Late Miocene exhumation and uplift of the Washington Cascade Range Peter W. Reiners Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA Todd A. Ehlers Division of Geological and Planetary Sciences, California 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, University of Washington, Seattle, 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 mountain 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 Mount Rainier (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 Oregon 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 Cascades ¯ank of the range record relatively rapid cooling in the early Tertiary, but slow exhu- region, apatite ®ssion-track ages of samples mation rates (;0.2 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 Mount Baker. 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 Mountains of British Columbia and southeast Alaska, ;1500 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 erosion. 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 North Cascades 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 ;1.1 and 1.7 km of relief do not vary systematically with elevation, and accretion episodes in the Cretaceous and Eo- over distances of ,;5 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 ;0.8 at 13.5 Ma, two samples from northeast of tonism, and large-scale strike-slip faulting km transect was collected across an ;25 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 6 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 ;70±180 8C) re- ents between 20 and 40 8C, the abundant 6± directed subduction 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 crust have been eroded from the volcanic rocks with ages dominantly between Washington Cascade Range north of Mount Rainier are 20±40 8C/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 volcanism. Instead, geothermal gradients at the time of cooling but if the Granite Mountain ages in the west the bedrock geology north of Snoqualmie Pass 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 Columbia River 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 ,;3±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 q 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 2s Mount Garibaldi, MBÐMount Baker, GPÐGlacier Peak, MRÐMount Rainier, MSHÐ for (U-Th)/He and 1s for AFT. See text foot- Mount St. Helens, IHÐIndian Heaven volcanic ®eld, MAÐMount Adams, SIMÐ note 1 for detailed data tabulation. MSBÐ Simcoe. Mount Stuart 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, ;0.05±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 ,;2±3 km since the early Tertia- subject to mean erosion rates of at least ;0.4 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 ,;3 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 8C/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 ;0.02±0.10 km/m.y.)Ð Late Miocene Uplift (Fig.