Evidence for Differential Unroofing in the Adirondack Mountains, New

Home , Adirondack Mountains, Dix Mountain, Mount Marcy

Evidence for Differential Unrooﬁng in the Adirondack Mountains, New York State, Determined by Apatite Fission-Track Thermochronology

Mary K. Roden-Tice, Steven J. Tice, and Ian S. Schoﬁeld1

Center for Earth and Environmental Science, Plattsburgh State University, Plattsburgh, New York 12901, U.S.A. (e-mail: [email protected])

ABSTRACT Apatite fission-track ages of 168–83 Ma for 39 samples of Proterozoic crystalline rocks, three samples of Cambrian Potsdam sandstone, and one Cretaceous lamprophyre dike from the Adirondack Mountains in New York State indicate that unroofing in this region occurred from Late Jurassic through Early Cretaceous. Samples from the High Peaks section of the Adirondack massif yielded the oldest apatite fission-track ages (168–135 Ma), indicating that it was exhumed first. Unroofing along the northern, northwestern, and southwestern margins of the Adirondacks began slightly later, as shown by younger apatite fission-track ages (146–114 Ma) determined for these rocks. This delay in exhumation may have resulted from burial of the peripheral regions by sediment shed from the High Peaks. Apatite fission-track ages for samples from the southeastern Adirondacks are distinctly younger (112–83 Ma) than those determined for the rest of the Adirondack region. These younger apatite fission-track ages are from a section of the Adirondacks dissected by shear zones and post-Ordovician north-northeast-trending normal faults. Differential unroofing may have been accommodated by reactivation of the faults in a reverse sense of motion with maximum

compressive stress, j1, oriented west-northwest. A change in the orientation of the post–Early Cretaceous paleostress ﬁeld is supported by a change in the trend of Cretaceous lamprophyre dikes from east-west to west-northwest.

Introduction The Adirondack Mountains are an elongate dome- Isachsen (1975) considered the Adirondack like exposure of Grenville (ca. 1.0–1.35 Ga; Mc- Mountains to be an anomalous feature on the east- Lelland et al. 1988; McLelland and Chiarenzelli ern North American craton because of their com- 1990; Mezger et al. 1991; McLelland et al. 1996) paratively large areal extent (∼27,000 km2) and pres- high-grade metamorphic rocks in northern New ent high elevation (up to 1600 m) compared with York State (ﬁg. 1; Isachsen and Fisher 1970; Mc- other uplifted areas on the craton. The Paleozoic Lelland and Isachsen 1986). On the basis of struc- unrooﬁng history of the Adirondack region has ture and physiography, the Adirondack Mountains been studied through subsurface mapping based on can be divided into two regions: (1) the northwest drill core data (Rickard 1969, 1973). Isopach maps Lowlands, which are composed mainly of meta- for Cambrian through Early Ordovician strata sedimentary rocks and are continuous with the (Rickard 1973) and the presence of Paleozoic inliers near Piseco Lake and the Sacandaga River (Fisher larger area of Grenville-age rocks across the Fron- et al. 1970) indicate sedimentation in the Adiron- tenac Arch in southern Ontario (McLelland and dack region during this time. A period of brief uplift Isachsen 1986; Mezger et al. 1991), and (2) the in the Early Ordovician was followed by burial dur- Adirondack Highlands, which consist of granulite- ing the Middle Ordovician (Rickard 1973). Isopachs facies metaplutonic rocks and metasediments for Silurian and Devonian strata (Rickard 1969, (McLelland and Isachsen 1986). 1973) showed north-northeasterly trends abruptly terminated at the southern border of the Adiron- Manuscript received July 13, 1999; accepted October 14, 1999. dack massif, suggesting that the isopachs originally 1 Current address: Department of Geology and Geophysics, crossed the Adirondacks during these times. This University of Utah, Salt Lake City, Utah 84112-0111, U.S.A. evidence and the occurrence of several down-

155 -of fission 10%ע Figure 1. Apatite fission-track ages in Ma for samples from the Adirondack region contoured to highlight fission-track age trends. Standard error is track age. Mapped normal faults, shear zones, and lineaments are indicated by dashed lines. Key for symbols for Precambrian, Cambrian, and Cretaceous samples is given on the map. The inset shows a location map for the Adirondack region and fission-track samples in New York State. Journal of Geology ADIRONDACK MOUNTAINS DIFFERENTIAL UNROOFING 157 dropped Paleozoic fault blocks within the south surfaces. Spontaneous fission tracks in apatite were Њ central Adirondacks indicates a post-Devonian up- revealed by etching for 20 s in 5M HNO3 at 21 C. lift for the Adirondack Mountains. The samples were irradiated at the Oregon State Heitzler and Harrison (1998) presented 40Ar/39Ar University TRIGA reactor, using a nominal flux of K-feldspar ages for the Adirondack region that sug- 8 # 1015 n/cm2 for apatite. The neutron flux was gest localized reheating in the eastern Adirondacks monitored by CN1 dosimeter glasses at the top and during the Ordovician. Their data indicate that this bottom of the irradiation tube. The Cd ratio (rela- reheating may result from the combined effects of tive to an Au monitor) for this reactor is 14, indi- burial and hot fluid migration along normal faults cating that the reactor is well thermalized (Green in the eastern Adirondacks during the Taconic and Hurford 1984). orogeny. In addition, 40Ar/39Ar K-feldspar ages from Fission-track ages were calculated using a the crystalline basement in eastern Adirondacks weighted mean z calibration factor (Hurford and (Heitzler and Harrison 1998) are consistent with Green 1983) based on Fish Canyon Tuff, Durango, studies that suggest Carboniferous burial in eastern and Mount Dromedary apatite standards (Miller et New York (Harris et al. 1978; Friedman and Sanders al. 1985). The following z factors were used in the 1982; Johnsson 1986; Miller and Duddy 1989). AFT age calculations (names in parentheses are the Preliminary apatite fission-track (AFT) ages rang- research assistants responsible for ascertaining the ,(M. Roden-Tice) 2.3 ע ing from 147 to 86 Ma determined by Miller and preceding z factor):103.4 ,(R. Sents) 6.5 ע I. Schofield), 88.7)4.7 ע Lakatos (1983) indicated an Early Cretaceous un- 98.3 -P. Rabi) 8.0 ע D. Michaud), and 140.7)5.0 ע roofing history for the Adirondack region. In ad- 87.1 dition, Isachsen (1975, 1981) has presented contro- deau). The z used for each age determination is in- versial evidence for a recent and ongoing rapid dicated on table 1. The central age was calculated uplift rate of ∼2–3 mm/yr in the Adirondacks. This according to Galbraith and Laslett (1993), using the conclusion is based largely on the results of optical x2 method of Brandon (1992). The AFT age deter- resurveys of survey lines across the eastern and cen- minations and track-length measurements were tral Adirondacks from Saratoga to Rouses Point made dry on an Olympus BMAX 60 microscope (1973) and Utica to Fort Covington (1981). Inferred at #1600. This microscope is equipped with a Quaternary neotectonic activity (Isachsen 1975, drawing tube for length measurements, which are 1981) is supported by the observation of offset bore- digitized on a Cal Comp Model 31120 Drawing holes (Fox et al. 1999) and historical low-magnitude Slate that is interfaced with a 386 Zenith PC com- (3.4–5.3) seismicity in the central and northern Ad- puter. Table 1 lists the number of track lengths irondack regions (Sbar and Sykes 1973, 1977; Seeber measured in each analysis. and Armbruster 1989; Revetta et al. 1999). Apatite Fission-Track Results. Apatite fission- The potential for active uplift in the Adirondack track thermochronology has become a standard area prompted a reexamination and expansion of technique for investigating thermal and burial his- the known Mesozoic unroofing history using AFT tories of sedimentary basins (Miller and Duddy ages as a baseline for the ancient uplift. This study 1989; Ravenhurst et al. 1994; Carter et al. 1998) presents AFT ages from Proterozoic crystalline and the unroofing histories of ancient (Crow- rocks, Cambrian Potsdam sandstone, and a Creta- ley 1991; Corrigan et al. 1998) and active orogens ceous lamprophyre dike that suggest differential (Blythe and Kleinspehn 1998; Brandon et al. 1998). unroofing occurred in the Adirondack region during Apatite fission-track analysis includes both the de- the Late Jurassic to Late Cretaceous (fig. 1; table termining of an AFT age and the modeling of time- 1). temperature histories based on the measured fission-track-length distributions. The fission-track technique is based on the formation of damage Methods zones resulting from the spontaneous fission of nat- Apatite for fission-track analyses was isolated by urally occurring 238U in U-bearing minerals such as standard heavy liquid and magnetic separation apatite. Fission tracks are retained in apatite below 20ЊC (Wagnerעtechniques after disaggregation from 2–3-kg sam- a closure temperature of100Њ ples. Sample preparation procedures for AFT age 1968; Naeser and Faul 1969; Naeser 1981). If apatite and track-length measurements are described by grains are heated above their closure temperature Gleadow (1984). The external detector method was for times on the order of 1 m.yr., then all existing used for AFT age determinations. Small aliquots, fission tracks will be annealed, and the fission- 10–20 mg of apatite, were mounted in epoxy on track age will be reset. In the case of complete an- glass slides and polished to expose internal grain nealing, the AFT age provides a cooling age re- 158 M. RODEN-TICE ET AL.

Table 1. Apatite Fission-Track Data for the Adirondack Region, New York

/conﬁdence Number of Mean track Standard Number of Latitude 95%ע AFT age Locality (Ma) interval grains U (ppm) length (mm) deviation (mm) tracks longitude Region

HP 44.12/73.92 70 2.0 2.עMount Marcy 168.1 ϩ19.9/Ϫ17.8 19 16.3 13.1 HP 44.08/73.79 88 1.1 1.עDix a 161.1 ϩ24.2/Ϫ21.1 20 18.3 13.7 HP 43.87/74.43 97 1.5 2.עBlue Mountain Lake 146.0 ϩ16.3/Ϫ14.7 20 48.5 13.0 HP 44.37/73.87 102 1.5 1.עWhiteface 141.8 ϩ15.8/Ϫ14.2 20 66.6 13.2 Van der Wacker 148.3 ϩ17.6/Ϫ15.7 20 16.0 ND ))43.90/74.09 HP HP 44.24/73.70 105 1.6 2.עHurricane 149.4 ϩ15.9/Ϫ14.4 20 58.1 13.5 Saranac Lake b 158.5 ϩ40.1/Ϫ32.1 20 10.0 ND ))44.27/74.36 HP Giant a 134.5 ϩ28.7/Ϫ23.7 10 15.9 ND ))44.16/73.70 HP SE 44.11/73.69 76 1.3 1.עNorth Fork Boquet a 106.8 ϩ17.0/Ϫ14.7 10 21.1 13.0 N 44.71/73.87 102 1.1 1.עLyon Mountain 138.7 ϩ13.5/Ϫ12.3 20 76.2 13.2 Terry Mountain 145.6 ϩ13.1/Ϫ12.0 20 56.1 ND ))44.57/73.63 N Vermontville 114.2 ϩ21.7/Ϫ18.2 10 31.1 ND ))44.55/74.05 N Alder Brook 122.2 ϩ11.7/Ϫ10.7 20 33.5 ND ))44.45/74.07 N N 44.86/73.67 61 1.4 2.עAltona Flatrock (ss) 124.5 ϩ17.5/Ϫ15.4 15 24.1 13.7 Chateguay (ss) 143.2 ϩ21.4/Ϫ18.6 20 22.0 ND ))43.93/75.38 N NW 44.44/75.30 102 1.7 2.עKents Corners 131.5 ϩ20.8/Ϫ18.0 15 20.1 13.0 Clifton 126.4 ϩ22.7/Ϫ19.3 12 68.2 ND ))44.23/74.78 NW Fine 135.2 ϩ12.8/Ϫ11.7 20 56.3 ND ))44.25/75.14 NW SW 43.59/75.84 94 1.5 2.עPort Leyden 135.9 ϩ15.1/Ϫ13.6 20 16.4 13.0 Brantingham Lake 140.7 ϩ17.9/Ϫ15.9 12 77.3 ND ))43.69/75.3 SW SW 43.36/75.04 100 1.0 1.עHinckley Reservoir 125.2 ϩ13.2/Ϫ11.9 20 43.1 13.6 Old Forge 143.1 ϩ15.8/Ϫ14.3 20 40.2 ND ))43.71/74.97 SW SW 43.04/74.86 101 1.5 1.עLittle Falls 130.1 ϩ12.9/Ϫ11.7 20 50.1 12.5 Lake Pleasant 138.8 ϩ17.2/Ϫ15.3 20 26.1 ND ))43.48/74.4 SW Gilmantown a 135.2 ϩ17.5/Ϫ15.5 20 65.8 ND ))43.43/74.31 SW Moose Mountain 130.8 ϩ14.5/Ϫ13.1 20 24.3 ND ))43.50/74.31 SW North Creek 132.5 ϩ19.2/Ϫ16.8 21 36.6 ND ))43.68/73.99 SC Minerva a 122.9 ϩ18.2/Ϫ15.9 19 22.8 ND ))43.81/74.01 SC Baker Brook 112.4 ϩ14.3/Ϫ12.7 20 32.6 ND ))43.60/74.06 SE Johnsburg c 104.1 ϩ18.8/Ϫ15.9 19 39.3 ND ))43.62/73.96 SE Pharaoh Mountaind 98.5 ϩ14.8/Ϫ12.9 20 46.4 ND ))43.82/73.65 SE Crane Mountain 96.1 ϩ12.5/Ϫ11.1 20 40.0 ND ))43.54/73.65 SE Fort Ann (ss) 91.9 ϩ12.2/Ϫ11.2 20 19.5 ND ))43.41/73.52 SE SE 43.4/73.71 101 1.3 1.עLake George 93.9 ϩ9.4/Ϫ8.6 20 64.3 13.6 SE 43.85/73.59 62 8. 1.עTiconderoga 82.7 ϩ12.5/Ϫ10.9 15 16.2 13.5 SE 43.79/73.79 80 1.3 1.עSchroon Lake 104.5 ϩ11.7/Ϫ10.5 20 38.0 13.3 North Hudson 98.1 ϩ11.4/Ϫ10.2 20 29.3 ND ))43.95/73.74 SE Blue Ridge 121.5 ϩ14.3/Ϫ12.8 20 16.9 ND ))43.96/73.78 SE E 44.05/73.46 99 1.4 1.עCraig Harbor 123.2 ϩ13.5/Ϫ12.2 20 36.2 14.0 E 44.27/73.32 103 1.4 1.עSplit Rock Point 119.6 ϩ12.8/Ϫ11.6 20 71.3 13.7 Willsboro Dike (l) 115.5 ϩ20.7/Ϫ17.6 20 11.0 ND ))44.45/73.38 E Mount Trembleau 127.0 ϩ14.4/Ϫ12.9 18 54.7 ND ))44.01/73.40 E E 44.40/73.52 40 9. 1.עPoke-O-Moonshine 106.7 ϩ12.9/Ϫ11.6 17 91.3 13.3 for CN1 glass (MR-T) except for samples 2.3 ע Note. Apatite FT ages given as central ages (Galbraith and Laslett 1993).z = 103.4 with footnotes a, b, c, and d.ND = not determined. Lamprophyre and sandstone are designated by (l) and (ss), respectively. Adirondack regions are indicated by HP (High Peaks), SE (southeast), N (north), NW (northwest), SW (southwest), SC (south central), and E (east). .I. Schoﬁeld, undergraduate assistant ;4.7 ע a z = 98.3 .P. Rabideau, undergraduate assistant ;8.0 ע b z = 140.8 .R. Sents, undergraduate assistant ;6.5 ע c z = 88.7 .D. Michaud, undergraduate assistant ;5.0 ע d z = 87.1

cording the time the rock containing the apatite the magnitude of denudation (Brown 1991; Brandon passed through its closure temperature. For the et al. 1998). most common type of apatite, F-rich apatite, the Confined track-length measurements in apatite, temperature range of ∼60Њ–100ЊC is referred to as combined with the AFT age, help constrain the the partial annealing zone, or PAZ. By assuming a cooling history of a rock below 100ЊC. Track-length geothermal gradient and a surface temperature, the distributions for apatite from rapidly cooled rocks, apatite PAZ can be used to infer burial depth and such as volcanics, yield characteristic long mean Journal of Geology ADIRONDACK MOUNTAINS DIFFERENTIAL UNROOFING 159 track lengths and small standard deviations ranging tively; fig. 2b; table 1). Dix Mountain gave a longer mm (Gleadow et al. 1986). mean track length (13.7 mm) and smaller standard (1.3–0.8ע) from 14 to 15 In contrast, apatite from rocks that have cooled deviation (1.1 mm) than the other High Peak sam- slowly through the apatite PAZ will yield shorter ples (table 1). These data suggest that Dix Mountain mean track lengths and larger standard deviations spent a shorter residence time in the apatite PAZ .mm (Gleadow et al. compared with the other High Peak samples (2–1ע) in the range of 12–14 1986). Experimental annealing studies of fission In the northwest Adirondack lowlands, three tracks in apatite (Laslett et al. 1987; Carlson 1990; samples of Proterozoic gneiss yielded slightly Crowley et al. 1991) have yielded the basis for al- younger AFT ages, 126–135 Ma, than those from gorithms used to calculate model thermal histories the central High Peaks region (fig. 1; table 1). Ap- for samples within the apatite PAZ (Green et al. atite yields from the west-central Adirondacks 1989; Crowley 1993; Issler 1996). An AFT age and were poor, and no AFT ages could be determined track-length distribution can be used to constrain for samples from this area. Samples from along the the low-temperature portion of the rock’s thermal southwestern margin of the Adirondack massif history by calculating model time-temperature gave a range in AFT ages from 125 Ma for a meta- paths from the rock’s cooling age to the present. sedimentary sample at Hinckley Reservoir to 141 Miller and Lakatos (1983) determined AFT ages Ma for a nelsonite at Port Leyden (fig. 1; table 1). ranging between 147 and 86 Ma for five anorthosite North of the High Peaks region, the AFT ages de- samples from the Adirondacks, three from the High crease slightly, ranging from 114 Ma near Ver- Peaks region, and two (one surface and one drill montville to 146 Ma at Terry Mountain (fig. 1; table core) from Wadhams in the Lake Champlain low- 1). Two samples of Cambrian Potsdam sandstone lands. This study determined AFT ages for 39 sam- from north of the Adirondack area yielded com- ples of Proterozoic gneisses, anorthosites, and parable AFT ages to those determined for the crys- metasediments; three samples of Cambrian Pots- talline rocks, 125 and 143 Ma (fig. 1; table 1). All dam sandstone; and one sample of an Early Cre- of the AFT ages determined for samples from areas taceous lamprophyre dike from throughout the Ad- north, northwest, and southwest surrounding the irondack region (fig. 1; table 1). This geographically central High Peaks are consistent with unroofing widespread survey was done to expose any areas of during the Early Cretaceous (fig. 1; table 1). differential uplift/unroofing within the Adiron- Track-length distributions for samples from the dacks. peripheral regions of the Adirondacks, Kents Cor- The AFT ages determined in this study range ners, Hinckley Reservoir, Little Falls, and Lyon Mountain yielded mean track lengths of 12.5–13.6 10%ע∽ from 168 to 83 Ma, with a standard error of of the FT age (fig. 1; table 1). The High Peaks and mm and standard deviations of 1.1–1.7 mm (fig. 3a, central portion of the Adirondack Mountains 3b; fig. 4a,4b; table 1). Most of these track-length yielded the oldest AFT ages, 168–135 Ma, suggest- distributions (e.g., Little Falls: fig. 4a; table 1), sug- ing that this region was unroofed earliest, begin- gest slow cooling and prolonged residence in the ning in the Middle Jurassic and extending into the apatite PAZ. The sample from Hinckley Reservoir, Early Cretaceous (fig. 1; table 1). Confined track- which is located along the southwestern border of length measurements for five samples from the the Adirondack crystalline terrane, yielded a longer central High Peaks region yielded mean track mean track length of 13.6 mm and a smaller stan- lengths ranging from 13.0 to 13.7 mm and standard dard deviation of 1.0 mm than the other samples deviations of 1.1–2.0 mm (table 1). Mount Marcy, (fig. 3b; table 1). These results suggest that the the highest peak, with an elevation of 1710 m, gave Hinckley Reservoir sample experienced a slightly the oldest AFT age of 168 Ma (fig. 1; table 1) and different cooling history from the others, including a negatively skewed track-length distribution with less time spent in the apatite PAZ. a distinct tail of short tracks (i.e., !10 mm; fig. 2a), Distinctly younger AFT ages, 112–83 Ma, were indicating slow cooling through the apatite PAZ. determined for samples from the southeastern Ad- Whiteface (1460 m) and Dix Mountains (1457 m), irondacks, including the summits of Pharaoh the fifth and sixth highest summits in the Adiron- Mountain (767 m, 99 Ma) and Crane Mountain (966 dack anorthosite massif, also yielded Middle to m, 96 Ma) and samples from lower elevations as Late Jurassic cooling ages, 142 and 161 Ma, re- far east as Fort Ann (92 Ma; fig. 1; table 1). This spectively (fig. 1; table 1). The track-length distri- part of the Adirondack massif is cut by numerous bution for Whiteface Mountain yielded a similar north-northeast-trending high-angle normal faults, mean track length and standard deviation to that shear zones, and lineaments (Isachsen and Fisher mm, respec- 1970), including the graben containing Lake George 1.5ע and 2.0 ע for Mount Marcy (13.1 160 M. RODEN-TICE ET AL.

Figure 2. Measured (histogram) and calculated (curve) apatite fission-track-length distributions at the 0.05 level, using AFTINV32 (Issler 1996) for samples from the Adirondack High Peaks region: a, Mount Marcy; b, Whiteface Mountain. Model thermal history at 0.05 level using Crowley et al. (1991) annealing model and AFTINV32 (Issler 1996) for samples from the Adirondack High Peaks region: c, Mount Marcy; d, Whiteface Mountain. and Schroon Lake (fig. 1). Samples, from opposite ture histories were calculated for eight Adirondack sides of mapped faults or shear zones that yielded samples, using measured track-length data, the offset AFT ages, include Baker Brook (112 Ma)/ Crowley et al. (1991) six-parameter annealing equa- Moose Mountain (131 Ma) and North Hudson (98 tion for Durango apatite, and the inverse model of Ma)/Blue Ridge (122 Ma; fig. 1; table 1), with the Issler (1996). The inverse model, AFTINV32, gen- younger age occurring on the eastern side of the erates a set of random forward-model thermal so- faults. Although these AFT ages overlap within 1j lutions that are used to calculate apatite fission- error, all of the AFT ages in the southeastern Ad- track age and length parameters to compare with irondacks are ≤112 Ma, indicating a regional trend the measured data (Issler 1996). Observed track- of young AFT ages (fig. 1; table 1). length distributions are compared with the pre- Samples from the southeastern section of the Ad- dicted cumulative track-length distributions by us- irondacks (e.g., Lake George and Schroon Lake) that yielded the youngest AFT ages gave short mean ing the Kolomogorov-Smirnov statistic at the 0.05 track lengths, 13.3–13.6 mm, and small standard de- significance level. The predicted fission-track ages viations, 0.8–1.3 mm (fig. 5a,5b; table 1). These are required to be within two standard deviations track-length distributions suggest a relatively short of the measured age. time spent in the apatite PAZ (∼60Њ–100ЊC). In this study, we chose to use coefficients for Thermal History Models. Model time-tempera- Durango apatite (Crowley et al. 1991) to solve the Journal of Geology ADIRONDACK MOUNTAINS DIFFERENTIAL UNROOFING 161

Figure 3. Measured (histogram) and calculated (curve) apatite fission-track-length distributions at the 0.05 level using AFTINV32 (Issler 1996) for samples from the northwestern and southwestern Adirondack regions: a, Kents Corners; b, Hinckley Reservoir. Model thermal history at 0.05 level using Crowley et al. (1991) annealing model and AFTINV32 (Issler 1996) for samples from the northwestern and southwestern Adirondack regions: c, Kents Corners; d, Hinckley Reservoir. empirical apatite fission-track-annealing equation iation being a factor in Adirondack AFT age dif- of Crowley et al. (1991) in the AFTINV32 model. ferences is shown in the comparison of the AFT Funding was not available for compositional anal- ages for Mount Marcy (168 Ma) and North Hudson ysis of the Adirondack apatites by electron micro- (98 Ma; fig. 1; table 1), both of which are from an- probe. On the basis of etch pit size and appearance, orthosite samples. it was qualitatively assumed that the apatites in Figures 2–5 show comparisons of the fit of the our samples were F rich and similar to Durango in measured track-length histograms and calculated composition (Crowley et al. 1991). In the south- track-length distributions (probability density eastern Adirondacks, where the youngest AFT ages functions) for the preferred thermal history calcu- are found, we sampled a variety of rock types in- lated by the model. Each figure also gives a plot of cluding anorthosite, metasedimentary rocks, char- the exponential mean (preferred) thermal solution nockitic and granitic gneisses, and amphibolites. plus the bounding temperature envelopes contain- Because the AFT ages from all these different rock ing all statistically acceptable solutions. The pre- types are within the 112–83 Ma age range, we sug- ferred thermal solution is the middle line on the gest that compositional variation of the apatites is time-temperature plot, and the top and bottom not a cause of AFT age variation in the Adiron- lines represent the limits of acceptable solutions dacks. Further evidence against compositional var- (fig. 2c,2d; fig. 3c,3d; fig. 4c,4d; fig. 5c,5d). The 162 M. RODEN-TICE ET AL.

Figure 4. Measured (histogram) and calculated (curve) apatite fission-track-length distributions at the 0.05 level using AFTINV32 (Issler 1996) for samples from the southern and northern Adirondack regions: a, Little Falls; b, Lyon Mountain. Model thermal history at 0.05 level using Crowley et al. (1991) annealing model and AFTINV32 (Issler 1996) for samples from southern and northern Adirondack regions: c, Little Falls; d, Lyon Mountain. corresponding calculated track-length distribution tiary. These hypotheses are beyond the scope of our for the mean temperature solution is the middle present investigation and will not be discussed in curve on the calculated track-length plot, and the this article. top and bottom curves indicate the range of ac- Best-fit preferred thermal model solutions were ceptable track-length distributions (fig. 2a,2b; fig. determined for High Peaks region samples from 3a,3b; fig. 4a,4b; fig. 5a,5b). Mount Marcy, 168 Ma, and Whiteface Mountain, The thermal history models for all of the Adi- 142 Ma, (fig. 1) by using a cooling-only thermal rondack samples show a period of relatively rapid history (fig. 2a–2d). The model yielded an average unroofing, 0.8ЊC/m.yr., from ∼20–25Matothe cooling rate of ∼0.6ЊC/m.yr. from 200–180 Ma to present (figs. 2, 3, 4, 5). This period of rapid Miocene the present. The mean thermal history solution and younger cooling has been interpreted to be the suggests continuous slow unroofing for the central result of the extrapolation of short-time-duration High Peaks region of the Adirondack massif since (!1 yr) laboratory annealing experiments, on which Late Jurassic time. the equations are based, to geologic time scales. The sample from Kents Corners, 132 Ma, in the Some authors, including Blackmer et al. (1994) and northwestern Adirondack Lowlands (fig. 1) was Boettcher and Milliken (1994), have suggested that used to model the time-temperature history for that this period of possible Miocene exhumation may region. As for the High Peaks samples, a cooling- be evidence for a major unroofing event in the Ter- only thermal history is the preferred model for the Journal of Geology ADIRONDACK MOUNTAINS DIFFERENTIAL UNROOFING 163

Figure 5. Measured (histogram) and calculated (curve) apatite fission-track-length distributions at the 0.05 level using AFTINV32 (Issler 1996) for samples from the southeastern Adirondack region: a, Lake George; b, Schroon Lake. Model thermal history at 0.05 level using Crowley et al. (1991) annealing model and AFTINV32 (Issler 1996) for samples from the southeastern Adirondack region: c, Lake George; d, Schroon Lake. northwestern Adirondack Lowlands (fig. 3a,3c). rapid cooling of 0.9ЊC/m.yr. (Hinckley Reservoir The best fit thermal solution yields an average cool- and Little Falls) to 1.3ЊC/m.yr. (Lyon Mountain) ing rate of 0.5ЊC/m.yr. from 140 Ma to the present. started earliest along the northern border at Lyon The model results suggest that the northwestern Mountain at ∼170 Ma and later from 140–150 Ma Lowlands have experienced continuous, slow un- in the south at Little Falls and Hinckley Reservoir roofing beginning in the Late Jurassic to Early (fig. 3b,3d; fig. 4a–4d). This is followed by varying Cretaceous. times spent in the apatite PAZ with little temper- Samples from Hinckley Reservoir, 125 Ma; Little ature change, 40 (Hinckley Reservoir) to 80 Ma Falls, 130 Ma; and Lyon Mountain, 139 Ma, along (Lyon Mountain). From 40–50 Ma to the present, the southern and northern margins of the Adiron- the model thermal histories suggest cooling rates dack region (fig. 1) yielded very different model of 0.4ЊC/m.yr. and 1ЊC/m.yr. (fig. 3b,3d; fig. 4a–4d). time-temperature histories from those of the High Model time-temperature histories for samples Peaks and northwestern Lowlands. All of the pre- from Lake George, 94 Ma, and Schroon Lake, 105 ferred thermal models for these samples required Ma, in the southeastern Adirondacks (fig. 1) also three-stage cooling histories, with the middle stage yield three-stage thermal solutions (fig. 5c,5d). being a period of long-term residence in the apatite Both samples show an initial rapid cooling rate, PAZ (fig. 3b,3d; fig. 4a–4d). An initial moderately ∼1.0ЊC/m.yr., from 110–120 Ma to 90 Ma (fig. 5c, 164 M. RODEN-TICE ET AL.

5d). At 90 Ma, there is an increase in the slope of The model time-temperature paths for all the pe- the cooling curve to ∼1.2ЊC/m.yr. This slight steep- ripheral Adirondack samples (fig. 3c,3d; fig. 4c,4d) ening of the time-temperature path may indicate a suggest renewed unroofing at ∼40–50 Ma, during reactivation of uplift coincident with the time of the Eocene, which is not a period of active tecton- changing compressive stress directions from east- ism along the Atlantic continental margin. This west to northwest-southeast in the southeastern possible pulse of unroofing that began during the Adirondacks, as suggested by McHone (1978). At Eocene may be still active at present and may sup- 80 Ma, both samples remain at ∼60ЊC in the apatite port evidence for ongoing uplift in the Adirondacks PAZ for 30–40 m.yr. (fig. 5c,5d), and at 40–50 Ma, (Isachsen 1975, 1981). the preferred thermal solutions for both samples Mechanisms for Differential Unroofing in the Adiron- indicate renewed unroofing at a rate of ∼0.8ЊC/ dack Mountains. Significantly younger AFT ages m.yr. from the Proterozoic metamorphic rocks and Cam- Initial unroofing along the northern and southern brian Potsdam sandstone in the southeastern Ad- Adirondack margins began ∼20–30 m.yr. later than irondacks suggest that differential unroofing oc- it did in central High Peaks region, based on the curred in this part of the massif in post–Early AFT ages and preferred thermal model results. This Cretaceous time. The AFT age data from the New suggests that the High Peaks area was at a higher York Appalachian basin (Miller and Duddy 1989) initial elevation than the peripheral regions. If the help define the post-Ordovician thermal history of topography of the Adirondack Mountains in the the Adirondack region. Miller and Duddy (1989) Late Jurassic to Early Cretaceous was similar to the presented AFT age results that indicated that the present topography, then the highest elevations Devonian clastic sedimentary wedge in the Cat- would have been in the High Peaks and south- skill region was cooled rapidly during the Early Cre- central regions. The 40–80 Ma residence times in taceous (120–140 Ma) as a result of the removal of the apatite PAZ suggested by calculated thermal 3–4 km of sediment at this time. The Mesozoic models for the peripheral Adirondack samples unroofing history of eastern North America has could indicate burial by sediments being eroded been further constrained by AFT analyses. Roden from the central and southern Adirondack massif. and Miller (1989) presented AFT age results sug- Deposition began earliest at ∼130 Ma in the most gesting that 3.4 km of sediment was removed from proximal location to the north of the High Peaks northeastern Pennsylvania during the Early Juras- at Lyon Mountain and slightly later at ∼90–100 Ma sic. Roden (1991) determined AFT ages for Devo- along the more distant southern margin at Little nian sedimentary rocks and bentonites that indi- Falls and Hinckley Reservoir (fig. 1). Model time- cated unroofing was regionally contemporaneous temperature histories for the northwestern Low- along the strike of the southern Appalachian basin lands sample from Kents Corners do not show any with Triassic-Jurassic extensional opening of the evidence of a long-term residence in the apatite Atlantic. On the basis of these AFT data, eastern PAZ (fig. 3c). This sample was located too far (∼120 North America appears to have undergone wide- km) from the areas of highest elevation to be buried spread unroofing during the Mesozoic, and south- significantly by the eroded sediment. In support of ern New York State underwent the same during this inference, the preferred thermal model for this the Early Cretaceous. sample shows only cooling. Preferred thermal mod- In their thermal history study of the Adirondack els for samples from Schroon Lake and Lake George Mountains, Heizler and Harrison (1998) found that suggest that the southeastern Adirondacks were the 40Ar/39Ar K-feldspar analyses from the east- unroofing at a cooling rate of ∼1ЊC/m.yr. from ∼120 ern Adirondacks indicated a thermal pulse of to 80 Ma. During most of this time, time-temper- ∼300–350ЊC during the Ordovician (∼450 Ma). ature histories calculated for samples from the Their data indicated the Ordovician thermal event northern, northwestern, and southwestern margins may be a combination of Taconic thrust sheet bur- of the Adirondacks indicate that these areas were ial and hot fluid migration focused along the net- already buried by sediment shed from the central work of north-northeast-trending normal faults in massif to temperatures of ∼60ЊC in the apatite PAZ. the eastern Adirondacks. On the basis of their data Sediment shed from the southeastern Adirondacks and the AFT ages presented in this article, it is would have been deposited to the east in Vermont reasonable to suggest that the southeastern Adi- or further south in New York State. At ∼80 Ma, rondack region has been periodically active both active unroofing in the southeastern Adirondacks tectonically and thermally from the Ordovician ceased for a significant period of time, ∼40 Ma (fig. through the Late Cretaceous. 5c,5d). Our AFT age data for the Adirondack region are Journal of Geology ADIRONDACK MOUNTAINS DIFFERENTIAL UNROOFING 165 in agreement with the idea that Late Triassic Champlain Valley and southeastern Adirondacks, through Early Cretaceous was a time of extensive the maximum compressive stress, j1, rotated from unroofing and erosion along the eastern coast of east-west to northwest-southeast. The change in North America. The Adirondack area appears to the stress field may have been in response to plate have seen two periods of unroofing, one in the Late motions associated with the final opening of the Jurassic to Early Cretaceous (∼160–120 Ma) North Atlantic. throughout most of the region, and a second in the Differential unroofing in the southeastern Adi- Early to Late Cretaceous (∼110–80 Ma) in the rondacks, as indicated by our AFT data, could have southeast. resulted from one of the following mechanisms: (1) The mapped division between AFT ages of the increased heat flow or hydrothermal fluid circula- High Peaks (∼170 Ma) and the southeastern Adi- tion associated with intrusion of the Monteregian rondacks (∼100 Ma) is sharply defined along a magmas during the Cretaceous; (2) differential Mes- north-northeast-trending lineament/fault system ozoic sedimentation in the Adirondack region, with from Baker Brook in the southwest to Poke-O- the central High Peaks region being less buried than Moonshine in the northeast. Most of the faults/ the southeastern margin; and (3) reactivation of lineaments along the southeastern and southern east-dipping normal faults and shear zones to a re- margin of the Adirondacks strike north-northeast. verse sense of motion during the Late Cretaceous. These faults are inferred to have originated as nor- The Monteregian Hills intrusives are located mal faults that initially accommodated the thermal ∼180 km north of the southeastern Adirondacks, subsidence of a passive margin after Late Protero- and although the Champlain Valley contains a zoic rifting (Zen 1967; Bird and Dewey 1970). Many number of Cretaceous lamprophyre dikes, no Cre- of these linear features were active steep, eastward- taceous dikes outcrop in the vicinity of our samples dipping normal faults in Ordovician time (Fisher that yielded young AFT ages (Isachsen and Fisher 1980; Cisne et al. 1982). The Lake George and 1970). Eby (1984a) determined that the Montere- Schroon Lake graben contain downfaulted Ordo- gian Hills are very shallow level intrusives (∼3–4- vician limestones as evidence of an extensional km depth) because their AFT ages are comparable stress field. At present, the higher topography re- to their Rb-Sr crystallization ages (∼120 Ma). Per- mains on the western side of these faults, stepping sonal field observation of the contact relationships down toward the east to the Champlain and Hud- between Mount Royal and Mount Johnson, Que- son River lowlands. bec, and the Ordovician sedimentary country rocks Early to Late Cretaceous uplift in the southeast- suggest that these magma bodies were relatively ern Adirondacks is contemporaneous with the in- dry and cool on intrusion. The contact metamor- trusion of the Monteregian Hills plutons in south- phic aureoles of Mount Royal and Mount Johnson ern Quebec and associated dikes in Quebec, (pyroxene and hornblende hornfels facies, respec- Vermont, and the Champlain Valley of New York tively; Eby 1984b) are relatively small, on the order between 140 and 118 Ma (Eby 1984a, 1987; Mc- of ∼20–40 m, and contain no evidence of hydro- Hone 1984; Foland et al. 1986). Emplacement of thermal alteration. Cretaceous lamprophyre dikes, the Monteregian plutons and dikes has been ex- which we have observed in the Champlain Valley, plained by the following mechanisms: (1) it is the are small (∼1 m wide) and show no evidence of result of a hotspot track that formed the Monte- metamorphism or hydrothermal mineral deposits regian Hills, the younger White Mountains, and the at their contacts with the country rocks. Our AFT New England Seamounts during the Mesozoic age of 116 Ma (fig. 1; table 1) for a Cretaceous lam- (Crough 1981; Morgan 1983; Duncan 1984; Sleep prophyre dike on Willsboro Point is probably a good 1990) or (2) it is the product of deep-seated alkali estimate of its crystallization age and also indicates basalt magmas intruded along tectonically active a shallow intrusion depth (3–4 km). All of these “weak zones” or deep fractures in the lithosphere observations suggest that our young AFT ages from (McHone 1981; McHone and Butler 1984; McHone the southeastern Adirondacks are not the result of et al. 1987). increased heat flow or hydrothermal activity con- McHone (1978) relates a change in the trend of centrated along north-northeast normal faults dur- Early Cretaceous lamprophyre dikes in the Cham- ing the Cretaceous. plain and Monteregian areas (from east-west to Direct evidence of Mesozoic sedimentation in northwest) to a clockwise rotation of the extension the northeast is only preserved in the Triassic- directions from north-south between ∼136 and 125 Jurassic arkosic sandstones of the Newark basin of Ma to north-northeast/south-southwest between New York and New Jersey and in the Hartford- 124 and 96 Ma. During the Early Cretaceous in the Deerfield basins of Connecticut and Massachu- 166 M. RODEN-TICE ET AL. setts. There is no record of Mesozoic sedimentation Ordovician faults characteristically occur in wide in the Adirondack region, and several AFT studies zones with much talus or other ground cover, and based throughout the Appalachian orogen (Miller the mapped fault trace is often inferred by con- and Duddy 1989; Roden and Miller 1989; Roden straints of bedrock type and surface topography. 1991) have indicated that during the Jurassic and Fault plane surfaces are rarely observable in the Cretaceous, widespread unroofing occurred in east- field, making it impossible to evaluate any kine- ern North America. matic indicators such as slickenlines that might be The AFT ages determined in this investigation present and indicate the direction of fault mo- indicate that at ∼170 Ma (Middle Jurassic), Mount tion. For example, in a detailed field study of Marcy was buried to a depth of ∼4 km, assuming the north-northeast-trending McGregor-Saratoga- a geothermal gradient of 25ЊC/km. Preferred ther- Ballston Lake fault system, Isachsen et al. (1981) mal histories based on the measured track-length found few kinematic indicators, and of those found, distribution for Mount Marcy suggest a slow, linear about one-half indicated a normal fault sense of cooling rate of 0.6ЊC/m.yr. from ∼170 Ma to the motion, while the rest indicated a reverse sense of present. Given these constraints, a denudation rate motion. of ∼0.024 km/m.yr. can be calculated. In contrast, Stanley (1980) described a north-trending fault the model time-temperature histories for both sam- system in northern Vermont that shows about 800 ples from the southeastern Adirondacks, Schroon m of displacement. He has suggested an early Mes- Lake and Lake George, yield an initial denudation ozoic age for the faulting because the faults cross- rate of ∼0.040 km/m.yr. assuming a cooling rate of cut Paleozoic compressional structures and are 1ЊC/m.yr. and a geothermal gradient of 25ЊC/km. crosscut by, in at least one instance, a Cretaceous At 90 Ma, the model thermal histories indicate a lamprophyre dike (Zen 1972). The stress field that slight steepening of the cooling curve to 1.2ЊC/ would be associated with these north-trending m.yr., which yields a denudation rate of ∼0.048 km/ Mesozoic faults is considered extensional (Stanley m.yr. 1980). In contrast, McHone (1978) described a com-

The difference in these denudation rates can be pressional stress field with j1 trending east-west demonstrated if we estimate and compare the initially and rotating west-northwest by 124–96 amount of erosion that occurred in the High Peaks Ma, based on a pattern of changing trends of lam- region with that for the southeastern Adirondacks prophyre dikes in the Champlain Valley. This con- after each area was uplifted through AFT closure flicting evidence suggests a changing orientation of temperature of 100ЊC. A depth of ∼4 km is esti- the paleostress field in the Champlain Valley and mated for the 100ЊC isotherm if a surface temper- southeastern Adirondacks post–Early Cretaceous. ature of 15ЊC and a geothermal gradient of 25ЊC/ Recent evidence for a compressive stress regime km are assumed. Thus 1.2 km of erosion would in the Adirondack region includes offset blasting have occurred from Mount Marcy between ∼170 boreholes exposed in modern roadcuts (Fox et al. and 120 Ma, the time at which the southeastern 1999), deep borehole breakouts, and earthquake Adirondacks began to unroof. In the next 30 m.yr., fault plane solutions (Sbar and Sykes 1977; Seeber 1.2 km would have been eroded from the south- and Armbruster 1989). Although these studies con- eastern Adirondacks and another 0.48 km in the clude that maximum stress, at present, is not west- following 10 m.yr. We suggest that these rapid de- northwest but approximately east- to east-north- nudation rates for the southeastern Adirondack re- east-trending, this stress orientation remains gion cannot be explained simply by erosional de- compressive. The rotation of the paleostress field nudation. Tectonic denudation processes are in the Adirondacks may have occurred during the required, and we favor Late Cretaceous reactivation Eocene (∼40 Ma), a period of renewed cooling and of east-dipping normal faults and shear zones. This unroofing, indicated in the model time-tempera- is supported by the coincidence of orientation ture histories of the samples from the southeastern of the trends of faults and shear zones with the margin of the Adirondack massif. trend of the boundary between the “older” and “younger” AFT ages. A compressional stress field Conclusions with j1 trending west-northwest would be most suitable to promote this reactivation in a reverse Apatite fission-track ages of 168–135 Ma for sam- sense of fault motion. Unfortunately, it is highly ples from the Adirondack High Peaks indicate that improbable that our fission-track method can lo- unroofing occurred in this region during the Late cate a specific fault along which movement could Jurassic to Early Cretaceous. Model time-temper- have occurred. The majority of mapped post- ature histories based on apatite track-length distri- Journal of Geology ADIRONDACK MOUNTAINS DIFFERENTIAL UNROOFING 167 butions suggest continuous slow unroofing with an Unroofing in the Adirondacks was initiated in average cooling rate of 0.6ЊC/km. The northern, the central High Peaks region during Late Jurassic northwestern, and southwestern Adirondack pe- and Early Cretaceous time and progressed outward rimeter samples yield generally younger AFT ages to the perimeter of the Adirondack region by the of 146–114 Ma, indicating unroofing during the end of the Early Cretaceous. In the southeastern Early Cretaceous. Calculated time-temperature area of the Adirondacks, unroofing was delayed un- paths from track-length measurements suggest til the Early Cretaceous and continued through the three-stage cooling histories, in which the second Late Cretaceous. Later, more rapid unroofing in the stage is a significant period of time (40–80 Ma) southeastern Adirondacks may have resulted from spent in the apatite PAZ with little temperature a change in the compressive stress field that reac- change. The younger AFT ages and model time- tivated east-dipping normal faults to a reverse sense temperature histories suggest that the delayed ex- of motion. humation of the peripheral Adirondack areas resulted from burial by detritus shed from the High ACKNOWLEDGMENTS Peaks. Samples from the southeastern Adirondacks This research was supported by a Plattsburgh yield distinctly younger and abruptly discontinu- State University Presidential Research Award, ous AFT ages ranging from 112 to 83 Ma, indicating 1998–1999. We thank R. Darling and F. Florence for unroofing occurred in the Early to Late Cretaceous. providing samples and J. Olmsted and J. G. McHone Preferred thermal models for these samples also for sample locations. We also recognize the signif- suggest three-stage cooling histories. There is a icant contributions of sample collection, pro- slight increase in slope of the model time-temper- cessing, and some AFT age determinations from M. ature paths at ∼90 Ma that supports our hypothesis Pickering, C. Schildge, J. O’Neill, P. Rabideau, E. of reactivated uplift along existing faults at this Hewson, J. Snider, D. Michaud, and R. Sents, Platts- time. The models also suggest that this region ex- burgh State University undergraduate research as- perienced a shorter residence time (30–40 Ma) in sistants. L. Gillett provided comments on an early the apatite PAZ, at temperatures of ∼60ЊC, than the version of the manuscript. Critical reviews pro- residence times calculated for other peripheral Ad- vided by J. Chiarenzelli and an anonymous re- irondack samples. viewer greatly improved the manuscript.

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