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2 Geothermal Resources Council, TRANSACTIONS Vol. 7, October 1983 THE CENTRAL OREGON HIGH CASCADE GRABEN: WHAT? WHERE? WHEN? Gary A. Smith and Edward M. Taylor Department of Geology Oregon State University Corvallis, OR ABSTRACT Study of late Miocene-Recent volcanism in the High Cascades indicates that regional extension culminated in the formation of the Central Oregon portion of the High Cascade graben about 4.5 m.y.b.p. Fault scarps on the east flank of the graben are discontinuous and probably reflect differential subsidence within the depression. Subsidence may have been greater where large volcanic edifices provided a load on thermally weakened crust. The graben is mostly filled by basalts erupted during the extensional episode, but some permeable volcaniclastic debris may be locally ponded against the Green Ridge fault. Silicic volcanism in the highland west of Bend was probably initiated prior to the Pleistocene and may obscure graben-bounding faults. INTRODUCTION Intra-arc volcano-tectonic depressions are recognized in several circum-pacific volcanic chains - e.g. Central and South America, Indonesia, Japan, Kamchatka. Allen (1966) was the first to postulate the existence of a similar structure in the Cascade Range. The occurrence of east-facing fault scarps in the McKenzie River- Horse Creek area and near Crater Lake, and complimentary west-facing scarps at Green Ridge and east of Mt. Hood suggest that at least portions of the Oregon High Cascades occupy a subsidence structure. This depression is referred to as the High Cascade graben (Priest et al., 1982). The future of geothermal exploration in the Oregon Cascade Range is strongly dependent on determining the nature of the High Cascades graben. Are the observed scarps related to large cauldron subsidence blocks? If so, then the faulting would be discontinuous and large shallow magma reservoirs might underlie the Cascade axis. Alternatively, is the intra-arc depression a true tectonic graben? If this is the case, then faulting should be fairly continuous along the Cascade trend. Another important consideration is the nature of the graben-fill. Is it possible that thick accumulations of ash-flow tuffs and volcaniclastic sediments have produced, at depth, conditions similar to the Wairakei geothermal system? And finally, when did the graben form and how recently has faulting occurred along and within the graben? Our efforts have been directed at studying the tectonic and magmatic development of the central Oregon Cascades from the vicinity of the Three Sisters northward to Mt. Jefferson (Fig. 1). In this area, the High Cascades are partly bounded by inward facing fault scarps. The Green Ridge fault scarp, in western Jefferson County, separates late High Cascades volcanics (less than 2.5 m.y.b.p.) on the west, from the late Miocene-early Pliocene Deschutes Formation to the east. The Deschutes Formation represents a large volume of ash-flow tuffs and lava flows erupted from ancestral High Cascade volcanoes over a short time interval ( m.y.b.p.). None of the Deschutes Formation source volcanoes are exposed in the modern High Cascade Range. Approximately 4.5 m.y.b.p. the ancestral High Cascade volcanoes subsided into a north-south volcano-tectonic depression, bounded locally by the Green Ridge fault, and were completely buried by a flood of late Pliocene-Pleistocene basalts and basaltic andesites. The prominent shield and composite volcanoes which dominate the present High Cascades skyline were constructed upon this mafic platform (Fig. 2). PRE-SUBSIDENCE VOLCANISM AND EAST FLANK STRUCTURE The Deschutes Formation provides the best exposed, most accessible, and most lithologically diverse record of the volcanic episode which culminated in the development of the central Oregon High Cascade graben. The type and distribution of erupted material and its major-element chemistry has been the subject of a number of Oregon State University graduate student theses in recent years. Presently, a volcanic stratigraphy of the Deschutes Formation is being compiled to illustrate the development of the pre-subsidence volcanic episode. Preliminary results of this work place several constraints on the questions posed by the title of this paper. When did the faulting at Green Ridge occur? The youngest rock, of Cascade provenance, dated from the Deschutes Formation, by K-Ar method, is 4.7k0.4 m.y. (Armstrong et al., 1975; recalculated by Fiebelkorn et al., 1982). This date is believed to represent the time of faulting and, consequently, the termination of Deschutes Formation deposition as the central Deschutes basin became 27 5

3 0 IO 20 KM +MJ HI GH ASCADE S GRABEN + + TS + /- SILK IC, ( HIGHLAND) \\ / L- 1 FIG. 1. Central Oregon Cascades region. Patterns show distribution of key units in the Deschutes Formation discussed in the text: 1 = northern Deschutes basin rimrock basalts (mafic platform lavas?); 2 = intraformational lavas with Cascade provenance; 3 = approximate known distribution of the McKenzie Canyon tuff member, MJ = Mt. Jefferson; TS = Three Sisters; GRF = Green Ridge Fault. 27 6

4 Deschutes basin. Mafic Hign I Western High Cascades Green Dechutes Cascades Graben Ridge Basin FIG. 2. Diagrammatic cross-section of central Oregon Cascade Range (modified from Taylor, 1981). Stippled pattern west of Green Ridge represents possible volcaniclastic graben-fill. isolated from the source volcanoes by the intervening Green Ridge scarp. The absence of known faults within the central High Cascade platform (Taylor, 1978) suggests that there has been no subsequent faulting in this area for at least the last 1.0 m.y. Where are the boundaries of the central Oregon High Cascade graben? The Green Ridge scarp, which rises over 700 m above the High Cascade platform, is not continuous, north or south, as a topographic feature (Fig. 1). This does not preclude the continuation of normal faults to the north and south of Green Ridge, but does strongly suggest that the displacement on such faults must have been of a lesser magnitude so that they were more easily eroded or buried by later volcanics. An east-facing topographic escarpment, complimentary to Green Ridge, divides the Western Cascades from the High Cascades between the,latitudes of Mt. Jefferson and South Sister (Taylor, 1978). This escarpment (Fig. 1) is capped by rocks, of age and lithology similar to the Qeschutes Formation, some of which were probably derived from volcanic centers now buried beneath the High Cascade graben-fill. What is the origin of the central Oregon High Cascade graben? Analysis of the volcanism preserved in the Deschukes Formation offers constraints, but no firm conclusions, about the structural character of the graben. The discontinuity of the Green Ridge fault scarp seems, at first glance, to suggest that the depression may be a local, albeit large, cauldron subsidence rather than part of a long, continuous graben. Such a caldera would be similar in scale as the Toba (Indonesia) or Lake Atitlan (Guatemala) depressions which are associated with ash-flow tuffs whose volumes are on the order of cubic kilometers. The pre-subsidence volcanism recorded in the Deschutes Formation includes several dozen ash-flow tuffs, but none with an estimated volume exceeding a few tens of cubic kilometers. Furthermore, there are no voluminous ash-flow tuffs exposed near the top of the Deschutes Formation stratigraphy which could be associated $th the formation of a giant caldera. It is unlikely that the record of such a paroxysm, if it occurred, would not be preserved somewhere in the Based on the nature of volcanism which has occurred in the central Oregon Cascades in the last 6 m.y., an extensional tectonic origin for the High Cascade graben is preferred. The flood of diktytaxitic basalts and basaltic andesites following, and contemporaneous with, graben formation suggests the passage of large volumes of primitive magma through the crust without fractionation. Many of these basalts contain Ti02 in excess of 1.2 wt.% which is atypical of subduction-related volcanism (Green, 1980), and have overall major-element composition similar to midocean ridge basalts. There are also textural and compositional similarities be tween these lavas and basalts erupted during contemporaneous Basin and Range extension to the east (Priest et al., 1982). Lavas of similar composition and texture are present in lesser volumes within the Deschutes Formation and appear in the Western Cascades as early as 9 m.y.b.p. (Priest et al., 1982). Onset of extension would have allowed for rapid evacdtion of differentiated crustal magma reservoirs resulting in the large volume of pyroclastic volcanism recorded in the Deschutes Formation. Primitive, mantle derived basalts began erupting at the same time and may have, in rare instances, mixed with more fractionated magmas to produce compositionally heterogenous ash-f low tuffs. As extension continued, structural depressions were formed along the axis of the arc and were partly or completely filled with diktytaxitic olivine basalts to produce the mafic platform. Analysis of the distribution of volcanic material within the Deschutes Formation suggests that greater subsidence occurred within the graben where more massive volcanic edifices had been constructed. The Green Ridge escarpment exposure is dominated by lavas of basalt to.rhyodacite composition. All but the most fels.ic of these Cascade-derived lithologies are represented in the Deschutes basin at least 12 km east of the Green Ridge crest. This suggests that one or more volcanic centers were located just west of Green Ridge during Deschutes Formation time and erupted sufficient volumes of lava that many of the flows were able to travel far from their source (Fig. 1). Numerous ash-flow tuffs are also represented from this source area with one channelled ash-flow extending at least 50 km beyond the site of Green Ridge. In contrast, in the area immediately to the north of Green Ridge, where no topographic expression of a major fault exists, the Deschutes Formation stratigraphy is characterized by a complete absence of intraformational lava flows which could have been erupted from the Cascades, at that latitude. Lava flows erupted in the Cascades adjacent to the northern Deschutes basin ere, therefore, not of sufficient volume to flow into the basin as was true of their southern counterparts. Reconnaissance mapping in the northern Deschutes basin suggests, also, that a much smaller volume of pyroclastic material was erupted from volcanoes north of the latitude of Green Ridge. It seems unlikely that a close correspondence between areas of large-volume eruptive activity and areas of exensive subsidence is coinci- 277

5 dental. Perhaps, under the influence of regional extension, subsidence along the Cascade axis was greater where the crust was loaded by large volcanic edifices. Therefore, a high scarp was produced at Green Ridge because of higher original constructional elevation and subsequent greater amount of subsidence. To the north, the crust was not as severely loaded and, thus, did not subside as much. The role of long-continued invasion by hot, sub-volcanic magmas in weakening the crust under volcanic centers is also probably important to this argument. mgals Any scarp which did form on strike with Green Ridge was subsequently buried by a flood of diktytaxitic olivine tholeiites which flowed out of the Cascades into the adjacent basin on top of the Deschutes Formation (Fig. 2). No reliable age data exist for these rimrock basalts in the northern Deschutes basin, but it is possible that they represent mafic platform lavas which were not confined to the graben by a fault scarp as is apparently true at the latitude of Green Ridge. WHAT FILLS THE HIGH CASCADE GRABEN? Although the stratigraphy of the Deschutes Formation offers important constraints on the timing and geometry of the central Oregon High Cascades graben, there are few clues as to what fills this depression. The oldest mafic platform lavas in this vicinity are approximately 2.5 m.y. old (Armstrong et al., 1975). If subsidence occurred around 4.5 m.y.b.p., as suggested earlier, then about 2 m.y. of record is lost to view beneath the oldest exposed platform basalts. Because extension was the likely mechanism for initiating eruption of the platform lavas, it is probable that diktytaxitic basalts form nearly all of the unexposed graben-fill as well. Residual gravity anomalies for the Oregon Cascades (calculated by Couch et al. 1982) show an elongate N-S gravity low west of Green Ridge which may represent several hundred meters of low-density volcaniclastic material (Fig. 3). One must be cautious in the interpretation of a single anomaly portrayed on a regional gravity map because of large density variations over such a large area (Couch et al., 1982). However, it is possible that this anomaly west of Green Ridge indicates the presence of volcaniclastic material, of pyroclastic or epiclastic origin, which was ponded against the fault scarp during subsidence and subsequently buried by mafic platform lavas (Fig. 2). Other than deep-drilling, there is no way to accurately predict the nature or permeability of possible graben-fill volcaniclastics. Volcaniclastic material in the Deschutes Formation km east of Green Ridge is largely ash-flow tuffs and coarse volcanic sandstones. These units show little evidence of diagenetic alteration (except incipient clay development) and are very permeable down to the deepest levels exposed - about 250 m. Welded zones in ash-flow tuffs, baked zones underlying lava flows, and rare, continuous lacustrine mudstone horizons create perched aquifers. Epiclastic material is rare in the Green Ridge es- FIG. 3. Residual gravity anomaly map (reduction density 2.43 gm/cc) of the central Oregon High Cascades and Deschutes basin (after Couch et al., 1982). MJ = Mt. Jefferson; TS = Three Sisters; GRF = Green Ridge Fault. carpment itself, so any epiclastic debris within the graben must represent graben-f ill and not lowdensity fault blocks of Deschutes Formation. This sediment would likely be dominated by lacustrine deposition processes developed as a response to the faulting. Faulted exposures of lake sediment near the west base of Green Ridge may represent the type of lacustrine sequences to be expected within the graben. The exposed lithologies are permeable diatomite interbedded with fine sand to silt rhythmites suggestive of turbidity currents in a large lake. SOUTHERN DESCHUTES BASIN AND THE "SILICIC HIGHLAND" The Green Ridge scarp terminates to the south within a zone of discontinuous, NW-SE, en echelon normal faults of the Brothers fault system (Fig. 1). The faults, with displacements rarely exceeding a few tens of meters, have been active through the Pleistocene, and possibly into the Holocene, but do not continue as young structures into the High Cascades. Southwest of the Brothers fault zone is a broad highland of volcanic domes, silicic lavas, and ash-flow tuffs, of early Pleistocene age, extending eastward from the High Cascade mafic plat- 278

6 form toward Bend (Taylor, 1978, 1980). This "silicic highland" (Fig. 1) may cover a southward extension of the Green Ridge fault. However, part of this highland may have originated before the Pleistocene. Sediment and ash-f low dispersal patterns within the Deschutes basin strongly suggest a highland east of the High Cascades, in this area, as early as late Miocene. A remnant of this older feature, not buried by younger volcanics, exists in limited outcrop on the lower flanks of the "silicic highland" about 15 km northwest of Bend. Numerous ash-flow tuffs entered the basin from the southwest and may have been derived from sources which are buried under "silicic highland" domes. The largest of these ash-flows, the McKenzie Canyon tuff member and Lower Bridge tuff member of the Deschutes Formation, each extend as sheets at least 50 km from potential sources under the "silicic highland" (Fig. 1). Exposure does not permit accurate estimates of the volumes of these ash-flows, but, if a source under the present is assumed, the minimum dispersal distance is about 50% further than for Mt. Mazama ash-flows east of Crater Lake. Therefore, it is quite likely that the volcanism characteristic of the "silicic highland" originated prior to the Pleistocene and suggests that high-level magma chambers of high-silica andesite to rhyolite composition have existed in this region for several million years. Eruption rates at this locus may have been great enough to obscure any faulting associated with graben development and, perhaps, large calderas. CONCLUSIONS Evaluation of the stratigraphy of the Deschutes Formation places several constraints on models for the origin of the central Oregon portion of the High Cascades graben. A period of regional extension began perhaps as early as 10 m.y.b.p. (Priest et al., 1982) but is best documented in the central Oregon Cascades by voluminous pyroclastic volcanism of the Deschutes Formation beginning at about 6.5 m.y.b.p. This extension culminated in the formation of the graben at about 4.5 m.y.b.p. Crustal extension released primitive basalt magmas from the upper mantle which partly filled the depression before the construction of the modern shield and composite volcanoes. The possibility that permeable volcaniclastic material also constitutes a significant portion of the grabenfill is suggested by a negative residual gravity anomaly (Couch et al., 1982) but is far from conclusive. More detailed geophysical exploration and drilling will be necessary to determine the possibility of a geothermal system similar to Wairakei. highland for over 5 m.y. and is atypical when compared to the dominance of mafic lavas within the modern High Cascades. REFERENCES CITED Allen, J. E., 1966, The Cascade Range volcanotectonic depression of Oregon, & Lunar Geological Field Conference, Transactions, p Armstrong, R. L., Taylor, E. M., Hales, P. 0. and Parker, D. J., 1975, K-Ar dates for volcanic rocks, central Cascade Range of Oregon: Isochron/West, no. 13, p Couch, R. W., Pitts, G. S., Gemperle, M., Braman, D. E., and Veen, C. A., 1982, Gravity anomalies in the Cascade Range of Oregon: Structural and thermal implications : Oregon Dept. of Geol. and Min. Ind. Open-File Rept , 66 p. Fiebelkorn, R. B., Walker, G. W., MacLeod, N. S., McKee, E. H., and Smith, J. G., 1982, Index to K-Ar age determinations for the State of Oregon: U.S. Geol. Surv. Open-File Rept , 40 p. Green, T. H., 1980, Island arc and continentbuilding magmatism - a review of petrogenic models based on experimental petrology and geochemitry: Tectonophysics, v. 63, p Priest, G. R., Woller, N. M., Black, G. L., and Evans, S. H., 1982, Overview of the geology and geothermal resources of the central Oregon Cascades, & Priest, G. R. and Vogt, B. F., eds., Geology and Geothermal Resources of the Cascades, Oregon: Oregon Dept. of Geol. and Min. Ind. Open-File Rpt , p Taylor, E. M., 1978, Field geology of the S.W. Broken Top quadrangle, Oregon: Oregon Dept. of Geol. and Min. Ind. Special Paper 2, 50 p., 1980, High Cascade ash-flow tuffs and pumice deposits in the vicinity of Bend, Oregon (abstr.): Geol. SOC. of America, Abst. with Prog., V. 12, p. 156., 1981, Central High Cascade roadside geology - Bend, Sisters, McKenzie Pass, and Santiam Pass, Oregon, & Johnson, D. A., and Donelly-Nolan, J., eds., Guides to some volcanic terranes in Washington, Idaho, Oregon, and Northern California: U.S. Geol. Surv. Circular 838, p Although the oldest rocks exposed in the "silicic highland" between the Three Sisters and Bend.are probably of early Pleistocene age (Taylor, 1978), a similar feature probably existed at least by late Miocene. Construction of silicic domes and eruption of moderate to large volume ash-flow tuffs has been characteristic of this 27 9