Right-lateral Shear Research Articles

New Evidence on the State of Stress of the San Andreas Fault System

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at approximately 2-kilometer depth on planes parallel to the San Andreas fault.

Kinematics of the Sinai triple junction and a two-phase model of Arabia-Africa rifting

Summary The breakup of Arabia from Africa started at 40–50 Ma in the East African Rift system and concentrated from about 20 Ma around the Red Sea. Presently, the most active system centres along the Gulf of Aden, Red Sea and Dead Sea shear. Two less active features, the Gulf of Suez and East African Rift meet the main boundary at what have often been considered to be active triple junctions (the southern tip of Sinai and Afar). This paper focuses on the continental Sinai triple junction near 27°30′N. Almost no reliable kinematic data can be found in the Red Sea. Only total motion based on a fit of coastlines is available. Yet, the mechanical basis for this is now known to be wrong. We propose a mechanical interpretation that retains the same pole of ARA-NUB motion, with a reduced amount of total opening. The best data come from the Dead Sea shear: 100 km of left-lateral displacement occurred over the last 20, or more likely 15 Ma (subsidence event in the Gulf of Suez). The pull-apart basins in the Gulf of Elat provide the best constraints on the azimuth of SIN-ARA motion. Four models of SIN-ARA-NUB kinematics are proposed depending on whether the Gulf of Suez is; (i) now extinct, (ii) still opening in the original direction, (iii) in a state of left-lateral shear, (iv) in a state of right-lateral shear. Total SIN-NUB motion is unlikely to exceed 1–2 mm yr −1 . We propose a two-phase history of break-up, first along the East African Rift/Red Sea/Gulf of Suez, superseded at ∼ 15 Ma by the present Gulf of Aden/Red Sea/Dead Sea shear. Distributed extension now occurs in the Afar and northern Red Sea deforming zones towards which three localized rifts propagate. The Sinai and Afar ‘triple junctions’ are actually kinks where plate boundaries that were not active at the same time are superimposed.

Recent activity and seismotectonics of the Eastern Pyrenees

Results from a recent earthquake in the Eastern Pyrenees are presented and the seismotectonics of the region is analyzed from the presently available data. On 26 September 1984 an earthquake ( M L = 4.4) took place in the area of the historical destructive earthquake of 1428. Several portable stations installed in the epicentral area to record aftershocks permitted of defining a precise location at 42°19.2′N, 2°10.2′E and 5 km depth. A maximum felt intensity of V (MSK) is obtained from macroseismic data. The epicentral location lies within a block bounded by E-W-trending structures and the focal solution shows right-lateral shearing with a NW-SE pressure axis. The seismicity in the Eastern Pyrenees shows a complex pattern which can be associated with both E-W fractures and NE-SW fault systems. Focal solutions of another two recent earthquakes of M L ~ 4, with differences in horizontal pressure axis, are also discussed.

Moving under, around, and through the Miocene Reconstruction of Southern California

Geological Survey Menlo Park. CA 94025 Modern southern California is frag- mented by faults that juxtapose blocks with contrasting topographies and dif- fering geologic histories. Many of the tectonic events that have shaped south- ern California were initiated during the Miocene, as subduction along the an- cient trench margin off southern Califor- nia was replaced by transform (strike- slip) faulting, such as that along the San Andreas fault. Miocene reconstruction of southern California was the subject of a Penrose Conference held in Oxnard, California, May 30-June 4. Sixty-four geologists, geophysicists, and paleontologists from academia, industry, and the govern- ment were brought together by A. Eugene Fritsche (Cahfornia State Uni- versity, Northridge) to discuss and de- bate the Miocene geologic history of southern California, south of 36N. A key event used in reconstructing southern California's geologic history was the anival of the ancestral East Pacific Rise at the trench margin off western North America. The model of Atwater (1970) suggests that this event occurred in the late Oligocene (28 Ma) at a latitude near or slightly south of Los Angeles (34N). A progressive shift of the convergent plate regime to one of right-lateral shear then followed, as the spreading center was split into two seg- ments, one of which migrated to the north, the other to the south. During the last 12 years various modifications of the Atwater (1970) model have been invoked by researchers such as Crowell (1974), Blake et al. (1978), Dickinson and Snyder (1979), and Crouch (1979) to explain basin formation, the cessation of volcanism, and the initiation of strike- slip faulting in different regions of cen- tral and southern California. As if the Miocene tectonic history of southern California were not compli- cated enough, the Transverse Ranges, a large segment of southern California trending east from Santa Barbara through northern Los Angeles to the Mojave Desert, apparently have been rotated clockwise by as much as 90 since the early middle Miocene, as a result of dextral shear between the North American and Pacific plates (Luyendyk et al., 1980, 1985). Furthermore, it seems that major strike-slip faulting in southern California has moved progressively inshore with time. The San Gabriel fault, west of the San Andreas fault, first became a major strike-slip surface in the late middle Miocene (11-12 Ma), while transform faulting was not initiated on the modern-

Seismicity and fault kinematics through the Eastern Transverse Ranges, California: Block rotation, strike‐slip faulting and low‐angle thrusts

Hypocentral locations and first‐motion data from the southern California seismic network are used to infer the present pattern of deformation along the southern San Andreas fault. The study area lies between the San Jacinto fault and Desert Hot Springs and includes the San Bernardino Mountains and San Gorgonio Pass. This region has some of the deepest earthquakes observed anywhere along the entire San Andreas system, exhibits a complex surface geology characterized by both right‐ and left‐lateral faults, has high topographic relief as a result of recent uplift, and is a potential site for the nucleation of a great earthquake. Although this area is unusually seismogenic, little activity can be directly associated with major throughgoing faults. Seismicity is also generally absent in the upper 5 km. The predominant style of faulting above 10–12 km is oblique slip with a large reverse component. The spatial distribution of relocated hypocenters and first‐motion data suggests the presence of a system of left‐slip faults striking northeast. This pattern of faulting, in conjunction with an unusual set of normal and reverse focal mechanisms, is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right‐lateral shear. At depths greater than about 10 km, seismicity defines a wedge‐shaped volume undergoing pervasive internal deformation on a combination of strike‐slip and low‐angle thrust faults. Velocity structures determined from earthquake arrival times suggest a low‐velocity zone at about 10 km below the San Bernardino Mountains but not below the San Jacinto Mountains. This is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the Mill Creek—Mission Creek branch of the San Andreas fault and the maximum depth of seismicity seen north of that fault. The low‐velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust San Bernardino Mountains are allochthonous. The present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right‐lateral displacements.

Japan Sea: a pull-apart basin?

Recent field work in the Hokkaido Central Belt and marine geology studies along the eastern margin of Japan Sea in addition to previously published data lead us to propose a new model of opening of the Japan Sea. The synthesis of both on-land and offshore structural data gives new constraints about the structural evolution of the system. The rhombohedral shape of the Japan Basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early Eo-Oligocene rifting of a pull-apart basin accommodated along two large right-lateral shear zones, east of Korea and west of northeast Japan and Sakhalin. It is followed, during Upper Oligocene/Lower Miocene, by the main opening of the Japan Basin as a mega pull-apart. Then a back-arc spreading probably related to the subduction process, induced the creation of the Yamato and Tsushima Basins at the end of Lower Miocene and in Middle Miocene. Clockwise rotation of southwest Japan larger than 20° or major bending of Honshu mainland deduced from paleomagnetic studies is unlikely at this time. Since 1 or 2 My B.P. to Present, compression prevails along the eastern margin of the Japan Sea. The generation of marginal basins as pull-apart basins along intracontinental strike-slip faults is a mechanism which has been proposed by other authors concerning the South China Sea, the question then is whether the fragmentation of the Asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one.

Additional paleomagnetic results from the Sierra Nevada: Further constraints on Basin and Range extension and northward displacement in the western United States

Modal analysis of new paleomagnetic results that have a skewed distribution from four plutons in the Kings Canyon National Park area, central Sierra Nevada, produces a pole position concordant with the results of Frei and others (1984) from the Yosemite area. These studies suggest (a) that stable magnetization resides in the more mafic plutons and inclusions in the Sierra Nevada and (b) that the amount of internal deformation within the central Sierra Nevada does not exceed a few degrees at the 95% confidence level. The mean pole of both studies limits the relative displacement between the Sierra Nevada and cratonal North America to 6° ± 8° of clockwise rotation and 6° ± 6° of poleward displacement. The clockwise rotation agrees well with the model of Frei and others (1984) for Basin and Range extension, which predicts 2° of clockwise rotation for the Sierra Nevada. The poleward displacement suggests right-lateral shear of ∼600 km within the Basin and Range between 90 and ∼35 Ma.

Tectonic and eustatic controls of late quaternary shelf sedimentation along the Central California (Santa Cruz) continental margin: high-resolution seismic stratigraphic evidence

A high-resolution “uniboom”, seismic stratigraphic investigation of a portion of the central California continental shelf has demonstrated that depositional patterns and sequences are controlled largely by an interplay of glacioeustatic sea-level fluctuations superimposed on local tectonics. Wrench tectonics, associated with active right-lateral shear along the San Gregorio fault zone, and the Pigeon Point Basement High control the location, distribution and overall geometry of depositional sequences via en echelon folding and differential subsidence. Areas of relatively thick and thin late Quaternary sediments conform in large part with structures produced during wrenching. Glacioeustatic sea-level oscillations have also shaped depositional patterns and sequences. Correlation of our seismic stratigraphic data with a southern California continental margin sea-level curve, suggests that during the last glacial maximum, approximately 18,000 yrs ago, a relative lowstand resulted in the erosion of a distinct unconformity upon which late Quaternary sediments have accumulated. A rapid rise of sea level to a relative stillstand, approximately 12,000 yrs ago, produced a concave-up, marine terrace profile across the mid-shelf, that has since been infilled with as much as 22 m of Holocene clastic sediments. A relative drop of sea level, approximately 11,000 yrs ago, allowed sediments to build seaward as a series of prograding clinoforms that form the basal sequences of the late Quaternary sediment fill. The succeeding Holocene transgression partially eroded the top of this earlier regressive sequence, and has now established a typical, wave-graded shelf along which sediments fine in a seaward direction to water depths of 90–100 m. At greater shelf water depths, surface sediments coarsen and appear to be relicts of previous relative sea-level lowstands. The presence of now submerged and buried marine terraces along both the central and southern California continental margins, at elevations lower than modern terraces actively forming today, suggests that flights of raised marine terraces may not always decrease in age with decreasing elevation, as is commonly assumed for relative dating. Because sea-level fluctuations during at least the past 700,000 yrs, have been both numerous and of unequal magnitudes, it is likely that age inversions will occur within any given flight of raised Quaternary marine terraces along tectonically active continental margins.

Two-Stage Laramide Orogeny in Southwestern United States: Tectonics and Sedimentation: ABSTRACT

The Laramide orogeny (80-40 Ma) in the southwestern United States is usually thought of as a single tectonic event with attention concentrated on its early stage because of its dramatic expression in the sedimentary record. The sharpest pulse of deformation, however, occurred in the latest Paleocene-early Eocene and was separated from the early stage by a tectonic lull and development of widespread lateritic weathering profiles, remnants of which are preserved in some early Tertiary basins. The first stage correlates plate tectonically with opening of the North Atlantic and Labrador Sea, the second stage with opening of the Norwegian Sea and Eurasian basin in the eastern Arctic. Rapid convergence between the North American and Farallon plates decreased the dip of the subd cting Farallon plate, until by early Eocene (55 Ma), strong viscous coupling was occurring between the Farallon plate and the overlying lithosphere. Change in direction of convergence from west-southwest-east-northeast during the early stage to southwest-northeast during the late stage brought the first-order shear direction into near parallelism with the north-northeast-trending southern Rocky Mountain deformed belt in New Mexico. This parallelism allowed the Colorado Plateau to decouple from the craton along right-lateral wrench faults. The Colorado Plateau was translated 100-130 km north-northeast with a comparable shortening across the Wyoming province. Both sedimentation rates and coarseness of synorogenic sediments increased dramatically from Wyoming to the Gulf Coast of Texas beginning in the early Eocene. Simultaneously, an en echelon series of strike-slip basins formed along the zone of decoupling from southern New Mexico to southern Wyoming. Deformation in the Wyoming province also increased sharply, resulting in as much as 21 km of overhang on range-front thrusts and up to 15 km of structural relief between adjacent uplifts and basins. As much as 2,500 m of Eocene orogenic sediments were deposited in rapidly subsiding basins. Deformation was so rapid that surface drainage was disrupted and runoff was impounded in huge lakes in which as much as 1,000 m of lacustrine sediments accumulated to form the oil shale province. Wrench faulting along the eastern margin of the Colorado Plateau was distributed across a 100-km wide belt that followed zones of weakness inherited from Precambrian and late Paleozoic deformation. Wrenching changed from nearly parallel in New Mexico to strongly convergent in Colorado and southern Wyoming because of a change in structural grain from north-northeast in New Mexico to north-northwest in Colorado. At least 25 km of shortening across the zone of decoupling during convergent wrenching caused conspicuous low-angle thrusting, which has tended to mask the lateral component of movement. Major north-northeast shortening across the Wyoming province and a lack of such shortening on the High Plains demand right-lateral wrench faulting. Offset of transverse aeromagnetic anomalies ac oss the zone of decoupling in Colorado and New Mexico, offset of Precambrian metavolcanic belts and age province boundaries, and offset of northeast-trending lineaments and distinctive rock types provide independent measures of the magnitude of right slip. Northward translation of the Colorado Plateau was a relatively minor part of regional right-lateral shear that extended through much of the North American cordillera in the early Tertiary. Recognition of the wrench-faulted nature of the eastern margin of the Colorado Plateau and Wyoming province creates new opportunities for petroleum exploration. These can be divided into: (1) subthrust plays beneath low-angle faults caused by convergent wrenching; (2) structural, stratigraphic, and fracture-controlled traps along wrench faults; and (3) fracture reservoirs along northeast-trending lineaments dilated during wrenching. Recognition of wrench faults paralleling the structural grain of the southern Rocky Mountains also means that isopach and facies maps drawn smoothly across the zone of decoupling should be reevaluated with careful attention to control points. End_of_Article - Last_Page 2044------------

Historical seismicity and rates of crustal deformation along the margins of the Ordos block, north China

AbstractEarthquakes in China show an empirical relation between seismic moment (M0) and the areal distribution of Modified Mercalli intensities VI and VIII (log M0 = 16.66 + 0.91 log AVIII and log M0 = 14.35 + 1.16 log AVI, where A and M0 are measured in squared kilometers and Newton-meter, respectively). The empirical relations may be used to estimate M0 for historical earthquakes in China to within a factor of three, on average, when sufficient isoseismal data exist. This observation and an extensive collection of isoseismal maps are used to estimate M0 for large earthquakes that occurred along the margins of the Ordos block during the last 700 yr. Focal parameters of the historical events are inferred from the orientation and displacements across Quaternary faults. Average rates of crustal deformation are then estimated from the 700-yr historical record with formulas that relate the occurrence rate of seismic moment in a region to rates of crustal strain. The Shanxi and northern Ningxia graben systems are situated along the eastern and northwestern edges of the Ordos block, respectively. Normal faults in the two systems trend northeasterly and are characterized by a component of right-lateral slip. The deformation resulting from slip during earthquakes in each of the respective fault systems is estimated at about 0.5 to 1.0 mm/yr of both right-lateral shear and north by northwest extension. The Weihe graben system bounds the southern edge of the Ordos block, strikes easterly and conjugate to the Shanxi and northern Ningxia fault systems, and exhibits left-lateral normal fault displacements. The average rate of deformation across Weihe is described by about 1.0 mm/yr of north by northwest extension and 1.5 mm/yr left-lateral east-west shear. The Hetao fault system delineates the northern edge of the Ordos block and displays Quaternary faults similar in orientation and mechanism to that observed in Weihe. Although mapped faults in Hetao exhibit evidence of Quaternary displacement, crustal deformation rates are not estimated because there exists no historical record of large earthquakes in the area. In southern Ningxia, at the southwest boundary of the Ordos block, deformation occurs by slip on left-lateral strike-slip faults oriented in an easterly azimuth and thrust faults with strikes ranging from southeast to south. The average deformation rate in southern Ningxia is found to be about 4.0 mm/yr of east by northeast contraction and 10.0 mm/yr of left-lateral shear. Deformation of each of the fault systems is consistent with a regional compressive stress that trends northeast and results in an average of about 3.0 mm/yr each of contraction at N70°E and extension of N160°E across the entire region. Inasmuch as uncertainties in estimates of M0 for historical earthquakes are about a factor of three, a similar uncertainty is attached to rates of crustal strain determined in this manner.

Regional deformation of the earth model for the Los Angeles region, California

A mathematical model for historical horizontal crustal deformation is presented for the Los Angeles region of California. Model parameters are estimated from triangulation, trilateration, and astronomic azimuth data observed during the past 120 years. Also geologic and seismic data are used indirectly in specifying the model which relates crustal deformation to strain rates over large geographic areas, secular slip rates along geologic faults, and to episodic movements associated with large ( M >- 6) earthquakes. The episodic movements are modeled in accordance with the theory of dislocation in an elastic halfspace. For the secular motion, the region is partitioned into a mosaic of proposed microplates or “districts” that are allowed to individually translate, rotate, and deform homogeneously as a linear function of time. The model thus accounts for the secular and episodic time variability of the geodetic markers coordinates. The results are compared with geodetic, geologic, and seismic studies by other investigators to evaluate the reliability of the procedure and the estimates. In some cases the model provides the first geodetic determinations of strain rates, slip rates, directions of maximum right-lateral shears, and values of coseismic displacements experienced within the region. The shear strain rates reveal that maximum right-lateral shears are oriented parallel to the San Andreas fault system and decrease in magnitude with distance from the central Transverse Ranges. The directions of shear strain form a funnel-shaped pattern that converges on the point outside the region where the San Andreas, Garlock, Big Pine, and San Gabriel faults meet. The velocity for a point near San Pedro relative to a point at the region's northeast corner is 1.8 ± 0.6 cm/yr in the direction N15 δW ± 11 δ. These results are discussed in light of data limitations and modeling inadequacies, subsidence due to oil and gas withdrawal, and postseismic relaxation following the 1971 San Fernando earthquake.

Rift Deformation Produced by Combined Extension and Shearing: ABSTRACT

Continental rifts form when lithospheric segments move apart. In response, the brittle, upper lithosphere faults and the ductile, lower lithosphere flows. Clay and analytical models suggest that rift-fault patterns depend on the angle between the rift axis and the relative displacement direction between the divergent lithospheric segments. If the axis and displacement direction are not orthogonal, both extension and shearing contribute to the rift deformation. In clay models of combined regional extension and right-lateral shearing, steeply dipping, planar, conjugate strike-slip faults form if the angle between the rift axis and the displacement direction is less than 30°. The dextral strike-slip faults trend subparallel to and the sinistral strike-slip faults trend at large angles to the rift axis. If the angle equals 30°, oblique-slip and normal faults form. The dextral oblique-slip faults trend subparallel to, the sinistral oblique-slip faults trend at large angles to, and the normal faults trend about 30° clockwise to the rift axis. The oblique-slip and normal faults have steep to moderate dips and are relatively planar. If the angle exceeds 30°, normal faults form. These faults strike obliquely to the rift axis and o the relative displacement direction between lithospheric segments, except if the rift axis and the displacement direction are orthogonal. In cross-sectional view, the moderately dipping, relatively planar normal faults commonly die out into other faults or splay out near gently dipping horizons within the clay. Analytical models support these experimental results. The models apply to the Gulf of California and Gulf of Aden, two continental rift systems produced by combined regional extension and right-lateral shearing. The modeling results can explain the presence of north-trending normal faults (striking obliquely to the Gulf of California trend and to the displacement direction between Baja California and mainland Mexico); northwest-trending, dextral strike-slip faults; and northeast-trending, sinistral strike-slip faults formed during the late Miocene/early Pliocene opening of the Gulf of California. The modeling results can also explain why many normal faults, which formed during the opening of the Gulf of Aden, trend west-northwest to west, obliquely to the Gulf of Aden trend and to the displacement direction between the Arabian Peninsula nd the Horn of Africa. End_of_Article - Last_Page 1838------------

Neotectonic deformation patterns in the convex-northwards arc of the North Anatolian fault zone

Summary The convex-northwards arc of the North Anatolian fault zone between Çerkeş and Erbaa contains structures and landforms permitting right-lateral displacements for several time intervals to be estimated. Within the Pontus Formation (Late Miocene-Early Pleistocene) in basins along the fault zone, an unconformity, representing a time interval from the latest Tortonian to the earliest Pliocene, is interpreted as marking the transformation of the structure from a broad shear zone to a narrow fault belt. Although the amount of displacement during the shear zone phase is unknown the offset of a sedimentary facies boundary in the Lower Pontus Formation of the Havza-Ladik basin demonstrates that since the latest Tortonian there has been 25 km of right-lateral slip. The offset of valleys and ridges suggests that there has been 8 km of Quaternary displacement, about 2 km of it in the late Quaternary and at least 500 m during the Holocene. Structures in the Pontus Formation indicate that the early history of the western Neogene basins was influenced by regional compression and that the later histories of all basins were dominated by strike-slip displacements. The main active trace is discontinuous, the 2 km-wide belt containing subordinate en échelon faults whose geometry is consistent with development during right-lateral shear.

Ages and stratigraphy of lower and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic history of the Straight Creek fault

In the central Cascade Range of Washington, three structural blocks of early Tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the Straight Creek fault. East of the fault, in the Teanaway River block, the early Eocene fluviatile feld-spathic sandstone of the Swauk Formation is interbedded with largely dacitic volcanic rocks of the Silver Pass Volcanic Member. Zircon fission-track ages on the Silver Pass are about 52 m.y. Overlying the tightly folded Swauk and Silver Pass is the Teanaway Formation, a gently dipping accumulation of basalt, andesite, rare dacite, and rhyolite that yields middle Eocene (about 47 m.y.) whole-rock K-Ar ages. Conformably overlying the Teanaway is the Roslyn Formation, a coal-bearing fluviatile feld-spathic sandstone of probable middle to late Eocene age. The late Eocene and Oligocene(?) Naches Formation, exposed west of the Straight Creek fault in the Cabin Creek block, is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite. The basal Guye Sedimentary Member of the Naches Formation underlies and is interbedded with the Mount Catherine Rhyolite Member in the Snoqualmie Pass area. On the basis of zircon fission-track ages on silicic tuffs and whole-rock K-Ar ages on basalt, the basal part of the Naches is about 40 to 44 m.y. old. The formation is tightly folded and complexly faulted along the Straight Creek fault. The Straight Creek fault intersects the Olympic-Wallowa lineament in the strongly deformed Manastash River block. In this block, fluviatile coal-bearing feldspathic sandstone of the Manastash Formation is overlain by principally dacitic volcanic rocks of the Taneum Formation. Overlying the Taneum is the basalt of Frost Mountain. We correlate this threefold sequence, feldspathic sandstone-dacite-basalt, with the sequence in the Teanaway River block. Small patches of rhyolitic ash-flow tuff, dated at 33 m.y. (late Oligocene) and interbedded feldspathic sandstone overlie the Roslyn and Teanaway Formations in the Teanaway River block and are probably correlative with the late Oligocene Wenatchee Formation exposed in the Chiwaukum graben east of the Teanaway River block as well as with the late Oligocene volcanic rocks (30 m.y.) along the Cascade crest to the southwest. These latter rocks are little deformed and unconformably overlie the Naches Formation. The ages and depositional record in the three blocks indicate a Tertiary history of dominantly vertical movement along the Straight Creek fault and its southeasterly splays that merge with the Olympic-Wallowa lineament. The horst-and-graben structure of the blocks, as well as enechelon fold axes in the Swauk Formation, suggests some post–early Eocene, right-lateral shear along the fault, although there is no direct evidence of lateral offset. Vertical movement, significant since early Eocene Swauk deposition, followed late Eocene Naches deposition but tapered off by late Oligocene time and ceased by Miocene time, when the fault was intruded by the 25-m.y.-old Snoqualmie batholith and other plutons. However, structures with Olympic-Wallowa lineament trends appear to have influenced folding of the Miocene Yakima Basalt Subgroup.

Rates of late Cenozoic tectonism in the Vallecito–Fish Creek basin, western Imperial Valley, California

Research Article| November 01, 1983 Rates of late Cenozoic tectonism in the Vallecito–Fish Creek basin, western Imperial Valley, California Noye M. Johnson; Noye M. Johnson 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Charles B. Officer; Charles B. Officer 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Neil D. Opdyke; Neil D. Opdyke 2Geology Department, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Geoffrey D. Woodard; Geoffrey D. Woodard 3Department of Geology, California State University, Rohnert Park, California 94928 Search for other works by this author on: GSW Google Scholar Peter K. Zeitler; Peter K. Zeitler 4Research School of Geology, Australian National University, Canberra 2601, Australia Search for other works by this author on: GSW Google Scholar Everett H. Lindsay Everett H. Lindsay 5Geoscience Department, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information Noye M. Johnson 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Charles B. Officer 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Neil D. Opdyke 2Geology Department, University of Florida, Gainesville, Florida 32611 Geoffrey D. Woodard 3Department of Geology, California State University, Rohnert Park, California 94928 Peter K. Zeitler 4Research School of Geology, Australian National University, Canberra 2601, Australia Everett H. Lindsay 5Geoscience Department, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1983) 11 (11): 664–667. https://doi.org/10.1130/0091-7613(1983)11<664:ROLCTI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Noye M. Johnson, Charles B. Officer, Neil D. Opdyke, Geoffrey D. Woodard, Peter K. Zeitler, Everett H. Lindsay; Rates of late Cenozoic tectonism in the Vallecito–Fish Creek basin, western Imperial Valley, California. Geology 1983;; 11 (11): 664–667. doi: https://doi.org/10.1130/0091-7613(1983)11<664:ROLCTI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The kinetics of continental rifting are recorded in the late Neogene sediments of the western Imperial Valley, California. The Gulf of California opened in its present form about 4 m.y. ago, capturing the Colorado River. Some 5 km of deltaic and fluvial sediments accumulated in the Vallecito–Fish Creek area during the interval 4.3−0.9 m.y. ago. Initial sedimentation rates were 5.5 mm yr−1, which diminished in an exponential fashion to 0.5 mm yr−1. When isostatic adjustments are accounted for, this implies tectonic subsidence rates of 1.5 mm yr−1, decreasing systematically to 0.1 mm yr−1. After 0.9 m.y. ago, right-lateral shearing along the Elsinore fault zone cut across the basin, causing it to be tilted and uplifted at a mean rate of 5.9 mm yr−1. The basin was concurrently rotated as a unit 35° clockwise. The dissected pediments and badlands that now characterize the area were formed and eroded in the past 0.9 m.y. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic interpretations of enigmatic structures in the North Anatolian fault zone

Two geometrically distinct groups of syn-sedimentary and post-depositional mesofaults and joints cut Neogene-Quaternary sediments in basins situated along the convex-northwards arc of the North Anatolian fault zone between Çerkes and Erbaa. One group comprises second-order fractures interpreted as having developed during episodes of right-lateral shear along the fault zone, while the morphologically identical fractures in the other group have been interpreted as secondary products of left-lateral shear; thus apparently implying one or more former episodes of eastwards motion of the Anatolian scholle. Because such a reversal of motion would be counter to the well-established westward escape of Anatolia the structures have been called anomalous or incompatible. Alternative hypotheses which have been advanced to explain the development of the anomalous mesofractures include: localized reversals related to displacements of rigid blocks acting as buttresses within basins; selective operation of intra-pull-apart strike-slip faults; stress release; the coincidence of the present western sector of the fault with an older left-lateral fault zone; and the influence of a North Turkish neotectonic stress regime.

P-velocity models and earthquake locations in the Livermore Valley Region, California

P-wave travel times from 223 well-recorded earthquakes and five timed explosions recorded by the US Geological Survey-Lawrence Livermore National Laboratory seismic network are inverted simultaneously for one- and three-dimensional velocity structures and hypocenter locations. The stability of earthquake locations between the one- and three-dimensional inversions demonstrates the adequacy of a layered earth model combined with station corrections for routine hypocenter locations in the Livermore Valley. Initial one-dimensional velocity inversions indicate a fairly strong positive velocity gradient in the upper crust of about 6 km. Three-dimensional models show that the strongest velocity contrasts occur along the eastern and southern edges of the valley where lower velocity down-dropped Tertiary sediments and basin-full alluvium are found in structural contact with higher velocity Late Mesozoic and Tertiary sediments of the Altamont anticline and the Diablo Range. The low-velocity basin structure is truncated along the western margin of the Livermore Valley by the Calaveras fault across which higher velocity aftershocks from the Livermore are exposed. Some of the relocated Cretaceous and Tertiary formations earthquake sequence show a good correlation with surface trace of the northwest-trending Greenville fault. In the southeast corner of the Livermore Valley, a number of short (5 km or less), linear, andmore » discontinuous epicentral zones are observed. These seismic lineations appear to define a conjugate set of right-stepping faults in a right-lateral shear zone. Focal mechanisms for these aftershocks show many complexities, but are generally consistent with this interpretation.« less

Late Palaeozoic basins of the southern U.S. continental interior

The timing and geometry of late Palaeozoic inhomogenous deformation in the southern U.S. continental interior from the Appalachian foreland of Kentucky-Tennessee through Oklahoma and Texas to the Ancestral Rockies of Colorado-Wyoming-Utah can be definitively linked with a discrete sequence of collisional events in the Appalachian-Ouachita-Marathon orogenic belt to the south (Permian coordinates). A progressive staged collisional sequence beginning in late Mississippian times in the southern Appalachians and culminating in early Permian times in the Marathons led to a progressive deformation of the adjacent craton in a wide swath dominated by right-lateral shear (e.g. Rough Creek Fault zone) by which a Mexican promontory of North America was displaced towards a Pacific ‘free face’. While the deformation in the Appalachian-Marathon belt was dominated by vertical plane strain leading to crustal thickening, the associated continental interior deformation can be averaged as a horizontal plane strain at or near sea level with localized deep extensions (Delaware) and flexural (Arkoma) basins and compressions uplifts (Amarillo-Wichita) giving local source areas.

Paleomagnetism and tectonic significance of Eocene basalts from the Black Hills, Washington Coast Range

The Black Hills, located about 15 km west of Olympia, constitute one of several large basement uplifts in the Washington Coast Range. The oldest rocks exposed in this west-dipping homocline consist of early to middle Eocene basalt flows and breccias with minor sedimentary interbeds, which are correlated with the Crescent Formation in the northern Olympic Peninsula. K/Ar geochronology of five basalt samples yields an average age of 53.1 ± 2.0 m.y. B.P., or early Eocene. A total of 263 paleomagnetic cores were collected from 37 sites. All sites were cleaned using AF demagnetization, and 35 sites were corrected for measured and inferred tilt. The mean direction of these sites was virtually unchanged by applying tectonic corrections, although the precision was slightly improved. No flattening or steepening of inclination is apparent, although the mean declination of the 35 sites is easterly discordant with respect to the expected early Eocene declination for “stable” North America by 28.7° ± 15.4°. This discordance indicates that the Black Hills have rotated clockwise about 29° since early Eocene time. The Black Hills basalts show significantly less rotation than Eocene rocks in the Oregon Coast Range, but they show nearly the same amount as analogous basalts in the southwestern Washington Coast Range and latest Eocene to late Oligocene volcanic rocks in the Oregon-Washington Cascade Range. Thus, the rotations of coastal Oregon and Washington, while identical in direction, differ significantly in amount, suggesting that the entire Coast Range block, from the Olympic Peninsula to north of the Klamath Mountains, has not been a coherent terrane since early Eocene time. Several tectonic models are discussed that involve accretion and independent clockwise rotation of two or more Coast Range blocks, or “microplates,” in response to episodic periods of underthrusting and extensive right-lateral shear along the Farallon-North America plate boundary during early and middle Tertiary time. The easterly discordances of declination that are observed in almost all Tertiary rocks sampled in the Oregon-Washington Coast and Cascade Ranges reflect a composite of differential rotation during accretion of the Coast Range seamount terrane, followed soon afterward by large-scale rotation of the western margin of the Pacific Northwest in response to oblique subduction and consequent distributed shear.

Strain accumulation near the epicenters of the 1978 Bishop and 1980 Mammoth Lakes, California, earthquakes

abstractThe Bishop, California, earthquake (ML = 5.8) of 4 October 1978 occurred virtually beneath one of the stations in a trilateration network used by the U.S. Geological Survey to monitor strain accumulation in northern Owens Valley. Subsequently (late May 1980), three earthquakes (ML = 6.0, 6.3, and 6.0), the Mammoth Lakes sequence, occurred on the edge of the trilateration network 10 to 20 km from the Bishop earthquake epicenter. The deformation of the network observed in the epoch 1976 to mid-1979 was not significantly different from that observed in the epoch 1973 to 1976. Thus, the Bishop earthquake itself did not produce a measurable deformation, nor was anomalous deformation detected prior to the Mammoth Lakes earthquakes. The deformation of the trilateration network between July 1979 and September 1980 indicates significant extension (50 mm in 25 km) in the direction N65°E, essentially perpendicular to the alignment of the epicenters of the Mammoth Lakes earthquakes. This deformation suggests normal faulting, whereas the preliminary nodal-plane solutions seem to require left-lateral slip. Secular strain accumulation was detected in northern Owens Valley (latitude 37°15′ to 37°45′) in the 1972 to 1979 interval. The principal strain rates with standard deviations were 0.08 ± 0.02 μstrain/a N83°E ± 15° and 0.02 ± 0.02 μstrain/a N07°W ± 15°, extension reckoned positive. The observed strain accumulation is consistent with normal faulting along the Sierra escarpment in Owens Valley north of Bishop. This accumulation appears to contrast with southern Owens Valley where strain accumulation is primarily right-lateral shear acorss the axis of the valley.