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Home , Batholith, Cascade Range, Mount Stuart, North Cascades

Late Miocene exhumation and uplift of the

Peter W. Reiners Department of and Geophysics, Yale University, New Haven, Connecticut 06511, USA Todd A. Ehlers Division of Geological and Planetary Sciences, Institute of Technology, Pasadena, California 91125, USA John I. Garver Department of Geology, Union College, Schenectady, New York 12308, USA Sara Gran Mitchell  Department of Earth and Space Sciences, , , Washington 98195, David R. Montgomery  USA Joseph A. Vance  Stefan Nicolescu Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA

ABSTRACT olith (Erikson, 1969; Tabor et al., 2000). Da- The Washington Cascade Range is a complex, polygenetic range that domi- citic intrusions with 4±8 Ma ages are also pre- nates the topographic, climatic, and cultural con®gurations of Washington State. Although sent northeast of (Armstrong et it has been the locus of ongoing arc magmatism since the Eocene, most of the range is al., 1976; Smith et al., 1989; P.E. Hammond, distinct from the southern part of the arc in and California in that bedrock uplift 2002, personal commun.), although our sam- has produced high surface elevations and topographic relief, rather than volcanic burial ples were collected tens of kilometers away or edi®ce construction. (U-Th)/He and ®ssion-track ages of bedrock samples on the east from these intrusions. In the north ¯ank of the range record relatively rapid cooling in the early Tertiary, but slow exhu- region, apatite ®ssion-track ages of samples mation rates (ϳ0.2 km/m.y.) through most of the Oligocene. Samples on the west ¯ank may have been affected by the 16±18 Ma suggest rapid cooling in the late Miocene (8±12 Ma), and age variations in vertical tran- Mount Barr pluton (Holunga, 1996) or Chil- sects are consistent with a pulse of rapid exhumation (0.5±1.0 km/m.y.) at that time. Ap- liwack batholith or younger intrusions near atite He ages as young as 1±5 Ma in several areas suggest that high cooling and possibly . In most of our samples, how- exhumation rates persist locally. Accelerated exhumation rates ca. 10 Ma are also observed ever, apatite He ages are signi®cantly younger in the Coast of and southeast Alaska, ϳ1500 km to the north, (by at least 5±10 m.y.) than higher tempera- suggesting a large-scale mechanism for the exhumation pulse at that time. ture ages of intrusions, suggesting that in most cases apatite He ages re¯ect cooling primarily Keywords: (U-Th)/He, thermochronology, apatite, Cascades, Washington. by surface .

INTRODUCTION for sample locations, data, and analytical de- RESULTS AND DISCUSSION The Cascade Range in Washington State tails1) from plutonic and metamorphic rocks West Flank and Regions (Fig. 1) forms an important regional geologic, in three regions in the Washington (and south- Apatite and zircon (U-Th)/He and ®ssion- climatic, and cultural boundary. Numerous ern British Columbia) Cascade Range, here track ages are plotted against elevation in Fig- studies focusing on the high-temperature geo- termed the west ¯ank, east ¯ank, and north ure 2 (see footnote 1). In the west ¯ank and logic history (especially of the northern part of Cascades regions (Fig. 1). Samples along ver- north Cascades sample groups, apatite He ages the range) have documented a series of terrane- tical transects of ϳ1.1 and 1.7 km of relief do not vary systematically with elevation, and accretion episodes in the Cretaceous and Eo- over distances of Ͻϳ5 km were collected in nearly all are between 6 and 12 Ma. Excep- tions are a sample from near the town of Index cene, accompanied by crustal thickening, plu- the east and west ¯ank groups; a shorter ϳ0.8 at 13.5 Ma, two samples from northeast of tonism, and large-scale strike-slip faulting km transect was collected across an ϳ25 km Mount Rainier at 5 Ma, and samples from the (e.g., Haugerud et al., 1994; Cowan et al., distance in the north Cascades region. 2.5 Ϯ 0.1 Ma Lake Ann pluton (James, 1979) 1997; Whitney et al., 1999). As is the case Use of low-temperature thermochronome- at 1.3±2.0 Ma. Assuming geothermal gradi- farther south in Oregon and California, east- ters (closure temperatures ϳ70±180 ЊC) re- ents between 20 and 40 ЊC, the abundant 6± directed has been ongoing off the quires consideration of potential spatial and 12 Ma apatite He ages in the north Cascades coast of Washington since ca. 40 Ma, resulting temporal variations in geothermal gradients. and west ¯ank regions require that at least in locally thick accumulations (3±5 km) of Modern gradients in most locations in the 1.5±3 km of have been eroded from the volcanic rocks with ages dominantly between Washington Cascade Range north of Mount Rainier are 20±40 ЊC/km with regionally surface since that time. The steep age- 15 and 36 Ma. Aside from ®ve Quaternary elevation correlations and scatter in both sam- stratovolcanoes however, little of the modern higher gradients largely restricted to a narrow zone between Mount St. Helens and Mount ple groups do not allow precise constraints on topographic expression of the Washington Adams (Blackwell et al., 1990). However, apparent exhumation rates in these regions, Cascade Range is due to . Instead, geothermal gradients at the time of cooling but if the Mountain ages in the west the bedrock geology north of through apatite He and apatite ®ssion-track ¯ank group are due primarily to cooling by is dominated by crystalline rocks uplifted rel- closure temperatures may have been higher in exhumation (rather than magmatic cooling of ative to the surrounding Puget lowlands and some areas, especially in the west ¯ank region the Snoqualmie batholith), they indicate an Columbia basin. Indirect evidence, such as in and near the 17±25 Ma Snoqualmie bath- apparent exhumation rate of 0.5±1.0 km/m.y. warping of ca. 15±16 Ma Ba- (Fig. 2). The much older apatite ®ssion-track salt Group lavas, suggests that at least some 1GSA Data Repository item 2002088, Apatite and age at a high elevation (43 Ma) in the west of this uplift is younger than middle Miocene. zircon (U-Th)/He and apatite ®ssion-track sample lo- ¯ank, however, limits total post-Eocene ero- We determined apatite and zircon (U-Th)/ cations, data, and analytical methods, is available from Documents Secretary, GSA, P.O. Box 9140, sion to Ͻϳ3±5 km. Differences between ap- He and apatite ®ssion-track ages (see GSA Boulder, CO 80301-9140, [email protected], or atite ®ssion-track and apatite He ages in the Data Repository supplementary information at www.geosociety.org/pubs/ft2002.htm. north Cascades group suggest that prior to the

᭧ 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; September 2002; v. 30; no. 9; p. 767±770; 3 ®gures; Data Repository item 2002088. 767 Figure 1. Map of Washington Cascade Range and surrounding region. Boxes en- Figure 2. (U-Th)/He and apatite ®ssion-track close three primary sample regions, and blue lines enclose speci®c regions, re- (AFT) ages of Washington (and southern ferred to in text, Figure 2, and GSA Data Repository item (see text footnote 1). British Columbia) Cascade Range samples, MSBÐMount Stuart batholith. Quaternary volcanoes of or near modern arc: MGÐ plotted against elevation. Error bars are 2␴ , MBÐMount Baker, GPÐGlacier Peak, MRÐMount Rainier, MSHÐ for (U-Th)/He and 1␴ for AFT. See text foot- Mount St. Helens, IHÐIndian Heaven volcanic ®eld, MAÐMount Adams, SIMÐ note 1 for detailed data tabulation. MSBÐ Simcoe. batholith, FOJÐFourth of July; a.s.l.Ðabove sea level. late Miocene, cooling and most likely exhu- These old cooling ages require that at least mation rates were relatively slow, ϳ0.05±0.2 locally on the east ¯ank, exhumation was gions with 2 km of local relief are generally km/m.y., depending on geothermal gradients. limited to Ͻϳ2±3 km since the early Tertia- subject to mean erosion rates of at least ϳ0.4 If the Granite Mountain transect re¯ects 0.5± ry. Most apatite ®ssion-track ages of east km/m.y. (e.g., Pazzaglia and Brandon, 1996). 1.0 km/m.y. exhumation at 10±12 Ma, exhu- ¯ank samples are relatively old, between 41 If this is the case on the east ¯ank of the Cas- mation rates must have since decreased by at and 83 Ma. With the exception of one sample, cades, these high exhumation rates could not least a factor of 2±4, because such rates would these ages are an average of 33 m.y., and a have begun earlier than ca. 8 Ma, because old produce ages of 2±4 Ma if ongoing today. minimum of 20 m.y., older than apatite He cooling ages at high elevations in this region ages on the same samples or on samples at limit total erosion to Ͻϳ3 km. Thus the in- East Flank Region similar elevations. These age differences re- ferred late Miocene uplift that affected the In the east ¯ank region, apatite He ages quire slow time-averaged cooling (1±2 ЊC/m.y.) west ¯ank may have also increased relief and range from 18 to 60 Ma and show a broad between apatite ®ssion-track and apatite He surface uplift on the east ¯ank. correlation with elevation, especially within closure in the early to mid-Tertiary (exhuma- the 1.7 km Fourth of July vertical transect tion rates of only ϳ0.02±0.10 km/m.y.)Ð Late Miocene Uplift (Fig. 2). This transect yields an apparent ex- consistent with the ϳ0.05 km/m.y. slope of Taken together, the distribution of (U-Th)/ humation rate of 0.15±0.25 km/m.y. through the east ¯ank apatite ®ssion-track data. He and apatite ®ssion-track ages in this study most of the Oligocene to early Miocene. A Although there is no direct thermochrono- supports a general model for the modern group of samples from various locations in the metric evidence in the east ¯ank samples for Washington Cascade Range involving slow southeast corner of the Mount Stuart batholith rapid late Tertiary exhumation, the combina- exhumation from the Eocene to late Miocene region falls in the vertical transect trend below tion of old cooling ages (60±80 Ma) and the at Յ0.25 km/m.y. Most west ¯ank and north an elevation of ϳ2 km, but those above 2 km modern high local relief (2±3 km) on the east Cascades samples show rapid late Miocene show no correlation with elevation. One of side is consistent with a recent increase in up- cooling that produced widespread 6±12 Ma these high-elevation samples has the oldest lift and exhumation rates that has removed He cooling ages and steep age-elevation cor- apatite He age (60 Ma)Ðonly slightly youn- Ͻ2±3 km of . Correlations between relief relations. It is dif®cult to rule out the in¯uence ger than its 64 Ma apatite ®ssion-track age. and erosion rates worldwide suggest that re- of postmagmatic thermal relaxation on these

768 GEOLOGY, September 2002 rapid cooling rates in some areas, but if the west ¯ank age-elevation data are interpreted as re¯ecting exhumation, they imply a pulse of 2±3 km of erosion in the late Miocene, at rates of 0.5±1.0 km/m.y. A variety of other geologic evidence sup- ports an episode of uplift and erosion in the Washington Cascade Range in the late Mio- cene. Eocene nonmarine sedimentary rocks on both sides of the range record drainage of mountainous regions hundreds of kilometers to the east, across the location of the modern Cascade Range (Tabor et al., 1984; Gresens, 1987; Vance et al., 1987). In addition, 3±5- km-thick sequences of late Eocene through early Miocene volcanic rocks are present in and adjacent to the Cascade Range, requiring lack of signi®cant erosion, and local subsi- dence and burial of the range, through this time. Middle Miocene (ca. 15 Ma) basaltic la- vas of the east-derived Columbia River Basalt Group are tilted and uplifted along the eastern edge of the range; near Mount Rainier these Figure 3. Apatite He ages (white circles) plotted against latitude for west ¯ank and north lavas reach elevations more than 1.5 km high- Cascades samples from Washington State, and sea-level samples from British Columbia er than correlative units to the east (Swanson, (Farley et al., 2001) and southeast Alaska (Hickes et al., 2000; Hickes, 2001). Gray box is 1997). Warping and uplift of Ellensburg and 12±6 Ma age range that encompasses regionally averaged apatite He ages (black circles). Simcoe units on the east side of the range also Other symbols represent cooling ages from other radioisotopic systems (Parrish, 1983; Donelick, 1986; Gehrels et al., 1991; Wood et al., 1991). AFTÐapatite ®ssion-track; ZFTÐ suggest an initiation of uplift between 10 and zircon ®ssion-track. 4.5 Ma (Hammond, 1979). Plant and pollen fossils from interbeds in the Columbia River Basalt Group throughout central and eastern and southeast Alaska, as far as 1500 km to the delamination of lower crust by convective in- Washington and northwestern consis- north (Fig. 3), indicate a Cenozoic uplift his- stability beneath the entire length of the north- tently indicate a warm, relatively mesic cli- tory similar to that of the Washington Cas- western coastal Cordillera at 12±6 Ma. Crustal mate, comparable to that in the southeastern cades, despite the subduction to transform thicknesses in the Washington Cascade Range today, through the late Miocene plate-boundary change in southern British Co- and British Columbia are (Chaney, 1938; Berry, 1929; Smiley and Rem- lumbia. Exhumation histories for the Coast only ϳ30±40 km, and no sign of a signi®cant ber, 1979; Barnett and Fisk, 1980; Wolfe, Mountains involve slow cooling and little to ma®c root is observed (Parsons et al., 1998; 1981); such a may imply the absence no exhumation from the Eocene through late Morozov et al., 1998), as would be required of the Cascades orographic barrier to Paci®c- Miocene, followed by accelerated exhumation for generation of intermediate-composition derived precipitation. beginning ca. 10 Ma (0.2±0.5 km/m.y.) (Par- volcanic rocks in the Cascade Range and the Although there are some ca. 5±15 Ma arc rish, 1983; Donelick, 1986; Hickes et al., massive Coast Plutonic Complex in British magmatic rocks in the Washington Cascade 2000; Farley et al., 2001; Hickes, 2001). Sim- Columbia from arc magmatism. Finally, evi- Range (e.g., Hagstrum et al., 1998; Smith, ilar to other geologic constraints in Washing- dence for accelerated exhumation rates in the 1988; Smith et al., 1989), they are relatively ton, the timing and rates of rock and surface late Miocene is common not just in the north- rare; in general, there is a distinct paucity of uplift in the Coast Mountains are also de®ned west Cordillera, but in many other locations volcanic rocks of this (or some similar) age by uplifted late Miocene basalts (Parrish, on the Paci®c rim and elsewhere. While recent range (Mattinson, 1977; Evarts et al., 1987; 1983) and paleobotanical climatic indicators attention has generally focused on to Swanson et al., 1989; Smith, 1993; Evarts and (Rouse and Mathews, 1979) east of the range. Holocene , most compilations Swanson, 1994). This gap is in spite of the If the uplift and exhumation of the Washing- show an earlier onset, with signi®cant accel- fact that plate tectonic reconstructions indicate ton Cascade Range is genetically related to eration in the late Miocene (e.g., Lear et al., a and subduction off the that of the entire northwest Cordilleran Coast 2000). It is not inconceivable, although it is west coast of Washington since at least the Mountains, the mechanism is of larger scale dif®cult to test at this point, that some com- Eocene (Brandon and Vance, 1992). As sug- than simply localized rotation in the U.S. Pa- ponent of climate change alone (such as the gested by others (e.g., Smith, 1993), the pau- ci®c Northwest or arc magmatic crustal onset of 103±105 yr climatic oscillations) ac- city of magmatic rocks of this age may be thickening. celerated erosional exhumation rates. In the simply due to lack of preservation because of One possible mechanism for continental- Cascades, however, this would mean that an accelerated uplift and erosion in the late Mio- margin±scale rock and surface uplift starting inference of late Miocene surface uplift from cene, rather than to an actual decline in arc at 12±6 Ma is simply a change in relative paleobotanical evidence east of the Cascades magmatic production. North American±Paci®c plate motion. Plate is incorrectÐpossibly an artifact of climate reconstructions show that such a change in change. Uplift of the Northwest Cordilleran relative plate motions ca. 8 Ma had a wide- Margin ranging effect on Neogene tectonics in the ACKNOWLEDGMENTS Thermochronometric and geologic studies southwestern United States (Atwater and We thank Mark Brandon and Paul Hammond for of the Coast Mountains in British Columbia , 1998). Another possible mechanism is helpful discussions on Cascades geology and Ham-

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770 GEOLOGY, September 2002