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Active Tectonics Along the Western Continental Margin of the Conterminous United States

JOHN C.CROWELL

University of California, Santa Barbara

ABSTRACT

Active-tectonic deformation along the continental margins of Baja California, California, Oregon, and Washington is the result of crustal mobility at the joining of the North American, Pacific, and Juan de Fuca lithospheric plates. South of the Mendocino triple junction, strike slip predominates along a broad and braided system of both major and minor faults with a roughly northwesterly strike. Blocks between faults are warped, folded, uplifted, depressed, and rotated as shown by deformed erosional surfaces and stratigraphic markers, by geodetic and geophysical measurements, and by earthquakes. On the southeast, this wide transform belt merges with the divergent boundary at the head of the Gulf of California. Farther to the northwest, the belt joins the convergent plate margin near Cape Mendocino. On northwestward, vertical tectonic movements predominate.

Geophysical, geologic, and geomorphic investigations of the coastal belt extending from the deep Pacific Ocean on the west to well within the continental cordillera on the east are revealing much concerning the manner of deformation along such a wide plate boundary. Data come both from intensive study of local sites, including those for engineering projects, and from broader regional studies. Melded results from many types of investigation, skillfully summarized on special maps depicting tectonic behavior, will aid in evaluating sites for construction projects and will contribute to basic tectonic understanding. Toward these ends, stimulation is needed to coordinate symbiosis among all geoscientists, ranging from engineering geologists to global tectonicists.

INTRODUCTION

The western margin of North America is technically active. Here rugged mountains and deep valleys are associated with earthquakes and ground warping, which attest to the mobility of the crust. This is the belt where giant lithospheric plates meet—the North American plate on the east and the Pacific and Juan de Fuca plates on the west. This mobility is measured and documented by employing many approaches, some instrumental and others through geomorphic and geologic mapping, which are described elsewhere in this volume. In this short paper I review briefly the nature of the crustal deformation taking place at present and back into the geologic past for the past half-million years or so along the western continental margin.

Studies of crustal deformation are important in that they provide information useful in the siting and design



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Active Tectonics: Studies in Geophysics 1 Active Tectonics Along the Western Continental Margin of the Conterminous United States JOHN C.CROWELL University of California, Santa Barbara ABSTRACT Active-tectonic deformation along the continental margins of Baja California, California, Oregon, and Washington is the result of crustal mobility at the joining of the North American, Pacific, and Juan de Fuca lithospheric plates. South of the Mendocino triple junction, strike slip predominates along a broad and braided system of both major and minor faults with a roughly northwesterly strike. Blocks between faults are warped, folded, uplifted, depressed, and rotated as shown by deformed erosional surfaces and stratigraphic markers, by geodetic and geophysical measurements, and by earthquakes. On the southeast, this wide transform belt merges with the divergent boundary at the head of the Gulf of California. Farther to the northwest, the belt joins the convergent plate margin near Cape Mendocino. On northwestward, vertical tectonic movements predominate. Geophysical, geologic, and geomorphic investigations of the coastal belt extending from the deep Pacific Ocean on the west to well within the continental cordillera on the east are revealing much concerning the manner of deformation along such a wide plate boundary. Data come both from intensive study of local sites, including those for engineering projects, and from broader regional studies. Melded results from many types of investigation, skillfully summarized on special maps depicting tectonic behavior, will aid in evaluating sites for construction projects and will contribute to basic tectonic understanding. Toward these ends, stimulation is needed to coordinate symbiosis among all geoscientists, ranging from engineering geologists to global tectonicists. INTRODUCTION The western margin of North America is technically active. Here rugged mountains and deep valleys are associated with earthquakes and ground warping, which attest to the mobility of the crust. This is the belt where giant lithospheric plates meet—the North American plate on the east and the Pacific and Juan de Fuca plates on the west. This mobility is measured and documented by employing many approaches, some instrumental and others through geomorphic and geologic mapping, which are described elsewhere in this volume. In this short paper I review briefly the nature of the crustal deformation taking place at present and back into the geologic past for the past half-million years or so along the western continental margin. Studies of crustal deformation are important in that they provide information useful in the siting and design

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Active Tectonics: Studies in Geophysics of critical engineering structures. But they also reveal much concerning the nature of crustal deformation and contribute to our understanding of the strength of the crust and the way structural flaws or ancient weak zones influence where faulting and folding take place. Local studies reveal the style, rates, episodicity, or clustering of deformational events. They document the style and distribution of strain features such as faults, folds, warpings, rotations, translations, and earthquakes. This information needs then to be dovetailed with regional investigations focused on arriving at better plate-tectonic generalizations. The latter research aids in arriving at improved geologic models, and these in turn will aid in understanding what is controlling deformation at local sites. To the extent that the models are applicable they will aid in extrapolating concepts to regions where engineering structures are needed and planned but where direct information on active deformation is missing because of the piecemeal and incomplete nature of the pertinent geologic record. The question arises: How far back into the geologic past are deformational data useful in understanding the ongoing flexings and breakings of the crust that affect human enterprises? A first answer to this question is: as far back as the deformational style and strain pattern are essentially the same as those operating today. Except within limited areas, the plate-tectonic control of deformation has been the same during the past million years or so. The limited areas where strain patterns and styles of deformation have changed significantly are at plate boundaries, and especially at junctures such as near the Mendocino triple junction. Locally within major plates, as along braided fault zones, deformation styles evolve through time but across limited areas. ACTIVE-TECTONIC REALMS OF CALIFORNIA, OREGON, AND WASHINGTON In general, the active-tectonic realms correspond to the physiographic provinces of the western conterminous United States because tectonic activity is primarily responsible for mountains and valleys, the shape of coastlines, and the boundaries of regions such as the Basin and Range province. An active tectonic realm is, therefore, defined as a region where the tectonic deformation in progress at present and for the past half million years or so has the same style and pattern. This means that the orientation of folds and faults undergoing growth and the locations and type of earthquakes and volcanic centers are nearly the same throughout the realm. Several maps have already been published that show features such as earthquake distribution, stress patterns, and elevation changes (e.g., Buchanan-Banks et al., 1978; Zoback and Zoback, 1980, 1981; Sbar, 1982; Gable and Hatton, 1983). The boundaries between some realms are sharp, but most are transitional. In addition, the concept involves scale. In working with an area the size of a city or county, small difference may be significant and may warrant separation into different subrealms. When dealing with a region as large as the three Pacific Coast states and adjacent portions of Baja California and British Columbia, however, generalizations are appropriate and local differences are smoothed. In Figure 1.1, only the main provinces are demarcated. They are described briefly from south to north because of the convenience of picturing the sliding of western North America obliquely away from the divergent plate boundary in the Gulf of California. The main splintered boundary is the San Andreas transform system, upon which most of this sliding takes place. The Basin and Range physiographic province is primarily a broad region of stretching in this scheme and is the region north of the Mendocino triple junction where oblique plate convergence is taking place. The floor of the Pacific Ocean west of northernmost California, Oregon, and Washington is both moving relatively northwestward with respect to the continent (Pacific plate) and also eastward beneath it (Juan de Fuca plate). All these movements are relative, however, with respect to the North American lithospheric plate, which is by no means fixed. For example, as the Atlantic Ocean widens, North America may be viewed as moving relatively westward. Gulf of California The peninsula of Baja California, as it moves northwestward, is in the process of being rifted from the mainland of Mexico, thereby opening the Gulf of California. The crust flooring the Gulf is expanding as new seafloor is formed at depth along a series of spreading segments defined by a pattern of oblique transform faults (Figure 1.1). The peninsula broke away from the Mexican mainland on the east about 4 million years (m.y.) ago (Larson et al., 1968; Moore and Buffington, 1968), and the spreading process is still continuing. This movement pattern results in several superimposed styles at a local scale within the region: strike-slip displacements near the major transform faults, sagging and warping over pull-apart basins where the rift floor is stretching, and downslope displacements along the margins of the Gulf of California where high-standing terrain of the old continent is relatively unsupported at the edge of the rift. The Salton Trough lies at the northwestern end of the

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Active Tectonics: Studies in Geophysics FIGURE 1.1 Map showing active-tectonic realms along the western margin of the conterminous United States. Boundaries between realms are transitional. Base map modified from Drummond et al. (1981).

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Active Tectonics: Studies in Geophysics Gulf of California rift and reflects transition into the San Andreas transform system (Crowell and Sylvester, 1979; Crowell, 1981; Johnson et al., 1982). Within this region several complex fault zones (Elsinore, San Jacinto, and San Andreas), characterized by braided slices, extend northwestward from spreading centers within the head of the Gulf. Crustal blocks and slices within and between these fault zones rise and sink (“porpoise structure”) as lithospheric displacements continue; some mountain blocks move upward and now stand high as the result of squeezing and shortening, whereas, not far away, other sectors are depressed to form basinal receptacles for sediments washed in from adjacent highlands. Even alluvial surfaces formed but a few millenia ago are warped and disturbed in this tectonically active region. Such a pattern of deformation is shown by earthquakes, geodetic measurements, and the interpretation of landforms (Sharp et al., 1972; Keller et al., 1982). The crustal deformation to be expected beneath a local site the size of a few city blocks—an area appropriate for most engineering structures—will therefore depend critically on exactly where it is sited in such a region. Baja California Peninsula and Peninsular Ranges West of the Gulf of California lies the peninsula of Baja California and its northwestern extension into southern California constituting the Peninsular Ranges (Gastil et al., 1981). This is primarily a region of deeply eroded basement rocks, broken into only a few long slices that are tipped westward. Compared with most of California the region is relatively intact and stable, except for belts along the major fault zones such as the Elsinore and San Jacinto. Its western margin is transitional into the California Borderland province along splays of the Newport-Inglewood Fault zone, which parallels the present shoreline just offshore. The northwestern margin of the Peninsular Ranges borders one of California’s most complex tectonic regions, a region abutting the Transverse Ranges. Here, bordering the Los Angeles Basin, Miocene and more recent crustal extensions and rotation of blocks have not yet been completely investigated. It is a region where at places folds are currently forming and faults are actively displacing alluvial surfaces (Harding, 1976). In rough terms, the northwestern margin of the Peninsular Ranges block is being forced into the Transverse Ranges as it is carried northwestward during the opening of the Gulf of California. California Borderland The topography of the seafloor to the west of southern California and the northern part of the Baja California Peninsula is characterized by basins and ranges (Moore, 1969; Howell and Vedder, 1981). Some of the ranges are surmounted by islands, others reach up to near sea level to form shoals. Many of the ranges are bordered by long, straight, discontinuous fault scarps as shown by subsea topography and the linear arrangement of earthquake epicenters. The region is one where the topography reflects the structure closely: the subsea mountains, formed mainly in post-mid-Miocene times, have been only slightly eroded. Basins, on the other hand, are being infilled by turbidity currents that are bringing sediment down submarine canyons from source areas on land well to the east. The flat floors of basins have been smoothed by sedimentation processes. Transverse Ranges One of the most active regions tectonically in western North America lies athwart the northwestern trend of mountain ranges fringing the continent and is appropriately named the Transverse Ranges. To the southeast the Peninsular Ranges parallel the trend of the continental margin, and to the northwest so do the Coast Ranges, Great Valley of California, and the Sierra Nevada. In this east-west trending province, marine terraces near the city of Ventura, for example, are moving upward at rates of as much as 7.5 mm per year (Lajoie et al., 1982). In the same region, strata laid down at marine depths of several thousand feet only a half-million years ago are now warped and uplifted well above sea level and are deeply eroded (Yeats, 1977). The region, which extends both east and west of the Big Bend in the San Andreas Fault system, is one where different parts have had different tectonic histories during the past million years or so (Jahns, 1973; Allen, 1981; Crowell, 1982; Yeats, 1983). Some faults root at depths where earthquakes originate, others are related to sliding of beds across each other during folding and are relatively shallow and not so likely to generate dangerous earthquakes. The region also includes the area of the enigmatic Palmdale Bulge, where geodetic measurements show either general uplift during the last few decades or episodic uplift, although the interpretation of the measurements is controversial (Castle et al., 1976; Jackson and Lee, 1979; Mark et al., 1981; Strange, 1981, 1984; Stein, 1984). Moreover, the Transverse Ranges and parts of bordering realms include tectonic blocks that have been rotated clockwise during Miocene

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Active Tectonics: Studies in Geophysics times (Luyendyk et al., 1980). Although these tectonic rotations are not known to be going on today, they have contributed to the heterogeneity of the bedrock, and so affect strain patterns now under development. The San Fernando earthquake of February 9, 1971, was located within the Transverse Ranges on a fault dipping northward beneath the San Gabriel Mountains (Grantz et al., 1971; Oakeshott, 1975a). The fault was one of many known to have broken young deposits and with fault landforms along it, such as the Raymond, Cucamonga, Sierra Madre, San Andreas, and San Jacinto. But it was not known beforehand that this fault, nor the segment where the ground was broken, was most likely to be the site of a disruptive earthquake. Most of the active faults in this region, and elsewhere around the Los Angeles metropolitan region, have now been demarcated, but tectonic understanding has not yet progressed far enough for geoscientists to pinpoint with confidence which fault is most likely to break next. Faults known to be recently active are most likely to rupture again, but more research to learn which are the most dangerous is required. The geologic situation in the vicinity of the San Fernando earthquake serves to illustrate problems in appraising future earthquake hazards. Young and datable sedimentary deposits, such as alluvium and terrace deposits, are needed to ascertain the time of movement of tectonic features that disrupt them. Faults are commonly known to be active where they cut deposits only a few thousand years old. Faults are suspected of being active where the youngest strata or geomorphic features cut are several tens of thousand or a hundred thousand years old. In the absence of datable young beds that are clearly cut by faults or warped and distorted by folding, there is at present no satisfactory way to determine whether a fault should be labeled as active and is likely to break again in the near future. Many faults are mappable in older rocks, but they may have been formed by geologically ancient deformations under tectonic regimes and stress situations long abandoned. The difficulties are illustrated in Figure 1.2, a cross section through the region of the San Fernando earthquake and extending northeastward into ancient rocks, including those of Precambrian age. Along the edge of the valley, transected young deposits prove that the faulting is young, but faults within the higher mountains underlain only by basement rocks may have been active at intervals any time since the rocks were consolidated over a billion years ago. These ancient rocks have also been tilted and folded as shown by layering in igneous rocks formed by settling of crystals on the flat floor of a magma chamber in Precambrian time. Whether the tilting of the layering, which was originally nearly hori FIGURE 1.2 Geologic cross section through the hypocenter of the San Fernando earthquake of February 9, 1971. Refer to text for discussion. Modified from Oakeshott (1975b, Fig. 4). zontal, took place not long after crystallization or but recently cannot be determined from examining local outcrops. Regional understanding of the tectonic history, however, may provide a basis for useful inferences. That the basement rocks in this region have been folded and displaced recently is also shown by the shape of the unconformity between young deposits and the basement (Figure 1.2; Oakeshott, 1975b). The syncline beneath the San Fernando earthquake fault is accompanied by an anticline above it, even though along the plane of the cross section there are no outcrops of the unconformity for about 8 km northward. The folded unconformity, dated as young because the deposits just above it are geologically young, proves that the latest deformation of the basement block took place in recent times. But this interpretation does not preclude many other deformational events, including several episodes of folding and faulting in the geologically remote past. During the earthquake the San Gabriel Fault did not rupture the ground surface so far as is known; this major ancient fault was ignored in the modern stress regime. Old faults may well become reactivated if they are oriented so that the resolved shear stress on them is sufficient for slip within the present-day tectonic situation. For this reason it is desirable for investigators to learn as much as they can concerning the style of deformation so that they can advance reasonable inferences concerning which faults are dangerous. Models of crustal structure and behavior that are well substantiated may be helpful in this task.

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Active Tectonics: Studies in Geophysics California Coast Ranges Northwest of the Transverse Ranges, and reaching into the area near the Mendocino triple junction, are several low but rugged ranges interspersed with long intermontane valleys. The region constitutes the Coast Ranges. They break off sharply both at the shoreline on the west and with the topographically flat Great Valley on the east. Throughout the mountain belt, the ranges are uplifted and deformed as shown by warped young terraces and surfaces (Buchanan-Banks et al., 1978; Page, 1981). The Coast Ranges are also constructed of ancient rocks that have been involved in a complicated history through geologic time. For example, many of the older strata were formed at a convergent tectonic boundary between lithospheric plates coming in against the continent from Pacific regions. Many faults in such areas have moved again and again, and some may now be active, although it is difficult to document the timing of the most recent displacements owing to the paucity of young offset strata. The stretch of the San Andreas Fault through the central Coast Ranges is straight and constitutes the near-surface boundary between the lithospheric plates. Activity along it is shown in two ways. First, it is the locus of continuous slow creep, and, second, from time to time noticeable earthquakes take place upon it, such as the Parkfield earthquake of 1966 (Brown et al., 1967). The reach of the fault subject to creep and frequent small earthquakes extends on northwestward from near the town of Parkfield into the San Francisco Bay area. This is the part of the fault system that is under close watch instrumentally by the U.S. Geological Survey. Seismographs and instruments of other types are monitoring the behavior of the walls on either side of the fault in a search for premonitory changes in strain, tilt, magnetism, electrical field, the depth and character of small earthquakes, and the gas contents and water levels in wells (Raleigh et al., 1982). When a major earthquake occurs along this segment of the San Andreas, geoscientists hope to have gathered many kinds of information pertinent to understanding when, where, and why the earthquake happened. The Coalinga earthquake of May 1983 took place in the central California Coast Ranges, with its epicenter near their margin with the Great Valley and about 30 km from the San Andreas Fault (Eaton et al., 1983; Namson et al., 1983; Wentworth et al., 1983). Although the tectonic setting of the earthquake is still under study, it appears that relatively rigid crustal layers are deforming differently from those at depth. The occurrence of the earthquake at distance from the San Andreas Fault attests to the mobility of a broad belt near the lithospheric plate boundary and also that geologists have much to learn concerning its dynamics. To the south of Parkfield the fault last ruptured during the Fort Tejon earthquake of 1857, one of the strongest earthquakes in California recorded history (Sieh, 1978a,b). This earthquake displaced ground features about 30 feet right laterally along the fault from the southernmost Coast Ranges, through the Big Bend region of the San Andreas Fault and the central Tranverse Ranges, to the vicinity of the city of San Bernardino. Within the San Francisco Bay region elongate crustal blocks are separated by several major faults of the San Andreas transform system, such as the San Gregorio, San Andreas (proper), Hayward, and Calaveras (Crowell, 1976; Page, 1981, 1982). These faults are neither exactly parallel to each other nor to the direction of movement between the Pacific and North American lithospheric plates. Where the faults diverge or converge in map view, blocks between them may be either squeezed or stretched. Where they are stretched, the terrain sags to form valleys and sedimentary basins, and where squeezed, terrain rises to make mountains. San Francisco Bay itself may well owe its origin to the sagging of the block between the San Andreas and Hayward Faults so that ocean waters from the Pacific are able to flood eastward upon the continent. In this region, terrains between major faults are undergoing deformation as shown by out-of-place old-erosional surfaces—some have been much uplifted and dissected; others, such as marine terraces, are now depressed far below the levels where they were formed (Atwater et al., 1977). Soft mélanges of the ancient Franciscan Complex stand high and are prone to severe landsliding. The region as a whole is undergoing deformation, but deformation is even more concentrated along the major fault zones (Brabb and Hanna, 1981; Prescott et al., 1981). North of San Francisco Bay several terrains between discontinuous active faults are recognized (Herd, 1978; Fox, 1983). Shear stress between the two lithospheric plates is apparently spread across a region running through broad lowlands near the city of Santa Rosa. The belt of displacements includes the San Andreas Fault on the west and extends well to the east into the high Coast Ranges, roughly on line northwestward from the active Calaveras and Concord Faults. This region is one where the active plate boundary is broad and diffuse and where crustal slices within it also show “porpoise structure.” Geologic mapping with an emphasis on documenting recent deformation is hampered in the northern California Coast Ranges by widespread dense forests and brush, steep terrain subject to landsliding, and the paucity of datable young rocks.

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Active Tectonics: Studies in Geophysics Mendocino Triple Junction The region onshore from the Mendocino triple junction, where the the Juan de Fuca, Pacific, and North American lithospheric plates meet, is complex tectonically (Atwater, 1970). It lies transitionally between terrains on the south undergoing transform displacements and those to the north undergoing oblique convergence. Moreover, the relative movement between the major plates is carrying the triple junction slowly northwestward relative to the continent on the east, so that through time, transform-belt tectonic features are being overprinted on those formed under a regime of oblique convergence. Investigation of a few patches of Quaternary strata show both folding and faulting. In fact, wrench faulting forming ahead of the arrival of the San Andreas transform system is recognized 120 km north of the triple junction (Kelsey and Cashman, 1983). Oregon and Washington Coast Ranges From the Mendocino triple junction, coastal mountain ranges extend on northward from within California through Oregon and Washington. Farther inland in Oregon lies the Willamette Valley and still farther eastward the Cascade Range. Active volcanoes surmount the Cascade Range, including Mount St. Helens (Lipman and Mullineaux, 1981), and are inferred to be the consequence of subduction of the Juan de Fuca plate beneath the continent. The belt of active volcanoes extends from Mount Lassen on the south, onshore to the southeast from the Mendocino triple junction, through Mount Shasta, and on northward through both Oregon and Washington and southernmost British Columbia. Along this trend, active deformation associated with volcanism therefore also requires evaluation, but the volcanic belt lies well to the east of the coastline. Along the coast, uplifted marine terraces attest to vertical tectonic movements, but active-tectonic investigations are sparse. The coastal ranges, including the Olympic Mountains in Washington, were uplifted beginning in Pliocene time and are still rising (Snavely and Wagner, 1963; Gable and Hatton, 1983). Fault scarps of Quaternary age have been identified on the southeastern part of the Olympic Peninsula (Wilson et al., 1979) and on the seafloor (Snavely et al., 1980). Earthquakes, such as those near Mount Rainier in 1973 and 1974 (Crosson and Frank, 1975; Crosson and Lin, 1975), and geodetic measurements prove recent deformation (Ando and Balazs, 1979). Investigations stimulated by the 1980 eruption of Mount St. Helens are adding much to understanding of the tectonics of the region (e.g., Weaver and Smith, 1983). In northwestern Washington the tectonic style of active deformation changes gradually from one of oblique convergence to one of transform displacements associated with the Queen Charlotte Fault zone. This zone, which lies off the coast of British Columbia, forms the boundary between the Pacific and North American plates northwest of the Juan de Fuca Rise. The transition zone lies inboard primarily of the fragmented Juan de Fuca plate (Figure 1.1; Clowes et al.; 1983). High river terraces and upland surfaces of Quaternary age attest to active uplift in the Coast Mountains of British Columbia, inferred to be due chiefly to vertical expansion after heating of a thick crustal slab; this active uplift is documented by the unroofing and setting of fission-track dates (Parrish, 1983). Active tectonics in this region appear to be the consequence of isostatic adjustments to a previous history of oblique subduction that has thickened the crustal slab. Other Active-Tectonic Realms Several other provinces lie to the east of the coastal belt and locally display active deformation but are not dealt with in detail here (Figure 1.1). They include the Great Valley of California, the Willamette-Puget Sound Lowlands of Oregon and Washington, Sierra Nevada, Cascade Ranges, and Basin and Range province. In fact, from a global viewpoint, the whole of western North America constitutes the deformed margin of the North American plate (Atwater, 1970). The eastern edge of this belt corresponds to the abrupt eastern escarpment of the Rocky Mountains where they meet the Great Plains. The relatively stable craton lies still farther east and underlies the middle of the continent. DISCUSSION These brief descriptions of active-tectonic realms along the Pacific Coast emphasize the mobility of the region. Earthquakes, geodetic surveys, other geophysical measurements of several types, geomorphic studies, and geologic observations document irregular ground movement both vertically and horizontally. Many small areas the size of cities have been intensively studied, so that their deformational history is well known, but these areas are scattered and unevenly distributed. In addition, the kind and quality of data documenting active deformation is unsatisfactorily variable. Documentation of recent deformation from earthquakes and from most geophysical measurements deals with the present and the past few decades only. It does not span time intervals long enough to reveal an understanding of average conditions—the time sampling is

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Active Tectonics: Studies in Geophysics just too short (Allen, 1975). Therefore basic documentation of recent deformation is lacking both areally and temporally. The former can be remedied only in part by adding many instruments throughout a region. Despite the huge cost, only short-time-span information can result. However, widespread studies in tectonic geomorphology—locally quantified by appropriate regional studies involving geophysical measurements and isotopic and soil dating—will add useful information on averages over several millenia. Investigations of these sorts have been most fruitful in local areas, especially around San Francisco Bay and in the Santa Barbara-Ventura regions (Atwater et al., 1977; Keller et al., 1982; Lajoie et al., 1982; Yeats, 1983). These types of study need to be extended throughout the coastal belt wherever there are young datable strata to work with. By means of such investigations, deformational histories extending back into geologic time for tens of thousands of years will provide better understanding of the episodicity or continuity of rates of uplift, sinking, warping, folding, and faulting. A long look at the late Cenozoic geologic history of the tectonically mobile belt shows that activity along major fault strands of the San Andreas Fault system, for example, has switched from fault to fault (Crowell, 1979). Studies of critical areas using geomorphic data should aid in understanding the timing of these switchings. They will also help in characterizing the style and timing of crustal movements on slices within fault zones themselves. Vast proportions of the mobile belt extending inland from the Pacific Coast are not continuously underlain by young datable strata or datable geomorphic surfaces and so do not lend themselves to geomorphic or geologic study aimed at demarcating episodes of active deformation through time. Documentation of deformation in such regions will therefore always be piecemeal and incomplete. Many of these intervening regions are underlain by older rocks deformed many times through the geologic ages. Tectonic overprinting of one style on another makes it difficult to separate the results of those crustal movements now under way from the results of ancient ones. Conclusions concerning deformation in these regions will therefore have to come largely from extrapolation from where data are available to intervening areas where there are none. These extrapolations need to be based on understanding of the regional tectonic behavior and on well-constrained tectonic models. With this information in hand we may be able to place geophysical instruments, such as strain gauges, at critical locations where they can provide warnings of impending earthquakes. The classification of active-tectonic realms in this brief paper is built on the plate-tectonic concept, and much local research at present is striving to add details to this concept and to improve models. In fact, maps at scales ranging from local areas to large regions, showing the style of active deformation, will provide a useful way to synthesize and portray data from both observations and models. More subsurface data are also needed to improve the models. Much crustal extension and shortening prevails in the upper few tens of kilometers of the crust. Profiles using the reflection seismograph will be especially helpful in outlining the structure at depth, including the position and character of décollement zones. The fine structure and behavior of small crustal units or blocks in three dimensions is not yet fully melded into global plate-tectonic theory. Progress in tectonic understanding will be strongly accelerated if there is better coordination among many types of geoscientist whose professional focus initially differs widely. Local studies by engineering geologists will improve understanding of the way small sectors behave through time. They reveal the style, rates, episodicity, or clustering of deformational events, that is, they reveal the strain pattern under development in limited areas. Investigations undertaken to evaluate the safety and suitability of sites for major engineering structures such as high-rise buildings, nuclear power plants, bridges, tunnels, and dams provide examples of such local studies. Data and inferences from these investigations will add significantly to tectonic models, which in turn will aid in extrapolating to areas where active-tectonic data are sparse or lacking. All geoscientists need to find ways to stimulate coordination leading to mutual symbiosis. During the decades ahead geoscientists foresee a marked improvement in the applicability of tectonic models, thanks to the integration of data from local detailed studies, both geologic and geophysical. Geoscientists are challenged to monitor these local studies and strive to place them into a regional synthesis. Both data and inferences can be depicted on maps and diagrams that are especially designed to show and discriminate between basic information and interpretation and to show active-tectonic realms more satisfactorily. Science and society will both benefit significantly. REFERENCES Allen, C.R. (1975). Geological criteria for evaluating seismicity, Geol. Soc. Am. Bull. 86, 1041–1057. Allen, C.R. (1981). The modern San Andreas Fault, in The Geotectonic Development of California, W.G.Ernst, ed., Prentice-Hall, Englewood Cliffs, N.J., pp. 511–534. Ando, M., and E.I.Balazs (1979). Geodetic evidence for aseismic

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