to dark-gray, fine-grained, muscovite-biotite-quartz-plagioclase metagraywacke and schist, with minor quartz-laminated metasiltstone. Garnet was identified in hand sample at locations identified as "Gt" on map. Shores complex named by Brown (1986) for exposures along the James River in Virginia Mafic and ultramafic rocks of Shores complex (Cambrian to Neoproterozoic?)—Dark-green, fine-grained, poorly foliated amphibolite; and well-foliated ±chlorite-amphibole schist. Contains rare bodies of massive, medium-grained gabbro ("Gb" on map). Includes isolated exposures within unit _Zsm of white, fine-grained, well-foliated talc schist ("Ts" on map) and two locations of ultramafic rock ("Uf" on map); and (in float only) massive, dark-green, fine-grained serpentinite(?) Byrd Mill formation (informal name) of Shores complex (Cambrian to Neoproterozoic?)—Gray, fine-grained, finely foliated to layered, ±muscovite±chlorite-biotite-plagioclase-quartz metasiltstone and metagraywacke Mafic body in Byrd Mill formation (informal name) of Shores complex (Cambrian to Neoproterozoic?)—Finely layered chlorite-epidote-amphibole schist. Gabbro is present in nearby float Hardware formation (informal name of N.H. Evans, written commun., 2017) (Cambrian to Neoproterozoic?)—Pale-yellow to white, fine-grained, sericite-quartz phyllite. The unit locally contains fine crystals of magnetite INTRODUCTION Bedrock geologic mapping of the Lahore, Va., 7.5-minute quadrangle was completed as part of a broader project, undertaken jointly between the U.S. Geological Survey, the Virginia Division of Geology and Mineral Resources, and other Federal and State agencies to better understand the causative mechanisms of the magnitude-5.8 (M5.8) earthquake that occurred near Mineral, Va., on August 23, 2011. This project involves detailed mapping of at least eight quadrangles in the epicentral region of the Mineral, Va., earthquake in order to improve our understanding of the geologic framework of the central Virginia seismic zone, which has a long record of historical and prehistoric seismicity (Tarr and Wheeler, 2006; Tuttle and others, 2015). Preliminary mapping results are summarized in Burton and others (2014, 2015). The Lahore 7.5-minute quadrangle contains the contact between Ordovician to Silurian, dioritic and granodioritic rocks of the Lahore (Olg, Old) and Ellisville plutons (SOeg) and older metasedimentary and metavolcanic rocks. The older metasedimentary and metavolcanic rocks include the Ordovician Chopawamsic Formation (Oc) to the east (of volcanic arc affinity) and informally named units to the west that were previously mapped as parts of mélange zones III and IV of the Mine Run Complex of Pavlides (1989), which include the following map units: (1) the Hardware formation (_Zh) of N.H. Evans, written commun., 2017; (2) the Byrd Mill formation (_Zbm and _Zb) of the Shores complex of Brown (1986); and (3) units _Zsm and _Zsum, also within the Shores complex of Brown (1986). The Lahore quadrangle is northeast of the Ferncliff and Louisa, Va., quadrangles, where the Shores complex is intruded by the Ellisville pluton (SOeg) along the pluton's southwestern margin (Burton and others, 2019). The new mapping in the Lahore quadrangle shows that the Shores complex continues northeast of the Ellisville pluton. A northeast-trending mafic-and ultramafic-bearing belt within the Shores complex (_Zb, _Zbm, and _Zh) is a fault-bounded accretionary zone (accretionary wedge) between rocks of the Chopawamsic Formation (Oc) and Laurentian slope-and-rise deposits (_Zsm and _Zsum) (Burton and others, 2019); this tectonic boundary extends from at least the James River to the south, to the Maryland Piedmont to the north (Burton and Southworth, 2017). In the Lahore quadrangle, this belt contains several mappable, northeast-to southwest-trending mafic bodies (_Zsum) and also includes small exposures of gabbro (Gb) and talc schist (Ts), as well as ultramafic float (Uf); just to the northeast of the Lahore quadrangle this belt contains an inactive serpentinite quarry in Verdiersville, Va. (Mixon and others, 2000). The Lahore quadrangle contains structures of both early-and late- Paleozoic age (fig. 1) that correspond to the Taconic and Alleghanian orogenies, respectively. Taconic (Late Ordovician) S1 schistosity in layered rocks is typically fine-grained and parallel to compositional layering, when EXPLANATION OF MAP SYMBOLS Contact—Approximately located. In cross section, dotted where projected above the ground surface FAULTS Fault—Approximately located; dashed where concealed by water. Harris Creek fault of late Paleozoic (Alleghanian orogeny) from Burton and other (2019) Taconic thrust fault—Location is inferred and based on the presence of ultramafic bodies; early Paleozoic age (Taconic orogeny) and overturned in the late Paleozoic (Alleghanian orogeny). In cross section, dotted where projected above ground surface and dashed below ground surface PLANAR AND LINEAR FEATURES Strike and dip of first-generation schistosity (S1)—Defined by prograde metamorphic minerals, typically biotite, during regional metamorphism of metasedimentary and metavolcanic rocks during the Late Ordovician (Taconic orogeny); biotite is parallel to compositional layering, where present. Late Ordovician age is based on 40Ar/39Ar mineral age data from Burton and others (2019) Inclined Vertical Strike and dip of foliation in plutonic rocks—Defined by planar-aligned mafic minerals, typically biotite or amphibole. Probably an igneous flow foliation of early Paleozoic age, but may have tectonically-influenced mineral orientation near contacts due to synkinematic timing of intrusion Inclined Vertical Strike and dip of second-generation schistosity (S2)—Defined by retrograde metamorphic micas, typically muscovite, during the late Paleozoic (Alleghanian orogeny). Late Paleozoic (Pennsylvanian) age is based on 40Ar/39Ar mineral age data from Burton and others (2019) Inclined Vertical Strike and dip of axial plane of second-generation fold (F2)—Defined as plunging open folds to tight crenulations that are late Paleozoic age (Alleghanian orogeny) Inclined Vertical Trend and plunge of F2 fold axis—Represented by mineral lineations, micro-crenulations, or mullion fabric. Combined with either S1 or S2 schistosity or the F2 axial plane. May indicate tectonic transport direction where steeply plunging Taconic biotite isograd—Represents the first appearance (southeast side of isograd) of biotite in regional metamorphic mineral assemblages Taconic garnet isograd—Represents the first appearance (southeast side of isograd) of garnet in regional metamorphic mineral assemblages Outcrop—Rock outcrop location examined in the map area present. Alleghanian (Pennsylvanian) S2 schistosity is coarser and more micaceous than S1, and is locally accompanied by a lineation that is represented by mineral lineations, micro-crenulations, or mullion fabric, and represents the hinges of F2 folds. These lineations are more steeply plunging (fig. 1D) than their counterparts in the Ferncliff and Louisa quadrangles (Burton and others, 2019) (fig. 1E), which is perhaps a function of the interaction of the Ellisville pluton (SOeg) with a dextral-transpressive stress field (Burton and others, 2019). A foliation in the plutonic rocks is represented by an equilibrium assemblage of aligned mafic minerals and is early Paleozoic in age, possibly representing an igneous flow foliation that was locally affected by tectonic stresses during synkinematic intrusion. Regionally, the most common trend and plunge of joints is northwest and subvertical, respectively, and orthogonal to the regional strike of foliation (Burton and others, 2019); however, the diversity of joint directions shown in the rose diagram of joint azimuths in the Lahore quadrangle (fig. 1F) resembles the joint pattern southeast of the "Ellisville neck" of the Ellisville pluton in the Ferncliff quadrangle, where early Mesozoic extension is thought to be a factor (Burton and others, 2019) (fig. 1G). Early Mesozoic extension may have also reactivated the Harris Creek fault, a late Paleozoic (Alleghanian orogeny) transpressional fault that marks the contact between granodiorite of the Ellisville pluton (SOeg) and the Chopawamsic Formation (Oc) (Burton and others, 2019). The Harris Creek fault, in the extreme southeast corner of the map, is a continuation of the fault mapped in the adjacent Mineral, Va., quadrangle by Carter and others (2019). Metamorphic grade in the non-plutonic rocks of the Lahore quadrangle ranges from lower-greenschist to the northwest to upper-greenschist to the southeast, as represented by mineral assemblages in non-plutonic rocks. The biotite isograd may be, in part, lithologically controlled by the contact between the informally-named Hardware (_Zh) and Byrd Mill (_Zb) formations, and locally affected by contact metamorphism by the Lahore pluton (Old and Olg, in western part of map). The Taconic garnet isograd is defined by the sparse presence of small (<1 millimeter), euhedral garnet crystals. Both the biotite and garnet isograds continue along strike to the southwest into the Ferncliff and Louisa quadrangles (Burton and others, 2019), where the isograds have been identified as Ordovician age (Taconic orogeny) based on muscovite, biotite, and amphibole 40Ar/39Ar cooling ages reported by McAleer and others (2017). 90° 180° 270° N 90° 180° 270° 90° 90° N FEET (VA S) FEET (VA N) FEET (VA N) FEET (VA S) 630000 FEET (VA N) FEET (VA N) FEET (VA S) FEET (VA S)