Steady rotation of the Cascade arc Ray E. Wells1 and Robert McCaffrey2 1U.S. Geological Survey, 345 Middlefi eld Road, MS 973, Menlo Park, California 94025, USA 2Department of Geology, Portland State University, PO Box 751, Portland, Oregon 97207-0751, USA ABSTRACT permitting westward escape and rotation of the Displacement of the Miocene Cascade volcanic arc (northwestern North America) from Pacifi c Northwest. the active arc is in the same sense and at nearly the same rate as the present clockwise block We examine the rotation of the Cascade arc in motions calculated from GPS velocities in a North American reference frame. Migration of light of recent GPS results which show that the the ancestral arc over the past 16 m.y. can be explained by clockwise rotation of upper-plate Pacifi c Northwest is still rotating clockwise at blocks at 1.0°/m.y. over a linear melting source moving westward 1–4.5 km/m.y. due to slab up to 2.0°/m.y. (Fig. 1B; McCaffrey et al., 2007, rollback. Block motion and slab rollback are in opposite directions in the northern arc, but 2013). The progressive offset of the ancient Cas- both are westerly in the southern extensional arc, where rollback may be enhanced by proxim- cade arc provides an opportunity to examine the ity to the edge of the Juan de Fuca slab. Similarities between post–16 Ma arc migration, paleo- utility of GPS for quantifying long-term block- magnetic rotation, and modern GPS block motions indicate that the secular block motions like motions and the relative importance of from decadal GPS can be used to calculate long-term strain rates and earthquake hazards. crustal rotation versus slab rollback in arc evolu- Northwest-directed Basin and Range extension of 140 km is predicted behind the southern tion. We show that geodetic rates agree with late arc since 16 Ma, and 70 km of shortening is predicted in the northern arc. The GPS rotation Cenozoic rates inferred from paleomagnetism poles overlie a high-velocity slab of the Siletzia terrane dangling into the mantle beneath Idaho and geology, and from that agreement we infer (United States), which may provide an anchor for the rotations. long-term rates of slab rollback and block rota- tion. We use the GPS-derived poles to recon- INTRODUCTION in response to changing slab buoyancy, ter- struct the Miocene arc at 16 Ma and examine The Cascade volcanic arc, which extends rane accretion, or mantle fl ow regime have all implications for back-arc extension and magma- from British Columbia (Canada) southward been proposed as possible contributors (e.g., tism, and arc deformation in Cascadia. into northern California (United States), is the Priest, 1990). Arc rotation occurs when subduc- product of subduction of the Juan de Fuca plate tion parameters vary along strike, for example, GPS VELOCITIES VERSUS GEOLOGIC beneath North America (Fig. 1A). Its 40 m.y. where rollback is locally impeded by collision DISPLACEMENT RATES history is recorded in volcanic and plutonic of buoyant, thickened crust, as in the northern rocks that are progressively offset clockwise New Zealand or Tonga-Kermadec arcs (Wallace Geodetic Rotation Rates Match from the modern arc axis. Cascade arc migration et al., 2005), or where proximity to a slab edge Paleomagnetic Rates is consistent with clockwise rotation of western may promote rollback and extension in the over- Paleomagnetic studies show that western Oregon determined from paleomagnetism (e.g., lying plate (Schellart, 2008). Humphreys and Oregon has rotated about a vertical axis at Magill and Cox, 1981; Wells, 1990; Wells et Coblentz (2007) suggested that Cascadia slab 1.19° ± 0.1°/m.y. for much of Cenozoic time, al., 1998), although slab rollback or fl attening rollback is a critical geodynamic component with the rate decreasing to the north, south, Pemberton GS N Miocene arc J CE Figure 1. A: Cascade volcanic arc and back- BC 49° N 49° N arc ages (northwest North America; Smith, belt B 0-2 Ma VI CS 1993; Sherrod and Smith, 2000; Hildreth, P 2007; Massey et al., 2005). Yellow triangles 42° N G 2-7 Ma are major Cascade volcanoes: Mount Baker (B), Glacier Peak (G), Mount Rainier (R), S A 7-17 Ma F 120° W Three Sisters (TS), and Mount McLoughlin (M). Red lines show Yakima fold-and-thrust R 17-45 Ma belt. BC—British Columbia; WA—Washing- WA ton; OR—Oregon; CA—California; NV—Ne- 46° 46° mid-Tertiary OR OF pole vada. B: Block velocities (arrows) from GPS plutons (McCaffrey et al., 2007) match sense of off- OF 1.0°/Ma set of ancestral, mostly Miocene Cascade Pacific Ocean arc plutons (red blobs) from active arc axis TS (green). Thick gray lines are block bound- 1 Ma aries; Oregon forearc block (OF) velocities and pole in red; Vancouver Island block (VI) in blue (pole is off fi gure). Inset: Pacifi c (P), Juan de Fuca (J), and North America (N) M SM10 Ma plates; yellow arrow is convergence direc- 42° 42° tion of Juan de Fuca plate. Garibaldi arc seg- CA NV ment (GS), Cascade arc segment (CS) after Age of Hildreth (2007); Mesozoic orocline (dots); 10-0 Ma silicic and Columbia embayment (CE). backarc magmatism Quaternary arc rhyolite in backarc 5 mm/yr AB120° W 200 km 120° W GEOLOGY, September 2013; v. 41; no. 9; p. 1027–1030 | doi:10.1130/G34514.1 | Published online 22 July 2013 GEOLOGY© 2013 Geological | September Society 2013of America. | www.gsapubs.org Gold Open Access: This paper is published under the terms of the CC-BY license. 1027 and east (Fig. 2A). A similar pattern emerges GPS Velocities Are Consistent With (Hildreth, 2007), but we argue the regional off- when vertical axis rotation rates are calculated Geologic Offset set is the result of a regional process. from GPS velocities estimated for the past Because the long-term geologic rotation rates To quantify the ancestral arc offset, we com- 15 yr across the Pacifi c Northwest (Fig. 2B). and decadal geodetic rotation rates are essen- pare the GPS rates to geologic rates of mag- An east-west profi le along the Columbia tially the same, we compare the evidence for matic migration in the direction of the GPS River (Fig. 2C) shows that GPS rotation rates geologic migration of Cascade arc magmatism vectors at several localities along the arc. North- increase toward the subduction zone (where over the past 16 m.y. to secular block motions east of Mount Baker (Washington) is a series of smaller block rotation occurs) and are nearly derived from the GPS velocity fi eld after remov- calderas and stocks that become progressively identical to the rates calculated from paleo- ing the short-term elastic compression due to older to the northeast (Fig. 3A). They include magnetic rotation of 12 Ma and 15 Ma fl ows of subduction (McCaffrey et al., 2007, 2013). We the Kulshan caldera at 1.1 Ma, the Lake Ann the Columbia River Basalt Group (McCaffrey suggest that the current surface velocities and stock at 2.8 Ma, and the Hannegan caldera at et al., 2007). The similar rotation rates at vastly their derived poles of rotation acting over mil- 3.7 Ma (Hildreth et al., 2003). At Mount Baker, different time scales and the great size of the lions of years can largely explain the offset of the shift of magmatic focus is consistent with rotating area indicate that the GPS-observed the Miocene arc. In Figure 1B, we superim- the observed N42°E plate motion with respect rotation is revealing permanent motions in the pose the block velocities (in the North Ameri- to NAM, with 3.5 mm/yr of the 6.0 mm/yr upper plate, as opposed to elastic strain from can [NAM] reference frame) calculated from westward magmatic migration rate explained the subduction zone or time-dependent effects the GPS data on a geologic map of Cascade by block motion. In Oregon, Miocene plutons of the earthquake cycle. volcanic and plutonic rocks grouped by age. lay 75–110 ± 15 km northwest of the High Cas- The axis of the Miocene arc inferred from the cade axis in the direction of current plate motion distribution of plutons aged 25–9 Ma (du Bray (Fig. 3B). Average geologic displacement rates and John, 2011) lies west of the modern arc in of the plutons from the present axis since 16 Ma A British Columbia Oregon and east of the modern arc in northern are 3.1 mm/yr at Mount Jefferson and 3.5 mm/yr 0 Washington State and British Columbia (Hil- at Mount McLoughlin (both in Oregon). GPS 7 dreth, 2007). Although the pluton distribution is rates are 4.1 and 7.7 mm/yr, respectively. 48° 25 0 0 complex in detail and not all plutons are dated, a 16 27 Washington broad swath through the plutons clearly demon- Slab Rollback 20 52 Pmag 2328 strates the pattern (Fig. 1B). When compared to In both Oregon and Washington, the differ- 0 rotation 46 36 the GPS velocity fi eld, the Miocene axis is dis- ence between geologic and GPS rates indi- 53 placed in a direction consistent with the veloc- cates a westward component of arc migration 45° 50-12 Ma 70 16 48 30 rocks Oregon ity fi eld, and where modern and ancient arcs in NAM of 1–4 mm/yr in addition to block 49 are nearly congruent, the GPS velocity is par- motions calculated from GPS. This additional 0 67 allel to the arc. Crustal structure and magmatic westward component of arc migration is most 30 km plumbing can cause shifts in magmatic focus easily explained as the result of slab rollback 14 42° 0 200 British Columbia 48.9° N B A V.I.