DARREL S. COWAN. CHARLES F. MANSFIELD. Department of Geology, Stanford University, Stanford, California 94305. Serpentinite Flows on Joaquin. Ridge ...
DARREL S. COWAN CHARLES F. MANSFIELD
Department of Geology, Stanford University, Stanford, California 94305
Serpentinite Flows on Joaquin Ridge, Southern Coast Ranges, California ABSTRACT Flows of serpentinite extend downslope more than 1.5 km from the northeastern margin of a large source body of serpentinite exposed on Joaquin Ridge in western Fresno County, California. The parent body, oval in plan, consists of almost completely serpentinized peridotite and dunite in fault contact with surrounding Franciscan metamorphic rocks and upturned strata of the Upper Cretaceous Panoche Group. The serpentinite flows have an external form which strikingly resembles the forms of debrisflow deposits and solidified lava flows. On the basis of topographic position and surface morphology, flows of several ages can be recognized. Remnants of older flows cap ridges or are perched above valley floors, whereas younger flows respect modern topography and fill present-day canyons which extend into the main serpentinite body. The flows consist of rounded blocks of serpentinized peridotite and dunite enclosed in a matrix of flaky, intensely sheared serpentine. Shear surfaces locally define a weak foliation that diverges around large, unsheared residual blocks. These internal features are strikingly similar to those encountered in the parent serpentinite body. The lower parts of flows of all ages unconformably overlie sandstones and shales of the Panoche Group. The geometric form of the Joaquin Ridge flows approximates the theoretical form of a rigid-plastic substance in critical equilibrium. Calculations involving the flow dimensions and the density of serpentine indicate that the sheared serpentinite comprising the flows had a shear strength on the order of 106 dynes cm~2 (1 bar) at atmospheric confining pressure and normal surface temperature. This relatively low value of strength contrasts significantly with much higher values determined experimentally by Raleigh and Paterson (1965) on solid, unsheared serpentinite. The low values indicate that tabular masses of sheared serpen-
tinite found along thrusts in the California Coast Ranges and Klamath Mountains could have been tectonically emplaced by plastic flow at modest stresses and temperatures that were much lower than hitherto suspected. Surface flows similar to those on Joaquin Ridge may have contributed to the formation of some enigmatic "sedimentary" or "detrital" serpentinites of Cretaceous and Miocene age found in the Coast Ranges. INTRODUCTION Serpentinized ultramafic rocks are widely distributed in the Coast Ranges of California. They commonly occur as: (1) irregular tabular masses along the major fault zones separating the Franciscan assemblage and Great Valley sequence; (2) sheared, elongate masses entirely surrounded by Franciscan rocks; and (3) rounded or oval bodies variously referred to as "piercements" or diapirs. Even though these ultramafic rocks may have been ultimately derived from the upper mantle (Irwin, 1964), the details of their origin and subsequent history have been obscured by serpentinization and tectonic emplacement. The tectonic environment of these rocks, their degree of serpentinization, their strongly sheared internal fabric, and the lack of high-temperature contact effects on surrounding rocks indicate that many of these ultramafic bodies assumed their present position by low-temperature intrusion in the solid state (Bailey and others, 1964, p. 86-87). This paper describes flows of serpentinite which extend downslope from a large oval body of serpentinized ultramafic rocks exposed on Joaquin Ridge in western Fresno County, California. Eckel and Myers (1946), and subsequently Coleman (1957), identified these features as local landslide deposits in general discussions of surficial deposits associated with the much larger parent serpentinite body. We prefer to call these features serpentinite flows because of their striking morphologic resem-
Geological Society of America Bulletin, v. 81, p. 2615-2628, 8 figs., September 1970 2615
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COWAN AND MANSFIELD—SERPENTINITE FLOWS, JOAQUIN RIDGE, CALIF.
blance and their apparent mechanical similarity to lava flows and debris flows. This paper includes a brief description of the main serpentinite body, a more detailed discussion of the flows and their mechanical properties, and, finally, our interpretation of the geologic significance of both the flows and the data on the properties of serpentinite obtained from them. Because these flows were derived from an exposed body of serpentinized peridotite and dunite, the problems regarding the ultimate origin of orogenic ultramafic rocks (Turner and Verhoogen, 1960; Wyllie, 1968, p. 407-414) are not directly applicable to the interpretation of the genesis, form, and internal structure of the Joaquin Ridge flows. These flows do provide, however, information on the behavior of sheared serpentinite at surface temperatures and pressures and, thus, may relate significantly to the low-temperature tectonic emplacement of similar sheared serpentinite masses in the Coast Ranges and other orogenic belts. Because they are surface deposits, flows such as these might have contributed to the formation of the enigmatic "sedimentary" or "detrital" serpentinites preserved in the geologic record
(Bailey and others, 1964, p. 164; Emerson and Rich, 1966). REGIONAL GEOLOGY The Southern Coast Ranges constitute the geologically complex part of California that lies between the Great Valley and the Pacific Ocean south of San Francisco and north of the Transverse Ranges. Although the ranges grossly trend parallel to the coast and to the San Andreas fault, several individual ranges that lie east of the fault and comprise the Diablo Range trend obliquely to the fault at N. 45° W. to N. 75° W. Joaquin Ridge is a prominent oblique anticlinal feature approximately 80 km southwest of Fresno in the central part of the Southern Coast Ranges (Fig. 1). Its crest is underlain by an elongate body of serpentinized ultramafic rock. The prolific New Idria mercury district lies at the northern end of this body. A variety of sedimentary and metamorphic rocks of Mesozoic and Tertiary ages are exposed in the area surrounding Joaquin Ridge, and the reader is referred to more complete descriptions of the regional geology by Anderson and Pack (1915) and Mielenz (1939). Joaquin Ridge Serpentinite Body
TERTIARY & QUATERNARY GREAT VALLEY SEQUENCE [•'.••VI FRANCISCAN ASSEMBLAGE [ •
J ULTRAMAFIC ROCKS SERPENTINITE FLOW 8, LANDSLIDE DEPOSITS
Figure 1. Generalized geologic map of the Joaquin Ridge area, California. Geology by E. B. Eckel, R. G. Yates, W. B. Myers, and A. E. Bradbury (Eckel and Myers, 1946, Pi. 8).
Eckel and Myers (1946) and Coleman (1957) have described in detail the mineralogy, petrology, and structure of the large serpentinite body and immediately adjacent rocks on Joaquin Ridge. Only a brief summary, largely based on their contributions, is included here. In plan view, the outcrop of the body is a fault-bounded, elongate, northwest-trending oval approximately 20 km long and 8 km wide (Fig. 1), that occupies the core of the northwest extension of the Coalinga anticline. The body is flanked by steeply dipping to locally overturned sandstone and shale beds of the Upper Cretaceous Panoche Group. This group is part of the Great Valley sequence, a thick sequence of Upper Jurassic to Upper Cretaceous clastic sedimentary rocks exposed along the western margin of the Great Valley of California. Blocks of Franciscan rocks discontinuously rim and overlie the outer margin of the main serpentinite body and are in fault contact with adjacent strata of the Panoche Group. At least some of the Franciscan sedimentary and igneous rocks have been regionally metamorphosed and contain blueschist-facies mineral assemblages. The contact between the serpentinite and surrounding sedimentary and meta-
SERPENTINITE FLOWS morphic rocks is a steep fault, and this feature, together with the complete absence of intrusive contact-metamorphic effects, indicates that the serpentinite body was tectonically emplaced into its present position. The parent ultramafic rock has been almost completely serpentinized. It consists of residual, rounded blocks of serpentinized dunite and minor peridotite enclosed in a strongly sheared matrix of serpentine. The residual blocks locally contain relict orthopyroxene and much rarer relict olivine grains (Coleman, 1957). Shear surfaces within the matrix locally define a foliation that diverges around unsheared blocks. Coleman (1957) and F. A. Mumpton and C. S. Thompson (unpub. manuscript) determined that chrysotile and lizardite are the most abundant serpentine minerals in the body; antigorite is found in certain areas. Brucite is also present in the sheared matrix and massive residual boulders (Hosteller and others, 1966). Short-fiber asbestos is being mined from the southeastern end of the serpentinite. Small fault-bounded inclusions of Franciscan metamorphic rocks are scattered throughout the main serpentinite body, and a few small syenite and camptonite bodies are also present. No definitive interpretations of the over-all shape and structure of the body have been made, even though it is exposed over 600 m of vertical relief. Byerly (1966), on the basis of a reconnaissance gravity survey, suggested that the body is approximately 1.5 km thick and does not contain any large masses of unserpentinized ultramafic rock at depth. The Joaquin Ridge serpentinite is commonly described as a "piercement" (Page, 1966, p. 274), but if the body does have a relatively shallow floor, it becomes difficult to ascribe a pluglike or diapiric geometry to it. SERPENTINITE FLOWS Several flows of serpentinite extend downslope as much as 1.5 km from the northeastern contact of the main serpentinite body (Figs. 1 and 2); other flows extend from the eastern and southern margins of this body, but are not discussed in this paper. The flows have a form similar to that of solidified lava flows and debrisflow deposits (Figs. 3 and 4). The flows range in width from a maximum of approximately 525 m to 80 m, or less, in their lower reaches. The lengths of recent flows range from 0.8 to nearly 1.5 km; the maximum flow relief, measured from flow head to toe, is nearly 275
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m. These measurements apply to those parts of the flows which extend downslope from the contact of the parent body with the surrounding country rocks. Several episodes of flowage can be recognized on the basis of topographic position and surface morphology of the flows. The youngest flows have a smooth, convex upward surface and are little modified by stream erosion except at their margins (Fig. 3A). Flows of intermediate age typically are hummocky and have been more dissected by intermittent streams (Fig. 3B). The oldest flows occur as isolated caps on low ridges or are perched above valley floors as dissected terrace-like features (C and D in Fig. 2, respectively). Flows of Young and Intermediate Age The youngest flows respect modern topography and fill present-day canyons (A and B in Fig. 2). The heads of modern canyons apparently extended toward the main serpentinite body and, with continued headward and downward erosion, eventually breached the contact between sedimentary rock and serpentinite to the extent required to allow flowage to begin. Larger flow deposits on the southern margin of the parent body are several square kilometers in area and are not confined to a single canyon (Fig. 1). These features indicate that continual or intermittent uplift of the parent body may also have initiated flowage. Thus, both uplift and canyon erosion seem to have been important processes in the development of serpentinite flows. The resulting flows, especially in their lower reaches, followed stream canyons as they moved downslope; one flow (A in Fig. 1), 80 m wide at its lower end, makes approximately a 100° bend where it enters a main trunk canyon from a side tributary (Figs. 3B and 4A). Drainage confined to a single channel in a valley bottom prior to filling by a younger flow is now constrained to two small channels on each side of the flow. The youngest flow, least modified by erosion, reaches a maximum width of approximately 240 m at a distance of 525 m from the main serpentinite body (B in Fig. 2, Figs. 3A and 4B). A maximum thickness of approximately 60 m can be estimated for this flow by projecting contour lines on the canyon walls to the inferred position of the preflovv canyon bottom and subtracting these elevations from the present elevation of the flow above these points. The deposits of this flow are draped across a flanking canyon wall where the flow
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COWAN AND MANSFIELD—SERPENTINITE FLOWS, JOAQUIN RIDGE, CALIF.
Figure 2. Detailed geologic map of area outlined in Figure 1. Letters indicate serpentinite flows described in this report and illustrated in Figures 3 and 4. Shaded area is underlain by undifferentiated Franciscan assemblage and Upper Cretaceous Panoche Group. Contacts are solid lines where accurately located, dashed lines where inferred, and dotted lines where concealed. Hachured lines indicate the positions of headwall scarps within the parent serpentinite body. Geology principally from aerial photographs supplemented by field work in some areas. Contour interval = 40 ft. crosses the contact between the Panoche Group and parent body. The surface of the flow is here concave upward. The cirquelike headwall scarp, which bounded the source area for the serpentinite now contained in the flow, is clearly visible in the parent body to the southwest of and above the flow (Figs. 2, 3A). The scarp is now slightly dissected by small streams, covered with a dense growth of chaparral, and has a maximum relief of about 5 0 m . Even though this headwall scarp is over 2 km long, the flow became restricted by the narrow upper part of the canyon down which it moved.
Flows of both young and intermediate ages have a surface morphology characteristic of many debris-flow deposits (Johnson, 1965). Those parts of the flows nearest the distal, or snout, end have a simple convex profile. Nearer the main body, the flows have a distinct medial depression flanked on each side by topographically higher lobes that parallel the longitudinal axes of the flows. Flows of intermediate age are hummocky and have been more severely dissected by intermittent streams. They tend to have well-developed medial streams as well as marginal ones. Surface morphology indicates
SERPENTINITE FLOWS
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- ------- -Figure 3A. Stereo pair of aerial photographs illustrating the youngest, morphologically most perfect serpentinite flow (B in Fig. 2). Headwall scarp bounding the source area for serpentinite in the flow is visible in lower right corner. Light-colored material on the ridge to the north of this flow is an isolated remnant of an earlier flow resting on the Panoche Group (C in Fig. 2).
--
- ------ -Figure 3B. Stereo pair of aerial photographs illustrating a serpentinite flow of intermediate age (A in Fig. 2). Note medial depression, flanking longitudinal ridges, marginal streams, and exhumed island of graywacke near head of flow. The flow makes a nearly 100° bend at its distal end. that one large deposit (A in Fig. 2) is actually a composite of contributions from several episodes of flow. The earliest flow extended down canyon approximately 1.2 km beyond the contact between the Panoche Group and the main serpentinite body and completely buried a low, longitudinal ridge of shale and graywacke near the contact. This earlier flow had a maximum width of about 525 m. After the first flow had stopped, a second flow accentuated the medial depression of the earlier flow, exposed the previously buried shale ridge, and advanced farther down canyon. Remnants of material from the earlier flow remain on the
crest of the ridge, 10m above the present surface of the flow. The material comprising this later flow was more mobile than that in the earlier flow and apparently had a lower strength. Older Flows Remnants of older flows cap ridges or are perched above valley floors (C and D, respectively, in Fig. 2, Fig. 4B). Those less severely dissected form irregular terrace-like features that flank present-day stream valleys, but their upper parts still reach the main serpentinite body. Remnants of still older flows that today
Figure 4A. Oblique aerial view of a flow of intermediate age (A in Fig. 2) showing hummocky surface, medial stream, and sharp bend at its distal end.
Figure 4B. Oblique aerial view of the youngest flow (B in Fig. 2). Source area for the serpentinite in the flow is at upper left; break in slope near the head of the flow is at the contact of the main serpentinite body. Isolated remnant of an older flow caps ridge to the right of this flow (C in Fig. 2).
SERPENTINITE FLOWS cap ridges are completely separated from the parent body. These older episodes of flowage clearly took place before the present-day topography and drainage system developed. Eroded remnants of the headwall scarps of these older flows are still visible within the parent serpentinite body. Even though the source of flow remnants on Joaquin Ridge is obvious, the origin of similar remnants later buried and incorporated in a sedimentary sequence might not be interpreted as easily. Contact Relationships Although their heads lie within the main serpentinite mass, in their lower reaches the flows unconformably overlie Upper Cretaceous sandstones, siltstones, and mudstones of the Panoche Group. This contact relationship, well exposed where small marginal streams have eroded downward a few meters, is everywhere distinctly angular (Fig. 5). Moderately to steeply dipping beds of the Panoche Group are truncated by the flows, and the parts of flows immediately above the unconformity are generally free of residual debris. A hard, wellcemented layer of "stream conglomerate" locally is observed lying on this uncontormity; it is particularly abundant in topographically
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low areas which presumably once were occupied by a stream that predated a flow. Stream conglomerate, a term introduced by Arnold and Anderson (1908), is a resistant layer of rounded alluvial debris cemented with calcite-dolomite solid solutions (Barnes and O'Neill, 1969, p. 1949), which is forming today and has formed earlier deposits in the beds of small streams flowing across and away from serpentinite bodies. An older ridge-capping flow (C in Fig. 2), now completely separated from the main serpentinite body, also overlies Upper Cretaceous sedimentary rocks in angular unconformity. This contact was excavated for a distance of 6 m to expose features obscured by weathering. Here, the flow lies on at least 60 cm of compact, sheared clay rather than on a layer of streamrock. This clay may well be part of a buried soil horizon of weathered shale that was on the side of a preflow canyon rather than in the stream bed itself. Near the contact, the serpentinite has a distinct, nearly horizontal foliation, that dips a few degrees upflow. Internal Structures Thick soil cover, local slide debris, and dense, brushy vegetation severely hamper examina-
Figure 5. Base of a serpentinite flow unconformably overlying sandstones and siltstones of the Upper Cretaceous Panoche Group.
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COWAN AND MANSFIELD—SERPENTINITE FLOWS, JOAQUIN RIDGE, CALIF.
Figure 6. Stream-cut outcrop of serpentinite flow exposing characteristic internal fabric of sheared matrix and residual massive blocks of serpentinized peridotite and dunite. tion oi the internal structures of the Joaquin flanks a nunatak-like shale ridge that was first Ridge flows. Only in stream cuts and scattered covered by the flow and later re-exposed when prospects can the fresh flow material be ob- flowage continued, the foliation steepens as the served. Exposures in stream cuts are also shale island is approached. Again, a systematic weathered, but the essential features of the measurement of shear-plane attitudes throughflow structures are preserved. out an entire flow is precluded by the limited, The serpentinite flows consist of rounded, nonrepresentative exposures. unshearcd blocks ol serpentinized peridotite These internal features of the flows are and dunite enclosed in a matrix of flaky, easily exactly like those encountered in the parent friable, and intensely sheared serpentine (Fig. serpentinite body, fn fact, the distinctive fabric 6). These resistant blocks vary in size; some consisting of shear foliation and residual blocks blocks up to 4.5 m in diameter have been is characteristic of other tcctonically emplaced found in flows nearly 1.2 km from the parent serpentinites (for example, Bailey and Everbody. The percentage oi blocks in any given hart, 1964, p. 47-50), serpentinite extrusions volume of matrix is usually less than 25 per- (Dickinson, 1966), and parts of the Franciscan cent, but vanes greatly. No systematic varia- tectonic melange throughout the Coast Ranges tion in block content of different parts of a (Ilsii, 1968). The consistent, ordered internal flow was determinable, because adequate expo- geometry indicates that flowage was charactersures of the internal structures of the flows are ized by systematic, rather than purely chaotic, restricted to their margins and bases. downslope movement. Shear surfaces within the matrix locally define a weak foliation that diverges around un- Other Features Several fundamental features of the flows, sheared blocks. Attitudes of the shear planes vary on a large scale, and locally the gross folia- including their absolute ages and rates of movetion has been deflected by large topographic ment, were not determined by the present obstacles in the path of a flow. This foliation study. While relative ages can be determined apparently is grossly parallel to the terrain over with ease from topographic expression and which the serpentinite flowed. In one flow that crosional modification ol the flows, their abso-
SHEAR STRENGTH OF SERPENTINITE lute ages remain unknown. One possible approach would be to date isotopically the carbonaceous material trapped within the flows or the calcareous streamrock deposits beneath the flows. Using inferred erosion rates in the region, an estimate of the time required to develop the present-day topography would give a very approximate age for only the youngest flows. Indirect evidence suggests the flows are not moving today. The toes of some flows showed no evidence of recent activity, whereas on the flows themselves the trunks of several digger pines, a distinctive conifer often found growing on Coast Range serpentinite, were neither strongly bent nor markedly inclined from the vertical. SHEAR STRENGTH OF SERPENTINITE Serpentinite flows, lava flows, and debris flows all have certain morphologic and geometric features in common. Most importantly, the general form of their snouts is similar to the form of the snout of a rigid-plastic substance in critical equilibrium (Fig. 7A). For these plastic substances, the height of the snout, Hs, can be related to the angle of inclination of the uniform slope behind the snout, 8, and to the shear strength, ^, and the unit weight, 7, of the material by the following equation: (1)
*(•-£)
where y equals the product of the acceleration due to gravity and the density of the plastic material. Johnson (in press) discusses in detail the derivation of this equation from rheological and equilibrium equations and the qualifications thereon. Actually, the strength determined by equation (1) is identical to the cohesive shear strength rc in the familiar Coulomb equation: on tan , (2) where T equals shear stress and trn equals the normal stress on a shear plane and tan tj> equals the coefficient of friction, if the angle of internal friction is zero. The relatively high snouts and low surface slopes of the flow deposits indicate that the effective friction angle of the serpentinite was low and, perhaps, negligible at the time of flow. These flows have surface slopes of between 10° and 13°, so that the effective friction angle during flowage was less than 13°. It can be shown that, if the effective friction angle were actually 10° and were ignored in calculations made below, the strength calculated with equation (1) would be 29 percent too high. This can be verified by considering the equation for the height of the snout of a Coulomb substance, obeying the law given in equation (2), (Johnson, in press): _7rC V ' (3) 7 For the purposes of our analysis, therefore, it is
30
\
20
IO
Figure 7A. Idealized cross section through the snout of a rigid-plastic body in equilibrium (Johnson, in press). H, = snout height, and $ = slope angle in degrees as used in equation (1).
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2.4 10°
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