West Fault Research Articles

Alkaline basalt from the Central Iran, a mark of previously subducted Paleo-Tethys oceanic crust

In western part of the CEIM (Central-East Iranian Microcontinent) (Bayazeh area, Isfahan province, Iran), a series of Paleozoic basaltic rocks, occur. Major minerals of these basalts are olivine, clinopyroxene (diopside, augite), plagioclase (albite), sanidine, amphibole (kaersutite), phlogopite, ilmenite and magnetite. Secondary minerals include epidote, pumpellyite, albite, calcite and chlorite. Olivine and clinopyroxene are as phenocryst, while feldspars are restricted to groundmass. Chemical composition of clinopyroxenes indicates crystallization during ascending of magma. Geochemical analysis of whole rock samples shows that these rocks are characterized by low SiO2 (43.21–48.45 wt %), high TiO2 (1.81–3.00 wt %) and P2O5 (0.18–0.34 wt %). Petrography, chemistry of clinopyroxenes and whole rock analyses reveal an alkaline nature of these basalts. They are enriched in alkalis (Na2O + K2O = 4.1–7.7 wt %), LILE, HFSE and LREE. The Bayazeh alkali-basalts present strong enrichment in LREE relative to HREE (La/Lu ratio = 77.6–119.6) and were dominantly derived from partial melting of a metasomatized asthenospheric garnet-amphibole lherzolite. Field relationships reveal that junction of faults in west of the Bayazeh prepared a suitable path for ascending of magma from deep regions to surface and intra-plate continental magmatism. The Paleo-Tethys subduction from lower to upper Paleozoic is too enough for mantle enrichment in volatiles and basaltic alkaline magmatisrn in upper Paleozoic of Bayazeh area.

Late Quaternary right-lateral slip rates of faults adjacent to the lake Qinghai, northeastern margin of the Tibetan Plateau

By combining terrace riser offsets with terrace ages dated by 14 C, optically stimulated luminescence (OSL), and 10 Be techniques, we determine average slip rates of 1.1 ± 0.3 mm/yr and 1.2 ± 0.4 mm/yr for the Elashan and Riyueshan faults, two north-northwest–trending, right-lateral, strike-slip faults west and east of the lake Qinghai in the northeastern margin of the Tibetan Plateau. These faults are conjugate to the major easterly trending, left-lateral Altyn Tagh, Haiyuan-Qilianshan, and Kunlun faults, and they contribute to the subdivision of the region between the Haiyuan-Qilianshan and Kunlun faults into small blocks tens to ∼100 km in dimension. The relatively low slip rates in this region reflect distributed deformation. The total right-lateral offsets of geological contacts are ∼9–12 km along the Elashan fault and ∼11–12 km for the northern segment of the Riyueshan fault. If long-term slip rates were constant during late Cenozoic time, dates of initiation of dextral movement would be 9 or 10 ± 3 Ma for the two strike-slip faults, concurrent with onsets or acceleration of tectonic deformation in Cenozoic basins nearby. Our study highlights a stage of tectonic deformation in the northeastern margin of the Tibetan Plateau beginning since ca. 8–12 Ma, tens of millions of years after the collision between India and Eurasia began.

Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003Mw= 6.9 Zemmouri, Algeria, earthquake

[1] The essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated, first through idealized examples and then by study of thrust faulting in Algeria. We calculated coseismic stress changes caused by the 2003 Mw = 6.9 Zemmouri earthquake, finding that a large majority of the Zemmouri afterslip sites were brought several bars closer to Coulomb failure by the coseismic stresses, while the majority of aftershock nodal planes were brought closer to failure by an average of ∼2 bars. Further, we calculated that the shallow portions of the adjacent Thenia tear fault, which sustained ∼0.25 m slip, were brought >2 bars closer to failure. We calculated that the Coulomb stress increased by 1.5 bars on the deeper portions of the adjacent Boumerdes thrust, which lies just 10–20 km from the city of Algiers; both the Boumerdes and Thenia faults were illuminated by aftershocks. Over the next 6 years, the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity. The stress also increased by 0.4 bar on the east Sahel thrust fault west of the Zemmouri rupture. Algiers suffered large damaging earthquakes in A.D. 1365 and 1716 and is today home to 3 million people. If these shocks occurred on the east Sahel fault and if it has a ∼2 mm/yr tectonic loading rate, then enough loading has accumulated to produce a Mw = 6.6–6.9 shock today. Thus, these potentially lethal faults need better understanding of their slip rate and earthquake history.

The Study on Ground Stress Measurement and the Stress Distribution Rule on E-W both Sides of Fault F17 of Ore Body II in Jingchuan Orefield

The comprehensive study results of variation rule of the stress in the orefield following the measurement depth had been done by mainly put forward the ground stress measurement of level 1200m, 1150m and 1000m of ore body II in Jinchuan orefield. The dimension and direction of ground stress of 16 points on E-W both sides of fault F17 are abtained by these measurements. The study result shows that the average of maximum principal stress of west fault F17 is about 25MPa, whereas that east of this fault is 20MPa. The general direction of maximum principal stress is NE-NNE, with somewhat local variation. The principal level of horizontal stress weakens progressively in the deep mining area. Nevertheless, the situation of different part of each level isn’t the same as one another. The increase of difference between dimensions of maximum and minimum principal stress shows the increase of shear stress in the deep mining area and the increase of intensity of shear damage. Above the buried depth 500-600m, the maximum principal and horizontal stress shows a linear increase following the increase of depth, whereas the tendency of increase becomes gentle beneath the buried depth 500-600m.

The tectonic evolution of Lake Eğirdir, West Turkey

The tectonic evolution of Lake Eğirdir, West Turkey Lake Eğirdir is one of the most important fresh-water lakes of Turkey. It has a tectonics-related origin. The area formed under a roughly N-S compressional tectonic regime during the Middle Miocene. The stresses caused slip faults west and east of Isparta Angle, and the lake formed at the junction of these faults. The area subsided between normal faults, thus creating the topographic condition required for a lake. The lacustrine sediments have fundamentally different lithologies. After the Late Miocene, central Anatolia started to move westwards, but western Anatolia moved in a SW direction along the South-western Anatolian Fault, which we suggest to have a left lateral slip, which caused that the Hoyran Basin moved t7 km towards the SW and rotated 40° counterclockwise relative to Lake Eğirdir.

Seismotectonics and Fault Structure of the California Central Coast

Abstract I present and interpret new earthquake relocations and focal mechanisms for the California Central Coast. The relocations improve upon catalog locations by using 3D seismic velocity models to account for lateral variations in structure and by using relative arrival times from waveform cross-correlation and double-difference methods to image seismicity features more sharply. Focal mechanisms are computed using ray tracing in the 3D velocity models. Seismicity alignments on the Hosgri fault confirm that it is vertical down to at least 12 km depth, and the focal mechanisms are consistent with right-lateral strike-slip motion on a vertical fault. A prominent, newly observed feature is an ∼25 km long linear trend of seismicity running just offshore and parallel to the coastline in the region of Point Buchon, informally named the Shoreline fault. This seismicity trend is accompanied by a linear magnetic anomaly, and both the seismicity and the magnetic anomaly end where they obliquely meet the Hosgri fault. Focal mechanisms indicate that the Shoreline fault is a vertical strike-slip fault. Several seismicity lineations with vertical strike-slip mechanisms are observed in Estero Bay. Events greater than about 10 km depth in Estero Bay, however, exhibit reverse-faulting mechanisms, perhaps reflecting slip at the top of the remnant subducted slab. Strike-slip mechanisms are observed offshore along the Hosgri–San Simeon fault system and onshore along the West Huasna and Rinconada faults, while reverse mechanisms are generally confined to the region between these two systems. This suggests a model in which the reverse faulting is primarily due to restraining left-transfer of right-lateral slip.

East–west faults due to planetary contraction

Contraction, expansion and despinning have been common in the past evolution of Solar System bodies. These processes deform the lithosphere until it breaks along faults. Their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface. While the search for despinning tectonics has not been conclusive, there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion, though the pattern is seldom isotropic as predicted. The cause of the non-random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction/expansion with another deformation (despinning, tidal deformation, reorientation). Another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation. Using thin elastic shells with variable thickness, I show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction/expansion into a pattern of faults striking east–west, preferably formed in the equatorial region. By contrast, lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike-slip faults and polar normal faults. If contraction is added to despinning, the despinning pattern first shifts to thrust faults striking north–south and then to thrust faults striking east–west. If the lithosphere is thinner at the poles, the tectonic pattern caused by contraction/expansion consists of faults striking north/south. I start by predicting the main characteristics of the stress pattern with symmetry arguments. I further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two, making it easy to determine fault orientation for combined contraction and despinning. I give two methods for solving the equations of elasticity, one numerical and the other semi-analytical. The latter method yields explicit formulas for stresses as expansions in Legendre polynomials about the solution for constant shell thickness. Though I only discuss the cases of a lithosphere thinner at the equator or at the poles, the method is applicable for any latitudinal variation of the lithospheric thickness. On Iapetus, contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge. On Mercury, the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation (north–south or east–west), which may be relevant to the orientation of lobate scarps.

Application of Invert-Emulsion Drilling Fluid and Aqueous Completion Fluids for Long Multilateral Wells

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 121905, "Design and Application of Invert-Emulsion Drilling and Aqueous Completion Fluids for Long Horizontal Multilateral Wells," by Alistair Hutton, SPE, Stephen Vickers, SPE, Marcus Davidson, SPE, and Jon Wharton, SPE, Baker Hughes; Andy Hatch, SPE, and Roger Simmonds, SPE, E.ON Ruhrgas UK North Sea Ltd.; and David Brankling, SPE, OCTL, originally prepared for the 2009 SPE European Formation Damage Conference, Scheveningen, The Netherlands, 27-29 May. The paper has not been peer reviewed. The first dual-lateral well aimed at carboniferous reservoirs drilled in the UK southern North Sea was in the Rita gas field. This method enabled both the east and west fault segments of the field to be produced from the same upper wellbore, thus reducing drilling costs and improving field economics. An invert-emulsion drilling fluid was chosen for this field application because of the risk of shale instability over the long horizontal sections of each wellbore. Introduction The industry has experienced lower-than-expected production rates from wells that have been suspended for long periods with oil-based, solids-laden fluids before a cleanup has been initiated. This raised the question of what type of fluid to leave below the whipstock because re-entry to this leg for cleanup or well remediation was not economically feasible if production was lower than expected. Because of the diametrically opposed shape of the well, with long horizontal sections, the drilling team was strongly in favor of using an oil-based reservoir drill-in fluid (OBRDIF). This fluid would give a stable wellbore and provide a low friction coefficient for drilling and running the completion assemblies. Using OBRDIF would reduce the risk of hole instability and minimize non-productive time. However if OBRDIF was used in the drilling phase, it would mean that a solids-laden invert emulsion would be left in the hole when the first leg was suspended with the sand screens below the whipstock. This presented several risks. The fluid would require fine screening to prevent blocking the completion equipment with drill solids. Also, there was a risk that the solid particles in the suspension fluid would agglomerate as the fluid remained static for an extended period. If agglomeration did occur, these solids would probably not pass through the mesh of the completion screens and could potentially reduce screen conductance. The size and concentration of the solids in the suspension fluid were recognized as areas of concern that should be addressed to ensure maximum productivity of this well.

Cenozoic palaeocanyon evolution, Ancestral Cascades arc volcanism, and structure of the Hope Valley–Carson Pass region, Sierra Nevada, California

We use new geologic mapping, geochronological, and geochemical data on Tertiary volcanic, volcaniclastic, and intrusive rocks to investigate the volcanic, stratigraphic, and structural evolution of the Carson Pass region south of Lake Tahoe in the central Sierra Nevada. Volcanic and volcaniclastic rocks were deposited in east–west-trending palaeocanyons carved into Mesozoic granitic and metamorphic basement rocks; sediments were transported westwards towards the present-day Central Valley of California. In the Carson Pass–Hope Valley area, two palaeotributaries are preserved in faulted terrane east of the present-day Sierran crest (Hope Valley area); these merge at the crest to form one large, >7 km wide palaeocanyon that is undisrupted by faults west of the crest (Carson Pass–Kirkwood area). This single, large palaeocanyon roughly coincides with the present-day Mokelumne River drainage. New 40Ar/39Ar dates and stratigraphic data east of the crest, integrated with previously published data west of the crest, constrain the ages of strata and unconformities within the Hope Valley–Carson Pass–Kirkwood palaeocanyon system. We interpret three major erosional unconformities to record uplift events at ca. 23–16, 13.5–11, and 10–7 Ma. In other parts of the central Sierra, these uplift events are inferred to correspond to range-front faulting events. We propose the term ‘Hope Valley Graben’ for the structural feature mapped immediately east of the Sierran crest at Carson Pass. It is a nearly symmetrical full graben that offsets volcanic rocks as young as 6 Ma at least 400 m (1300 ft) on each of its bounding faults – herein named the Red Lake Fault on the west and the Hope Valley Fault on the east. However, we infer that faulting began before eruption of the 6 Ma volcanic rocks for three reasons: (1) the graben-localized emplacement of one of the largest volcanic centres in the Sierra, the 6.34 ± 0.14–6.18 ± 0.14 Ma Markleeville Peak Center; (2) andesite lava flows erupted at 6.22 ± 0.14 Ma from the Red Lake Fault, and abut it within the graben; and (3) brecciated granite along the Red Lake Fault is intruded by altered andesite, indicating that the fault started slipping before magmatism ceased. Our stratigraphic and geochronologic data do not permit an estimate of the amount of pre-6 Ma displacement in the Hope Valley graben. The geometry of the palaeocanyon system indicates that a dextral component of slip demonstrated for transtensional faults in other parts of the region did not operate in the Hope Valley graben.

A GPS and modelling study of deformation in northern Central America

SUMMARY We use GPS measurements at 37 stations in Honduras and El Salvador to describe active deformation of the western end of the Caribbean Plate between the Motagua fault and Central American volcanic arc. All GPS sites located in eastern Honduras move with the Caribbean Plate, in accord with geologic evidence for an absence of neotectonic deformation in this region. Relative to the Caribbean Plate, the other stations in the study area move west to west–northwest at rates that increase gradually from 3.3 ± 0.6 mm yr −1 in central Honduras to 4.1 ± 0.6 mm yr −1 in western Honduras to as high as 11–12 mm yr −1 in southern Guatemala. The site motions are consistent with slow westward extension that has been inferred by previous authors from the north-striking grabens and earthquake focal mechanisms in this region. We examine the factors that influence the regional deformation by comparing the new GPS velocity field to velocity fields predicted by finite element models (FEMs) that incorporate the regional plate boundary faults and known plate motions. Our modelling suggests that the obliquely convergent (∼20 ◦ ) direction of Caribbean–North American Plate motion relative to the Motagua fault west of 90 ◦ W impedes the ENE-directed motion of the Caribbean Plate in southern Guatemala, giving rise to extension in southern Guatemala and western Honduras. The FEM predictions agree even better with the measured velocities if the plate motion west of the Central American volcanic arc is forced to occur over a broad zone rather than along a single throughgoing plate boundary fault. Our analysis confirms key predictions of a previous numerical model for deformation in this region, and also indicates that the curvature of the Motagua fault causes significant along-strike changes in the orientations of the principal strainrate axes in the fault borderlands, in accord with earthquake focal mechanisms and conclusions reached in a recent synthesis of the structural and morphologic data from Honduras. Poor fits of our preferred models to the velocities of GPS sites near the Gulf of Fonseca may be an artefact of the still-short GPS time-series in this region or the simplifying assumptions of our FEMs.

Stress change and effective friction coefficient along the Sumatra‐Andaman‐Sagaing fault system after the 26 December 2004 (Mw= 9.2) and the 28 March 2005 (Mw= 8.7) earthquakes

The 2004 Aceh and 2005 Nias events are the two greatest earthquakes of the past 40 years with a total rupture of 1700 km long and a coseismic slip reaching up to 25 m. These two earthquakes have caused large stress perturbations which significantly altered seismic activity in the Sumatra‐Andaman region. Using both detailed mapping of failure planes and various slip distributions, we calculate this stress change along the Sumatra‐Andaman‐Sagaing fault system from central Sumatra to southern Myanmar. The static Coulomb stress change ΔCFFand the observed seismic activity are in very good agreement with a Coulomb index ∼ 20% greater than the one obtained for random events. Compared to previous studies, this high Coulomb Index confirms two important issues on the use of static stress change criterion: unsuited to study near‐field aftershocks and only relevant for aftershocks analysis on large and mature faults at a time scale of several months. The calculated ΔCFFdistribution suggests that the 2004 and 2005 earthquakes inhibit failure on the North Andaman rift and on the Sagaing fault, while failure is encouraged along the transform Andaman zone, the central Andaman rift, the West Andaman fault, the Sumatra fault system, and the offshore thrust faults west of Sumatra Island. The maximum value of ∼15–20 bar (1.5–2 MPa) for ΔCFFis reached in the northern part of the Sumatra fault system. This high value together with the lack of major earthquake in the last 170 years result in a high seismic hazard for this region. Our results are also consistent with temporal evolution of both earthquakes’ location and focal mechanism prior to and after the events. In particular, we explain the occurrence and the mechanism of seismic swarms observed in the central Andaman rift and along the west Andaman fault. Finally, our calculations reveal that the seismicity in the Andaman rift zone can only be explained ifμ′ > 0.5. This result leads to two end‐member models: one with a constant and high fault friction and one with spatial variations, for which friction may depend on either the nature of the lithosphere (oceanic versus continental) or the fault type.

Palaeoseismology of the North Anatolian Fault near the Marmara Sea: implications for fault segmentation and seismic hazard

Abstract We conducted palaeoseismic studies along the North Anatolian fault both east and west of the Marmara Sea to evaluate its recent surface rupture history in relation to the well-documented historical record of earthquakes in the region, and to assess the hazard of this major fault to the city of Istanbul, one of the largest cities in the Middle East. Across the 1912 rupture of the Ganos strand of the North Anatolian fault west of the Marmara Sea, we excavated 26 trenches to resolve slip and constrain the earthquake history on a channel–fan complex that crosses the fault at a high angle. A distinctive, well-sorted fine sand channel that served as a marker unit was exposed in 21 trenches totaling over 300 m in length. Isopach mapping shows that the sand is channelized north of the fault, and flowed as an overflow fan complex across a broad fault scarp to the south. Realignment of the feeder channel thalweg to the fan apex required about 9±1 m of reconstruction. Study of the rupture history in several exposures demonstrates that this displacement occurred as two large events. Analysis of radiocarbon dates places the age of the sand channel as post ad 1655, so we attribute the two surface ruptures to the large regional earthquakes of 1766 and 1912. If each was similar in size, then about 4–5 m of slip can be attributed to each event, consistent with that reported for 1912 farther east. We also found evidence for two additional surface ruptures after about ad 900, which probably correspond to the large regional earthquakes of 1063 and 1344 (or 1354). These observations suggest fairly periodic occurrence of large earthquakes (RI= c . 283±113 years) for the past millennium, and a rate of c . 16 mm/a if all events experienced similar slip. We excavated six trenches at two sites along the 1999 Izmit rupture to study the past earthquake history along that segment of the North Anatolian fault. One site, located in the township of Köseköy east of Izmit, revealed evidence for three surface ruptures (including 1999) during the past 400 years. The other trench was sited in an Ottoman canal that was excavated (but never completed) in 1591. There is evidence for three large surface rupturing events in the upper 2 m of alluvial fill within the canal at that site, located only a few kilometres from the Köseköy site. One of the past events is almost certainly the large earthquake of 1719, for which historical descriptions of damage are nearly identical to that of 1999. Other earthquakes that could plausibly be attributed to the other recognized rupture of the Izmit segment are the 1754, 1878 or 1894 events, all of which produced damage in the region and for which the source faults are poorly known. Our palaeoseismic observations suggest that the Izmit segment of the North Anatolia fault ruptures every one and a half centuries or so, consistent with the historical record for the region, although the time between ruptures may be as short as 35 years if 1754 broke the Izmit segment. Release of about 4 m of seismic slip both west and east of the Marmara Sea this past century (1912, 1999) support the contention that Istanbul is at high risk from a pending large earthquake. In that historical records suggest that the last large central Marmara Sea event occurred in 1766, there may be a similar 4 m of accumulated strain across the Marmara basin segment of the North Anatolian fault.

Two Post-Glacial Earthquakes on the Saddle Mountain West Fault, Southeastern Olympic Peninsula, Washington

Abstract Two earthquakes ruptured the western strand of the Saddle Mountain fault after the Cordilleran ice sheet began its retreat from the Puget Sound Lowland about 17 ka. The fault system includes multiple northeast-trending thrust faults on the southeastern Olympic Peninsula delineated on light detection and ranging topography by prominent, east-side-up scarps that deform Vashon till and extend over 6–7 km. The Cargill Creek Trench on the Saddle Mountain West fault exposed faulted drift and postglacial colluvium below a 1–2 m high scarp. Steeply dipping northeast-striking faults form a 10 m wide zone beneath the scarp. Structural relations in the trench indicate that two earthquakes produced at least 1 m of vertical separation. Additional folding or faulting beyond the trench raised the scarp to a total height of 1.7 m. Aligned cobbles dipping approximately 40° to the north attest to earthquake-related tilting. A cobble firmly lodged in till and split by a secondary fault indicates that hanging-wall deformation involved a dextral component of slip. However, slip on the master fault may have involved a left-lateral component based on slip indicators at adjacent trench sites in the region. A single charcoal age and estimates of the onset of ice retreat limit the time of the oldest earthquake to 17–7.7 ka. The most recent event occurred after 1.7 ka, and may be the same earthquake that dammed Price Lake about 1100 yr ago known from related studies. Empirical relations suggest that the most recent earthquake was about M 7±0.2 and produced an estimated total slip of 1.3–1.7 m. The total postglacial slip rate ranges from 0.1 to 0.3 m/k yr. These results support the hypothesis that the Saddle Mountain fault is part of a larger fault system that accommodates transpressive strain as the Puget Lowland crust migrates around the eastern flank of the Olympic Mountains.

A first deep seismic survey in the Sea of Marmara: Deep basins and whole crust architecture and evolution

Increased source strength, streamer length and dense spatial coverage of seismic reflection profiles of the SEISMARMARA Leg 1 allow to image the deep structure of the marine North Marmara Trough (NMT) on the strike-slip North Anatolian Fault (NAF) west of the destructive Izmit 1999 earthquake. A reflective lower crust and the Moho boundary are detected. They appear upwarped on an E-W profile from the southern Central Basin eastwards, towards more internal parts of the deformed region. Thinning of the upper crust could use a detachment suggested from an imaged dipping intracrustal reflector that would allow upper crustal material to be dragged from beneath it and above the lower crust, accounting for the extensional component but also southwest motion of the southern margin of the NMT. Sections across the eastern half of the NMT, crossing the Cinarcik and Imrali basins, reveal several faults that are active reaching into the basement and have varying strike and proportions of normal and strike-slip displacement. They might be viewed as petals of a large scale negative flower-structure that spreads over a width of 30 km at surface and is rooted deeper in the lithosphere. Under the Central Basin a very thick sediment infill is revealed and its extensional bounding faults are active and imaged as much as 8 km apart down to 6 km depth. We interpret them as two deep-rooted faults encompassing a foundering basement block, rather than being merely pulled-apart from a jog in a strike-slip above a décollement. The deep-basin lengthening would account for only a modest part of the proposed 60 km finite motion since 4 Myr along the same direction oblique to the NMT that sidesteps the shear motion from its two ends. Thus differential motion occurred much beyond the deep basins, like subsidence involving the NMT bounding faults and the intracrustal detachments. The complex partitioned motion localized on active faults with diverse natures and orientations is suggested to represent the overburden deformation induced from horizontal plane simple shear occurring in depth at lithospheric scale, and in front of the North Anatolian Fault when it propagated through the region.

Importance of small-block rotations in damage zones along transcurrent faults. Evidence from the Chuquicamata open pit, Northern Chile

Chuquicamata, in northern Chile, is one of the largest porphyry copper deposits in the world; the western side of its orebody is bounded by a major longitudinal fault, the West fault. We report paleomagnetic results from surface sites and drill cores from different geological units at Chuquicamata, especially within the late Eocene Fiesta granodiorite of the western block of the West fault. Characteristic remanent magnetizations (ChRM) were determined after detailed thermal or alternating field demagnetization. Soft components carried by multidomain magnetite crystals in the Fiesta granodiorite were removed by AF demagnetization at 10–20 mT. The ChRMs, not demagnetized by alternating fields up to 100 mT, have unblocking temperatures above 580 °C with ~75% of the magnetization removed in the temperature range of 580–590 °C. Optical and SEM mineralogical observations, and microprobe data indicate the occurrence of multidomain magnetite formed during a late magmatic stage of alteration coeval with strong oxidation of primary titanomagnetite and formation of ilmenite, hematite, pseudobrookite, and rutile. The characteristic directions have negative inclinations and declinations (330° to 230°); strongly deflected from the expected Eocene direction. Anisotropy of magnetic susceptibility (AMS), with degree up to 1.4, is carried by multidomain magnetite. AMS ellipsoids have subvertical foliations with azimuth varying strongly from N280° to N20°. We show that both the ChRMs and the AMS fabrics record the same apparent relative rotations between sites. Although the AMS anisotropy is high, there is no evidence for a solid-state deformation and the apparent rotation of the magnetic fabric is interpreted to be the consequence of small-block rotation. The apparent large (>100°) counterclockwise rotations of small blocks within the Fiesta granodiorite suggest a wide damaged zone related to sinistral displacement along the West fault. This interpretation is consistent with previous models indicating that the Fiesta granodiorite was sinistrally translated and brought in front of the early Oligocene porphyry copper deposit during the Oligocene–early Miocene. This study shows that paleomagnetic markers are useful for improving the quantification and understanding of small-scale deformation within plutons adjacent to major fault zones.

Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon U-Pb ages from the Magdalena fan and adjacent areas

ABSTRACTThe Magdalena fan is an apparently beheaded submarine depocenter that has fi g-ured prominently in reconstructions of mid-dle to late Miocene Pacifi c–North American plate interactions. The deposit accumulated rapidly at the base of the continental slope on top of newly formed oceanic crust of the Magdalena microplate from 14.5 to 13 Ma. Subduction of this crust ceased as the Pacifi c-Magdalena spreading center encountered the trench. The widely accepted two-phase kine-matic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacifi c and North Ameri-can plates occurred along faults west of Baja California prior to the onset of dextral-trans-tensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to character-ize the provenance of the fan, determine its source region, and defi ne the cumulative dextral slip along faults offshore of south-western Baja California. Zircons from the fan are dominantly 120–65 Ma with subor-dinate 15–35 Ma grains. Excellent matches to the fan can be obtained by mixing Magda-lena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zir cons from the east-draining portion of the Los Cabos block and mainland Mex-ico. Our results favor a western Baja source region for the fan and suggest that cumula-tive dextral slip along faults west of Baja was <150 km, much less than previously believed. We propose that the fan was fed by erosional denudation of the Magdalena shelf produced by increased mantle buoyancy due to the ridge-trench juxtaposition. The fan’s source was cut off when faults west of Baja Califor-nia began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.Keywords:

Style of the surface deformation by the 1999 Chichi earthquake at the central segment of Chelungpu fault, Taiwan, with special reference to the presence of the main and subsidiary faults and their progressive deformation in the Tsauton area

We describe the style of surface deformation in the 1999 Chichi earthquake in the central segment of the Chelungpu Fault. The study covers the Kung-fu village, north of Han River, to the south of Tsauton area. A characteristic style of the surface deformation is a convex scarp in profile and sinuous plan view, due to the low angle thrust fault. Two subparallel faults, including the west facing Tsauton West fault, and the east facing Tsauton East fault, limit the western and eastern margin of the Tsauton terraced area. The Tsauton West fault is the continuation of the main Chelungpu fault and the Tsauton East fault is located about 2 km apart. Both faults record larger amounts of vertical displacement on the older terraces. The 1999 surface rupture occurred exactly on a pre-existing fault scarp of the Tsauton West and East faults. Thus, repeated activities of these two faults during the Holocene, possibly since the late Quaternary, are confirmed. The amount of vertical offset of the Tsauton East fault is smaller, and about 40–50% of that of the Tsauton West fault for the pre-existing fault. This indicates that the Tsauton East fault is a subsidiary fault and moved together with the main fault, but accommodated less amount.

Finite-frequency traveltime tomography of San Francisco Bay region crustal velocity structure

SUMMARY Seismic velocity structure of the San Francisco Bay region crust is derived using measurements of finite-frequency traveltimes. A total of 57 801 relative traveltimes are measured by cross-correlation over the frequency range 0.5–1.5 Hz. From these are derived 4862 ‘summary’ traveltimes, which are used to derive 3-D P-wave velocity structure over a 341 × 140 km2 area from the surface to 25 km depth. The seismic tomography is based on sensitivity kernels calculated on a spherically symmetric reference model. Robust elements of the derived P-wave velocity structure are: a pronounced velocity contrast across the San Andreas fault in the south Bay region (west side faster); a moderate velocity contrast across the Hayward fault (west side faster); moderately low velocity crust around the Quien Sabe volcanic field and the Sacramento River delta; very low velocity crust around Lake Berryessa. These features are generally explicable with surface rock types being extrapolated to depth ∼10 km in the upper crust. Generally high mid-lower crust velocity and high inferred Poisson's ratio suggest a mafic lower crust.

Structural variation along the Devil's Mountain fault zone, northwestern Washington

The eastern Juan de Fuca Strait is subject to long-term, north–south-oriented shortening. The observed deformation is interpreted to result from the northward motion of the Oregon block, which is being driven north by oblique subduction of the oceanic Juan de Fuca plate. Seismic data, acquired during the Seismic Hazards Investigation in Puget Sound survey are used, with coincident first-arrival tomographic velocities, to interpret structural variation along the Devil's Mountain fault zone in the eastern Juan de Fuca Strait. The Primary fault of the Devil's Mountain fault zone developed at the northern boundary of the Everett basin, during north–south-oriented Tertiary compression. Interpretation of seismic reflection data suggests that, based on their similar geometry including the large magnitude of pre-Tertiary basement offset, the Primary fault of the Devil's Mountain fault west of ~122.95°W and the Utsalady Point fault represent the main fault of the Tertiary Devil's Mountain fault zone. The Tertiary Primary fault west of ~122.95°W was probably kinematically linked to faults to the east (Utsalady Point, Devil's Mountain, and another to the south), by an oblique north–northeast-trending transfer zone or ramp. Left-lateral transpression controlled the Quaternary evolution of the Devil's Mountain fault zone. Quaternary Primary fault offsets are smaller to the east of ~122.95°W, suggesting that stress here was in part accommodated by the prevalent oblique compressional structures to the north. Holocene deformation has focussed on the Devil's Mountain, Utsalady Point, and Strawberry Point faults to the east of ~122.8° but has not affected the Utsalady Point fault to the west of ~122.8°W.

Acoustic stratigraphy of Bear Lake, Utah–Idaho—Late Quaternary sedimentation patterns in a simple half-graben

A 277-km network of high-resolution seismic-reflection profiles, supplemented with a sidescan-sonar mosaic of the lake floor, was collected in Bear Lake, Utah–Idaho, in order to explore the sedimentary framework of the lake's paleoclimate record. The acoustic stratigraphy is tied to a 120 m deep, continuously cored drill hole in the lake. Based on the age model for the drill core, the oldest continuously mapped acoustic reflector in the data set has an age of about 100 ka, although older sediments were locally imaged. The acoustic stratigraphy of the sediments below the lake indicates that the basin developed primarily as a simple half-graben, with a steep normal-fault margin on the east and a flexural margin on the west. As expected for a basin controlled by a listric master fault, seismic reflections steepen and diverge toward the fault, bounding eastward-thickening sediment wedges. Secondary normal faults west of the master fault were imaged beneath the lake and many of these faults show progressively increasing offset with depth and age. Several faults cut the youngest sediments in the lake as well as the modern lake floor. The relative simplicity of the sedimentary sequence is interrupted in the northwestern part of the basin by a unit that is interpreted as a large (4 × 10 km) paleodelta of the Bear River. The delta overlies a horizon with an age of about 97 ka, outcrops at the lake floor and is onlapped by much of the uppermost sequence of lake sediments. A feature interpreted as a wave-cut bench occurs in many places on the western side of the lake. The base of this bench occurs at a depth (22–24 m) similar to that (20–25 m) of the distal surface of the paleodelta. Pinch-outs of sedimentary units are common in relatively shallow water on the gentle western margin of the basin and little Holocene sediment has accumulated in water depths of less than 30 m. On the steep eastern margin of the basin, sediments commonly onlap the hanging wall of the East Bear Lake Fault. However, no major erosional or depositional features suggestive of shoreline processes were observed on acoustic profiles in water deeper than about 20–25 m.