W.F. BARBAT California Academy of Sciences. San Francisco. California 94118

Megatectonics of the Coast Ranges, California

ABSTRACT dence of the formation of deep-ocean trenches or island arcs has been found. The Coast Ranges are considered in the light INTRODUCTION of the "new global tectonics." It is proposed that at the end of the Nevadan orogeny, a large This paper attempts to explain the geology of prism of sediments, ranging in age from Late the Coast Ranges in terms of relative move- Jurassic to Late Cretaceous, was deposited ment of the North American plate away from across the boundary of the oceanic and the the Mid-Atlantic Ridge, toward, and over the steep-sided sialic crusts. Westward movement Pacific plate. It proposes a model to explain the of North America, away from the mid-Atlantic Coast Range orogeny, subsequent transcurrent rise, caused the oceanic block to underthrust movements on the San Andreas fault system, the continental block, in the Coast Range and Tertiary crustal shortening. orogeny. The thrust fault, here named the Coast Ranges thrust, passed through the post- GENERAL SETTING OF THE COAST Nevadan sediment prism at a gentle angle and RANGES separated it into two blocks. The hanging-wall From east to west, northern California may block is relatively undisturbed, and its sedi- be divided into three geomorphic provinces ments are identified as the Great Valley se- (see Fig. 1). They are the Sierra Nevada- quence. The footwall block is greatly disturbed Klamath Mountains, the Great Valley, and the into a tectonic jumble of sediment and ocean Coast Ranges. North of the latitude of Red- crustal igneous rocks, collectively identified as ding, the Great Valley ends and the Klamath the Franciscan. Rocks in the footwall block are Mountains and Coast Ranges merge. Each pre- subjected to various facies of high-pressure and serves its geological identity, and they are sepa- low-temperature metamorphism. rated by the North Fork Mountain fault which On approaching the steep-sided sialic crust, extends into southern Oregon. the footwall block folded the crust downward and backward and continued under it at a steep- The Sierra Nevada-Klamath Mountains ened angle. Peridotite from the oceanic crust or Plutonic rocks are most characteristic of the from the mantle, mobilized by serpentinization, Sierra Nevada-Klamath Mountains. They in- worked up the thrust plane and was dispersed trude isoclinally folded, thrust-faulted, and in the footwall melange. No batholithic instru- variously metamorphosed rocks that have been sions and no mountain making accompanied dated by fossils as Paleozoic and up to early the Coast Range orogeny. Late Jurassic (Kimeridgian) age. Early workers The tectonic event of the Coast Range considered the deformation and intrusion as a orogeny terminated with the propagation of single event and termed it the Nevadan the right-lateral transcurrent San Andreas fault. orogeny. It now appears that there has been a This change is estimated to have taken place in westward shifting in time of the thrusts (Davis, Eocene time. The San Andreas fault has offset 1968) and that there have been five epochs of the Coast Ranges thrust fault approximately plutonism ranging in age from Triassic to Late 310 mi since its inception. The relative pressure Cretaceous (Evernden and Kistler, 1970). of the ocean basin toward the continent was The extensive work of Irwin (1964, 1966) partly alleviated by strike-slip of the ocean ba- and Davis (1968, 1969) suggests that the sin, relatively northwesterly with respect to the Nevadan orogeny produced a series of east- continent. The orientation of the San Andreas dipping thrust faults, separating different lithic is not ideal for the relief of this strain and com- types and invaded by serpentinized ultramafic pressional folding has also occurred. No evi- rocks derived from the mantle. The faults

Geological Society of America Bulletin, Vol. 82, p. 1541-1562, 6 figs., June 1971 1541

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steepen with depth. In the Klamath Mountains, referred to as "eugeosynclinal," but recognized where erosion has exposed higher structural as the Franciscan Formation by California levels, the dips are moderate, but in the western geologists from its type locality in the San Fran- Sierra Nevada where lower structural levels are cisco Bay area. The term "formation" is used in exposed, they are steep to nearly vertical. default of an appropriate term for such an as- Davis (1968) suggested the possibility that semblage of characteristic rocks that have un- the tectogenesis resulted from the interaction dergone nonsystematic disturbance and have of a "passive" continental margin with an "ac- no physical, spatial, and temporal coherence tive" underthrusting oceanic plate. He be- (Page, 1966). Hsu (1968) proposed calling lieved that underthrusting would explain the this complex a melange, which is appropriate westward shifting in time of the thrusting. because the stratigraphic code makes no provi- The results of the Nevadan Orogeny were sion for this connotation. In this paper, these accretion to the continent and mountain build- rocks will be called the Franciscan. Although ing. Speculations by Bateman and others they are not numerous, identifiable fossils of (1963) and by Bateman and Wahrhaftig marine invertebrates, ranging in age from Late (1966) led to the belief that a large thickness of Jurassic (Tithonian) to Late Cretaceous (Cam- rocks (±9 to ±11 mi) has been stripped off the panian) have been found in the Franciscan. Sierra Nevada in the post-Nevadan to middle Apparently overlying the Franciscan is an or- Eocene. Such a thickness is required to provide derly sequence of sedimentary rocks of late for the size of the prism of epiclastic sediments Mesozoic age (Tithonian to Maestrichtian), fre- deposited at the western foot of this range. quently called "miogeosynclinal" that are known as the Great Valley sequence (Bailey The Great Valley and others, 1964). The Franciscan and the The Great Valley is a large wedge of sedi- Great Valley sequence were deposited contem- ments covering a peneplane cut on the plutonic poraneously in a common sedimentary prism, and metamorphic rocks of the Sierra Nevada but have subsequently existed in different tec- complex. Smith (1964) shows that the pene- tonic environments. The physical differences plane dips nearly uniformly westerly at an aver- between these contemporaneous rocks and age rate of about 475 ft per mi and strikes about their spatial relationships give valuable clues as N. 15° W. at the north and curves gradually to to their tectonism. about N. 55° W. toward the south. The surface is gently folded into a few transverse arches and THE GREAT VALLEY SEQUENCE a major depression, the "Dark Hole," south of VERSUS THE FRANCISCAN the Bakersfield Arch. Along the western mar- gin of the Valley, the surface steepens on a The Great Valley sequence, exposed along gentle curve. Reflection seismograph traverses the west edge of the Great Valley, consists of a indicate that its dip increases to approximately thick succession of sedimentary rocks, sand- 20°. Seismic dip components in the overlying stones, shales, and conglomerates. Sections sediments show that they onlap the surface at measuring 40,000 ft or more are exposed, but angles near 10°. Thus, the surface may be inter- the total thickness of the deposit may exceed preted to have had a westerly dip near 30° when this, because the bottom and top of the se- the onlapping beds were deposited (see Fig. 2). quence have not been recognized in a single section. West of the Sacramento Valley, the Coast Ranges oldest beds depositionally overlie a typical The Coast Ranges are tripartite. There is a ophiolite sequence that extends downward northwest-trending central mass between the through local impure chert, mafic pillow lava Sur-Nacimiento and San Andreas faults that is and breccia, and basal serpentinized ultramafics made up of granitic and high-grade meta- interpreted to be oceanic crust (Bailey and oth- morphic rocks and overlying sediments of late ers, 1970). The contact between the Creta- Upper Cretaceous and younger ages (Fig. 1). ceous and the oldest Tertiary is gradational This was named Salinia by Reed (1933). where exposed. Northeast and southwest of Salinia are two Exposures of the Great Valley sequence are masses referred to for convenience as the generally good; bedding is distinct, and beds Northeast Franciscan block, and the Southwest may be traceable for many miles; dips are sys- Franciscan block. These two blocks contain the tematic, and unbroken vertical continuity is characteristic assemblage of lithologies often characteristic; its gross structure is simple and is

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SUNRAY MID CONTINENT SOCAL'HEDFERN" ESCOBAR 28-11S-I2E DIABLO RANGE 9-11S-IOE GREAT VALLEY

COMPARISON BETWEEN PRESENT STRUCTURE AND STRUCTURE AT END OF CRETACEOUS NORTHERN SAN JOAQUIN VALLEY CALIFORNIA

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Figure 2. Upper: Cross section of boundary between a cross valley reflection seismic depth-section. Lower: the Coast Ranges and Great Valley at east side of Diablo Same cross section restored on top of Cretaceous, made Range. The Great Valley portion is the western end of horizontal. D-1C toj-l? are Cretaceous faunal zones.

reflected in a strike-ridge and valley topogra- distance. Contacts between different rock types phy. are often fault contacts; dips and strikes fre- The Franciscan sediments are predominantly quently may not be apparent and are often non- sandstone with shale and conglomerate rela- systematic; fold structures, except on a small tively minor. Jumbled with sediments are scale, are difficult to recognize if they do exist. masses of an ophiolite suite of rocks, including Shearing on a large scale is evidenced by wide chert, pillow lava and breccia, mafic rocks, and zones of diverse rock types with smeared sur- serpentinized ultramafics. These may be inter- faces. Rocks of differing degrees of metamor- preted as fragments of the oceanic crust upon phism are found in close association. which the sediments were deposited. The Fran- The Great Valley sequence rocks are less ciscan is widely intruded by peridotite, dunite, dense than the Franciscan. Bailey and others and some gabbro, usually well serpentinized. (1963, 1964) have found that the average Except for small intrusions of "diabase," (Ches- median specific gravity of Upper Cretaceous terman, I960) there is an absence of contact sandstones of the Great Valley sequence is effects around the intrusions. The contacts of 2.5 5; of Lower Cretaceous, 2.5 7; and of Upper these "cold" intrusions may be readily inter- Jurassic, 2.59. In the Franciscan, they found preted as fault contacts. median densities of sandstones associated with Franciscan rocks are generally pervasively younger fossils to be 2.60; with older fossils, jointed, fractured, and sheared, making it diffi- 2.65. cult to obtain satisfactory hand specimens. This Bailey and Irwin (1959) and Bailey and oth- promotes deep and rapid weathering, heavy ers (1964) report differences in the K-feldspar soil cover, downhill soil creep, and landsliding. content of the sandstones of the Great Valley Except in actively eroding stream channels and sequence and of the Franciscan. The latter pa- sea cliffs, natural outcrops are discontinuous per reports that the median K-feldspar content and are often erratically disposed erosion rem- of the Upper Cretaceous unit of the Great Val- nants. Only in a few places can intact sequences ley sequence is 13 percent, and Lower Creta- be traced horizontally or vertically for any great ceous unit, 1.1 percent; also, the Upper Jurassic

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unit, 0.5 percent. Franciscan sandstones have and others, 1969). This possibility has been virtually 0.0 percent K-feldspar, except in the confirmed experimentally by Brace (1969) and western part of the Northeast Franciscan block may be the explanation for the "upside-down" and in parts of the Southwest Franciscan block or inverted metamorphic facies zonation below where percentages of K-feldspar are similar to major thrust faults observed by Kilmer (1961), those of the Great Valley sequence. Blake and others (1967), Blake and Cotton (1969), and Blake and others (1969). The tec- Franciscan sandstones are generally de- tonic overpressuring explanation is favored, scribed as "graywacke" because they have a considering the temporal aspects of the tectonic high ratio of matrix to grains. Sericite makes up model of this paper. High fluid pressure along much of the Franciscan graywacke matrix and or below the faults would also assist the thrust appears to have developed from the K-feld- movements and shield the hanging-wall blocks spars. Chlorite, developed from volcanic rock from destructive deformation. detritus and tuffaceous elements, is another common constituent. Sandstones of the Great Volcanic rock masses are scattered through- Valley sequence are usually described as ar- out much of the Franciscan. Pillow lavas and kosic, although graywackes are noted increas- breccias may be identified. The volcanic rocks ingly in the lower measures of the sequence. are mafic and are somewhat altered by low- grade metamorphism of several kinds, includ- Rock-type differences between the sand- ing spilitization and zeolitization. Collectively, stones of the Franciscan and those of the Great they are called greenstones. Bailey and others Valley sequence can be attributed to relative (1964) estimate that these rocks make up about depth of burial. The less deeply buried sand- 10 percent of the Franciscan. stones are arkosic, have K-feldspar, and have a lower specific gravity. With increased burial, Thinly bedded radiolarian cherts and shale the K-feldspar is sericitized, increasing the ma- constitute about .5 percent of the Franciscan, trix to grain ratio and the specific gravity of the but this rock type is widely distributed and is rock. These related phenomena may be seen by often in contact with volcanic rocks or in close careful examination of Plate 2 of Bailey and association with them. Radiolarian chert has others (1964). This map shows megafossil been observed in cavities between pillows. localities, K-feldspar content, and specific gravi- There was considerable speculation as to the ties of upper Mesozoic sedimentary rocks in origin of the cherts until cores recovered from western California. Furthermore, albitization the ocean bottoms (by the JOIDES expedi- and zeolitization increases with depth of burial. tions) demonstrated that they are biogenous, Dickinson and others (1969) have found albiti- low-energy level deposits on the volcanic ocean zation starting at a stratigraphic depth of 7500 crust below the depth of limestone compensa- ft in the Great Valley sequence, almost com- tion. They obviously were deposited before the plete albitization at a depth of 20,000 ft, and high-energy level sandstones were laid down. albite replaced by zeolite below 30,000 ft. Mild In summary, the Franciscan rocks and those metamorphism is characteristic of the bulk of of the Great Valley sequence are of different the Franciscan. In addition, the Franciscan has character, but of the same age. The former has higher-grade metamorphic rocks assigned to been called "eugeosynclinal" and the latter the blueschist facies. This most common miner- "miogeosynclinal" by many authors. These als of these rocks are glaucophane, lawsonite, terms may be retained as a matter of conven- jadeite, stilpnomelane, pumpelleyite, riebec- ience only, since no evidence has been uncov- kite, crossite, and aragonite; these are the pro- ered to indicate that they were deposited in two ducts of high-pressure and low-temperature separate bodies. Rather, they were deposited as metamorphism. The depth of burial required to a single prism of sediment in a single orthogeo- attain these pressures is 70,000 ft or more (Tay- syncline that straddled the late Mesozoic lor and Coleman, 1968); but at such depths, the boundary between the continent and the ocean temperature would be too high for their stabil- floor. Their main difference is that they have ity. It has been suggested that quick burial and not had the same tectonic histories. They are a prompt uplift, before normal geothermal gradi- different tectofacies and not a different sedi- ents set in, were necessary conditions. Others mentary facies. Satisfactory terms do not appear suggest that tectonic overpressuring may have to exist in the geologic literature for these con- lessened the burial depth requirement (Blake trasting tectofacies.

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DISTRIBUTION OF THE contacts between the Franciscan rocks and FRANCISCAN AND GREAT VALLEY Great Valley sequence have been of slow corn- SEQUENCE prehension. For many years, published cross As may be seen on Figure 1, the Franciscan sections tended to show them as high-angle re- appears to lie chiefly in the cores of large verse faults. If the fault encircled a Franciscan uplifts. The largest areas of Franciscan in the mass, such as at New Idria, it was shown (see Northeast Franciscan block are (a) in the Dia- Fig. 3) as a high-angle reverse fault on one flank blo Range, which is a doubly plunging antiforrn and a normal fault on the other side (Eckel and about 90 mi long, and (b) in the northern Coast Myers, 1946), or they may have been depicted Ranges, which is a complex southerly plunging (Fig. 4) as piercement structures such as at Mt. antiform. Smaller areas of Franciscan are ex- Diablo (Pampeyan, 1963). Bailey and others posed in anticlines ringed or partially ringed by (1964) cautiously advanced the hypothesis that Great Valley sequence rocks in the Diablo large-scale thrusting or gravity sliding along Range, such as at Mt. Diablo, New Idria, and serpentine masses may account for the juxtapo- Table Mountain. In addition, there are small sition of the two facies. Brown (1964) showed uplifts or intrusions of Franciscan, often serpen- the Stony Creek fault, which separates the Fran- tinite, extending from Sulphur Springs Moun- ciscan rocks and Great Valley sequence in the tain, north of the Bay Area (Weaver, 1949), to northern Coast Ranges, to be a folded thrust the southern tip of the Diablo Range (Marsh, fault. Dickinson (1965) recognized that the I960) and into the Bitter Creek area of the contact in the Diablo Range was a folded thrust Temblor Range (Heikkila and MacLeod, fault and documented the nature of it in the 1951). In the northern Coast Ranges, which are Table Mountain area (Dickinson, 1966). exposed by erosion to deeper structural levels Page (1966) came to the conclusion that "It than is the Diablo Range, Franciscan rocks are is probable that a great thrust, or series of widespread, but Great Valley sequence rocks thrusts, carried rocks of the Great Valley se- are confined to the Great Valley border and to quence over the Franciscan assemblage for a isolated patches. It may be interpreted from the distance of tens of miles"; and "The overriding mapping by Clark (1940), Yates and Hilpert sheet is believed to have moved from east to (1946), Manning and Ogle (1950), Gealey west relative to the underlying Franciscan (1951), Brice (1953), Ogle (1953), McNitt rocks. The total displacement exceeds 50 miles, (1968a, 1968b), Swe and Dickinson (1970), judging from the position of probable out- and others, that these patches are essentially liers." Most California geologists now accept grabens, half-grabens, or downfolds. The Great the folded thrust fault relationship of the Fran- Valley sequence also extends into the sad- ciscan and Great Valley sequence because it dleform of the Bay Area between the northern satisfies all of the conditions. Coast Ranges and the Diablo Range antiforms. Thus, the relative superposition of Great Valley THE COAST RANGES THRUST sequence over Franciscan rocks is attested. FAULT Until Cretaceous fossils were found in the The fault separating the Franciscan and the type locality of the Franciscan (Schlocker and Great Valley sequence called the Coast Range others, 1954; Hertlein, 1956), and until it was thrust by Bailey and others (1970), may be clearly demonstrated (Irwin, 1957; Bailey and appropriately called the Coast Ranges thrust others, 1964) that the Franciscan and Great fault because it occurs throughout the Coast Valley sequence were in part coeval, the latter Ranges in both the Northeast Franciscan block was frequently mapped as being in depositional and the Southwest Franciscan block. Where it is contact with the former. However, no deposi- exposed along the west side of the Great Val- tional contact has been truly documented. The ley, the strike of the fault is nearly parallel to contacts of Franciscan rocks and the Great Val- beds that are mappable in the Great Valley se- ley sequence are frequently obscure, but where quence for many miles. Near parallelism of dip best exposed, they are clearly fault contacts, or is also suggested by field observation. How- more typically, fault zones as much as several ever, if structure cross sections involving the miles wide. Serpentinite, along with tectonic Coast Ranges fault and the Great Valley se- inclusions of deep-seated rocks, mark much of quence are restored to pre-folding flatness, as in the fault zones, particularly in the eastern Coast Figure 5, it is seen that the thrust dips at a low Ranges. The nature and significance of the fault angle in an easterly direction. Thus, in each of

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Figure 3. Cross section of Coalinga anticlines in the New Idria district, modified from Eckel and Myers (1946), showing folded thrust fault interpretation.

LEVEL

Figure 4. Cross section of Mt. Diablo anticline, modified from Pampeyan (1963), showing folded thrust fault interpretation.

the Northeast Franciscan block Coast Ranges fault slices through the sialic crust and under- antiforms, beds of Great Valley sequence rest- thrusts it. An alternative possibility is that the ing in fault contact on Franciscan are older on fault steepens to a high-angle reverse fault the eastern flanks and younger on the western. which carries the oceanic crust downward It is similarly true that the Franciscan rocks of against a folded-back and abraided continental the eastern Coast Ranges are older, on the basis crust. of fossils reported, than those found in the west- In the Klamath Mountains where the Coast ern Coast Ranges. Other indices of depth of Ranges thrust fault emerges from beneath the burial and perhaps indirectly of age, show the Great Valley sequence, it is mapped as the east- same pattern. The median bulk densities of dipping South Fork Mountain fault, with two sandstones are higher in the eastern Coast klippen north and northeast of Eureka (Fig. 1). Ranges than the western; the K-feldspar con- The fault continues into Oregon for nearly 80 tent, virtually 0.0 percent in the eastern Coast mi where it disappears under a cover of Terti- Ranges, may approach the K-feldspar content ary rocks (Irwin, 1966; Hotz, 1969). Metamor- of the Great Valley rocks in the western Coast phosed and intruded pre-Nevadan orogeny Ranges. rocks, similar to the Sierran sialic crust, are in If the unfolded Coast Ranges thrust fault is the hanging wall and Franciscan rocks are in the projected easterly under the sediments of the footwall.Thus, the Coast Ranges thrust is inter- Great Valley, it must impinge upon the Sierra preted as being a folded and faulted, easterly Nevadan sialic crust (Fig. 2) in relatively few dipping, low-angle thrust fault where the over- miles from its outcrop. In the model illustrated riding Great Valley sequence and Franciscan by Page (1966, Fig. 7), the Coast Ranges thrust are in fault contact. This may be regarded as the

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sediment segment of the fault. When the Coast 1970) show the relationships of these younger Ranges thrust involves the Sierran-Klamath faults and the sediment segment of the Coast Mountains crystalline rocks in the hanging wall, Ranges thrust fault. The relationship of the it steepens to a high-angle fault. This may be younger faulting and the basement segment ex- regarded as the basement segment of the fault. posed near Point Sur, about 25 mi south of The boundary between the Southwest Fran- Monterey Bay, is not as clear-cut. As mapped by ciscan block and Salinia has long been known as Oakeshott (1951), there is a thin elongate the Sur-Nacimiento fault zone. It is a compara- sliver of Miocene sedimentary rock between tively complex structure which may be broken the Salinian basement and the Franciscan. This down into three elements. The simplest and may be interpreted as a graben, or possibily it best known element is where the Franciscan was emplaced by strike-slip on the Sur fault, as rocks of the Southwest Franciscan block are suggested by Page (1970b). In any event, it overridden by the plutonic and high-grade strongly suggests that, at least locally, late metamorphic rocks of Salinia by an east-dipping Cenozoic fault movements have taken place on high-angle thrust. This is analogous to the a section of the Coast Ranges thrust fault that South Fork Mountain fault and can be inter- provided a suitable strain relief zone. preted as the basement segment of the Coast Southwest of the Sur-Nacimiento fault zone Ranges thrust fault. The second element of the at the Cuyama River, east of Santa Maria, Sur-Nacimiento fault zone is a wide band of Brown (1968) described a broad anticline in Great Valley sequence Upper Cretaceous sedi- the core of which Franciscan rocks are exposed. mentary rocks in patches allochthonous with Overlying the Franciscan in fault contact are respect to the Franciscan. As shown by Page Upper Jurassic and Cretaceous rocks of the 1970b) these patches are bounded by folded Great Valley sequence, with successively and faulted, mostly low-angle thrusts. This is younger rocks resting on the Franciscan in a analogous to the folded and faulted thrust de- southwesterly direction. This is a counterpart of scribed by Dickinson (1966) in the Diablo the Mt. Diablo anticline with its east-dipping Range and can be interpreted as the sediment folded thrust fault. segment of the Coast Ranges fault. Wentworth (1968) described what may be The third element of the Sur-Nacimiento fault interpreted as another folded thrust with pillow zone adds greatly to its complexity. This ele- spilite, resembling the common spilites of the ment consists of high-angle faults that cut Fran- Franciscan, in the core of Black Point anticline ciscan, Great Valley sequence, and Cenozoic west of the San Andreas fault and north of Point rocks. These faults cut the Coast Ranges thrust Reyes. The spilite is apparently overlain by a fault, are younger, and not directly related to it. conformable sequence of Later Upper Creta- Recently published maps (Page, 1970b; Loney, ceous, Paleocene, and Eocene sediments. He Ci

Turn, BASE OF Tsp, ,Tum

-T\lk f-'~^~. 2000 Tmk :400o

Jf+Sp

Figure 5. Restoration of cross section of Mt. Diablo the mid-Eocene unconformity discussed in the text; and anticline shown in Figure 4, by "unfolding" to make the for a lower order of tectonism that continued in the base of the San Pablo Formation (T sp ) horizontal. The Coast Ranges, after the Coast Ranges thrusting, and restoration shows evidence for the easterly (toward prior to the intense Plio-Pleistocene folding, right-hand side) dip of the Coast Ranges thrust fault; for

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points out that all spilite contacts are faults, structure. One would normally expect the gra- some of which are low angle. nitic and metamorphic rocks to extend to great The physical relations of Franciscan and depth. A rather detailed refraction profile has Great Valley sequence are the same in the been reported by Stewart (1968). An unex- Southwest Franciscan block as in the Northeast pected feature of the survey is the extremely Franciscan block. In both blocks, they are sepa- rapid attenuation of first arrivals, being as great rated by a folded and faulted, low-angle thrust as the inverse sixth power of distance, at dis- fault. The physical relations of Franciscan and tances between 40 km and 80 km. Assuming Sialic crust are also the same in both blocks. that the attenuation is due to a low velocity Despite the fact that the Klamath Mountain zone, it begins at a depth of no more than 10 basement is older than the Nevadan orogeny km (6.2 mi). Stewart speculated that the low- and the plutons of Salinia are Upper Cretaceous velocity zone could be the effect of increasing in age (Curtis and others, 1958), the Franciscan temperatures with depth, or that the granitic is separated from the sialic crust by an easterly material is underlain by low-velocity Franciscan dipping reverse fault. It may be concluded that material. it is a single fault that is offset by the San An- Heat-flow studies on opposite sides of the dreas fault. San Andreas fault near Hollister (Roy and The Coast Ranges thrust fault has some well- Brune, 1968) show that about 50 percent more defined imbrications (Swe and Dickinson, heat is flowing out of the earth east of the fault 1970) that are particularly apparent in the in the Franciscan terrain than is flowing out northern Coast Ranges. Much of the internal west of it in the granitic terrain. From eight disorder of the Franciscan is likely due to sec- holes as much as 1000 ft deep, temperature ond-order imbricate thrusting in the footwall projection to a depth of 6 mi east of the fault block. In the Great Valley sequence, where fos- is 660°C, as compared to 430°C west of the sil zones are recognized, second-order imbri- fault. It is not likely, therefore, that tempera- cate thrusting also has been recognized. ture is the cause of the low-velocity zone. The hanging-wall block has several oblique It has been noted that there is no characteris- tear faults, that are well defined by offsets of tic gravity anomaly associated with the San An- zones of the fossil pelecypod Buchia, near the dreas fault (Clement, 1965; Chapman, 1966). north end of the Great Valley (Jones and oth- This is despite the fact that the granitic and ers, 1969). These tear faults have a left lateral metamorphic rocks on the west side of it have sense of movement of many miles. They shift specific gravities of the order of 2.70, while the the crystalline basement rocks of the hanging- Franciscan sedimentary rocks east of the fault wall block northwesterly, from a position at have specific gravities averaging 2.65. This sug- depth along the west side of the Great Valley, gests that the low-velocity zone is less dense to a position at the surface near the coast in the than the average Franciscan sedimentary rock. Klamath Mountains. It would seem that only serpentine could meet the geophysical requirements. SALINIA AND THE SAN ANDREAS FAULT The Relationships of the San Andreas Salinia is composed of granitic and meta- Fault and the Coast Ranges Thrust Fault morphic rocks described by Compton (1966), The San Andreas fault is an active, right-lat- which are overlain by Late Upper Cretaceous eral, strike-slip fault that has been responsible and Cenozoic sediments. Franciscan rocks are for several major earthquakes in historical absent on Salinia. The southwestern boundary times and innumerable lesser shocks. In addi- of Salinia is the Sur-Nacimiento fault zone and tion, there are sections of it that exhibit slow, its offshore continuation. Its northeastern progressive, essentially aseismic creep. Geolo- boundary is the San Andreas fault. Geologi- gists and geophysicists have studied its ancient cally, Salinia is definitely out of place and its and modern history perhaps more than any reason for being so has been the subject of other fault. Since the notable publication of much discussion. By now, it is almost univer- Hill and Dibblee in 1953, it has become appar- sally agreed that it is out of place by virtue of ent that the San Andreas fault has had a long right lateral movement of hundreds of miles history of movements and a cumulative offset along the San Andreas fault. measurable in hundreds of miles since its incep- Recent geophysical studies have indicated tion. Lithologies and faunal comparisons of that Salinia may have a very unusual internal material from opposite sides of the fault have

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been made by many authors, who suggest the Andreas fault. These resemble the second- fault has been active through most of the Ceno- order drag folds of Moody and Hill (1956). On zoic. The later offsets are more discernible, and progressing easterly, the fold axes curve and there is less question as to their history. It is parallel the strike of the Sierran basement con- suggested that the relationships of the San An- tours shown on Smith's (1964) map. The Coa- dreas fault and the Coast Ranges thrust fault linga anticline-Lost Hills line of folding shown may contribute to an understanding of both. on Figure 1 is an example. Since the Coast Ranges thrust fault separates Faults other than shallow biextensional tear, the Franciscan from the granitic rocks, its down- features and high-angle reverse faults at depth, ward projection to this contact has been approx- are not common in this fold belt. They do in- imated along the west side of the Great Valley crease as the San Andreas fault is approached shown on Figure 1. Although control at the and become common in the Bay Area sad- south end of the Valley is not adequate for close dleform and the northern Coast Ranges an- positioning, it is evident that its trace must in- tiform. The larger faults have San tersect the San Andreas fault between the Bitter Andreas-like, strike-slip features. Folds on Sa- Creek area of the Temblor Range where Fran- linia are largely confined to local areas of thick ciscan rocks have been mapped and the west- sediments, chiefly Miocene shales. Faults are erly extension of the granites of the San apparently basement controlled except for a Emigdio Range. few thrust faults originating in folds. Folds and Previous authors have placed the offset gra- faults in the Southwest Franciscan block tend to nitic-Franciscan contact on the west side of the parallel the granitic-Franciscan boundary along San Andreas fault, at sea, north of the 39th the Sur-Nacimiento fault zone, suggesting that parallel. As a result, they have estimated a dis- it might be a line of weakness at depth. Thus, placement of 350 to 400 mi. However, the it appears that the lesser structures of the Coast Black Point anticline described by Wentworth Ranges are influenced by the depth and prox- (1968) appears to resemble Franciscan terrain imity to granitic basement and to the San An- rather than Salinian terrain. On this basis, the dreas fault system. offset contact is placed between Cordell Bank The age of the folding and faulting of the and Bodega Head on one side and Black Point Coast Ranges cannot be determined in all areas. anticline on the other. Under this interpreta- 1 However, when Pliocene-Pleistocene beds are tion, the offset is approximately 310 mi. present, they have been affected. At least lo- cally, as in the Buena Vista Hills oil field east CENOZOIC STRUCTURES of the Temblor Range, active thrusting related Folds and faults are numerous in the Coast to folding is shearing well casings and buckling Ranges. The larger folds occur east of the San surface pipelines. There are, however, numer- Andreas fault and west of the Franciscan-Sier- ous unconformities and sedimentary wedges in ran crystalline rock boundary. However, near Coast Ranges folds that indicate structural the southern end of the Great Valley where the growth starting in Eocene time. The major crystalline rocks have been greatly depressed to movements, however, have been very recent. form the "Dark Hole" which has received a great thickness of Cenozoic sediments, large, TECTONIC EVENTS high-amplitude folds persist beyond the bound- It is the basic thesis of this paper that there ary. The more westerly folds have axes ori- have been two separate tectonic events in the ented at an acute angle (10° to 30°) to the San Coast Ranges. The first event was the Coast Ranges thrusting during which the continent 1 The aggregate amount of lateral displacement, so far overrode the oceanic crust. The second event recognized, south of the Coast Ranges is only about one-half was the lateral faulting of the San Andreas fault of this distance. This disparity may be due to several factors, system and folding and faulting associated with such as possible absorption of some of the movement by this event. Even though the two events may crustal shortening limted to one side of the San Andreas Fault have had a single and continuing mechanical system or by faults not presently known to be part of this fault cause, the mode of failure differed, and they system. Further, conflicting left-lateral displacements on the transverse Ranges faults and right-lateral movements on the were not contemporaneous. The first event ter- San Andreas system tend to confuse and confound. Suppe minated at the onset of the second event. A (1970b) is attacking the problem, and his efforts are ap- modeling of the tectonism will be attempted in plauded. later paragraphs.

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Age of the Coast Ranges Thrust Fault gin of the Coast Ranges, paralleling the Coast Fossils as young as Campanian have been Ranges thrust. However, as the lower Tertiary found in the footwall Franciscan rocks of the swings westerly around the plunges of the Coast Ranges thrust. This has led to the belief folded thrust folds, as at Coalinga anticline, a that the thrusting took place in Late Cretaceous mid-Eocene unconformity is observed, and time (see Irwin, 1966) or perhaps in latest lower Eocene and uppermost Cretaceous are Cretaceous or early Tertiary time. One of the truncated. Also, on restored sections, as shown most convincing pieces of evidence for dating on Figure 5, the mid-Eocene unconformity is the start of the Coast Ranges thrusting has been apparent. The unfolded mid-Eocene uncon- found by Keith D. Berry (1970, personal com- formity has the same geometric character as the mun.). Berry recognized paleobathymetric unfolded Coast Ranges thrust fault and appears foraminiferal facies in the Cretaceous of the to be related to it. No similar Oligocene phe- Great Valley. Identification was not made on nomenon has been recognized, and so it is con- the basis of supposed modern counterparts but cluded that mid-Eocene is the most probable empirically on the basis of faunal changes in a date for the close of the period of thrusting. bed traced basinward from a fossil shoreline. This date happens to coincide with the end of By his method, Berry reports that there is an the early period of uplift and erosion of the orderly progression from "shallow-water" to Sierra Nevada as determined by Bateman and "deep-water" facies in well samples taken as Wahrhaftig (1966). As will be discussed in fur- one goes westerly across the Great Valley ther paragraphs, the two events may go hand in throughout the Upper Cretaceous, until a dis- hand. tinct wedge of "shallower" facies appears from Age of the San Andreas Fault the west side. This wedge is about 500 ft below the top of the Cretaceous and is correlated with As has been indicated above, the San An- the Maestrichtian. dreas fault postdates the Coast Ranges thrust It is obvious that large-scale thrusting must fault and is active today. Attempts to date the take considerable time, but the dating of the inception of movement on it have been made end of the thrusting is less certain. In the Coast by numerous geologists. Comparisons of li- Ranges, rocks as old as lower Miocene lay in thologies and faunas on the two sides of the depositional contact across the Coast Ranges fault have indicated progressively greater offset thrust, and so it is certain that thrusting was with age of the rocks compared. As has been pre-Miocene. In the Santa Lucia Range, Jen- pointed out by Dibblee (1966), the Oligocene nings (1958) shows a patch of nonmarine Refugian stage rocks of the two sides of the San Oligocene resting on the Nacimiento fault, in- Andreas fault offer a good comparison. On the dicating pre-Oligocene movements on this seg- east side of the fault in the San Emigdio Moun- ment of it. If this segment is related to the Coast tains, is a fossil strand line between marine and Ranges thrust fault, as seems likely, it suggests nonmarine beds. The apparent continuation of pre-Oligocene termination of movements on it. this strand line on the west side of the fault lies A further suggestion of pre-Oligocene age of east of Monterey Bay and indicates a displace- the thrusting may be obtained from the folded ment of about 175 mi. Using Bandy and Ar- thrust of Brown (1968) in the Cuyama River nal's (1969) approximation of the age of the area. Here, conglomerates of the Oligocene Refugian-Zemorian boundary of approxi- Sespe Formation rest unconformably on the mately 28 m.y., a rate of movement of 6.25 Upper Cretaceous above the folded thrust. In mi/m.y. is suggested. other places in the Coast Ranges, conformable If there was an equivalent rate of movement sequences of Great Valley sequence, Paleo- since the Coast Ranges thrust was offset, the cene, and Eocene rocks overlie the folded 310-mi displacement would have taken place Coast Ranges thrust fault. As has been pointed about 50 m.y. ago, or about mid-Eocene time. out by Swe and Dickinson (1970), this suggests This is in harmony with the dating of the end that the thrusting ceased no earlier than Oligo- of activity on the Coast Ranges thrust. cene. This apparent dilemma may yet be resolved SUMMARY OF GEOLOGY if finer divisions of the Eocene and Oligocene Evidence has been cited in support of a differ- are considered. In a gross sense, there is essen- entiation of the late Mesozoic rocks of the tial conformity from the youngest Cretaceous Coast Ranges on the basis of their tectonic his- through the Oligocene along the eastern mar- tories. The Great Valley sequence rocks, al-

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though compacted, slightly metamorphosed by A PROPOSED MODEL loading pressure, and folded, are essentially un- An explanation of the geology of the Coast disturbed. Their time equivalents, the Francis- Ranges has been sought and attempted for can rocks, may be considered as a giant tectonic many years. The seemingly ambiguous nature zone. The two tectonic facies are everywhere in of the Coast Ranges has led to many diverse fault contact with the Great Valley sequence interpretations and frustrating contradictions. overriding the Franciscan along a gently east- King (1959, p. 171) could not have better erly dipping thrust fault that has subsequently stated the existing bafflement when he wrote, been folded. For convenience, this has been "...all the King's horses, and all the King's men, called the sediment segment of the Coast cannot put the Coast Ranges back together Ranges thrust fault. Where the Great Valley again." sequence rocks have been removed by erosion, The models offered have been useful, how- the Franciscan rocks are in fault contact with ever, in setting forth a pattern that would fit, or plutonic and high-grade metamorphic rocks not fit, geological conditions as they were along a high-angle, easterly dipping thrust known or subsequently determined. Models fault. This has been called the basement seg- have been discarded, revised, or improved as ment of the Coast Ranges thrust fault. This seg- new data were obtained and new concepts ment is exposed in the Klamath Mountains derived. With the explosive advances in (South Fork Mountain fault) and along the geology and geophysics of the last decade, we coast south of Monterey Bay (Sur-Nacimiento have seen revolutionary explanations of Coast fault). It is buried under sediments of the Great Ranges geology and drastic improvement of Valley between the 40th and 31st parallels the models. Some late models of interest are: where it intersects the San Andreas fault. Its Bailey and others (1964), Ernst (1965, 1970), offset continuation intersects the San Andreas Hamilton and Myers (1968), Hamilton fault at sea south of Black Point anticline, which (1969), Lowe (1969), and Page (1966, 1969, exposes the sediment segment of the Coast 1970a, 1970b). These have been carefully con- Ranges thrust, and Bodega Head, where gra- sidered in fashioning the proposed model. nitic rock is exposed. It underlies the Pacific between there and the onshore Sur-Nacimiento The Late Jurassic Setting fault. It is envisioned that the continental-oceanic A westward shoaling of the seas in Late boundary at the close of the Nevadan orogeny Cretaceous (Maestrichtian) is the first clear evi- was a sharp demarcation between newly dence of the inception of the Coast Ranges formed mountains and a relatively flat and un- thrusting. Continued uplift and erosion, cul- disturbed sea floor. In central California, this minating in mid-Eocene unconformities along boundary approximates the eastern border of the eastern border of the Coast Ranges, gives a the present Coast Ranges and lies more than reasonable date for its termination. 100 mi east of the present continental margin. A continental shelf, under these conditions, The San Andreas fault offsets the Coast would not exist. The first debris derived from Ranges thrust fault and postdates it. The end of the continent would be deposited as a continen- the thrusting and the beginning of the strike- tal rise deposit at the foot of the continent and slip movement are coincident. Gentle folds, would build upward against the continent and evidenced by crustal thinning and minor un- outward onto the pre-existing sea floor. conformities, developed adjacent to the San Andreas fault through the remainder of the The Sediment Prism Tertiary. In Plio-Pleistocene time, the folding The Franciscan and the Great Valley se- intensified and many large anticlines and an- quence were deposited in a single, laterally con- tiforms were developed. This was the time of tinuous sedimentary prism. The dimensions of maximum folding of the sediment segment of the prism can be reasonably approximated. At the Coast Ranges thrust fault. Faulting of this least 40,000 ft of sediment can presently be segment also took place with an increase in measured in undisturbed sections and an addi- number and intensity toward the San Andreas tional 15,000 ft needs to be accounted for. A fault, and the larger faults exhibit San Andreas- width on the order of 200 mi may be derived type movements. Faulting also occurs along or from the present width in addition to that which near the Sur-Nacimiento fault zone and rejuve- may be seen by restoring the deformed part by nates portions of it. "unfaulting" and "unfolding" the prism.

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A prism of sediment 10 or more miles thick, cene time, folding and advancing uplift caused at its thickest part, and 200 or so miles wide is the sea to retreat from much if not all of the necessary to balance the amount of uplift and Great Valley. The continued uplift and erosion erosion of the post-Nevadan Sierra Nevada- of the Sierra Nevada ceased about mid-Eocene Klamath Mountains (see Bateman and Wahr- time with the ending of the depositional and haftig, 1966). A sketch of this model is shown tectonic thickening of the Coast Ranges. as Figure 6A. It will be noted that continental It is difficult to determine the width of the slope deposition against the continental crust Coast Ranges fault zone, because the San An- gives way to continental shelf deposition in dreas fault system and the waters of the Pacific Cretaceous time. The downwarp of the oceanic Ocean obliterate its western border. There is a crust, under the prism of sediments, and the width of about 90 mi between the basement complementary uplift and erosion of the adja- segment and the most westerly unobliterated cent continental crust, became more stabilized sediment segment in the northern Coast and Upper Cretaceous beds lapped upon an Ranges. This does not indicate that there has erosion-cut continental shelf. been a minimum of 90 mi of dip-slip move- ment. The amount of movement has to be con- The Coast Range Orogeny siderably less than this to preserve the relations The Coast Range Orogeny of Irwin (1964) of younger hanging-wall rocks on younger can now be more precisely dated as Late Creta- footwall rocks on the west, and older hanging- ceous to mid-Eocene, or from about 75 to 50 wall rocks on older footwall rocks on the east. m.y. ago. In the model of the Coast Range To preserve this relationship, the actual move- Orogeny, the outstanding feature is the relative ment must have been 50 mi or less. overthrusting of the continental crust over the The Coast Ranges thrust fault is not limited oceanic crust along the Coast Ranges thrust to the Coast Ranges of California. As has been fault. Figure 6B is a simplified cross section of stated above, it extends into Oregon where it the model. When compared to Figure 6A, it disappears under Tertiary rocks. In southern shows that the fault cuts the prism of sediments California, south of the Transverse Range, the along a gentle east-dipping plane dividing them steep basement segment underlies the west side into a hanging-wall Great Valley sequence and of the Los Angeles Basin between the Catalina a footwall Franciscan. It shows the thrust plane Schist and the eastern basement complex. This steepening as it approaches the continental is near the Newport-Inglewood fault, a more crust. Unlike the model illustrated by Page recent feature that probably takes advantage of (1966), it does not cut the continental crust, but the old line of weakness (see Gardett, 1970). pushes it down and back, possibly by "mantliz- South of the Los Angeles Basin, it is inferred ing" it. The boundary between the two offers that it continues off the west coast of Baja Cali- the avenue for ultramafic rocks to rise and fornia to south of Cedros Island from dredgings become the serpentinite intrusions along the of Franciscan-like rocks (Normark, 1969) and Coast Ranges thrust fault and the scattered tec- the occurrence of like rocks on certain islands tonic inclusion in the Franciscan. (Allison, 1964). The fault may actually be ex- Since this is a simplified cross section, it does posed on Cedros Island which contains both not show these cold intrusions, or the frag- Franciscan and granitic rocks (Kilmer, 1969). ments of oceanic crust that are incorporated in The Coast Ranges thrust fault is unknown in the the Franciscan. It also cannot show the intri- Transverse Ranges, but it should lie somewhat cately imbricated and tectonically disorganized west of Santa Cruz Island where granitic rocks nature of the footwall block. are exposed. As thrusting proceeded easterly, the hang- Although it is very speculative, the geology ing-wall block was lifted, causing shoaling of of the Olympic Peninsular suggests that it may the water westerly and finally emergence. The be an antiform, exposing the footwall equiva- sediments, which in Late Cretaceous time could lents of the Coast Ranges thrust fault. At pre- spread widely over the continental shelf and sent, it seems highly likely that the Coast slope, became confined to progressively more Ranges thrust fault may be of a continental scale easterly areas. The lower Eocene sediments and may some day be called the North Ameri- have dark shales, suggesting stagnant bottom can thrust fault. It seems reasonable that the conditions, and sand distribution patterns, sug- Laramide thrusts of the eastern Cordillera are gesting source areas to the west as well as to the backthrusts of the Coast Ranges thrust (see east (Hackel, 1966). By the end of early Eo- Burchfiel and Davis, 1968, for comparisons).

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C. Cenozoic rocks 0 5 10 Ku Upper Cretaceous rocks MILES Ki L'ower Cretaceous Ju Upper Jurassic G.V.5. Great Valley Sequence F. Franciscan

Pacific Ocean Salinia Sialic Crust with young Ku * C cov 5-

10- 0 5 10 MILES

Figure 6. Model cross sections across the Coast ary of oceanic and sialic crusts. B. The Coast Range tion, showing folding of the Coast Ranges thrust and Ranges. A. Prism of sedimentary rocks deposited from orogeny showing a schematic representation of the Coast right-lateral displacement on the San Andreas fault. Late Jurassic to Late Cretaceous time across the bound- Ranges thrust. C. Post-Coast Ranges orogeny deforma-

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The Coast Ranges orogeny was not accom- genic activity through the post mid-Eocene, it panied by mountain making or plutonism as was of a low intensity until Pleistocene time. were the similar Nevadan orogenies. These Then the intensity compounded to form the two phenomena may be late-stage features of an Coast Ranges as they presently are, and to orogeny of this type. The Coast Ranges renew the upward movement by tilt, and ero- orogeny is viewed as an arrested orogeny that sion of, the Sierra Nevada. failed to reach such a late stage. Instead, the While the San Andreas fault was relieving accumulating strain of the continent and ocean, strain by simple strike-slip movement, crustal pushing against each other, found relief along shortening took place in the Coast Ranges. Res- a newly formed fracture that was rapidly propa- tored cross sections of the Diablo Range show gated into the San Andreas fault system. that this area has been shortened from 50 mi wide to 30 mi wide. Post-Coast Range Orogeny Continuous strike-slip movement along the DISCUSSION San Andreas fault from mid-Eocene to the pre- Since the concepts of sea-floor spreading are sent has been the outstanding feature of the so widely used to explain tectonism today, it is second tectonic event. In the model (Fig. 6C), worthwhile to examine the tectonics of the the San Andreas fault is a right lateral (dextral), Coast Ranges in this light. The Coast Ranges strike-slip fault. When movement started on it, have been at the boundary of the North Ameri- a portion of the North American block parted can and Pacific Ocean plates since sedimenta- from the remainder of it, and, together with the tion began in Late Jurassic time. Further, they Pacific Ocean block, moved relatively north- have within them a remarkably well-preserved westerly. Salinia is the sialic addition to the Pa- record of their sedimentary and tectonic histo- cific Ocean block in the Coast Ranges. It has ries. moved from a former position in line with the The two most obvious spreading zones that Cretaceous plutons of the Sierra Nevada which could have affected the Coast Ranges are the have the same K-Ar age dates (Curtis and oth- mid-Atlantic rise and the East Pacific rise. Be- ers, 1958; Evernden and Kistler, 1970). cause of the proximity of the East Pacific rise, Folding and faulting in the Coast Ranges was it has been almost unanimously singled out as contemporary with San Andreas fault move- the active agent in recent papers. Circumstan- ment. Local structural basins developed and tial evidence to support this idea is the apparent were filled with thick lenses of predominantly absence of certain of the "isochrons" (Heirt- marine sediments. The Coast Ranges thrust zler, 1968) of the East Pacific rise opposite Cali- fault was folded, and the anticlines and large fornia. In the sea-floor spreading theory, an antiforms of the Coast Ranges took shape. ocean rise is a symmetrical system of magnetic These were elevated and subjected to erosion, anomalies of increasing age outward from the providing the chief source of the sediments. By median axis of the rise. Since a good part of the Miocene time, erosion reached the footwall East Pacific rise is missing, it has been inferred block of the folded thrust fault in certain areas that it has vanished under California. The sea and Franciscan detritus started to be a recogniz- floor over the missing part of the rise is pre- able component of the sediments. Folding and sumed to have underthrust the continent (Bai- faulting affected the crystalline rocks of Salinia ley and others, 1964; Irwin, 1964; Ernst, 1965, and under the Great Valley, particularly the 1970; Hamilton and Meyers, 1968; Page, "Dark Hole" at its southern end, and deep 1966, 1969, 1970a; Crowell, 1968; Davis, depositional basins were formed on them. 1968; Hamilton, 1969; Lowe, 1969). Benioffs The erogenic movements in the Coast (1954) model, determined by seismic studies, Ranges were not neatly systematic. Although has been applied to the lost Pacific ocean crust marine formations were deposited from upper and the Coast Ranges have been envisioned as Eocene to late Pliocene, there are no continu- overlying a fossil "Benioff zone." ous columns. Interruptions in deposition and There are reasons why the mid-Atlantic rise unconformities, not everywhere simultaneous, rather than the East Pacific rise may be consid- occur. Many of the formations are restricted in ered the active agent. The North American area, and marked changes in lithofacies occur plate is essentially a single plate, extending un- within short distances. broken from the Mid-Atlantic Ridge westerly While the sedimentary record indicates oro- to the Pacific Ocean plate. From Late Jurassic to

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Holocene, the continental plate has shown a supported. The East Pacific rise off California remarkable consistency and durability. Seem- is not a mid-ocean rise, however. It is snuggled ingly, the Mid-Atlantic Ridge also shows simple up to a very large and durable continental persistence. On the other hand, the eastern Pa- block. Could it be that movement of new crust cific magnetic lineations have been fragmented in such a situation was unidirectional? It may be and rotated. Attempts to put parts (like the Juan noted on published maps of ocean rises (for de Fuca ridge, Gorda ridge, and East Pacific example, Heirtzler, 1968) that the magnetic rise of the Gulf of California) back together anomalies, or the "isochrons," are spaced about again have led to visionary extensions of the twice as far apart off California as they are at logic of transform faulting. Further, the East equivalent latitudes in the Atlantic Ocean. This Pacific rise is inactive today, in or near the could happen if each successive newly formed Coast Ranges, and has been inactive for some band of crust could only move westerly, and if tens of millions of years. The younger isochrons the spreading rates off California and in the have been extrapolated to a position under the Atlantic were somewhere near equal. If the East continental crust. If continental crust were to be Pacific rise off California was unidirectional in pushed over an actively spreading rise, there its spreading and kept pace with the westerly appear to be only two things that could happen. drifting continent, the combined results of the The continental crust could lock the two sides mid-Atlantic and East Pacific spreading would, of the spreading oceanic crust together and stop together, move the Pacific plate as down-goin the spreading, or the continental crust could slabs of crust and lithosphere into the astheno- separate into two blocks that would move away sphere in the western Pacific as modeled by from each other. The geological evidence of sacks and others (1968). True Benioff zones such a separation would be a rift such as the occur in the western Pacific. Gulf of California. Since no rift occurs in or The geology of the Coast Ranges suggests near the Coast Ranges, inactivity is indicated. that the motions of the Pacific and North In the period of time from approximately 7 5 American plates, relative to each other, varied to 50 m.y. ago, the oceanic crust underthrust with time. As the late Mesozoic prism of sedi- the Coast Ranges. The Coast Ranges show no ments was being deposited across the boundary evidence of large-scale underthrusting before between the plates, there appears to have been or after that period of time. Based on the esti- little or no differential movement between the mate of Hamilton (1969), about 1250 mi of the two. During the Coast Range orogeny, the east limb of the East Pacific rise moved under North American plate overtook the Pacific the Coast Ranges in this ± 25 m.y. period. This plate and underthrusting ensued. With the ad- would be at a seemingly incredible rate of vent of the San Andreas fault, simple compres- nearly 1 1/3 ft per year. If this amount of crust sion gave way to lateral shear. actually passed beneath the continent, there The genesis of the San Andreas fault has al- was a "Benioff zone" under the Coast Ranges ways raised perplexing questions. It is a con- — an exceedingly active one too. The on-land tinuous fault from Cape Mendocino to a point geology suggests that the sediment deposited approximately 800 mi southeasterly, where it on the east limb underthrust a maximum of 50 enters the head of the Gulf of California. Prior mi during this period of time. The amount of to the opening of that gulf in late Cenozoic basalt, preserved in the footwall block of the time, it probably extended the full length of the Coast Ranges thrust fault cannot account for the gulf as well. As so reconstructed, it had a length additional 1200 mi. Only about 10 percent of of nearly 1500 mi. It has continental dimen- the footwall rock is basalt, which is consistent sions and must be a factor in global tectonics. In with a 50-mi rather than a 1250-mi under- the Coast Ranges, it mostly bounds granitic and thrusting. If 1250 mi of ocean crust moved un- Franciscan rock; however, it has granitic rocks der the Coast Ranges, it left no evidence of such on both sides of it for 70 percent of its recon- movement. structed length. The granitic rocks west of it Perhaps alternatives should be considered. It have parted from the continental plate and has been contended that genesis of mid-ocean welded to the Pacific plate, they now move rises requires symmetry of magnetic patterns in northwesterly independently of the rest of the the crust formed between plates diverging from continent. From these relationships, it would a spreading center. This symmetry is shown by seem almost certain that the site of the incep- all of the mid-ocean rises and seems to be well tion of faulting was within the granitic crust of

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western North America. The strike of the fault the detached block of continental crust south- is oblique to other major plate features: the west of the San Andreas fault, is riding on a magnetic lineations of the East Pacific rise; the shallow layer of asthenic material, probably ser- transverse fracture zones of the eastern Pacific; pentine. Movemant of this block is by essen- and the mid-Eocene plate boundary, the Coast tially aseismic creep or by very shallow Ranges thrust fault. The San Andreas fault cuts earthquakes. A reasonable assumption is that all these features indiscriminately and none the block is riding on the serpentine layer of a could have occasioned it. pre-Upper Jurassic ophiolite sequence similar The true origin of the San Andreas fault may to the one underlying the Great Valley se- not be known for some time, but for now there quence. Southeast of the Coast Ranges, the seems to be a reasonable possibility that it was fault is less well known. In the Tranverse fortuitous. When the ocean and continent were Ranges, the detached block probably rides on in an extreme state of compression, it formed. schist as depicted by Yeats (1968a, 1968b). In Perhaps, by some vagary, strike-slip movement the Salton Trough and in the Gulf of California, along some minor fault in the continental crust the San Andreas fault becomes a rift. The East was a step in the right direction to relieve this Pacific rise, made dormant under Baja Cali- strain and this new strike-slip fault propagated. fornia and most of California, is reactivating up The orientation of the San Andreas fault is the Gulf along the San Andreas line of weak- not ideal for relieving the strain of ocean ness. This is a late Cenozoic development, since against continent. In addition to strike-slip early Pliocone, as marine faunas from the Im- movements in the Coast Ranges, there has been perial Formation of the Salton Trough suggest structural shortening, by Tertiary and Quater- —or within the last 4 m.y., as paleomagnetic nary folding, on the order of three-fifths of its data suggest (Larson and others, 1968). The original width. Recent movements also indicate right-lateral direction of offset is preserved by this. An analysis of horizontal movements short northeasterly trending spreading centers within a U.S. Coast and Geodetic Survey trian- and northwesterly striking transform faults. gulation network in the Coast Ranges from Northwest of Cape Mendocino, paleomag- 1930 to 1950 was made by Burford (1967). netic data suggest that active uncovered rem- He found that simple right-lateral shear oc- nants of the East Pacific rise occur. To date, curred within a band nearly 20 mi wide, cen- satisfactory interpretations of the relationship tered on the fault. Northeast and southwest of of these features to the San Andreas fault have this band, crustal shortening dominates the not been found. There are reasons to believe strain pattern. South of the Transverse Ranges, that they tend to interfere with each other. The the San Andreas fault is under tension. San Andreas trace is doubly bowed south of There has been and there undoubtedly will Cape Mendocino. The near offshore magnetic continue to be much speculation concerning anomalies (Raff and Mason, 1961) are sharply the role the San Andreas fault has played in flexed between Cape Mendocino and Cape global tectonics. In this paper, an attempt shall Blanco, and the eastern anomalies north of be made to explain its influence. It has already there are rotated to the northeast and frag- been suggested that there was a change in the mented into a number of separate blocks as pattern of Pacific plate movement relative to shown by Vine (1966). Various structural in- the North American plate with the inception of terpretations have appeared subsequently, in- the San Andreas fault.Together with similar cluding transform faults; but the flexing, strike-slip faults in Canada and Alaska, such as rotation, and fragmentation could result from the Queen Charlotte, Denali, and Fairweather the northwest movement of the Pacific Ocean, faults, it forms the fault type of boundary be- relative to North America, that is concentrated tween blocks at which crustal surface is neither along the San Andreas fault system to the south. created nor destroyed (Morgan, 1968). It has The discussion would not be complete with- separated two trench types, the Aleutian and out considering the age dates that have been mid-America trenches. More than 80 percent found in Franciscan metamorphic minerals. of this boundary is under water and is known Suppe (1970a) compared 30 new K-Ar dates of chiefly from earthquake seismology. The larg- coexisting minerals from well-studied parts of est on-land segment is in the Coast Ranges the Franciscan. He found two apparent times of where it has been studied in many disciplines. metamorphism: Late Jurassic (150 to 140 m.y.) As pointed out in a preceding section, Salinia, from white mica, and mid-Cretaceous (115 to

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80 m.y.) from other minerals and whole-rock block exposures where b + tl + to is greatest, samples. Both time periods are difficult to relate and a belt of lower grades along the westerly to geological events as interpreted. The older belt where the pressure sum is least. period conflicts with the view that Franciscan The sediment segment of the Coast Ranges rocks postdate the Nevadan Orogeny. The thrust fault dips beneath a portion of the younger period is more tenable, but does not oceanic crust (ophiolite sequence) along the coincide with the span of the Coast Ranges west side of the Sacramento Valley. Elsewhere, Orogeny. As Suppe has pointed out, white mica it encounters the crystalline rocks of the conti- dates may record the time of metamorphic re- nental crust along a steepening curve, called the crystallization in low-temperature, high-pres- basement segment, without penetrating the sure metamorphism rather than time of cooling base of the column. The total amount of dip-slip in high temperature rocks. This recrystalliza- on the fault is limited by the ages of juxtaposed tion may take place very early in the arkose-to- hanging-wall-and footwall-block rocks on the graywacke stage, but even so, the dates two sides of the Coast Ranges and their geo- obtained are too old. Can they be inherited graphical dimensions. It appears not to exceed from an older orogeny? Are the younger dates 50 mi. The model is unlike the Benioff model. from other minerals reflecting some early phase The Coast Ranges thrust fault is cut by the of burial metamorphism rather than tectonic San Andreas fault and predates it. The time of pressure? This is difficult to reconcile with the cessation of movement on the Coast Ranges low-temperature, high-pressure requirement of thrust fault coincides with the initiation of their metamorphism. Finally, is the geological movements on the San Andreas fault. Move- dating at fault? It is hoped that good thinking ments on the strike-slip San Andreas fault re- will be applied to this quandary. quired less energy than movements on the dip-slip Coast Ranges fault. CONCLUSIONS The distance between the projected trace of It is concluded that a large prism of sedi- the basement segment of the Coast Ranges ments was deposited across the boundary of the thrust fault on the two sides of the San Andreas Pacific Ocean and North American continent at fault approximates 310 mi. A 310-mi displace- the site of the present Coast Ranges in Late ment, since mid-Eocene time, is in agreement Jurassic to Late Cretaceous time. During the with rates of displacement, based upon younger Coast Range Orogeny, Late Cretaceous to mid- geological features, widely reported in the liter- Eocene, the prism was bisected by a gentle east- ature. erly dipping thrust fault, the sediment segment In addition to right-lateral strike-slip on the of the Coast Ranges thrust fault. Sedimentation San Andreas fault, the Coast Ranges have un- during the Coast Range Orogeny was progres- dergone compression and crustal shortening. sively confined to the east due to shoaling and The Coast Ranges thrust fault has been folded emergence of the Coast Ranges; however, into large antiforms and smaller anticlines and there was no mountain building, no volcanism, synclines. Folding activity probably started in and no plutonism—nor were island arcs and the Eocene, for by Miocene time, erosion trenches formed. breached the folded Coast Ranges thrust fault The sediment segment of the Coast Ranges and exposed Franciscan rocks. However, the thrust separates the prism of sediments into two most intense folding has been in late Pliocene tectofacies. The hanging-wall block contains and Pleistocene time and folding is continuing the relatively undisturbed Great Valley se- today. Faulting accompanied the folding, par- quence. The footwall block contains the tec- ticularly where basement or basementlike rocks tonically jumbled Franciscan. The Great Valley are shallow (as in Salinia, the Southwest Fran- sequence is essentially unmetamorphosed ex- ciscan block, and the northern Coast Ranges cept by burial metamorphism. The Franciscan is antiform). At least some of the faults have ab- mildly to intensely metamorphosed. It is high- sorbed some of the differential strike-slip move- pressure, low-temperature metamorphism re- ments of the Pacific and against North sulting from burial (b) + tectonic loading (//) American plates. It is possible that this has + tectonic overpressuring (to). Original pat- developed because of the interference to free terns of metamorphism have been largely lost movements on the San Andreas fault proper, as in the jumble, but there is a concentration of in the Cape Mendocino area. higher grades along the easterly belt of footwall It is concluded that spreading from the Mid-

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Atlantic Ridge and westerly movement of Blake, M. C., Jr.; and Cotton, William R. Inverted North America, relative to the Pacific Ocean, metamorphic mineral zones in Franciscan meta- can account for the tectonism of the Coast graywacke of the Diablo Range, northern Cal- Ranges. Finally, it is suggested that the "new fornia [abstr.]: Abstracts with Programs for 1969, Part 3, Geol. Soc. Amer., p. 6-7, 1969. global tectonics" should be looked at in the Blake, M. C., Jr.; Irwin, W. P.; and Coleman, R. light of the geology of the Coast Ranges. G. Upside down metamorphic zonation blues- chist facies along a regional thrust in California ACKNOWLEDGMENTS and Oregon: U.S. Geol. Surv., Bull. 575-C, p. Grateful acknowledgment is made to the C1-C9, 1967. Blake, M.C., Jr.; Irvin, W.P.; and Coleman, R. G. Standard Oil Company of California for having Blueschist-facies metamorphism related to re- given the author gainful employment as a gional thrust faulting: Tectonophysics, Vol. 8, geologist in the Coast Ranges of California No. 3, p. 237-246, 1969. from 1927 to 1970 and for permission to pub- Brace, W. F. Tectonic overpressure in Franciscan lish this paper. Thanks are due to my fellow rocks [abstr.]: Amer. Geophys. Union, Trans., geologists with Standard Oil Company of Cali- Vol. 50, No. 4, p. 325, 1969. fornia, Western Operations, Inc., for sugges- Brice, James C. Geology of the Lower Lake quad- tions and criticisms, and to Margaret M. Schultz rangle, California: Calif, Div. Mines Geol., and D. V. Williamson for special assistance. Bull. 166, 72 p., 1953. Brown, James A. Probable thrust contact between Franciscan formation and the Great Valley se- REFERENCES CITED quence northeast of Santa Maria, California Allison, Edwin C. Geology of areas bordering Gulf [abstr.]: Geol. Soc. Amer., Spec. Paper 115, p. of California: Amer. Ass. Petrol. Geol., Mem., 23-24; 1968. No. 3, p. 3-29, 1964. Brown, Robert D. , Jr. Thrust fault relations in the Bailey, E. H.; and Irwin, W. P. K-feldspar content northern Coast Ranges, California: U.S. Geol. of Jurassic and Cretaceous graywackes of north- Surv. , Prof. Paper 475-D, p. D7-D13, 1964. ern Coast Ranges and Sacramento Valley, Cali- Burchfiel, B. C.; and Davis, G. A. Two-sided na- fornia: Amer. Ass. Petrol. Geol., Bull., Vol. 43, ture of the Cordilleran Orogen and its tectonic No. 12, p. 2797-2809, 1959. implications: 23rd Internal. Geol. Congr., Rep., Bailey, Edgar H.; Irwin, William P.; and Jones, Czechoslovakia, Proc. Sect. 3, p. 175-184, David L. Geol. Soc. Sacramento Ann. Field 1968. Trip, Guidebook, p. 39-46, 1963. Burford, Robert Oliver. Strain analysis across the Bailey, Edgar H.; Irwin, William P.; and Jones, San Andreas fault and Coast Ranges of Cal- David L. Franciscan and related rocks and their fornia: Ph. D. dissert., Stanford Univ., Stanford, significance in the geology of Western Cali- Calif, 1967. fornia: Calif, Div. Mines Geol., Bull. 183, 177 Chapman, Rodger H. The gravity field in northern p., 1964. California: Calif. Div. Mines Geol, Bull. 190, Bailey, Edgar H.; Blake, M. C., Jr.; and Jones, p. 395-405, 1966. David L. Character and significance of the Chesterman, C. W. Intrusive ukrabasic rocks and ophiolitic oceanic crust that forms the base of their metamorphic relationships at Leech Lake the Great Valley sequence in western California Mountain, Mendocino County, California: 21st [abstr.]: Abstracts with Programs, Vol.2, No.2., Internat. Geol. Congr., Rep., Copenhagen, Proc. p. 68-69, Geol. Soc. Amer., 1970. Part 3, p. 208-215, I960. Bandy, Orville L.; and Arnal, Robert E. Middle Clark, Samuel G. Geology of the Covelo District, Tertiary basin development, San Joaquin Val- Mendocino County, California: Calif, Univ., ley, California: Geol. Soc. Amer., Bull, Vol. 80, Publ. Geol. Sci., Vol. 25, p. 119-142, 1940. No. 5, p. 783-820, 1969. Clement, W. G. Complete Bouguer gravity map of Bateman, Paul C.; and Wahrhaftig, Clyde. the northern part of the San Francisco Bay area Geology of the Sierra Nevada: Calif, Div. and its geological interpretation: U.S. Geol. Mines Geol., Bull. 190, p. 107-172, 1966. Surv., Map GP-468, 1965. Bateman, P. C., Clark, L. D.; Huber, N. K; Moore Compton, Robert R. Granitic and metamorphic J. G.; and Rinehart, C. D. The Sierra Nevada rocks of the Salinian Block, California: Calif, batholith—a synthesis of recent work across the Div. Mines Geol., Bull. 190, p. 277-287, 1966. central part: U.S. Geol. Surv., Prof. Paper 414- Crowell, John C. Movement histories of faults in the D, 46 p., 1963. Transverse Ranges and speculations of the tec- Benioff, Hugo. Orogenesis and deep crustal struc- tonic history of California: Stanford Univ. ture—additional evidence from seismology: Publ., Geol. Sci., Vol. 11, p. 323-341, 1968. Geol. Soc. Amer., Bull., Vol. 65, p. 385-400, Curtis, G.H.; Evernden, J. F; and Lipson, J. Age 1954. determination of some granitic rocks in Cali-

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