Presidential Range Research Articles

Consistent slow exhumation in a late Cenozoic glaciated landscape: The Presidential and Carter ranges of the White Mountains in New Hampshire, USA

To investigate the ability of the onset of Quaternary glaciations to drive an acceleration in erosion rates we compare short timescale (103–104 yr) erosion rates using the in situ terrestrial cosmogenic nuclide (TCN) 10Be to longer time scale exhumation rates (106–107 yr) using the apatite (UTh)/He method in the Presidential and Carter ranges of the White Mountains in New Hampshire, USA. Our results are generally consistent with but refine previous studies using similar techniques. Resulting (UTh)/He ages and thermal modeling from Mt. Washington in the Presidential Range (n = 4, 91.7 to 117.3 Ma) suggest that exhumation rates from the Late Cretaceous to the present were slow, <0.01–0.02 mm yr−1. Be-10 analyses of high elevation bedrock and talus from the Presidential Range (n = 4; 1.5–1.8 km above mean sea level [amsl]; 18–130 ka) and Carter Range (n = 4; 1.0–1.4 km amsl; 14–22 ka) yield exposure ages that dominantly predate the retreat of the last glacial maximum ice sheet (13.9 ka) and indicate inheritance or inefficient glacial erosion of peaks. A non-linear positive relationship between exposure age and elevation suggests a component of elevation dependent polythermal basal ice conditions. A comparison of 10Be concentrations measured in modern fluvial sand and flood deposits in the region (n = 14; 5–19 × 104 atoms g−1 SiO2) to till (n = 5; ~2–7 × 104 atoms g−1 SiO2), colluvium (n = 2; 2–7 × 104 atoms g−1 SiO2), bedrock, and talus suggests that the modern day sediment budget continues to be dominantly sourced from remobilized glacial sediment rather than postglacial weathering. As such, late Cenozoic glaciation and postglacial processes modified existing topography at lower elevations but did not significantly accelerate exhumation rates in the higher elevation regions of White Mountains of New Hampshire. These results refute the idea of a global acceleration in erosion rates resulting from late Cenozoic climatic cooling and glaciation as a result of frozen-based minimally erosive ice cover in conjunction with low rates of tectonic uplift.

Comparison of logistic regressions and snowfall intensity–duration threshold as forecasting tools for direct-action snow avalanches in the Presidential Range, New Hampshire, USA

Snow avalanches are an important natural hazard for backcountry skiers and alpinists in the Presidential Range (New Hampshire, United States). Over the past 20 years, over 72 people were involved in avalanche accidents. Direct-action avalanches resulting from recent snowfall dominate the avalanche regime of this region. Given this peculiarity, avalanche activity should be influenced by the intensity and duration of snowfalls. This study compares logistic regressions (LR) and a novel approach based on snowfall intensity–duration (ID) thresholds to analyse the weather patterns favourable to the triggering of snow avalanches. LR models suggest that 72-h solid precipitation, 24-h liquid precipitation, 72-h average daily maximum temperature and 72-h average winds peed are the best predictors of avalanche activity. The accuracy of the snowfall ID threshold to predict snow avalanches was similar to the accuracy of rainfall ID thresholds to identify landslide activity. However, a Monte Carlo cross-validation procedure suggests that the LR model is more accurate and more robust in predicting avalanche activity, since they allow the inclusion of more meteorological predictors. Finally, we discuss the relevance of this novel approach and the need for replicate studies in similar, direct-action avalanche-dominated contexts.

Density and Habitat Relationships of the Endemic White Mountain Fritillary (Boloria chariclea montinus) (Lepidoptera: Nymphalidae).

We conducted point counts in the alpine zone of the Presidential Range of the White Mountains, New Hampshire, USA, to estimate the distribution and density of the rare endemic White Mountain Fritillary (Boloria chariclea montinus). Incidence of occurrence and density of the endemic White Mountain Fritillary during surveys in 2012 and 2013 were greatest in the herbaceous-snowbank plant community. Densities at points in the heath-shrub-rush plant community were lower, but because this plant community is more widespread in the alpine zone, it likely supports the bulk of adult fritillaries. White Mountain Fritillary used cushion-tussock, the other alpine plant community suspected of providing habitat, only sparingly. Detectability of White Mountain Fritillaries varied as a consequence of weather conditions during the survey and among observers, suggesting that raw counts yield biased estimates of density and abundance. Point counts, commonly used to study and monitor populations of birds, were an effective means of sampling White Mountain Fritillary in the alpine environment where patches of habitat are small, irregularly shaped, and widely spaced, rendering line-transect methods inefficient and difficult to implement.

Large‐scale teleconnection patterns and synoptic climatology of major snow‐avalanche winters in the Presidential Range (New Hampshire, USA)

ABSTRACTThe relationships between the synoptic climatology, large‐scale teleconnections and the regional avalanche activity index (RAAI) inferred from tree‐rings were evaluated for the Presidential Range in the White Mountains (New Hampshire, USA). During the period 1936–2012, 18 years of regional avalanche activity were compared with the winter‐scale prevailing joint temperature/climatic modes (cold/wet (CW), cold/dry (CD), warm/wet (WW) and warm/dry (WW)), the North Atlantic Oscillation (NAO), the El Niño‐Southern Oscillation (ENSO), the 500‐mbar composite anomaly maps and the ratio of snow from different storm tracks. The total winter snowfall and the NAO negatively correlate with the RAAI. There is also a significant difference in avalanche activity between winters with a NAO under or above −3. Winters of regional avalanche activity were present in the four climatic modes, albeit the ratio of avalanche/non‐avalanche years is superior for CW winters compared to the three other modes, as well as for wet winters compared to dry winters. CW, CD and WW winters exhibit a negative NAO anomaly, which is eastbound for the wet years. Cold winters (CW, CD) receive more snow from the Great Lakes, whereas coastal depressions are more important during wet winters (CW, WW). The NAO is an adequate predictor of snowfall, but does not provide information about the storm tracks. On the contrary, the ENSO is poorly correlated with snowfall, but its relationship is significant with the ratio of snow produced by coastal depressions (positive relationship) and Great Lakes storms (negative relationship). These are the first results quantifying the atmospheric circulation – synoptic meteorology – snow avalanche relationships in Northeastern North America.

Dendrogeomorphic reconstruction of snow avalanche regime and triggering weather conditions

While dendrogeomorphology has been recognized as a useful tool to identify past avalanche activity, there is only a handful of papers that focus on the assessment of weather or climatic triggers of tree-ring reconstructed avalanche events. This paper compares the potential of logistic regression and classification tree algorithms to highlight weather scenarios responsible for the occurrence of high-magnitude avalanche activity in the Presidential Range of the White Mountains, New Hampshire (USA). Our tree-ring procedure improves the modern GD- It threshold with the implementation of a second criteria based on the Moran index. 450 trees sampled in seven different avalanche paths allowed us to reconstruct 45 avalanches that occurred during 19 different years for the period 1936–2012. The results show that while statistically significant, the logistic regression models are less accurate than classification trees to assess avalanche activity based on annual and monthly weather variables. Moreover, even if snow related covariates are located at the root node of every classification tree model, the addition of temperature and wind predictors increases their robustness. This suggests that high-magnitude avalanches in the Presidential Range not only respond to snow, but also to atmospheric conditions responsible for the creation of weak layers within the snowpack.

Can we discriminate snow avalanches from other disturbances using the spatial patterns of tree-ring response? Case studies from the Presidential Range, White Mountains, New Hampshire, United States

Dendrogeomorphology has been recognized as a useful tool to reconstruct past snow avalanche chronologies, especially in remote areas where archives are non-existent. In recent years, there have been a multiplicity of snow avalanche studies based on tree-ring analysis. Yet, the dendrogeomorphic procedure applied to snow avalanches still lacks consensus within the scientific community. This paper illustrates four issues regarding this method encountered on a dataset encompassing 293 trees sampled from 4 sites in the White Mountains (New Hampshire, United States). (1) Separating a sample in an upslope and downslope subgroup allowed to reconstruct a more thorough avalanche chronology. (2) On the other hand, a strong response at a site sheltered from any avalanche track was attributed to extreme snow loadings with a return period well above 100 years. (3) In addition to climatic disturbances, ecological disturbances such as windthrows can cause an anatomical response in the trees similar to snow avalanches. An avalanche track might act as a wind tunnel, making the underlying runout zone a suitable site to windfalls. Sampling in transects can assist in determining the limit between avalanche-related and wind-related disturbances. (4) Early-spring torrential floods and avalanche activities at a multi-process site exhibit distinct spatial patterns in the dendrogeomorphological response that allow discrimination between the two processes in the reconstruction of past chronologies. While the dendrogeomorphologist should be cautious of these issues, their acknowledgement is an opportunity to understand the interactions between the different ecological, climatic and geomorphological processes operating on the forested slopes in the alpine–subalpine environment.

Alpine Soil Parent Materials and Pedogenesis in the Presidential Range of New Hampshire, U.S.A.

Previous work has explored the contribution of bedrock weathering to soil formation in alpine environments of the northeastern United States. In contrast, the role of surficial sediments as a parent material has received little attention. This study investigated the development of alpine soils in relation to surficial sediments and bedrock in part of the Presidential Range of the White Mountains, New Hampshire, U.S.A. Published mapping of the study area indicates that five contrasting bedrock units are thinly mantled by glacial sediment, providing the opportunity to evaluate the relative importance of these two potential soil parent materials. Samples collected from 25 pedons were analyzed for a variety of physical, chemical, and mineralogical properties. Soil profiles have distinct Oa, A, Bw, and BC horizons. X-ray diffraction analysis reveals the presence of chlorite in soils above all bedrock formations, despite the absence of this mineral in several bedrock types. Trace element concentrations are relatively uniform for all soils with no clear relationship to underlying bedrock. Differences in median grain size are significant in soils over different bedrock formations; however, extractable cations, cation exchange capacity, exchangeable acidity, and other soil properties exhibit no significant differences. The overall similarity of soils above strongly contrasting bedrock formations indicates that these soils are forming in a homogenous mantle of glacial sediment deposited in the late Wisconsin, and that bedrock weathering does not contribute to pedogenesis to measurable degree in this setting. Preservation of glacial sediment in the study area may be a function of the amount originally deposited, reduced erosion rates on relatively gentle slopes, or another unidentified factor.

Notes on the Demography, Life History, and Behavior of the White Mountain Arctic Butterfly (Oeneis melissa semidea)

ABSTRACT. The White Mountain Arctic butterfly [WMA; Oeneis melissa semidea (Say, 1828)] is endemic to the alpine zone of the Presidential Range of the White Mountains, New Hampshire, USA. Although it has been listed as “imperiled”, many biological characteristics of the WMA important for its conservation assessment and management are unknown. We conducted field studies in 2011 and 2012 to further characterize the WMA's demography, life history, and behavior. In both years, adults emerged in mid-June and occurred on Mts. Washington and Jefferson in association with Bigelow's sedge (Carex bigelowii). On both mountains, adult numbers generally were very low, suggesting that the population has declined considerably since its first description. Adults dispersed among some of the meadows on Mt. Washington, but we were unable to confirm if they moved between Mts. Washington and Jefferson. Adults generally congregated on rocky ledges and out-croppings, where males employed both perching and patrolling mate-locati...

Apatite fission-track evidence for the Cretaceous development of kilometer-scale relief and steady-state Tertiary topography in New England

Apatite fission track (AFT) ages in samples collected along the 5000 foot relief (1500m) exposed at Mt. Washington in the Presidential Range of New Hampshire are used to constrain the Cretaceous cooling history of this area in the northern Appalachians. Nine AFT ages for samples of the Littleton and Rangeley formations collected along this profile range in age from ~150Ma at the highest elevations of ~1900m to ~100Ma at the base (~500m). Thermal histories based on these results and on model time–temperature histories based on the distribution of fission track lengths in the higher elevations samples show three stages of cooling, from 1.5–2.0°C/m.y. (170–130Ma) to 0.2°C (130–65Ma), to ~0.6°C/m.y. (65Ma to the present). In contrast, the thermal history calculated for the lower elevations sample shows cooling delayed until ~120Ma at ~2°C/m.y. (120–100Ma), followed by monotonic cooling of ~0.6°C/m.y. from 100Ma to the present. The convergence of these histories from high, intermediate, and low elevations suggests a common cooling history independent of elevation differences of >1km. Structural/tectonic explanations for this thermal convergence are implausible, and we conclude that the most likely explanation for the common cooling history across >1km of relief is that the relief was established by the end of the Cretaceous and has persisted with steady-state topography through the Tertiary to the present. The AFT results are consistent with an earlier relief method study employing 40Ar/39Ar muscovite cooling ages. Geothermal gradients calculated from the results of both studies yield ~40°C/km suggesting that this gradient persisted throughout Permian and Mesozoic times.

Occurrence of Pond-Breeding Amphibians at Alpine Ponds in the White Mountains, New Hampshire

Ponds in the White Mountains of Grafton and Coos Counties, NH provide some of the highest elevation breeding habitat for amphibian populations in the north- eastern United States. Between 2007 and 2009, we conducted fi eld surveys of alpine ponds above 1100 m on the Presidential Range and Franconia Ridge in the White Moun- tain National Forest. These ponds include the only currently known amphibian breeding sites in tundra-dominated landscapes reported from the eastern United States. Four spe- cies of anuran and one species of salamander were detected at elevations ranging from 1180 to 1546 m, with direct evidence of breeding observed at every pond complex. This preliminary study contributes to available baseline data for amphibian populations in alpine and subalpine environments in the northeastern United States. Amphibian populations may be strong indicators of climate change and environmental stressors in alpine ecosystems; these populations appear to warrant long-term monitoring.

A comparison of fine particle and aerosol strong acidity at the interface zone (1540 m) and within (452 m) the planetary boundary layer of the Great Gulf and Presidential-Dry River Class I Wildernesses on the Presidential Range, New Hampshire USA

Mount Washington, NH in the White Mountain National Forest, is flanked to the north-northeast and south by two Class I Wilderness areas, the Great Gulf and Presidential Range-Dry River Wildernesses, respectively. The Clean Air Act protects Class I Area natural resource values from air pollution. Aerosol sulfate, a fine particulate component that is often transported long distances, is a known contributor to visibility degradation and acidic deposition. We examined summertime fine particulate aerosol mass and sulfate, strong acidity and ammonium concentrations from 1988 to 2007 on Mount Washington at two elevations, 452 and 1540 m (msl). The former site is often within, and the latter at the interface of, the planetary boundary layer. Comparisons of sampling interval durations (10 and 24 h) and site vs. site are made. We also examine the extent to which aerosol sulfate is neutralized. Ten hour (daytime) compared to 24 h samples have higher mass and aerosol sulfate concentrations, however paired samples are well correlated. Fine mass concentrations compared between the 452 m and 1540 m sites (standard temperature and pressure corrected) show a weak positive linear relationship with the later being approximately 32% lower. We attribute the lack of a strong correlation to the facts that the 1540 m site is commonly at the interface of and even above the regional planetary boundary layer in summer and that it can intercept different air masses relative to the 452 m site. Sulfate is ∼18% lower at the higher elevation site, but comprises a greater percentage of total fine mass; 42% compared to 37% for the high and low elevation site, respectively. Aerosol strong acidity was found to increase with increasing sulfate concentrations at both sites. Further the ratio of hydrogen to sulfate ion was greater in 24 h than 10 h samples at the higher elevation site likely due to overnight transport of fresh acidic aerosols.

Constraining the relative strengths of high-grade metamorphic rocks using foliation refraction angles: an example from the Northern New England Appalachians

Foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite-facies metaturbidites in the Presidential Range of the White Mountains, in eastern New Hampshire. An early-formed foliation, developed during km-scale nappe folding, consistently displays larger bedding-foliation angles in metapelitic units than metapsammitic units, suggesting that the metapelitic units had higher effective viscosities during this deformation. We collected bedding-foliation angle measurements from a combination of outcrops and locally-derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades. Our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers, which we attribute to the presence of large (up to 15 cm long), abundant (up to 30 vol.%) effectively rigid andalusite porphyroblasts in the metapelitic layers. We present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types, address the practical limitations we encountered, and suggest this method is an easy way of estimating the strength contrast between different rock types. Finally, we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis.

Cirques of the Presidential Range, New Hampshire, and surrounding alpine areas in the northeastern United States

Evidence for rejuvenation of cirque glaciers following wastage of continental ice remains elusive for the Presidential Range and Mount Moosilauke of New Hampshire, Mount Katahdin and the Longfellow Mountains of Maine, and the Adirondack Mountains of New York. At Ritterbush Pond in the northern Green Mountains of Vermont, radiocarbon ages from lake sediment cores suggest that a low-altitude valley head, located up- valley of a series of cross-valley moraines, was ice-free by 11,940 14 C yrs BP (Bierman et al. , 1997). Although some workers argue that these moraines in Vermont are evidence for cirque glaciation, the moraines could have been formed by a tongue of continental ice during deglaciation. At Johnson Hollow Brook valley in the Catskill Mountains of New York, a radiocarbon age from basal sediments in a pond dammed by a moraine suggests that glacier ice may have persisted until 10,860 14 C yrs BP (Lederer and Rodbell, 1998). Because this moraine appears to have been deposited by a cirque glacier, the radiocarbon age provides the best evidence in the northeastern United States for cirque glaciation post-dating recession of continental ice. Cirque morphometric data, compiled from newly available topographic maps, add to the conundrum that these two sites in the Green and Catskill Mountains should not be nearly as favorable for maintaining local glaciers post- dating icesheet recession as higher-altitude and better-developed cirques in the Presidential Range and Mount Katahdin, where evidence for post-icesheet cirque glaciers is lacking.

History of research on glaciation in the White Mountains, New Hampshire (U.S.A.)

The glacial geology of the White Mountains in New Hampshire has been the subject of many investigations since the 1840's. A series of controversies evolved during this period. First was the question of what geologic processes were responsible for eroding the bedrock and depositing the cover of surficial sediments. By the 1860's, the concept of glaciation replaced earlier theories invoking floods and icebergs. Research in the late 1800's concerned the relative impact of continental versus local glaciation. Some workers believed that surficial deposits in northern New Hampshire were the product of valley glaciers radiating from the White Mountains, but in the early 1900's continental glaciation was established as the most important process across the region. Debate over the extent and timing of alpine glaciation in the Presidential Range has continued until recent years. The most intensely argued topic has been the manner in which the Late Wisconsinan ice sheet withdrew from the White Mountains: whether by rapid stagnation and downwastage, or by progressive retreat of a still-active ice margin. The stagnation model became popular in the 1930's and was unchallenged until the late 1900's. Following a research hiatus lasting over 40 years, renewed interest in the glacial history of the White Mountains continues to inspire additional work.

Classification and ordination of plant communities along an altitudinal gradient on the Presidential Range, New Hampshire, USA

An analysis of vegetation along an altitudinal gradient on the Presidential Range, New Hampshire, USA, using the Braun–Blanquet approach followed by multivariate data analysis is presented. Twelve main plant communities have been distinguished. Floristic information is presented in twelve tables and one appendix. The relationships of the communities to complex environmental gradients are analyzed using Correspondence Analysis. Floristic composition and community structure are controlled primarily by the altitudinal gradient (temperature, precipitation), and by mesotopographic conditions (snow accumulation, exposure and cryoturbation, slope position, and soil moisture).

Timing of the Acadian Orogeny in Northern New Hampshire.

New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian; (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.

High Spatial Resolution Ar-Ar Age Profiles in Micas — Slow Cooling in the Presidential Range, New Hampshire, USA

Apparent age gradients in single crystals of muscovite have been detected in a sufficient number of cases to indicate that they are common. The various studies have suggested one or more dominant process controlling the development of 'fossil' age gradients in micas, including; volume diffusion, multipath diffusion, deformation, chemical variations, and heterochemical recrystallisation. If we can discriminate between age profiles formed by volume diffusion and those formed by other means, they may be very useful for evaluating thermal histories. Moreover, thermal histories, determined using high spatial resolution UV laser originate from single minerals and single grains, circumventing to a large extent the uncertainties resulting from the various estimates of closure temperatures. The Presidential Range of New Hampshire, USA, offers an ideal site to study argon age profiles in muscovite. Regional geochronological studies and the depositional age of the Littleton formation, suggest the main stage of the Acadian metamorphism in the Presidential Range occurred in the interval 412-405 Ma. Rocks are exposed along the eastern flank of Mount Washington from the summit at 1917 m to the base at 411 m. In earlier work Eusden and Lux (1994) found 30 m.y. of variation in bulk-sample muscovite ages from the top to the base of Mt. Washington, from 305 Ma at the summit to 275 Ma at the base. Eusden and Lux interpreted these results to indicate the present day relief of Mt. Washington exposes ancient, deep crustal levels that passed through the argon closure temperature for muscovite (sensu stricto Dodson 1973) during a 30 m.y. interval of post Acadian unroofing and cooling. Their results and previous studies by West and Lux (1993), and Harrison et al. (1993), corroborated a proposed slow cooling history in the Late Devonian to Permian time in portions of northwestern New England. Eusden and Lux (1994) recorded argon release spectra deviating slightly from ideal plateaux though none recorded staircase which might be expected to indicate the effects of argon loss. Single crystal laser fusions yielded ages ranging from 267_ 3 to 327 ___ 4 Ma, an age range of 60 Ma, in a locality where Eusden and Lux recorded a total gas age of 304 • 1.5 Ma. To further test the distribution of ages, micas were collected from a pegmatitic granite dyke chosen for their exceptional size (up to 1 cm) and clarity. A total of 31 spot fusions using an Ar Ion laser resulted in ages varying from about 390 to 305 Ma with a systematic age distribution from centre to edge. Ages reflecting early cooling from the Acadian metamorphism in the grain centre (380-390 Ma) gave way to ages reflecting late cooling at the grain edges though the spatial resolution of the Ar Ion technique prevented resolution of the age gradient in the outer 100 I-tm. Further work using a UV laser to achieve high spatial resolution close to the edge of one grain, revealed that ages varied by up to 50 Ma in the outer 100 lam of the grain (Fig. 1). The larger grains yielded much older ages than the smaller grains analysed by Eusden and Lux (1994) so in order to study the effects grain size upon the ages we analysed some large grains with cores rich in fibrolitic sillimanite. The sillimanite needles acted as macroscopic analogues for smaller scale defects in

Stratigraphy and ductile structure of the Presidential Range, New Hampshire: Tectonic implications for the Acadian orogeny

The Presidential Range of New Hampshire with its unique high relief contains exceptionally well exposed rocks of the Acadian orogenic hinterland. These rocks are within the Central Maine terrane, a belt of complexly metamorphosed and deformed Silurian and Devonian metasedimentary rocks. The Presidential Range lies between regions to the south dominated by refolded nappes and synkinematic high-grade regional metamorphism and regions to the northeast dominated by overlapping multiple deformation and contact metamorphism produced by late- and post-Acadian plutons. To better understand these complex tectonic variations and, in particular, to clearly and accurately determine the effects of the Devonian Acadian orogeny in a well-exposed region, the bedrock geology in the alpine zone of the Presidential Range was mapped in great detail. The results of the stratigraphic and ductile structural analyses are presented here. Five metasedimentary formations have been recognized and constitute a conformable stratigraphy, which, from oldest to youngest, are the Silurian (?) Rangeley, Perry Mountain, Smalls Falls, and Madrid Formations, and the Devonian (?) Littleton Formation. The Littleton Formation has been subdivided into fifteen different members and three submembers based on variations in bedding style of the schists and quartzites and other lithologic peculiarities. The Madrid, Smalls Falls, and Perry Mountains Formations are extremely thin, consistent with their deposition near the Silurian tectonic hinge. The Rangeley Formation is interpreted as an olistostromal melange and has been subdivided into three different members and six submembers on the basis of lithologic variations in the gneiss and subordinate units. The Presidential Range preserves three phases of ductile folding (F1, F2, and F3) and a single phase of thrust faulting (T1). Well-constrained southeast-, east- or northeast-facing directions and consistent asymmetry of F1 nappe limbs suggest that nappe vergence was easterly. Severe refolding of some nappes resulted during F2 collapse of the nappe pile. The west-vergent (?) Greenough Spring thrust fault truncated F1 and F2 structures and juxtaposed nonmigmatized and migmatized rocks. F3 east-vergent folding dominates the alpine zone and represents the final tightening of Acadian structures, which occurred after early granitic plutonism and prior to the emplacement of postkinematic granitic sheets. The structural and sedimentological evidence supports the existence of a subduction system that was active beginning in the early Silurian on the east flank of the Bronson Hill Anticlinorium with a west-dipping subducting slab geometry. This system persisted throughout the Acadian.

Slow late Paleozoic exhumation in the Presidential Range of New Hampshire as determined by the 40Ar/39Ar relief method: Comment and Reply

Slow late Paleozoic exhumation in the Presidential Range of New Hampshire as determined by the 40Ar/39Ar relief method: Comment and Reply

Slow late Paleozoic exhumation in the Presidential Range of New Hampshire as determined by the 40Ar/39Ar relief method

40 Ar/ 39 Ar mineral ages from muscovite were used to determine exhumation rates in the multiply deformed and metamorphosed Silurian and Devonian metasedimentary rocks exposed on Mount Washington, New Hampshire. The muscovite ages increase progressively from the bottom to the top of the mountain; the oldest (Mount Washington summit, elevation 6200 ft [1900 m]) is 304 ± 3 Ma and the youngest (Route 16, elevation 1350 ft [410 ml) is 274 ± 3 Ma. The age and elevation data yield an exhumation rate of 0.04 mm/yr. The exhumation rate indicates that (1) the Middle Pennsylvanian through Early Permian was a period of very slow exhumation; (2) rapid uplift associated with the Acadian orogeny must have terminated by at least 305 Ma; (3) sometime after 274 Ma, renewed uplift created the present topography of the Mount Washington massif; and (4) the relief method can be used successfully in older, more deeply eroded orogenic belts, if the topographic relief is adequate and cooling was slow.