In 1981, the U.S. Geological Survey con- ducted a seismic-refraction experiment in northeastern California designed to study the. Klamath Mountains, Cascade ...
A geologic interpretation of seismic-refraction results in northeastern California G. S. FUIS J. J. ZUCCA* W. D. MOONEY B. MILKEREIT*
U.S. Geological Survey, Menlo Park, California 94025
ABSTRACT In 1981, the U.S. Geological Survey conducted a seismic-refraction experiment in northeastern California designed to study the Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range provinces. Key profiles include 135-km-long, north-south lines in the Klamath Mountains and Modoc Plateau provinces and a 260-km-long, eastwest line crossing all of the provinces. The seismic-velocity models for the Klamath and Modoc lines are comparatively homogeneous laterally but are quite different from each other. The Klamath model is finely layered from the surface to at least 14-km depth, consisting of a series of high-velocity layers (6.1-6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. A layer with an unreversed velocity of 7.0 km/s extends from 14 km to an unknown depth. The Modoc model, in contrast, is relatively thickly layered and has lower velocities than does the Klamath model at all depths down to 25 km. An upper layer, 4.5 km thick, of low-velocity material (2.1-4.4 km/s) overlies a basement with a considerably higher velocity (6.2 km/s). Velocity increases slowly with depth, with a small velocity step (to 6.4 km/s) at 11 km and a 7.0-km/s layer beginning at 25-km depth. Moho is probably 38-45 km deep under the Modoc Plateau, but its depth is unknown under the Klamath Mountains. A combined velocity-density model for the eastwest line consists of a western part similar in configuration to the Klamath velocity model, an eastern part similar to the Modoc velocity
•Present addresses: (Zucca) Lawrence Livermore National Laboratory, Livermore, California; (Milkereit) Earth Physics Branch of Energy, Mines and Resources Department, Ottawa, Canada.
model, and laterally changing velocity-density structure in between, in the Cascade Range. Beneath its upper layer, the velocity model for the Modoc Plateau is similar to that determined by other researchers for the adjacent Sierra Nevada. The velocity model is unlike those for rift areas, to which the Modoc Plateau has been compared by some authors. We theorize that beneath a veneer of volcanic and sedimentary rocks (the upper layer), the Modoc Plateau is underlain by a basement of granitic and metamorphic rocks that, like rocks in the Sierra Nevada, are the roots of one or more magmatic arcs. The fine layering in the Klamath seismicvelocity model is consistent with the geologic structure of the Klamath Mountains, characterized by imbricate thrusting of oceanic rock layers of various compositions and ages. Independent modeling of aeromagnetic data indicates that the base of the Trinity ultramafic sheet, the second major rock layer down in the structural sequence, corresponds to a velocity step to 6.7 km/s at 7-km depth in our model. The 6.7-km/s layer beneath the Trinity ultramafic sheet apparently corresponds to rocks of the central metamorphic belt, which are mafic schists. Rock units structurally deeper than rocks of the central metamorphic belt can be correlated with velocity layers below the 6.7-km/s layer, but with less certainty. In the model for the east-west line, the region of laterally changing velocity structure beneath the Cascade Range includes a 10-km step down to the east in the top of the 7.0km/s layer. This region of lateral velocity change we interpret to be a fault, fold, or intrusive contact (or some combination of the three) between the stack of oceanic rock layers that underlie the Klamath Mountains and the buried roots of magmatic arcs inferred to underlie the Modoc Plateau. Magmas form-
Geological Society of America Bulletin, v. 98, p. 53-65,11 figs., 1 table, January 1987. 53
ing the modern Cascade Range arc apparently rise through this region. INTRODUCTION In 1981, the U.S. Geological Survey conducted a seismic-refraction survey of northeastern California designed to characterize the structure in four of the geologic provinces of that region (Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range provinces, Fig. 1) and to establish the relationships among these provinces. The survey was also intended to define the structural setting of two large Cascade Range volcanoes, Mount Shasta and Medicine Lake volcano. The seismic-refraction data and geophysical interpretation of the survey are reported elsewhere (Zucca and others, 1986). We here present a geologic interpretation of these results. Two 135-km-long, north-south, seismic-refraction lines were laid out parallel or nearly parallel to the strike of the geological structures in the Klamath Mountains and Modoc Plateau (Fig. 2). Two 130-km-long, northwest-southeast lines were centered on Mount Shasta and Medicine Lake volcano and coincided with lines of a teleseismic P-delay experiment (Evans, 1982). A 260-km-long, east-west line linked all of the profiles together. Each seismic-refraction line consisted of 100 portable seismographs with spacing ranging from 0.5 km near shotpoints to 1.75 km at greater distances. Most lines also had widely spaced (5-km) stations near one end for reconnaissance recording of arrivals from deeper layers. The seismographs record the output of 2-Hz seismometers in FM-analog form on cassette tapes (Healy and others, 1982); they are capable of rapid deployment, programmable turn-on, and rapid playback in the field or laboratory. Most lines recorded at least three shots, one at either end and one in the middle. The east-west
54
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Figure 1. Index map showing layout of seismic-refraction survey in northern California. Geologic provinces surveyed include the Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range. line was recorded in two halves with each half recording 3 - 4 local shots plus a distant shot (at shotpoints 2 and 1.3) to provide coverage of deeper layers. GEOLOGIC SETTING The Klamath Mountains province is characterized within the ¡survey area (Figs. 1, 2) by rugged mountains and narrow valleys. The province is underlain by several north-trending lithotectonic belts, or subprovinces, that are arcuate and convex to the west. Within the region of our experiment, these include, from east to west, the eastern Klamath belt, the Trinity ultramafic sheet of Irwin (1977), the central metamorphic belt, and the western Paleozoic and Triassic belt, the last of which is further subdivided into several terrenes (Fig. 2; Irwin, 1966, 1972, 1977). All contacts except that between
the first two belts are clearly thrust faults. The eastern Klamath belt consists of sedimentary and volcanic rocks, of forearc or arc affinity, with a probable aggregate thickness of more than 13 km and ages ranging from Middle Jurassic to Ordovician (Irwin, 1966, 1977, 1981). In the outcrop area east of Trinity reservoir and south of Mount Shasta (Fig. 2), these rocks dip generally eastward. Between Callahan and Yreka, however, the rocks are cut by thrust faults, including the Mallethead thrust fault, and are metamorphosed in places to lower greenschist fades. The Trinity ultramafic sheet of Irwin (1977) consists mostly of serpentinized ultramafic rocks with lesser intrusive bodies of gabbro that are Ordovician in age (Lanphere and others, 1968; Mattinson and Hopson, 1972). The central metamorphic belt consists of the Salmon Hornblende Schist and Abrams Mica
Schist (Davis and others, 1965; Irwin, 1966). These rocks are isoclinally folded, and their structural thickness ranges from less than a kilometre to several kilometres (Irwin, 1977; Irwin and Dennis, 1979; Davis and others, 1980). The western Paleozoic and Triassic belt is composed of at leasi; two terranes within the region of our survey: the Stuart Fork Formation, greenschistfacies metasedimentary and metavolcanic rocks of oceanic affinity (Davis and Lipman, 1962; Davis and others, 1965); and the North Fork terrane of Ando and others (1983), composed of similar but unmetamorphosed rocks in its upper part and late Paleozoic ophiolite in its lower part. All of the lithotectonic belts of the Klamath Mountains are intruded by Upper Jurassic and Lower Cretaceous plutons that are dominantly quartz diorite (Irwin, 1966; Hotz, 1971; Irwin, 1985, orail commun.). These lithotectonic belts
55
INTERPRETATION O F SEISMIC-REFRACTION RESULTS, CALIFORNIA OREGON
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SO KM EXPLANATION KLAMATH MOUNTAINS PROVINCE
V O L C A N I C BOCKS (OENOZOIC) - • DIVIDED INTO : YOUNGER BASALT (QUATERNAHF AND PLIOCENE) - - YOUNGER THAN S U.Y,
EASTERN KLAMATH BELT (JURASSIC TO ORDOVICIAN) - - SEDIMENTARY AND VOLCANIC HOCKS
YOUNGER ANDESITE [QUATE RNAR V AND PLIOCENE) - • YOUNGER THAN i u . v .
UNDIVIOEO ROOKS (JURASSIC TO PALEOZOIC)
OLDER BASALT (MIOCENE AND OLDER) * • OLDER THAN S M Y.
STUART FORK FORMATION AND UPPER PARI OF NORTH FORK TEHRANE OF ANDO AND OTHERS 0 9 8 3 ) TJUHASSIC TO UPPER PALE07CHC) - • METAMORPHOSED ANO JNMETAMORPHOSED SEDIMENTARY AND VOLCANIC ROCKS
OLDER ANDESITE (MIOCENE AND OLDER] * • OLDER THAN 5 M.Y
LOWER PART OF NORTH FORK TERHANE (UPPER PALEOZOIC) • • OPHIOLITE
OLDER RHYOLITE AND DACITE [MIOCENE AND OLDER) - - OLDER THAN S U . V .
SALMON HORNBLENDE SCHIST ANO ABRAMS MICA SCHIST, UNDIVIDED (DEVONIAN METAMORPHIG AGE)
YOUNGER « H Y O L I T E AND DACITE ¡QUATERNARY AND PLIOCENE) - • YOUNGER THAN I M.Y.
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UNDIVIDED VOLCANIC AND SEDIMENTARY HOCKS (TERTIARY)
CONTACT FAULTS: HIGH ANGLE AND THRUST (TEETH ON UPPER PLATE) DASHED » H E R E UNCERTAIN. DOTTED WHERE BURIED VOLCANIC VENTS LESS THAN 100.000 YR. OLO ABBREVIATIONS GBF
TRINITY ULTRAMAFIC SHEET OF IRWIN ( 1 9 7 J ) (ORDOVICIAN) - - UMT, AS MAPPEO. INCLUDES OTHER ULTRAMAFIC ROCKS OF UNKNOWN AGES
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SEDIMENTARY HOCKS (TERTIARY AND UPPER CRETACEOUS) INTRUSIVE HOCKS (CRETACEOUS AND JURASSIC) * - CHIEFLY GRANITIC ROCKS, LESSER OABBRO
Figure 2. Geologic map of study area, compiled from Jennings (1977), Davis and others (1965), Potter and others (1977), Hotz (1977), Ando and others (1983), and Luedke and Smith (1981). Seismic-refraction lines are solid lines and shotpoints are large solid dots with numbers. and plutons are overlain by Lower and Upper Cretaceous strata of the Great Valley sequence. The Cascade Range in northern California is a southeastward-trending chain of shield and small composite volcanoes. This chain is flanked on the southwest by Mount Shasta, a stratovolcano, and on the northeast by Medicine Lake volcano, a shield volcano. Both volcanoes are much larger and longer lived than other individual volcanoes of the Cascade chain (Christiansen, 1982; Donnelly-Nolan, 1983). Broad valleys also flank the Cascade chain. Rocks exposed in this province include Upper Cretaceous and Eocene sedimentary rocks, and volcanic
rocks divisible into the Eocene to Miocene Western Cascade sequence and the Pliocene to Holocene High Cascade sequence (Peck and others, 1964; Macdonald, 1966). Both sequences consist of lava flows and volcaniclastic deposits that are chiefly pyroxene andesite. Rocks of the older sequence are faulted and tilted northeastward and are unconformably overlain by rocks of the younger sequence. The boundaries between the Cascade Range, Modoc Plateau, and Basin and Range provinces are not distinct because block faulting and similar rocks characterize all three provinces. The Modoc Plateau province south of latitude
41°30' is characterized chiefly by northwesttrending ranges, and north of that latitude, by a plateau. The part of the Basin and Range province that is in the region of our survey includes Surprise Valley and the Warner Mountains, the westernmost basin and range, respectively, of the province. The Modoc Plateau province and the Warner Mountains are underlain by a section of rocks that includes at its base volcaniclastic deposits of early Tertiary age overlain by interbedded Miocene tuffaceous continental and lake deposits and flood-basalt flows (Macdonald, 1966; Duffield and Weldin, 1976). The basal volcaniclastic deposits are largely andesitic
56
in composition; the overlying tuffaceous deposits are largely rhyolitic; and the lava flows are largely high-alumina basalt similar to that erupted at mid-ocean ridges or back-arc basins (McKee and others, 1983). Rocks of the Modoc Plateau are cut by northwest- to north-trending normal faults, but major right-lateral strike-slip is reported for the Likely fault (Fig. 2; Macdonald, 1966). In the Basin and Range province, the Warner Mountains are tilted gently westward, and the Surp rise Valley block is dropped downward by more than a kilometre along the Surprise Valley fault (see below) and covered by thick Quaternary alluvium and lake deposits.
FUIS AND OTHERS
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Seismic-refraction data were recorded in 1962 along a line from Lake Shasta (Fig. 2) southeastward through the southern Cascade Range and northern Sierra Nevada provinces to Mono Lake, on the east edge of the Sierra Nevada (Roller anc. others, 1963; Roller and Gibbs, 1964). The interpretation of these data by Eaton (1966) and Prodehl (1979) differ to a large degree at the: north end of the line. Eaton indicated the shoaling of an intermediatevelocity layer northward under the Cascade Range province (a. 6.8-km/s layer rising to less than 10-km depth), whereas Prodehl's results indicate a similar shoaling of an intermediatevelocity layer (6.4- to 6.5-km/s layer rising to less than 10-km depth) coupled with an underlying low-velocity zone (LVZ; as low as 6.1 km/s between 10- and 20-km depth). The interpretations are in closer agreement on velocity structure in the northern Sierra Nevada (both show a 6.4-km/s layer beginning at 10-20-km depth, no LVZ, and a crustal thickness of 41-47 km).
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PREVIOUS GEOPHYSICAL WORK The Klamath Mountains and the Cascade Range have been the subjects of several geophysical studies in the past two decades. The earliest work revealed an eastward decrease in the Bouguer gravity across the Cascades, which suggested an eastward thickening of the crust (LaFehr, 1965, 1966). In addition, the gravity work provided evidence of a large volume of low-density material beneath a large area, including Mount Shasta and the Cascade chain of volcanoes. In contrast to this gravity low, a gravity high is seen over Medicine Lake volcano. This high is more pronounced when a Bouguer reduction density more appropriate for extrusive rocks is used (2.2 g/cm 3 ; Finn and Williams, 1982). Finn and Williams (1982) modeled the gravity data over Medicine Lake volcano with a shallow intrusive complex in the upper crust that is 0.4 g/cm 3 denser than surrounding rocks. This interpretation is supported by the electromagnetic sounding work of Stanley (1982).
MTS
VELOCITY ( k m / s e c )
Figure 3. Velocity model for Klamath line (A) and velocity-depth sections through model (B). Numbers on model are average velocities, in km/s, for layers. Parentheses indicate velocity that is uncertain (inferred from second arrivals; see Zucca and others, 1986). Asterisk indicates that velocity is supported by data on cross line. Layers with no velocity gradient or reversal in velocity gradient (LVZ's) are indicated by diagonal lines. Dotted areas indicate strong lateral velocity change. Layer boundaries are dashed where no ray coverage. Inverted solid triangles indicate shotpoints and locations of velocity-depth sections in Figure 3B. Names in parentheses are projected onto line. Vertical exaggeration is 2x.
Leaver and others (1984) used a combination of earthquakes and artificial sources recorded at permanent and portable seismograph stations to model a north-south profile along the axis of the Oregon Cascades. Their velocity-depth model is quite similar to the model for the northern Sierra Nevada obtained by Eaton and Prodehl, including a crustal thickness of 42 km, but an intermediate crustal velocity of 6.4 km/s is encountered everywhere at shallower depths (less than 10 km). RESULTS FROM MODELING SEISMIC-REFRACTION AND GRAVITY DATA We summarize here the velocity models for the Klamath and Modoc lines, and a combined velocity-density model for the east-west line. None of the models discussed below are unique, because they contain lateral velocity changes and LVZ's. We estimate that velocities are accu-
rate to a few percent except in LVZ's, where they are poorly known. Depths to velocity boundaries are probably accurate to 5%-10%. Below LVZ's, depths to boundaries are more uncertain. Choosing the type of velocity boundary to be used in a given model (for example, a step in velocity versus a velocity gradient) accounts for most of the variability possible in this type of forward modeling. We have chosen the types of boundaries that seem to provide the best match to secondary arrivals in our record sections, but in some cases alternate models (not discussed here) are equally plausible (Zucca and others, 1986). Because of station spacing (1 km) and signal frequencies (5-10 Hz), we cannot in most cases resolve layers with thicknesses less than 14-1 km or offsets less than 14-1 km. Klamath Line The Klamath line is 130 km long, extending from just north of Yreka, California, to Trinity
INTERPRETATION OF SEISMIC-REFRACTION RESULTS, CALIFORNIA CA/OR border North ^
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generally more thickly layered. A 7-km/s velocity is encountered first at 24.5-km depth. Lateral velocity variation is stronger as well, particularly in the upper part of the model. For example, at shotpoint 11, a local high-velocity layer (5.5 km/s) overlies a local LVZ (2.5? km/s), and the aggregate thickness of the upper layers decreases from 4.5 km north of the shotpoint to nearly half that to the south. Note that the model is "unreversed" and hence poorly controlled south of shotpoint 12. An important feature of the model is the strong velocity contrast between the 4.4-km/s layer and the "basement" (6.2 km/s), giving rise to a clear reflection in the data (see Zucca and others, 1986). A thin (2-km) LVZ appears to be present at the transition from the 6.2 to the 6.45-km/s layer. East-West Line
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304 6 VELOCITY
8 (KM/SEC)
Figure 4. Velocity model for Modoc line (A) and velocity-depth sections through the model (B). Refer to caption for Figure 3 for explanation.
reservoir, and southward at increased instrument spacing (Figs. 1,2). Between shotpoints 1 and 2, the profile lies primarily on the sedimentary and volcanic rocks of the eastern Klamath belt, but near its north end, it crosses obliquely several underlying thin lithotectonic belts. Between Callahan and Trinity reservoir, the profile traverses the Trinity ultramafic sheet of Irwin (1977) and ends in the eastern Klamath belt, which is faulted against the Trinity sheet near shotpoint 3. The velocity model for the Klamath line (Fig. 3) is finely layered from the surface to a depth of 14 km, where depth is measured below sea level. Below the uppermost layer (4.7 km/s), it consists of a series of high-velocity layers (6.1-6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. Layers with negative velocity gradients are LVZ's. A layer with an unreversed (and hence uncertain) velocity of 7.0 km/s extends from 14 km downward to an unknown depth. Relatively little structure, or lateral velocity variation, is seen along this profile.
Modoc Line The Modoc line is 140 km in length, extending from the Oregon border to Adin, California, and beyond with increased station spacing (Figs. 1,2). The profile lies entirely within the Modoc Plateau province. It crosses flood basalts between shotpoints 10 and 11, and, south of shotpoint 11, horsts, grabens, and tilted blocks exposing interbedded flood basalts, lake deposits, pyroclastic beds, and the basal andesitic volcaniclastic series of this province. Faults bounding these blocks strike northwest, parallel to the Likely fault, a major strike-slip fault passing approximately beneath shotpoint 11 (Figs. 1, 2). The velocity model for the Modoc line (Fig. 4) is quite different from that for the Klamath line. Generally lower velocities are found at all depths. The upper part of the model has a greater thickness (3 layers with velocities of 2.1-4.4 km/s, aggregate thickness of 4.5 km) compared to the Klamath model (1 layer with a velocity 4.7 km/s, thickness of 1 km). The lower part of the model (6.2-km/s layer and deeper) is
The east-west line is 260 km long and connects shotpoints 2, 4, 5, 14, 11, and 13 (Fig. 1), crossing varied terrain and geology (Fig. 2). Shotpoint 2 is located in the Klamath Mountains, and shotpoint 4 is located near the boundary of the Klamath Mountains and Cascade Range provinces. Shotpoint 5, at Yellow Butte, is on a horst and inlier of Klamath Mountains rocks exposed on the north flank of Mount Shasta. Shotpoint 14 is located on top of Medicine Lake volcano. Shotpoint 11 is on the Modoc Plateau, and shotpoint 13 is in Surprise Valley, in the Basin and Range province. Major mapped geologic structures crossed include (1) the faults bounding the horst at Yellow Butte; (2) the fault zone near Tennant, California, which is a zone of north-striking, obliqueslip faults that is currently generating earthquakes; (3) the terminus of the Likely fault; and (4) the Surprise Valley fault. In addition to exposed structures, the profile crosses a very steep gravity gradient near shotpoint 4; furthermore, several north-striking faults east of Tennant project beneath Medicine Lake volcano and may be buried structures there (J. M. DonnellyNolan, 1983, oral commun.). Among these, the Gillem Bluff fault shows the largest offset. We converted the east-west, seismic-velocity model (Zucca and others, 1986) to a density model (Fig. 5) to allow comparison with observed Bouguer gravity. For the "basement" layer (second layer down), densities cited by LaFehr (1965) were used initially: 2.71 g/cm 3 for Klamath province rocks ("Bedrock Series" of LaFehr) and 2.55 g/cm 3 for volcanic basement near Mount Shasta ("volcanic basement rocks" of LaFehr). The latter density was subsequently reduced to improve the fit to the data. For other layers, the velocity-density relationships of J. E. Nafe in Dobrin (1976) and Birch
58
FUIS AND OTHERS WEST
EAST
DISTANCE
(KM)
Figure 5. (A) Gravity data for east-west line. Open squares are observed complete Bouguer gravity values read at 5-mGal contour intervals from Kim and Blank (1970) and Chapman and Bishop (1968); solid line is calculated gravity. (B) Two-dimensional velocity-density model for east-west line. Densities, in g/cm 3 , are obtained as described in text; velocities, in km/s, are listed after densities. Areas where densities had to be changed significantly from initial values to match observed gravity values are indicated by shading. Densities are changed in these areas by -0.07 g/cm 3 (light shading), -0.15 g/cm 3 (moderate shading), and +0.30 g/cm 3 (heavy shading). Short-dashed lines separate layers with velocity differences but no density differences. Long-dashed lines indicate no ray coverage. Inverted solid triangles indicate shotpoints. Vertical exaggeration is 2*.
(1961) were followed approximately (Zucca and others, 1986). Density adjustments were made in limited box-like regions in the Klamath Mountains to approximate the effect of both low- and high-density plutons (Fig. 5B). Our two-dimensional gravity calculation for the east-west line provides in most places a reasonable approximation to the gravity effects of features crossed by the line, because most features, including province boundaries, geologic contacts, and faults, have nearly north-south strikes (Figs. 1,2). Some features, such as Medicine Lake volcano and plutons in the Klamath Mountains, clearly are not well approximated by two-dimensional structure, however. Finn and Williams (19&2) discussed the gravity modeling of Medicine Lake volcano in detail. The combined velocity-density model for the east-west line is similar in configuration to the Klamath model (Fig. 3) in the west and to the
Modoc model (Fig. 4) in the east, with strong lateral velocity and density variation in between, in the Cascade Range province. Strong lateral variation occurs in the upper two layers, which thicken and become lower in velocity and density east of the boundary between the Klamath Mountains and Cascade Range provinces, and also in the lowest crustal layer (7 km/s, 2.85 g/cm 3 ), which deepens eastward by 10 km in the center of the Cascade Range province. Other features of note include (1) the effects of two granitic plutons near the profile (Figs. 2, 5, larger shaded boxes); (2) the strong effect of an unmapped body of rocks (gabbroic pluton?) within the Trinity ultramafic sheet of Irwin (1977) (Fig. 5, smallest shaded box); (3) a horstlike rise of velocity and density boundaries at Yellow Butte; (4) a body of rocks of moderately high velocity and density (5.7 km/s, 2.67 g/cm 3 ) under Medicine Lake volcano; (5) a
local LVZ under shotpoint 11, similar to the one seen in l:he Modoc model (Fig. 4); (6) a basin under Alturas, California; (7) a basin under Surprise Valley; (8) a 3° westward dip on interfaces in the upper crust under the Modoc Plateau (below the 4.4-km/s, 2.4-g/cm 3 layer); (9) a regional LVZ at 5-10 km depth beneath the Modoc Plateau and Basin and Range provinces (6.0? km/s); and (10) a horizontal(?) Moho. The last feature is of special note. Moho arrivals are not clearly observed in our seismic-refraction data. A series of weak reflections observed on the east half of the east-west line, if from the Moho, indicate a mantle depth of 38-45 km for the Modoc Plateau (Zucca and others, 1986). We have no seismic control on the depth of the Moho elsewhere, but, interestingly, a horizontal Moho in the velocity-density model (Fig. 5) is consistent with the Bouguer gravity data. We have thus modeled the observed eastward de-
INTERPRETATION OF SEISMIC-REFRACTION RESULTS, CALIFORNIA
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VELOCITY
Figure 6. Velocity-depth curves from this study compared with curves from other areas. (A) Curves for Modoc Plateau and Klamath Mountains (in area underlain by Trinity ultramafic sheet of Irwin, 1977) are shown along with curves for Sierra Nevada (Eaton, 1966) and Gabilan Range (Walter and Mooney, 1982). (B) Curve for Modoc Plateau is shown along with curves for Salton Trough (Fuis and others, 1984), a modern rift, and western and eastern Snake River Plain (Hill and Pakiser, 1967; Braile and others, 1982), an inferred rift. (C) Curve for Modoc Plateau is compared with curves for west-central and eastern Basin and Range province (Prodehl, 1979, Eureka to Fallon, Nevada; Braile and Smith, 1975, model BR1). See text for discussion.
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crease in gravity by an eastward decrease in crustal density.
25 -
DISCUSSION
30 •
The refraction survey was laid out so that at least two lines, the Klamath and the Modoc lines, would not cross geologic province boundaries and might lead to relatively straightforward interpretation. The velocity models for those two lines are complicated but are indeed the simplest of the experiment, showing the least lateral velocity variation (Zucca and others, 1986). In our efforts to relate rock type to velocity layers, we use surface geology, laboratory velocity data, independent modeling of aeromagnetic data, and comparisons with velocitydepth curves in other areas where the geology is better known. Velocity-depth curves for the Klamath and Modoc lines are reproduced together (Fig. 6A)
1
I I I II
2
3
4
VELOCITY
for ease of comparison. The Modoc curve (at shotpoint 10) includes three layers of relatively low velocity, totaling - 4 . 5 km in thickness, that overlie a basement with a velocity of 6.2 km/s. The Klamath curve is quite different. Higher velocities are seen at all depths from the surface downward. The intercalation of LVZ's gives this curve a distinct layered appearance that is generally consistent with the imbricate structure of the Klamath Mountains. A velocity of 6.9 km/s is reached at a depth of only 14 km,
I I I I' 6
7
Ckm/s)
compared to the velocity step to 7.0 km/s at 24.5 km on the Modoc curve. These velocitydepth curves clearly indicate different types of crust beneath the Modoc Plateau and the Klamath Mountains. Modoc Plateau The velocity-depth curve for the Modoc Plateau (Fig. 6A) indicates at least two major rock divisions in the upper 10 km, separated by the
60
FUIS A N D OTHERS
marked velocity step at 3.5 km below sea level. As discussed below, the upper division is volcanic and sedimentary rocks, and the lower division is inferred to be granitic and metamorphic rocks that, like in the Sierra Nevada a short distance to the south, may be the roots of one or more magmatic arcs.. Rocks exposed in the Modoc Plateau and adjacent Basin and Range province are volcanic and sedimentary rooks that are chiefly Miocene in age but are as old as Oligocene at the base (east side) of the Warner Mountains (Duffield and Weldin, 1976; W. A. Duffield and E. H. McKee, 1983, written commun.). Basement is only a kilometre or so deep in the Warner Mountains, dipping 3 degrees to the west (Fig. 5); thus, the exposed Oligocene rocks are near the base of a ti lted section of volcanic and sedimentary rocks underlying the Warner Mountains and Modoc Plateau. Considerable thought has been {¡iven to the possibility that sedimentary and (or) volcanic rocks older than Oligocene might be present beneath the Modoc Plateau, such as the Upper Cretaceous Hornbrook Formation [Nilsen, 1984), Jurassic to Ordovician rocks of the eastern Klamath belt, and Jurassic to upper Paleozoic rocks of western Nevada (Speed, 1977, 1978; Hamilton, 1978). We cannot rule out the presence of any of these rocks in our unit of volcanic and sedimentary rocks. Furthermore, some of these rocks may be present as metamorphic rocks in the basement.
E X P L A N A T I O N j
The velocity-depth curve for the Modoc Plateau below the marked step to 6.2 km/s lends itself to a relatively simple geologic interpretation by comparison to quite similar curves for the Sierra Nevada of east-central California (Eaton, 1966) and the Gabilan Range of westcentral California (Walter and Mooney, 1982) (Fig. 6A). Both the Sierra Nevada and Gabilan Range are underlain by the roots of magmatic arcs. The basement velocity of 6.0-6.1 km/s for the Sierra Nevada and Gabilan Range agrees fairly well with the Modoc basement velocity of 6.2 km/s; the velocity step to 6.4 km/s occurs at similar depths (9-10 km); the step to a higher velocity in the Sierra Nevada and Gabilan Range (6.9-8.1 km/s) brackets in depth the step to 7.0 km/s at about 25-km depth on the Modoc Plateau. The only significant difference among the curves is the lack of an LVZ above the velocity step to 6.4 km/s in the Sierra Nevadan and Gabilan curves. The most straightforward interpretation from this comparison (Fig. 6A) is that, below a volcanic and sedimentary cover, the Modoc Plateau is underlain by granitic and metamorphic rocks similar to those of the Sierra Nevada and Gabilan Ranges— rocks generated in i magmatic-arc environment. As indicated by surface exposures, a basement velocity of 6.0-6.1. km/s in the Sierra Nevada and Gabilan Ranges corresponds to granitic
| O u t c r o p of T r i n i t y u l t r a m a f l c s h e e t of l r w l n ( 1 9 7 7 )
©
C o n t o u r of t o t a l a e r o m a g n e t i c I n t e n s i t y , f r o m B l a k e l y and A o t h e r s t 1 9 8 5 ) ; c o m p l i e d at 4 5 7 1 m « v a l u e in g a m m a s : contour interval 2 5 0 gammas C o n t o u r of t o t a l
Seismic-refraction
profile
and shotpoint M a g n e t i c p r o f i l e m o d e l e d by A' G r l s c o m ( 1 9 7 7 )
aeromagnetic
intensity, s k e t c h e d from
California
D i v i s i o n of M i n e s and G e o l o g y ( 1 9 7 8 ) ; somewhat different
elevation
rocks, mostly granodiorite, with pendants and (or) septa of metamorphic rocks. The somewhat higher basement velocity on the Modoc Plateau (6.2 km/s) may indicate a somewhat higher fraction of metamorphic rocks. In the foothills of the Sierra Nevada, such rocks range in velocity from 6.0 to 6.3 km/s (Spieth and others, 1981). A velocity step to 6.4 km/s at 10-km depth on all three velocity-depth curves (Fig. 6A) presumably represents a density increase caused by a composition or metamorphic change. Possible rocks include plutonic rocks of intermediate composition, such as tonalité or diorite, metamorphic rocks of amphibolite or possibly granulite facies, or migmatites (see Smithson and Brown, 1977; Kay and Kay, 1980). The velocity step to 7.0 km/s in the Sierra Nevada and Modoc Plateau may be caused by a composition change to mafic plutonic rocks, such as gabbro
and anoithosite (see discussion of deep magmatic arc processes in Hildreth, 1981), or metamorphic rocks of granulite facies. Some additional evidence indicates that basement in the Modoc Plateau is composed of granitic and metamorphic rocks, although other evidence is contradictory. Models of magnetotelluric soundings taken along our east-west line between Medicine Lake and Surprise Valley show resistivity boundaries very similar to our seismic boundaries to depths of 15 km (Stanley, 1982; V/. D. Stanley, 1983, 1984, written communs.). Resistivities for the basement are moderate to high (greater than 100-300 ohmm), consistent with a granitic and metamorphic rock composition and in sharp contrast to the low resistivities (5-20 ohm-m) in the layers of volcanic and sedimentary rocks above. Another bit of evidence for our proposed basement com-
INTERPRETATION OF SEISMIC-REFRACTION RESULTS, CALIFORNIA
position is the occurrence of granitic cobbles in an Oligocene conglomerate at the base of the Warner Mountains (Duffield and Weldin, 1976; W. A. Duffield and E. H. McKee, 1983, written commun.). The proximity of the Sierra Nevada to our study area (60 km south of shotpoint 12) also makes it plausible to propose that granitic and metamorphic rocks underlie on the Modoc Plateau. Evidence that would appear to contradict our thesis and favor a rift filled with a new crust under the Modoc Plateau (see below) is the chemical and isotopic affinities of the basalts on the Modoc Plateau: the basalts are primitive and have been little contaminated by sialic crust (McKee and others, 1983). Basement on the Modoc Plateau may be a composite of the roots of several magmatic arcs. The oldest arc(s) for which there is evidence in this region persisted from the Silurian through the Middle Jurassic in an area east of the present Klamath Mountains (Irwin, 1966; Condie and Snansieng, 1971; Potter and others, 1977). The eastern Klamath sedimentary and volcanic rocks were laid down in forearc basins. It is, of course, not known where rocks generated in these arcs are to be found today, but the Modoc Plateau must be allowed as a possibility. Magmatism continued in the Late Jurassic and Early Cretaceous at the site of the present Klamath Mountains. In a number of episodes from Triassic to Late Cretaceous, magmatism occurred at the site of the present Sierra Nevada (Evernden and Kistler, 1970), and in the Late Cretaceous it also extended northeastward into Nevada and Idaho (Smith and others, 1971). During any of these periods, magmatism may also have occurred in the Modoc Plateau. The only epochs for which there is clear evidence for magmatism in the Modoc Plateau are the Oligocene and Miocene (see "Geologic Setting"). In summary, a series of magmatic arcs may have been accreted and (or) superposed across the site of the present Modoc Plateau. Other authors (Blake and Jones, 1977; Hamilton, 1978) have proposed that the Modoc Plateau is underlain by a rift filled with new crust, but comparison of the velocity-depth curve for the Modoc Plateau with curves for known and inferred rifts does not bear this out (Fig. 6B). These curves, including one for the Imperial Valley (Fuis and others, 1984) and two for the Snake River Plain (Hill and Pakiser, 1967; Braile and others, 1982), are characterized by velocities generally less than 6.1 km/s to 7- to 11-km depth and velocity steps to 6.5-7.2 km/s at those depths. This velocity structure apparently reflects the infilling of a rift from above by low-velocity sedimentary and volcanic rocks and from below by high-velocity mafic intrusive rocks (see Fuis and others, 1984). Although the Modoc Plateau does not appear to be underlain by a rift, Basin and Range-style
extension is clearly occurring, as indicated by block faulting and tilting (see discussion of geologic cross section). A comparison of the velocity-depth curve for the Modoc Plateau with curves for the Basin and Range province (Prodehl, 1979; Braile and Smith, 1975) shows some similarities, including an LVZ separating upper and middle crust and a middle crustal velocity of 6.4-6.5 km/s. Differences in the Basin and Range province include a lower-velocity upper crust, a deeper (14- to 16-km) middle crust, no 7-km/s layer, and a shallower Moho. In summary, of the comparisons we have made, the Modoc Plateau is most similar in velocity structure to the Sierra Nevada and least similar to rifts; some analogies are seen with Basin and Range-type extended crust. The tabular low-velocity body beneath the Modoc Plateau has no clear geologic interpretation. It may be analogous to reflectors observed by Lynn and others (1981) at 6- to 10-km depth in batholithic terranes. The origin of the reflectors is not clear, but they may be associated with the bases of batholiths. It is also possible that the LVZ is a body of rocks bounded by one or more faults. In particular, it may represent subducted sedimentary and volcanic rocks, and the 6.4km/s rocks below it, a subducted plate, in accord with Silberling's (1973) and Speed's (1977) notion of an east-facing arc that collided with the North American continent in the Early Triassic along the Golconda thrust fault of northwestern Nevada. On the other hand, it may correlate with a subhorizontal detachment fault zone such as is seen in the eastern Basin and Range province (Allmendinger and others, 1983; Gants and Smith, 1983).
Klamath Mountains In the Klamath Mountains, we are able to relate the velocity-depth structure to the geologic layering with the aid of aeromagnetic and other data. The aeromagnetic anomaly associated with the Trinity ultramafic sheet extends beyond the outcrop of the sheet into areas underlain by rocks of the eastern Klamath belt between Callahan and Yreka and also south of Mount Shasta (Fig. 7), indicating the presence of the sheet at depth in these areas (Griscom, 1977; Blakely and others, 1985). Extension of a gravity anomaly associated with gabbros in the sheet into the Callahan-Yreka area also supports the presence of the sheet at depth in that area (Griscom, 1980). Griscom (1977) modeled several profiles through the aeromagnetic anomaly, including profile A-A', which coincides approximately with our east-west line between shotpoints 4 and 5, and B-B', which crosses our Klamath line near shotpoint 3 (Fig. 7). The base of the modeled magnetic body, 6.5-8 km below
61
sea level between shotpoints 4 and 5 and 8 km deep near shotpoint 3, appears to correlate with a velocity step in our models to 6.7 km/s (Fig. 8; 6.5 km/s at Yellow Butte). Our interpretation, then, is that in the Klamath Mountains four of the upper five layers in the velocity models (Figs. 3, 5), having velocities ranging from 6.1-6.5 km/s, correlate with the Trinity ultramafic sheet. The uppermost layer, 4.7 km/s, corresponds in some places to weathered ultramafic rocks (for example, at shotpoint 2) and in other places to rocks of the eastern Klamath belt (for example, at shotpoint 3). Velocities of 6.1 to 6.5 km/s compare favorably with independent laboratory velocity measurements of ultramafic and serpentinized ultramafic rocks (Fig. 9; Table 1); in fact, one infers that from its 6.5-km/s maximum velocity that the Trinity sheet is more than 35% serpentinized. The implication of velocity layering within the Trinity sheet is discussed below. The velocity-depth curve below the Trinity ultramafic sheet can be interpreted several ways. The following interpretation fits the geology, but is by no means unique given the complexity of thrust-faulting in the Klamath Mountains. The velocity step to 6.5-6.7 km/s below the sheet is consistent with laboratory velocities for rocks of the central metamorphic belt (Fig. 9; Table 1). Estimated structural thickness for rocks of this belt is as much as a few kilometres (Irwin and Dennis, 1979). The velocity reversal (LVZ) beneath the 6.7 km/s layer might correlate with the Stuart Fork Formation, structurally below the central metamorphic belt (Davis and others, 1965), and with the upper sedimentary and volcanic sequence of the North Fork terrane of Ando and others (1983). The Stuart Fork Formation may be a metamorphosed version (lower greenschist facies) of the upper part of the North Fork terrane, which is bedded chert, argillite, limestone, and pillowed basalt flows (Ando and others, 1976, 1983). The sedimentary and metasedimentary rocks in this sequence at the appropriate pressure (3 to 4 kb) and temperature (150-250 °C; Lachenbruch and Sass, 1977) probably have velocities ranging from 5.5 to 6.5 km/s (see Stewart and Peselnick, 1977). The volcanic rocks, that make up 10% to 20% of the sequence (Davis and others, 1965), probably have velocities ranging from 6.1-6.7 km/s (Roger Stewart, 1983, written commun.). This sequence, therefore, would on the average form a low-velocity zone relative to the central metamorphic belt above it (Fig. 9). The structural thickness of these rocks is poorly known. The velocity step to 6.9 km/s has been tentatively correlated with the ophiolitic basement of the North Fork terrane of Ando and others (1983), consisting of gabbro, diabase, pillowed basalt flows, and interleaved tectonitic harzburgite. These rocks crop out west of Cecilville,
62
FUIS A N D OTHERS
Klamath
Mountains
40
Yellow Butte SP5
SP4
50
EXPLANATION
KLAMATH MOUNTAINS
F 7 T v 7>5
VOLCANIC ANO SEDIMENTARY
BOCK
( C E N O Z O I C TO UPPLH C R E T A C E O U S )
SYMBOLS
PROVINCE
3
EASTERN K L A M A T H B E I T (JURASSIC TO
3
O R O O V I C I A N ) — S E D I M E N T A R Y AND VOLCANIC
GEOLOGIC CONTACT, OASHED WHERE UNCERTAIN
DOCKS INTRUSIVE
ANO METAMOHPHIC
ROCKS
r*Tr7T~i UNOIVIOED ÛRANITIC ANO METAMORPHIC T * * » « II RR3 - ^
nn
R O C K S (CENOZOIC TO P A L E O Z O I C ? )
. ..
OH O L D E R )
MOTION
AND UPPER PART OF NORTH FORK
M I MALLETHEAO THRUST FAULT
TERRANE OF ANCO ANO OTHERS M S B 3 )
ST S I S K I Y O U THRUST FAULT
(JURASSIC TO UPPER P A L E O Z O I C ) - - M E T A .
GRANITIC R O C K S ( C R E T A C E O U S ANO JURASSIC)
MORPHOSED ANO UNMETAMQRPHOSED
F A U L T ; TEETH ON UPPER PLATE OF THRUST FAULTS, DOTTED WHERE BURIED; ARROW SHOWS RELATIVE
STUART FORK F O R M A T I O N (TBIASSIC AND/
-
SEDIMEN-
I*
SEISMIC-REFRACTION PROFILE: SC. SHOTPOINT
TARY AND V O L C A N I C ROCKS
mm
6. 1
LOWER PART OF NORTH FORK TERRANE OF ANDO AND O T H E R S < 1 9 8 3 ) (UPPER
PALEOZ040--
't
SEISMIC VELOCITY ( I N KM/5) ANO VELOCITY BOUNDARY (HORIZONTAL LINE): DIAGONAL PATTERN. LOW-VELOCITY ZONE
OPHIOLITE MODELED UAQNETIC BODY OF ORISCOM ( 1 9 7 7 ; S A L M O N HORNBLENOE SCHIST ANO ASRAMS MICA SCHIST UNDIVIDED ( D E V O N I A N METAMORPHIC
AGE)
Jim
WRITTEN COMMUN.. I B I » ; K . MAGNETIC SUSCEPTIBILITY
TRINITY ULTRAMAFIC SHEET OF IRWIN ( TOT?) (OROOVICIAN)
Figure 8. Block diagram showing magnetic models, seismic velocities, and inferred geologic cross sections through Trinity ultramafic sheet of Irwin (1977). Magnetic models were constructed independently along profiles A - A ' and B-B' (see Fig. 7) and are shown on faces of two cut-away blocks of crust. Between shotpoints 4 and 5, refraction profile coincides roughly with A--A'. Refraction profile crosses B-B' approximately at shotpoint 3. California, the end of our east-west line. Laboratory velocity for gabbros and metagabbros bracket our refraction velocity for this layer (Fig. 9; Table 1). Alternatively, the Hayfork Bally meta-andesite (a mafic schist) or some other body of high-velocity rocks may truncate the North Fork terrane at relatively shallow depth (less than 1 km) west of our east-west line (Davis and others, 1980; Ando and others, 1983) and may project downward to become the 6.9-km/s layer. We can explain only in part the apparent velocity layering within the Trinity ultramafic
sheet. Our velocity models (Figs. 3, 5) indicate two LVZ's within the sheet: one in the center of the body, above a velocity step to 6.5 km/s, and one at the base of the body, above the velocity step to 6.7 km/s. At least at shotpoint 3, there appears to be a correlation between the deeper velocity reversal and the magnetized part of the Trinity body (Fig .8), which in that location is only its basal part. Magnetic susceptibility is generally correlated with the degree of serpentinization (see Carmichael, 1982), but seismic velocity is inversely correlated (Fig. 9). The zone of velocity reversal and the velocity step that
occur in the middle of the Trinity sheet, consistently at a depth of 2 - 3 km, do not correlate with features seen either in the magnetic models or in outcrops. Because this feature appears to crosscut geologic contacts on the northsouth cross section (Fig. 10), it may be caused by some physical process operative in all lithologic units, such as a pore-pressure effect (see Brace, 1972). Geologic Cross Sections We have constructed a north-south geologic cross section of the Klamath Mountains
INTERPRETATION O F SEISMIC-REFRACTION RESULTS, CALIFORNIA
A 0 Trinity ultramafic
body
(serpentinized)
Central metamorphic (hornblende
belt
schists)
Stuart Fork F o r m a t i o n a n d upper p a r t of N o r t h F o r k t e r r a n e of A n d o a n d o t h e r s ( 1 9 8 3 ) ^(sedimentary and v o l c a n i c r o c k s ) L o w e r p a r t of N o r t h F o r k
t e r r a n e of
a n d o t h e r s ( 1 9 8 3 ) ( o p h l o l i t e ) or high-velocity
6
7
VELOCITY ( K M / S )
B
IDEAL SERPENTINE-OLIVINE AGGREGATE A T 1 0 Kbar
>
O O LU >
LABORATORY MEASUREMENTS A T 2 Kbar
J
u
20
40
60
80
PERCENT SERPENTINIZATION
ioo
Ando
other
rocks
Figure 9. (A) Comparison of velocity-depth curve in Klamath Mountains with laboratory data for selected rocks at appropriate pressures (and, for curves 13 and 14, appropriate temperatures). (B) Graph of laboratory velocity versus degree of serpentinization in peridotites and d unites and curve for ideal serpentine-olivine aggregate at 10 kbar (Birch, 1961, Table 3). Numbered curves in Figure 9A are described in Table 1. Data points for serpetinized peridotites in Figure 9B (squares) are from Christensen (1978); data points for unserpentinized dunites and one harzburite (labeled H) (triangles) are from Birch (1960).
TABLE 1. DESCRIPTION OF NUMBERED CURVES IN FIGURE 9A
Curve 1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11 •
12. 13. 14.
Rock type and refrence Klamath Mountains velocity-depth curve (this study). Peridotite, 71% serpentinized, Stonyford, California (Chirstensen, 1978, Table II, sample 1). Peridotite, 95% serpentinized, Stonyford, California (Christensen, 1978, Table II, sample 2). Peridotite, 10% serpentinized, Mt. Boardman, California (Christensen, 1978, Table II, sample 1). Peridotite, 45% serpentinized, Vallecitos, California (Christensen, 1978, Table II). Peridotites, 90% to 96% serpentinized: a, Paskenta, California; b, Canyon Mtn., Oregon; c, Black Mtn., California (Christensen, 1978, Table II; b is sample 1). Haraburgite, Bushveld Complex (mean of three directions) (Birch, 1960, Table S). Amphibolite, mica-plagiodase-hornblende (last-named mineral most abundant), S. -Alta, Finnmark, Norway (Kern and Richter, 1981, Table 1, sample 1387); similar in modal composition to sample Dll-52d of Grouse Ridge Formation (Abrams Schist) of Davis and others (1965, Table 1). Amphibolite, quartz-plagioclase-homblende, Karasjok, Finnmark, Norway (Kern and Richter, 1981, Table 1, sample 1396); similar in modal composition to sample DO-87 of Salmon Hornblende Schist (Davis and others, 1965, Table 1). Amphibolite, epidote-plagioclase-hornblende, Hoher Bogen, Bayr. Wald, Germany (Kern and Richter, 1981, Table 1, sample 1454); similar in modal composition to sample H4277 of Salmon Hornblende Schist (Davis and others, 1965, Table 1). Metasedimentary rocks (graywackes, prehnite to pumpellyite fades) of the Franciscan assemblage, Coast Ranges, California (Stewart and Peselnick, 1977, Figs. 3-5, samples IV5-P5); and metavolcanic rocks (andesitic), Sierra Nevada foothills, California (R. Stewart, 1983, written commun.). Metagabbro, epidote-amphibole-ptagioclase, Trinity ultramafic sheet, Klamath Mountains, California (Christensen, 1978, Table II, sample 1). Hornblende gabbro, homblende-plagiodase, Point Sal, California (Lin and Wang, 1980, Table 1 and Fig. 8, using T = 200° C). Olivine gabbro, augite-olivine-plagioclase, Point Sal, California (Lin and Wang, 1980, Table 1 and Fig. 7, using T = 200° C).
(Fig. 10) and an east-west geologic cross section of northeastern California (Fig. 11), using our velocity models (Figs. 3, 5) as frameworks. In the velocity models, boundaries are best constrained near points of critical reflection (see Figs. 10, 11). In drawing the cross sections, an effort was made to keep geologic contacts parallel to velocity boundaries, when warranted by
additional evidence, especially near points of critical reflection. In the north-south cross section (Fig. 10), the base of the Trinity ultramafic sheet is constrained by Griscom's (1977) magnetic models to be the velocity step to 6.7 km/s, as discussed above (Fig. 8). Near shotpoint 1, however, the base of the sheet crops out at the surface and
63
must crosscut velocity layers. The fault contacts between the Trinity sheet and the central metamorphic plate also apparently crosscut velocity layers, although the details of this structural complexity are conjectural. The fact that the geologic cross section must necessarily depart from the velocity model in the region south of shotpoint 1 is a reminder that unless other data are available, as near shotpoints 2 and 3, velocity models alone may be poor guides for drawing geologic cross sections. Other units are drawn in the north-south cross section according to the correlations made in the last section. Two granitic plutons on or near the profile are shown. The shape of the Trinity ultramafic sheet in the Klamath Mountains on the east-west cross section (Fig. 11) is inferred, using the block diagrams (Fig. 8) as guides. Units deeper than the Trinity sheet are drawn in a fashion analogous to the north-south cross section. In the Cascade Range, we show dips and discontinuities in geologic units to reflect the strong lateral velocity changes seen (Figs. 5, 11). Most detail is intended to be schematic and is shown with dashed lines. The uppermost unit of volcanic and sedimentary rocks is probably largely Cenozoic in age, but it also includes Upper Cretaceous sedimentary rocks (Hornbrook Formation), as indicated by outcrops north of Yellow Butte (Fig. 2). The uppermost unit cannot be separated from rocks of the eastern Klamath belt on the basis of velocity, as both have similar composition; hence the contact is shown dashed. The contact between rocks of the eastern Klamath belt and the unit of undivided granitic and metamorphic rocks is shown schematically to be interfingered to indicate the possibility that the former rocks continue eastward as metamorphic pendants and septa. Inclusions of ultramafic rocks in Cenozoic volcanic rocks are found as far east as Mount Shasta (R. L. Christiansen, 1983, oral commun.) indicating, along with the aeromagnetic data, the presence of the Trinity ultramafic sheet at depth in this area. The configuration of units deeper than the Trinity sheet in the Cascade Range is highly speculative. We have shown both truncation by intrusion and by faulting. The 10-km downward step in the 7.0km/s layer is modeled as offset along the major thrust fault separating rocks of the central metamorphic belt from the North Fork terrane (Siskiyou thrust fault). This major discontinuity, however, could also be a transform or normal fault, a fold, an intrusive contact, or a combination of all of these. In any case, it appears to represent a region where the stack of oceanic rock layers that underlie the Klamath Mountains are sutured to or in intrusive contact with the roots of magmatic arcs inferred to underlie the Modoc Plateau. Composition of the "basement" in the Cascade Range (6.1-6.3 km/s; Fig. 5) and deeper
FUIS A N D
64
KLAMATH
OTHERS
rocks (6.3-7.0 km/s) is unknown but presumed to be an igneous and metamorphic complex similar to that discussed above for the Modoc Plateau. One explicit pluton, a high-velocity body consisting presumably of congealed dikes and sills, is shown at Medicine Lake volcano. Units shown on the Modoc Plateau are, from top to bottom, volcanic and sedimentary rocks (2.5-4.4 km/s; Fig. 5), granitic and metamorphic rocks (6.2-6.4 km/s), and high-velocity (7.0-km/s) lower crustal rocks. Moho is placed at a minimum depth of 38 km. Structure in the Modoc Plateau and westernmost Basin and Range provinces consists chiefly of westward tilting of a large block bounded by the Surprise Valley fault and an inferred fault under Medicine Lake volcano whose surface expression may be the Gillem Bluff fault, to the north (Figs. 1, 2).
MOUNTAINS
(Yr
— J Sea L o v e l
0
-
10
•vl^W?,
Figure 10. Geologic cross section along Klamath line constructed from velocity model (Fig. 3). For explanation, see Figure 11. Note velocity boundaries are best constrained near points of critical reflection (large dots with arrows; see text).
BASIN
AND
0
Kfco . «V - v
>.•*. «•-.*.».; - * •
SL
• - - . - • * «*.'. •
30H
•30
Vertical Exaggeration 2
4020
20
40
60
80
—\— 100
120
140 160 Oisiance (km]
—r~ 180
—r* 220
200
"—I— 240
—]— 260
280
-40 300
EXPLANATION KLAMATH MOUNTAINS PROVINCE VOLCANIC ANO SEDIMENTARY ROCKS (CENOZOIC TO UPPEH CRETACEOUS) INTRUSIVE ANO METAMORPHIC ROCKS BASALT THROUGH RHYOLITE (CENOZOIC) UNDIVIDED GRANITIC ANO METAMORPHIC ROCKS (CENOZOIC TO PALEOZOIC ?) - - PATTERN DENSITY CORRELATES SCHEMATICALLY WITH VELOCITY: LOW OENSITY. 8 2 K M / S (FELSIC AND/OR LOW-GRADE METAMORPHIC ROCKS). HIGH DENSITY. 6.4 KM/S (INTERMEDIATE - COMPOSITION ROCKS AND/OR HIOM-ORAOE METAMORPHIC ROOKS)
GRANITIC ROCKS (CRETACEOUS AND JURASSIC)
LOWER CRUSTAL ROCKS
EASTERN KLAMATH BELT (JUHASSIC TO OROOVICIAN) - - SEDIMENTARY ANO VOLCANIC ROCKS
m
IBS
3TUART FORK FORMATION (TRIASSIC AND/OR OLDER) ANO UPPER PART OF NORTH FORK TERRANE OF ANDO AND OTHERS ( 1 9 8 3 ) (JURASSIC TO UPPER PALEOZOIC) • METAMORPHOSED ANO UNMETAMORPHOSED SEDIMENTARY AND VOLCANIC ROCKS LOWER PART OF N O R T H FORK TERRANE OF A N D O AND OTHERS ( 1 9 8 3 ) (UPPER P A L E O Z O I C ) - - OPHIOLITE
MAFIC A N O / O H GRANULITE-FACIES ROCKS (AGE UNKNOWN)
--
GEOLOGIC C O N T A C T , DASHED WHERE UNCERTAIN F A U L T , DASHED WHERE UNCERTAIN
— *""•* INTERNAL REFLECTOR
S A L M O N H O R N B L E N D E SCHIST AND ABRAMS MICA SCHIST, UNDIVIDED ( D E V O N I A N METAMORPHIC A G E ) TRINITY U L T R A M A F I C SHEET OF IRWIN ( 1 9 7 7 ) (OROOVICIAN)
OPHIOLITE OR OTHER HIOH-VELOCITY (7.0 K M / S ) ROCKS ( A G E UNKNOWN)
0RADATIONAL C O N T A C T - POSTULATED
G ,
,
' / / / ,
POINTS OF CRITICAL REFLECTION. HAY DIRECTIONS INDICATED; BOX WITH X INDICATES RAY INTO OR OUT OF PAGE. PARENTHESES INDICATE POORLY OBSERVED CRITICAL POINT LOW-VELOCITY ZONE
Figure 11. Geologic cross section along east-west line constructed from velocity-density model (Fig. 5). Klamath Mountains are underlain by stack of oceanic rock layers. Modoc Plateau is inferred to be underlain, beneath layer of volcanic and sedimentary rocks, by crystalline igneous and metamorphic rocks, constituting roots of magmatic arcs. Cascade Range, in between, is complex suture region, currently being intruded by magmas.
INTERPRETATION OF SEISMIC-REFRACTION RESULTS, CALIFORNIA
CONCLUSIONS Our seismic-refraction survey of northern California indicates at least two different types of crust. (1) An igneous and metamorphic crust similar to that of the Sierra Nevada appears to underlie the volcanic and sedimentary cover of the Modoc Plateau. This crust may represent the superposition or accretion of a number of magmatic arcs in this region. (2) An imbricated stack of oceanic rock layers underlies the Klamath Mountains. The complex region of suturing or intrusion between these two types of crust occurs beneath the Cascade Range. Apparently this region is an avenue for magmas of the most recent arc of the region, the Cascade Range. ACKNOWLEDGMENTS A large number of people, acknowledged in Zucca and others (1986), assisted with field work or expedited permitting. In the office, we were assisted by S. Johnson, R. Rebello, J. Ellefson, M. Sanders, R. Eis, L. Hollis, and the late R. Buszka. The geophysical interpretation benefited from reviews by P. A. Spudich and D. A. Stauber. The geologic interpretation benefited from reviews by W. P. Irwin, R. L. Christiansen, G. A. Thompson, and G. P. Eaton, from a field trip conducted for us by R. L. Christiansen, and from discussions at a workshop in December 1982 attended by J. P. Albers, M. C. Blake, Jr., R. L. Christiansen, P. Coney, J. M. DonnellyNolan, R. J. Blakely, R. C. Jachens, D. L. Jones, N. J. Silberling, and R. C. Speed. In addition, discussions with A. Griscom were crucial in developing our geologic interpretation.
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