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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. - 4-1 r, Quaternary extrusion rates from the Cascade Range, northwestern United States and British Columbia David R. Sherrod and James G. Smith U.S. Geological Survey, Menlo Park, California 94025 ABSTRACT abruptly along the length of the volcanic arc. The The Quatemary extrusion rate and style of eruption Quaternary rate north of Mount Rainier is about 0.21 for the Cascade Range of thePacific Northwest changes km3/km/m.y.; in southern Washington and northern 13d 125• 120• - \ 1'.X 1 ..1-1• '' Mt. Garibaldi - 50' Explorer .0-9.RM&£14'Wl,6 . Plate BRITISH -------COLUMBIA / 11<13» 0,41 -------0 Mt Baker WASH 00-0 . C 0 1"('.. 4 '\QK-/ t. Glacier Pk e / 4# ·' -'i:-:* E;/ t. ; 94 . ..(4. 01., 4-.P 00490•,2 ... /1 1iF• Mt. Rainier 6 f Juan de Fuca 2 Plate I. 1'•St. Helens 1,J-- 4 ys i .-_-!.1--'·«'- Convergent ,, I Mt. Hood 4140 Margin e Mt JeffersoL9.1•10 - 45' 6/-"00o ;: k..04 U /'C)404. 4,6 2- Three Sisters11 0 0 ** 4394, Pacific ge Newberry volcano . ip . Plate /) ««'2, • Crater Lake Ae, 0 150 km l 1 / r ('100 mW/m2OREG bC, --,-- C< --- 3 CALIF. | NEV. ,* Mt. Shasta • Medicine Lake S,1 1 /! S. Lassen Pk.1 I 1 Figure l. General setting ofthe QuaternaryCascadevolcanic arc inwesternNorthAmerica, modifiedfromGulfanti andWeaver (1988). Dots locatemajorQuaternaryvolcanoes. Arrow on the JitandeFucaplate indicates directionof present-day convergence betweentheJuande FucaandNorthAmericanplates. Dashedlinesare depthcontoursdrawn on top oftheJuan de Fucaplate underpart ofthe Coast rangesas locatedbyWeaver andBaker(1988): showndotted whereinferred. Linewithsaw teethshowslocationofsubductionzone marking the boundary betweenthe Juan de Fuca andNorthAmericanplates: sawteethOn upperplate. Line A-A'fromWeaver andMichaelson(1985) divides the arc in Washington state into two segments. Heat-flow contours of 70 and 100 mWim• from Blackwell and others (1989). Brothersfault zonefrom Walker and King (1969); its postulatedextension shown dotted through CascadeRange(fromSherrodandConrey, 1988). 94 Oregon as far south as Mount Hood, the rate is about newly published and unpublished mapping and radio- 1.6 km3/km/m.y. In contrast, the rate is 3 to 6 metric determinations completed since 1980. km3/km/m.y. in central Oregon and 3.0 km3/km/m.y. We report here some surprising conclusions about in northern California. For central Oregon, short- and the rates and volumes of Quaternary volcanism in the long-term Quaternary extrusion rates have not varied Cascade volcanic arc. In comparing different parts of significantly over intervals as short as 0.25 m.y. the volcanic arc from British Columbia to California, Volcanic style changes at different latitudes than we find that the extrusion rate changes abruptly along rate. Volcanic style at the ends of the arc is character- the arc's length, diminishing by an order of magnitude ized by volcanism focused near isolated intermediate to northward from Mount Rainier across a zone that silicic central volcanoes. These central volcanoes are matches an apparent seismic boundary suggested by -30 percent of the total volcanic production along the Weaver and Michaelson ( 1985). Consequently, arc arc. Mafic volcanic fields partly ring some central vol- segments north and south of this boundary may have canoes, more so in the south than in the north, but they different resource potential for igneous-related geother- contribute less volume than central volcanoes along a mal energy. corresponding length of arc. In contrast, diffuse volcanism characterizes the middle of the arc, a 260-km- GEOLOGIC SETTING long segment in central Oregon stretching from north of The Cascade volcanic arc reaches from British Mount Jefferson to soutli of Mount Mazama. Columbia to northern California. It parallels the Pa- Numerous overlapping mafic shields and a few central cific coastline and convergent margin between the North volcanoes have built a broad ridge. American and Juan de Fuca plates (fig. 1). In northern Quaternary extrusion rate correlates closely with Washington and British Columbia, the arc lies 300 to regional heat flow. That part of the arc in Washington 400 km east of the margin and is built largely upon and Oregon where extrusion rate is greater than 1.6 lower Eocene and preTertiary sedimentary and crystalline km3/km/m.y. lies within the 70-mW/m2 regional heat- rocks. In northern California, Oregon,+ and southern flow contour. Different extrusion rates likely indicate Washington, the Quaternary Cascade arc lies 250 to 300 zones of differing heat input. Contrasting volcanic km east of the convergent margin and is built upon up- style may signify diffuse versus focused heat sources or per Eocene and younger volcanogenic rocks. crustal changes in permeability to ascending magma In Washington, the most important components of along the arc. Both extrusion rate and volcanic style Quaternary Cascade volcanism are the Indian Heaven and may be significant in the search for exploitable southern part of the Simcoe volcanic fields and six geothermal energy. composite stratovolcanoes (Mount Baker, Glacier Peak, Mount Rainier, Goat Rocks, Mount Adams, and Mount INTRODUCTION St. Helens). In the Oregon part of the arc, Quaternary In igneous-related geothermal systems, the rate of volcanic rocks form a continuous outcrop belt (High upper crustal magmatism is an important component of Cascades) that extends from Crater Lake northward to the shallow-level heat budget. Although the balance near Mount Hood (fig. 2). The northern and southern between volcanism and intrusion is difficult to assess, parts of Oregon have only a few small, discrete volcanic the extrusion rate may be a first-order approximation of centers. In California, Quaternary volcanic rocks are the overall rate of magmatism. areally extensive in a roughly triangular area whose Until the middle 1980's, reliable estimates of vol- corners are formed by Mount Shasta, Medicine Lake ume, timing, and extrusion rate for the Cascade Range volcano, and Lassen Peak. volcanic arc of the United States could not be made for In the following discussion, the extrusion rate is lack of sufficient isotopic ages and detailed geologic normalized to cubic kilometers per kilometer of arc maps from which to draw the necessary stratigraphic and length per million years (km3/km/m.y.). Except for structural details. Our rate calculations are based on low-density air-fall tephra, we did not adjust volumes of volumes measured while constructing 1:250,000- and different eruptive products to dense-rock-equivalent 1:500,000-scale geologic compilation maps of the magma; all volumes are shown as 1 or 2 significant Cenozoic volcanic rocks of the Cascade Range in figures. Thus the calculated extrusion rates are only Washington, Oregon, and California (Smith, 1987, in approximate. However, as even these approximate rates press-a, in press-b; Sherrod, 1987; Sherrod and Smith, differ by close to an order of magnitude for different 1989). These maps are not merely reworkings of segments of the arc, we predict our conclusions will not previously published data but instead rely largely on be modified greatly as more detailed mapping, 95 geochronology, andrefined volume calculations become 124 1220 available. 'f -6 BRITISH E---27 . COLUMBIA Quaternary volcanism in northern Washing- Vancouver ton and southern British Columbia: Volume Island . Mt. Baker and extrusion rate 1...C Quaternary volcanogenic rocks are not volumetri- " cally abundant in this segment of the Cascade Range, Glacler Pk..•3 which contains Mount Baker, Glacier Peak, Mount + 4 -48• Garibaldi, and a few basalt flows and cinder cones be- tween Glacier Peak and Mount Rainier. All known Quaternary volcanic rocks were erupted less than 1 Ma; volcanic rocks 1 to 2 Ma are unknown (table 1). This distribution suggests that volcanic production in this arc I.. '1».7 segment is episodic over intervals less than one million gv«9¢,. years. Volumes for some volcanoes are published. For 65 »taner·..,2.-F others we estimated volumes by conventional methods, either subdividing the volcano into approximate regular geometric solids or using a planimeter to measure areas yASH. ,») »0*, at successive elevations (Church, 1981). T••-•t==-St Hele•n•9A . »ro•\,,• Mt Adams The present cone of Mount Baker and underlying OREG.7• «-''' ''fir...,1 - 46 Pleistocene volcanoes erupted 72 km3 of mainly an- 0 4#up 0 desite flows during the last 700 ka. The summit forms L-. -- an impressive snow-covered peak over 3,200 m high, D»<-3 15 7 but the volcano is perched on a high bedrock ridge and 161 7Mt Hood thus its volume is much less than its summit elevation 3 09 rj 200 2 would suggest. A few flank flows, volcanic debris 0 +B#,3 + flows and tephra layers contribute less than 2 km3 to g • ' Mt Jefferson the total volume. The next stratovolcano south along 0 tl-f ...,-, the arc, Glacier Peak, also sits on a bedrock ridge.