Segmentation of the Cascade volcanic chain - GeoScienceWorld

Segmentation of the Cascade volcanic chain John M. Hughes, Richard E. Stoiber Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755

Michael J. Carr Department of Geological Science, Rutgers University, New Brunswick, New Jersey 08903

ABSTRACT Aligned volcanoes can be used to divide the Cascade volcanic chain into six segments averaging 175 km long. The petrology and eruptive style of volcanoes vary with their position on the segmented structure. Large volcanoes along the volcanic front and in the interiors of segments have been called "coherent," relatively uncomplicated composite volcanoes that erupt uniform porphyritic andesite with only a small range of composition. Volcanoes near segment boundaries are "divergent" volcanoes that erupt porphyritic basalts, andesites, dacites, and rhyolites. Volcanoes near the segment boundaries but behind the volcanic front are distinguished by a predominance of basaltic and rhyolitic volcanism and phenocryst-poor lavas, as compared with those near the segment boundary but on the volcanic front. INTRODUCTION The Cascade volcanic chain is the clearest evidence for plate convergence between the Juan de Fuca and North American plates. This region lacks several attributes of convergent plate margins such as a well-defined Benioff zone, great shallowthrust earthquakes, and a well-developed trench. These features may be absent for several reasons, including the proximity of the mid-ocean ridge and consequent young age and thermal history of subducted material, and the slow rate of convergence and recent changes in the rate and pattern of plate movements (Atwater, 1970). Overall, the geophysical evidence for plate convergence in this region is convincing (Atwater, 1970; Silver, 1971; Riddihough, 1978). H y n d m a n and others (1980) and Herd (1978) have described microplates and other complications that confuse the northern and southern terminations of this convergence zone. The distribution of large volcanoes in the Cascade chain is similar to distributions described at other convergent plate margins such as Central America and Mexico (Stoiber and Carr, 1973) and Japan (Carr and others, 1973). At these margins volcanoes occur either in discrete lineaments delineating segments that form the volcanic front or in clusters near the segment boundaries and behind the volcanic front. In Central America seismic GEOLOGY, v. 8, p. 15-17, J A N U A R Y

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and geologic evidence suggests that the breaks between volcanic segments occur at zones of transverse faulting both in the crust and in the underthrust slab (Carr, 1976; Dean and Drake, 1978). We outline here the segmented nature of the Cascade volcanic chain and suggest some petrological relationships to the segmented structure. SEGMENTATION OF CASCADES CHAIN Segmentation in the Cascades is indicated by the distribution of large recent volcanoes. Although the Cascade Range has long been viewed as a simple linear belt of volcanoes, it apears to consist of six linear volcanic segments that make up the volcanic front and clusters of volcanoes that are offset to the east (Figs. 1, 2). All the m a j o r Cascade volcanoes fit this pattern and are in one of three groups: (1) along the volcanic front within a segment, (2) along the volcanic front at segment boundaries, and (3) near segment boundaries and behind the volcanic front. The proposed volcanic segments strike subparallel to the coastline and vary between 110 and 240 km in length. The strike of the transverse structures is not well defined by m a j o r topographic or structural features. However, the tectonics of the segmentation model demands that the

1968) showing principal Quaternary volcanoes of High Cascades (dots), lineaments defining segmented volcanic front (lines), and segmentboundary zones (stippled areas). L = Lassen Peak; S = Mt. Shasta; ML = Medicine Lake; M = Mt. McLoughlin; C = Crater Lake; N = Newberry volcano; T = Three Sisters; J = Mt. Jefferson; H = Mt. Hood; SV = Simcoe volcanic field (added by us); A = Mt. Adams; SH = Mt. St. Helens; R = Mt. Rainier; GP = Glacier Peak; B = Mt. Baker; G = Mt. Garibaldi. 15

transverse segment boundaries be parallel to the direction of plate convergence, as noted in other areas of the circumPacific where this model has been applied (Carr and others, 1974). The direction of plate convergence is N50°E (Atwater, 1970; Riddihough, 1977). Lines connecting points of change in strike in the volcanic chain, when extended in this direction of plate convergence, intersect changes in strike and offsets of the base of the continental slope (Fig. 2). Couch and Lowell (1971), on the basis of studies by Couch and MacFarlane (1971) and Dehlinger and others (1971), characterized a regional stress field in Oregon as east-west compression, which would give rise to three kinds of faults: (1) northwest-trending right-lateral strikeslip faults, (2) northeast-trending leftlateral faults, and (3) north-trending normal faults. Wells and Peck (1961) and Walker and King (1969) have mapped surface traces of faults in Oregon which are consistent with this postulated stress field. These faults also are consistent with our Cascade model. This pattern of one major fault direction parallel to the volcanic: chain and one direction parallel to the direction of plate convergence is also noted in Indonesia (Fitch, 1970; Ranneft, 1972) and Central America (Stoiber and Carr, 1973). This suggests a common mechanism related to the underthrusting process: that the alignments of certain recent faults and vents in the Cascades is caused by the plate convergence process rather than being a modification of the Basin and Range fault system (Higgins, 1973) along pre-existing basement trends (Zietz and others, 1971). TECTONIC SETTING OF DIFFERENT PETROLOGIC TYPES OF VOLCANOES McBirney (1968) identified two categories of volcanic centers in the Cascades: "coherent" volcanoes that erupt and.esite or basaltic andesite of relatively constant composition with a notable absence of rhyolite, and "divergent" volcanoes that erupt siliceous andesites and dacites with later basalts and rhyolites. From the segmentation model thai: is defined by the lineaments in the volcanic chain, a correlation between McBirney's (1968) volcano categorization and volcano position on the segmented structures becomes apparent. Figure 3 is a projection of volcano position and segment-boundary position on a line perpendicular to the strike of the segment boundaries. The positions of these twelve volcanoes classified by McBirney can be divided into two groups: (1) those located within a segment, and (2) those located at or near segment boundaries. 16

50°

Figure 2. Volcanic segments of Cascades. Active volcanoes (black dots) after tectonic map of North America (U.S. Geological Survey and American Association of Petroleum Geologists, 1961). Boundaries of segments are dashed lines striking about N55°E. Convergent plate boundary (thrust fault symbol) taken as base of continental rise (after Chase and others, 1970); other plate boundaries after Atwater ( 1 9 7 0 ) for Juan de Fuca plate; Herd ( 1 9 7 8 ) for Humboldt plate (Hp); and Hyndman and others ( 1 9 8 0 ) for Explorer plate (Ep). Heavy line is spreading ridge.

40°

The coherent volcanoes (Baker, Rainier, Hood, Jefferson) are large stratovolcanoes that erupt dominantly porphyritic pyroxene andesites. Without exception, the coherent volcanoes are located within segments and on the volcanic front. The divergent volcanoes, however, all lie at or near a segment boundary. The greatest variety of volcanic activity occurs at segment boundaries, either on or behind the volcanic front. These divergent (segment boundary) volcanoes can be further subdivided into a group of volcanoes that lie on the volcanic front and another group to the east of the volcanic front. The Cascade volcanoes at segment boundaries exemplify many of the characteristics known to occur at seg-

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Figure 3. McBirney's ( 1 9 6 8 ) classification of petrologic type of major Cascade volcanoes. Volcano and segment boundary positions are projected o n t o line perpendicular to segment boundaries. Letters indicate volcanoes as in Figure 1.

rnent boundaries elsewhere in the circumPacific region (Carr and others, 1974). For example, recent tephra deposits from paroxysmal eruptions are found in the Cascades region at Crater Lake, St. Helens, and Glacier Peak, all of which are volcanoes on segment boundaries. The divergent volcanoes on the volcanic front include Mt. Garibaldi (Mathews, 1958), Glacier Peak volcano (Tabor and Crowder, 1969), Mt. St. Helens, Three Sisters, and Crater Lake (Williams and McBirney, 1968), Mt. Shasta (Blakely and Christiansen, 1978), and Lassen Peak (Williams, 1932). A recurring theme of these volcanoes is a wide range of lava compositions, as noted by McBirney. Siliceous domes, commonly dacite, are present at these volcanic centers as well as numerous basaltic cinder cones. Three complex volcanic centers that lie to the east of the Cascade volcanic front were recognized by Waters (1962). These are the Medicine Lake Highlands (Anderson, 1941; Mertzman, 1977), Newberry volcano (Higgins, 1973; MacLeod, 1978), and the Simcoe volcanic field (Sheppard, 1960). Waters noted that these centers differ from the Cascade stratovolcanoes in that porphyritic hypersthene andesites are few or absent, and rhyolite is present at all three centers. Medicine Lake and Newberry are large shield-shaped volcanoes, in contrast to the stratovolcanoes along J A N U A R Y 1980

the volcanic front. The recent volcanism at these two centers is bimodal, consisting of large obsidian flows, commonly rhyolite, and numerous basalt or basalticandesite cinder cones. Both these centers were classified as divergent by McBirney (1968). Mertzman (1977) further distinguished the Medicine Lake lava flows from those of the other Cascade volcanoes on the basis of the greatly increased iron enrichment. There is also a low Sr content at a given value of K 2 0 for the Medicine Lake lava flows. The similarity of the Medicine Lake and the Newberry volcanic rocks is striking (Higgins, 1973; N. MacLeod, 1979, personal commun.). Any grouping of Cascade volcanoes must recognize these similarities and the difference between these two centers and the other Cascade volcanoes. The Simcoe volcanic field, which lies to the east of the volcanic front, was not classified by McBirney (1968). On the basis of Sheppard's (1960) work, however, the divergent nature of the lavas of the area is apparent. Sheppard noted that the Simcoe Mountain volcanic area differs from the High Cascades but is similar in many respects to the Medicine Lake and Newberry volcanoes. The most recent activity in the area (compare Medicine Lake, Newberry) was olivine basalt cinder cone and rhyolite eruptions. SUMMARY The alignment of volcanoes in the Cascade volcanic chain suggests a segmented convergent plate margin. The volcanic segments are divided by transverse breaks that strike in the direction of plate convergence. The volcanoes are in three locations: (1) along the volcanic front but not on the segment boundaries, (2) along the volcanic front but at segment boundaries, and (3) behind the volcanic front at segment boundaries. The petrology of those volcanoes on the volcanic front but not on the segment boundaries is that of McBirney's (1968) coherent volcanoes. All the volcanoes classified as divergent by McBirney are located at or near segment boundaries. Further distinctions become apparent between those boundary-zone volcanoes that lie on the volcanic front and those located behind the volcanic front. Those on the volcanic front are stratovolcanoes that have erupted a wide range of lava compositions and at which dacite domes are common. Those centers behind the volcanic front are shield-shaped centers with a wide range of lava compositions, with rhyolite eruptions being common.

GEOLOGY

REFERENCES CITED A n d e r s o n , C. A., 1 9 4 1 , V o l c a n o e s of t h e Medicine Lake H i g h l a n d , C a l i f o r n i a : University of C a l i f o r n i a P u b l i c a t i o n s in Geological Sciences, v. 2 5 , p. 3 4 7 - 4 2 2 . A t w a t e r , T., 1 9 7 0 , I m p l i c a t i o n s of plate tectonics for the Cenozoic tectonic evolution of w e s t e r n N o r t h A m e r i c a : Geological Society of A m e r i c a Bulletin, v. 8 1 , p. 3 5 1 3 - 3 5 3 6 . Blakely, R. J., a n d C h r i s t i a n s e n , R. L., 1 9 7 8 , T h e m a g n e t i z a t i o n of M o u n t S h a s t a a n d imp l i c a t i o n s f o r virtual g e o m a g n e t i c poles det e r m i n e d f r o m s e a m o u n t s : J o u r n a l of Geophysical R e s e a r c h , v. 83, p. 5 9 7 1 - 5 9 7 8 . Carr, M. J., 1 9 7 6 , U n d e r t h r u s t i n g a n d Q u a t e r n a r y f a u l t i n g in n o r t h e r n C e n t r a l A m e r i c a : Geological S o c i e t y of A m e r i c a Bulletin, v. 8 7 , p. 8 2 5 - 8 2 9 . Carr, M. J., S t o i b e r , R. E., and D r a k e , C. L., 1 9 7 3 , D i s c o n t i n u i t i e s in t h e d e e p seismic zones u n d e r t h e J a p a n e s e a r c : Geological S o c i e t y of A m e r i c a Bulletin, v. 8 4 , p. 2 9 1 7 - 2 9 3 0 . — 1 9 7 4 , T h e s e g m e n t e d n a t u r e of s o m e c o n t i n e n t a l margins, in B u r k , C. A., a n d D r a k e , C. L., eds., T h e geology of c o n t i n e n t a l margins: N e w Y o r k , Springer-Verlag, p. 1 0 5 - 1 14. Chase, R. E., M e n a r d , H. W., a n d M a m m e r i c k x , J., 1 9 7 0 , B a t h y m e t r y of t h e N o r t h Pacific [ w i t h d i a g r a m a t i c abyssal t o p o g r a p h y ] : Scripps I n s t i t u t e of O c e a n o g r a p h y and Instit u t e of Marine Resources, 10 c h a r t s . C o u c h , R. W „ a n d Lowell, R. P., 1 9 7 1 , E a r t h q u a k e s and seismic energy release in O r e g o n : O r e Bin, v. 3 3 , p. 6 1 - 8 4 . C o u c h , R. W., and MacKarlane, W. T., 1 9 7 1 , A f a u l t plane s o l u t i o n of t h e O c t o b e r , 1 9 6 9 Mt. Rainier e a r t h q u a k e and t e c t o n i c m o v e m e n t s in t h e Pacific n o r t h w e s t f r o m f a u l t plane a n d first m o t i o n studies | a b s . ] : E O S ( A m e r i c a n G e o p h y s i c a l Union Transact i o n s ) , v. 5 2 , p. 4 2 8 . D e a n , B. W., a n d D r a k e , C. L., 1 9 7 8 , Focal m e c h a n i s m s o l u t i o n s a n d t e c t o n i c s of t h e Middle A m e r i c a arc: J o u r n a l of G e o l o g y , v. 8 6 , p. 1 1 1 - 1 2 8 . Dehlinger, P., a n d o t h e r s , 1971, N o r t h w e s t Pacific s t r u c t u r e , in The sea, V o l u m e 4 : N e w Y o r k , J o h n Wiley & S o n s , p. 1 3 3 - 1 8 9 . F i t c h , T . J., 1 9 7 0 , E a r t h q u a k e m e c h a n i s m s a n d island arc t e c t o n i c s in t h e I n d o n e s i a n Philippine r e g i o n : Seismological S o c i e t y of A m e r i c a Bulletin, v. 6 0 , p. 5 6 5 - 5 9 1 . Herd, D. G., 1 9 7 8 , l n t r a c o n t i n e n t a l plate b o u n d a r y east of Cape M e n d o c i n o , California: G e o l o g y , v. 6, p. 7 2 1 - 7 2 5 . Higgins, M. W., 1 9 7 3 , Petrology of N e w b e r r y v o l c a n o , c e n t r a l O r e g o n : Geological Society of A m e r i c a Bulletin, v. 8 4 , p. 4 5 5 - 4 8 8 . H y n d m a n , R. D., R i d d i h o u g h , R. P., a n d Herzer, R., 1 9 8 0 , T h e N o o t k a f a u l t z o n e - A n e w plate b o u n d a r y : G e o p h y s i c a l J o u r n a l (in press). M a c L e o d , N. S., 1 9 7 8 , N e w b e r r y v o l c a n o , Oreg o n : Preliminary results of n e w field investig a t i o n s : Geological S o c i e t y of A m e r i c a Abs t r a c t s w i t h P r o g r a m s , v. 10, p. 115. M a t h e w s , W. H., 1 9 5 8 , Geology of t h e M o u n t Garibaldi m a p - a r e a , s o u t h w e s t e r n British C o l u m b i a , C a n a d a : Geological Society of A m e r i c a Bulletin, v. 6 9 , p. 1 7 9 - 1 9 8 . M c B i r n e y , A. R., 1 9 6 8 , P e t r o c h e m i s t r y of Cascade a n d e s i t e v o l c a n o e s , in A n d e s i t e c o n f e r ence g u i d e b o o k : O r e g o n D e p a r t m e n t of Geological and Mineralogical I n d u s t r i e s Bulletin, n o . 6 2 , p. 1 0 1 - 1 0 7 .

P R I N T E D IN U.S..

M e r t z m a n , S. A., 1 9 7 7 , T h e p e t r o l o g y a n d geoc h e m i s t r y of t h e Medicine Lake v o l c a n o , C a l i f o r n i a : C o n t r i b u t i o n s t o Mineralogy a n d P e t r o l o g y , v. 6 2 , p. 22 1 - 2 4 7 . R a n n e f t , T.S.M., 1 9 7 2 , T h e e f f e c t s of c o n t i n e n tal d r i f t on t h e p e t r o l e u m geology of western I n d o n e s i a : A u s t r a l i a n P e t r o l e u m E x p l o r a t i o n A s s o c i a t i o n J o u r n a l , v. 12, p t . 2, p. 5 5 - 6 3 . R i d d i h o u g h , R. P., 1 9 7 7 , A m o d e l f o r r e c e n t plate i n t e r a c t i o n s o f f C a n a d a ' s west c o a s t : C a n a d i a n J o u r n a l of E a r t h Sciences, v. 14, p. 3 8 4 - 3 9 6 . 1 9 7 8 , T h e J u a n de Fuca p l a t e : E O S ( A m e r i can G e o p h y s i c a l Union T r a n s a c t i o n s ) , v. 59, p. 8 3 6 - 8 4 2 . S h e p p a r d , R. A., 1 9 6 0 , Petrology of t h e S i m c o e M o u n t a i n s area, W a s h i n g t o n [ Ph.D. dissert. | : B a l t i m o r e , J o h n s H o p k i n s University, 153 p. Silver, E., 1 9 7 1 , T r a n s i t i o n a l t e c t o n i c s and late C e n o z o i c s t r u c t u r e of t h e c o n t i n e n t a l margin o f f n o r t h e r n m o s t C a l i f o r n i a : Geological Society of A m e r i c a Bulletin, v. 8 2 , p. 1 - 2 2 . S t o i b e r , R. E., a n d Carr, M. J., 1 9 7 3 , Q u a t e r nary volcanic and t e c t o n i c s e g m e n t a t i o n of C e n t r a l A m e r i c a : Bulletin V o l c a n o l o g i q u e , v. 3 7 , p. 3 0 4 - 3 2 5 . T a b o r , R. W„ a n d C r o w d e r , D. F „ 1 9 6 9 , On b a t h o l i t h s and v o l c a n o e s — I n t r u s i o n a n d e r u p t i o n of late C e n o z o i c m a g m a s in t h e Glacier Peak area, N o r t h Cascades, Washingt o n : U.S. Geological Survey Professional Paper 6 0 4 , 67 p. U.S. Geological Survey and A m e r i c a n Association of P e t r o l e u m Geologists, 196 1, Tect o n i c m a p of t h e United S t a t e s , exclusive of Alaska and H a w a i i : U.S. Geological Survey, scale 1 : 2 , 5 0 0 , 0 0 0 , 2 s h e e t s . Walker, G. W „ and King, P. B., 1 9 6 9 , Geologic m a p of O r e g o n : U.S. Geological Survey Misc e l l a n e o u s Geologic Investigations Map 1-595, scale 1 : 2 , 0 0 0 , 0 0 0 . Waters, A. C., 1 9 6 2 , Basalt m a g m a t y p e s a n d their t e c t o n i c a s s o c i a t i o n s : Pacific N o r t h west of t h e U n i t e d S t a t e s , in T h e crust of t h e Pacific Basin: A m e r i c a n G e o p h y s i c a l U n i o n M o n o g r a p h 6, p. 1 5 8 - 1 7 0 . Wells, F. G., a n d Peck, D. L., 1 9 6 1 , Geologic m a p of O r e g o n west of the 121st m e r i d i a n : U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-325, scale 1:500,000. Williams, H., 1 9 3 2 , G e o l o g y of t h e Lassen Volcanic N a t i o n a l Park, C a l i f o r n i a : University of California P u b l i c a t i o n s in Geological Sciences, v. 2 1 , p. 1 9 5 - 3 8 5 . Williams, H „ a n d M c B i r n e y , A . R „ 1 9 6 8 , An investigation of volcanic d e p r e s s i o n s , Part I: Geologic a n d geophysical f e a t u r e s of calderas: N A S A Progress R e p o r t , N A S A Research G r a n t N G R - 3 3 - 0 3 3 - 0 1 2 , 87 p. Zietz, I., a n d o t h e r s , 1 9 7 1 , I n t e r p r e t a t i o n of an a e r o m a g n e t i c strip across t h e n o r t h w e s t e r n U n i t e d S t a t e s : Geological S o c i e t y of A m e r i c a Bulletin, v. 8 2 , p. 3 3 4 7 - 3 3 7 2 .

ACKNOWLEDGMENTS R e v i e w e d by A. R . McBirney a n d S. A. M e r t z m a n . S u p p o r t e d by N a t i o n a l S c i e n c e F o u n d a t i o n Grant EAR-76-09377. J. Kelleher a n d R . H e r w i n s p r o v i d e d valuable criticism o n early versions of t h e m a n u s c r i p t . MANUSCRIPT

RECEIVED

A P R I L 2, 1 9 7 9

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OCT. 27, 1979

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