Arthur G. Sylvester

Strike-slip faults

The importance of strike-slip faulting was recognized near the turn of the century, chiefly from investigations of surficial offsets associated with major earthquakes in New Zealand, Japan, and California. Extrapolation from observed horizontal displacements during single earthquakes to more abstract concepts of long-term, slow accumulation of hundreds of kilometers of horizontal translation over geologic time, however, came almost simultaneously from several parts of the world, but only after much regional geologic mapping and synthesis. Strike-slip faults are classified either as transform faults which cut the lithosphere as plate boundaries, or as transcurrent faults which are confined to the crust. Each class of faults may be subdivided further according to their plate or intraplate tectonic function. A mechanical understanding of strike-slip faults has grown out of laboratory model studies which give a theoretical basis to relate faulting to concepts of pure shear or simple shear. Conjugate sets of strike-slip faults form in pure shear, typically across the strike of a convergent orogenic belt. Fault lengths are generally less than 100 km, and displacements along them are measurable in a few to tens of kilometers. Major strike-slip faults form in regional belts of simple shear, typically parallel to orogenic belts; indeed, recognition of the role strike-slip faults play in ancient orogenic belts is becoming increasingly commonplace as regional mapping becomes more detailed and complete. The lengths and displacements of the great strike-slip faults range in the hundreds of kilometers. The position and orientation of associated folds, local domains of extension and shortening, and related fractures and faults depend on the bending or stepping geometry of the strike-slip fault or fault zone, and thus the degree of convergent or divergent strike-slip. Elongate basins, ranging from sag ponds to rhombochasms, form as result of extension in domains of divergent strike slip such as releasing bends; pull-apart basins evolve between overstepping strike-slip faults. The arrangement of strike-slip faults which bound basins is tulip-shaped in profiles normal to strike. Elongate uplifts, ranging from pressure ridges to long, low hills or small mountain ranges, form as a result of crustal shortening in zones of convergent strike slip; they are bounded by an arrangement of strike-slip faults having the profile of a palm tree. Paleoseismic investigations imply that earthquakes occur more frequently on strike-slip faults than on intraplate normal and reverse faults. Active strike-slip faults also differ from other types of faults in that they evince fault creep, which is largely a surficial phenomenon driven by elastic loading of the crust at seismogenic depths. Creep may be steady state or episodic, pre-seismic, co-seismic, or post-seismic, depending on the constitutive properties of the fault zone and the nature of the static strain field, among a number of other factors which are incompletely understood. Recent studies have identified relations between strike-slip faults and crustal delamination at or near the seismogenic zone, giving a mechanism for regional rotation and translation of crustal slabs and flakes, but how general and widespread are these phenomena, and how the mechanisms operate that drive these detachment tectonics are questions that require additional observations, data, and modeling. Several fundamental problems remain poorly understood, including the nature of formation of en echelon folds and their relation to strike-slip faulting; the effect of mechanical stratigraphy on strike-slip-fault structural styles; the thermal and stress states along transform plate boundaries; and the discrepancy between recent geological and historical fault-slip rates relative to more rapid rates of slip determined from analyses of sea-floor magnetic anomalies. Many of the concepts and problems concerning strike-slip faults are derived from nearly a century of study of the San Andreas fault and have added much information, but solutions to several remaining and new fundamental problems will come when more attention is focused on other, less well studied strike-slip faults.

Papoose Flat pluton: A granitic blister in the Inyo Mountains, California

The Papoose Flat pluton (75 to 81 m.y. old) is one of several Mesozoic granitic bodies in the White-Inyo Range which are regarded as satellites of the Sierra Nevada batholith. The pluton, composed chiefly of quartz monzonite with K-feldspar megacrysts, crops out as an elongated east-trending dome, 16 km long and 8 km wide. It was emplaced within the southwest limb of the major southeast-plunging Inyo anticline, consisting of virtually unmetamorphosed late Precambrian and Cambrian sedimentary rocks. Stratigraphic units around the western half of the pluton have a regionally metamorphosed aspect and are tectonically and concordantly thinned to as little as 10% of their regional thicknesses without loss of Stratigraphic identity or continuity. Around the discordant eastern contact, however, wall-rock textures are hornfelsic, and there is little or no attenuation of the stratigraphic succession. Foliation, lineation, and preferred orientation of minerals are well developed in the western half of the pluton and its wall rocks and, together with the boudinaged wall rocks, are consistent with the geometry of strain required for attenuation of the stratigraphic succession. The structural evidence indicates that the granite penetrated discordantly through lower formations in the anticline and formed a “blister” in the fold limb beneath the stretched higher formations. Metamorphic mineral assemblages of the wall rocks suggest a maximum temperature of metamorphism of less than 600 °C and a pressure of less than 2.5 kb.

Nature and distribution of deep crustal reservoirs in the southwestern part of the Baltic Shield : Evidence from Nd, Sr and Pb isotope data on late Sveconorwegian granites

Late Sveconorwegian (‘postorogenic’) granites (c. 1.0–0.93 Ga) make up a voluminous and widespread suite of intrusions across south Norway. From radiogenic isotope data, three groups of late Sveconorwegian granites can be distinguished: (1) granite with more than 150 ppm Sr, 87Rb/86Sr<5,87Sr/86Sr0.93 Ga<0.710 and εNd<0; (2) granite with less than 150 ppm Sr, 87Rb/86Sr>5, 87Sr/86Sr0.93 Ga>0.710 and εNd<0; (3) juvenile granite with 87Sr/86Sr0.93 Ga<0.705 and εNd>0. Granite plutons belonging to Group 1 (‘normal-Sr concentration granite’) occur all over south Norway and include the largest batholiths (Østfold, Flå, Herefoss). Granite plutons of Group 2 (‘low-Sr concentration granites’) are restricted to north-central Telemark (Rjukan rift), and are associated with c. 1.5 Ga Rjukan Group rhyolite. Group 3 is represented by one intrusion only, but still suggests input of mantle-derived magma, or the presence of young, mantle derived rocks in the deep crust in the region at c. 0.93 Ga. The Group 1 granites are similar in Sr and Nd characteristics to some of the older (1.05 Ga) Sveconorwegian granitic intrusions (‘augen gneisses’) in the region in terms of radiogenic isotope systematics, but Pb isotopes suggest that the magmas did not form by simple remelting of augen gneiss. The Nd, Sr, and Pb isotopic systematics of the late Sveconorwegian granites indicate mixing between a depleted-mantle derived component and two or more major components with an extended crustal history. One of the crustal end members is present throughout south Norway and has an isotopic signature similar to older granitic rocks of the Trans Scandinavian Igneous Belt (TIB). The other crustal end member is indistinguishable from Rjukan Group rhyolites and slightly younger intrusions associated with these, and is restricted to areas within the mid-Proterozoic Rjukan rift. The available data suggest that the deep continental crust of south Norway, both east and west of the Permian Oslo Rift, is an integral part of the Baltic Shield, with a common history back to 1.7–1.9 Ga, that is, to the end of the Svecofennian orogeny and the TIB magmatism.

Structure and neotectonics of the western Santa Ynez fault system in southern California

Abstract Geologic, geomorphic and seismologic data indicate that west of Lake Cachuma the Santa Ynez fault branches into several major W- and NW-trending splay faults. Two of the faults bracket the wedge-shaped Santa Maria basin. The most compelling evidence for the existence of these two faults is the fact that the Santa Maria basin is floored by Franciscan basement overlain only by Miocene and younger sedimentary rocks, whereas across the inferred traces of each of these faults, the adjacent terrains consist of Franciscan basement overlain by thick sequences of Early Tertiary strata, as well as by Miocene and younger rocks. The third splay fault strikes northwestward through the central Santa Maria basin. Narrow zones of tightly appressed, left-stepping en-echelon folds are locally adjacent to the faults along the south edge, and through the center of the basin. The geometrical arrangement of these folds is indicative of formation over buried sinistral wrench faults. Evidence for Holocene surface rupturing is lacking or nebulous at best, but epicenters of damaging historical earthquakes are spatially, and by inference, genetically related to the central Santa Maria basin faults, indicating that they comprise the presently active strands among the several splay faults.

Monzonites of the White-Inyo Range, California, and their relation to the calc-alkalic Sierra Nevada batholith

Monzonitic rocks are found in two plutons in the Deep Springs Valley area of the White-Inyo Range, California. These intrusive rocks, among the oldest in the Sierra Nevada region (170 m.y.), are characterized mineralogically by their abundance of potassium feldspar and paucity of quartz, and chemically by exceptionally high concentrations of alkalis (K 2 O + Na 2 O generally greater than 8% by weight) and strontium (greater than 1,000 ppm) and low silica (generally less than 60%). Both quartz-bearing and feldspathoidal varieties are present. Modal and chemical data clearly show that these alkalic intrusive rocks are not petrogenetically compatible with the far more abundant quartz-rich, calc-alkalic rocks that characterize the Sierra Nevada batholith. Instead, the monzonitic rocks constitute part of a discrete petrogenetic unit that is in part contemporaneous but certainly not comagmatic with the Sierra Nevada batholith. The available data are compatible with an origin for the Inyo monzonites during an early Mesozoic magmatic episode related to an arc-trench–subduction system that tapped an upper-mantle source beneath the western edge of the North American plate.

Wall Rock Decarbonation and Forcible Emplacement of Birch Creek Pluton, Southern White Mountains, California

Birch Creek pluton is a small body of quartz monzonite and granodiorite of Cretaceous age that was emplaced within Precambrian argillite and dolomite comprising part of the steep eastern limb of the White Mountain anticline, a major south-plunging fold. The granitic magma was intruded initially along a nearly vertical fault parallel to and within the eastern limb of the anticline. The dolomitic rocks were heated and decarbonated during intrusion of the granite. We suggest that the liberated CO 2 entered the vapor phase of the magma, causing isothermal crystallization of the magma over and around most of the pluton. Moreover, the viscosity of the magma increased in the upper part of the magma column so that a relatively rigid shell of granite and metamorphosed wall rocks was formed which prevented continued upward and eastward intrusion. Yielding to the pressure of rising magma beneath, the viscous magma bulged laterally in a northwest direction where structural interpretations suggest the dolomite shell was very thin or absent. The forcible nature of this stage of emplacement is clearly shown in the core of the anticline by the curvilinear traces of strata, crestal axes of folds, and pre-pluton faults, which wrap concordantly around the northwest corner of the pluton.

Strike-slip basins; Part 1

Various types of sedimentary basins that contain a great variety of sedimentary successions and structural attributes form along strike‐slip faults. These basins have commonly been called “pull‐apart basins.” However, we believe that the term “strike‐slip basin” is more suitable because “pull‐apart basins” are only one of many varieties that may develop along strike‐slip faults.

INVESTIGATION OF NEARFIELD POSTSEISMIC SLIP FOLLOWING THE MW 7.3 LANDERS EARTHQUAKE SEQUENCE OF 28 JUNE 1992, CALIFORNIA

Repeated precise surveys of six nearfield trilateration arrays reveal that minor horizontal displacements occurred across the several fault ruptures in the five months following the Landers earthquake sequence. The displacements range from 0±2 mm on the Camp Rock and Emerson faults, to 3±2 mm on the Landers fault, to 9plusmn;2 mm on the Johnson Valley fault, to 40±5 mm on the Eureka Peak fault, consistent in magnitude and duration with GPS and creepmeter measurements by other investigators. The relative lack of afterslip at all sites except across the Eureka Peak fault is probably due to the lack of a thick alluvial cover. Concomitant precise resurveys of level lines across the San Andreas fault in Coachella Valley and of a trilateration array at Durmid Hill, south easternmost San Andreas fault, failed to reveal vertical or horizontal displacements greater than the allowable uncertainty (<1 mm for leveling; 2 mm±2 ppm for trilateration). Therefore the Landers earthquake sequence did not perturb the San Andreas in any way that was evident from our measurements of near-surface strain.

SA Microfabric Study of Calcite, Dolomite, and Quartz around Papoose Flat Pluton, California

Post-crystalline strain features were studied in quartzite and dolomite-bearing marble around the Papoose Flat pluton to compare various hypotheses regarding stress orientations obtained from deformation lamellae in quartz, calcite, and dolomite. Analyses of twin lamellae in calcite and dolomite yield maxima of “compression” (C) and “tension” (T) axes that are not symmetrically related to the foliation, lineation, or patterns of preferred orientations of minerals. The C-T maxima are oriented consistently with respect to geographic coordinates and record a post-crystalline state of stress considerably different than the para-crystalline stress previously inferred for the same rocks. The quartz deformation lamellae are unusual in their crystallographic orientation: most of them are inclined between 20° and 80° to the {0001} planes; no near-basal and few prismatic lamellae are present. Statistically, the lamellae define two planes in most specimens, and are believed to have formed in planes of high shear stress, consistent with experimental evidence. The acute and obtuse bisectrices of the two planes of lamellae are parallel to the maxima of C- and T-axes, respectively, determined from twinning in calcite and dolomite. Thus, these axes derived from the orientation of nonbasal quartz lamellae appear to reflect the orientation of the principal stresses during their formation, as other investigators have demonstrated for the more common near-basal lamellae. The maximum principal (compressive) stress was horizontal and NNE, and the least principal stress was horizontal and WNW. These orientations are consistent with those determined from a conjugate system of faults which also formed soon after emplacement of the pluton.

The nature and polygenetic origin of orbicular granodiorite in the Lower Castle Creek pluton, northern Sierra Nevada batholith, California

Mafic and granodioritic magmas mingled to produce a swarm of microdiorite enclaves in the granodiorite. The enclaves and rare hornfels inclusions were carried upward in an oval-shaped pipe, 30 m long and 15 m wide, probably as a gas-driven mass to a point where an H 2 O-rich, superheated felsic melt intruded the pipe and the enclave mass, and upon abrupt undercooling, deposited orbicular shells of tangentially oriented microcrystals of sodic plagioclase, quartz, K-feldspar, and biotite on 20% of the enclaves and inclusions now located only against the north margin of the pipe. The mass of orbicular and nonorbicular enclaves was then injected by a quartz monzodiorite magma that now comprises the matrix among the enclaves.