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.

Strike-Slip faults

Granitic—rhyolitic liquids were produced experimentally from moderately hydrous (1.7–2.3 wt% H2O) medium-to-high K basaltic compositions at 700 MPa and fO2 controlled from Ni-NiO −1.3 to +4. Amount and composition of evolved liquids and coexisting mineral assemblages vary with fO2 and temperature, with melt being more evolved at higher fO2s, where coexisting mineral assemblages are more plagioclase- and Fe–Ti oxide-rich and amphibole-poor. At fO2 of Ni–NiO +1, typical for many silicic magmas, the samples produce 12–25 wt% granitic–rhyolitic liquid, amounts varying with bulk composition. Medium-to-high K basalts are common in subduction-related magmatic arcs, and near-solidus true granite or rhyolite liquids can form widely, and in geologically significant quantities, by advanced crystallization–differentiation or by low-degree partial remelting of mantle-derived basaltic sources. Previously differentiated or weathered materials may be involved in generating specific felsic magmas, but are not required for such magmas to be voluminous or to have the K-rich granitic compositions typical of the upper continental crust.

Voluminous granitic magmas from common basaltic sources

SUMMARY Plate motions relative to the hotspots over the past 4 to 7 Myr are investigated with a goal of determining the shortest time interval over which reliable volcanic propagation rates and segment trends can be estimated. The rate and trend uncertainties are objectively determined from the dispersion of volcano age and of volcano location and are used to test the mutual consistency of the trends and rates. Ten hotspot data sets are constructed from overlapping time intervals with various durations and starting times. Our preferred hotspot data set, HS3, consists of two volcanic propagation rates and eleven segment trends from four plates. It averages plate motion over the past ≈5.8 Myr, which is almost twice the length of time (3.2 Myr) over which the NUVEL-1A global set of relative plate angular velocities is estimated. HS3-NUVEL1A, our preferred set of angular velocities of 15 plates relative to the hotspots, was constructed from the HS3 data set while constraining the relative plate angular velocities to consistency with NUVEL-1A. No hotspots are in significant relative motion, but the 95 per cent confidence limit on motion is typically ±20 to ±40 km Myr −1 and ranges up to ±145 km Myr −1 . The uncertainties of the new angular velocities of plates relative to the hotspots are smaller than those of previously published HS2-NUVEL1 (Gripp & Gordon 1990), while being averaged over a shorter and much more uniform time interval. Nine of the fourteen HS2-NUVEL1 angular velocities lie outside the 95 per cent confidence region of the corresponding HS3NUVEL1A angular velocity, while all fourteen of the HS3-NUVEL1A angular velocities lie inside the 95 per cent confidence region of the corresponding HS2-NUVEL1 angular velocity. The HS2-NUVEL1 Pacific Plate angular velocity lies inside the 95 per cent confidence region of the HS3-NUVEL1A Pacific Plate angular velocity, but the 0 to 3 Ma Pacific Plate angular velocity of Wessel & Kroenke (1997) lies far outside the confidence region. We show that the change in trend of the Hawaiian hotspot over the past 2 to 3 Myr has no counterpart on other chains and therefore provides no basis for inferring a change in Pacific Plate motion relative to global hotspots. The current angular velocity of the Pacific Plate can be shown to differ from the average over the past 47 Myr in rate but not in orientation, with the current rotation being about 50 per cent faster (1.06 ± 0.10 deg Myr −1 ) than the average (0.70 deg Myr −1 ) since the 47-Myr-old bend in the Hawaiian‐Emperor chain.

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Young tracks of hotspots and current plate velocities

Several methods have been developed to assess the thermal state of the mantle below oceanic ridges, islands, and plateaus, on the basis of the petrology and geochemistry of erupted lavas. One leads to the conclusion that mantle potential temperature (i.e., TP) of ambient mantle below oceanic ridges is 1430°C, the same as Hawaii. Another has ridges with a large range in ambient mantle potential temperature (i.e., TP = 1300–1570°C), comparable in some cases to hot spots (Klein and Langmuir, 1987; Langmuir et al., 1992). A third has uniformly low temperatures for ambient mantle below ridges, ∼1300°C, with localized 250°C anomalies associated with mantle plumes. All methods involve assumptions and uncertainties that we critically evaluate. A new evaluation is made of parental magma compositions that would crystallize olivines with the maximum forsterite contents observed in lava flows. These are generally in good agreement with primary magma compositions calculated using the mass balance method of Herzberg and O'Hara (2002), and differences reflect the well-known effects of fractional crystallization. Results of primary magma compositions we obtain for mid-ocean ridge basalts and various oceanic islands and plateaus generally favor the third type of model but with ambient mantle potential temperatures in the range 1280–1400°C and thermal anomalies that can be 200–300°C above this background. Our results are consistent with the plume model.

/pdf/temperatures-in-ambient-mantle-and-plumes-constraints-from-t2dh8hln70.pdf

Temperatures in Ambient Mantle and Plumes: Constraints from Basalts, Picrites, and Komatiites

Systematic rare earth and minor element concentration variations in basalt recently erupted along the post-glacial Reykjanes Ridge Axis and its northward extension over Iceland reflect the existence, spatial influence, and primordial nature of the Iceland hot mantle plume.

Iceland Mantle Plume: Geochemical Study of Reykjanes Ridge

The rifts of the 14 basaltic shield volcanoes that extend from Kauai to Hawaii are composed of thousands of dykes that were fed laterally by periodic leakages from central volcanic con­duits. Individual dykes are believed to be thin, steeply dipping blades, several kilometres from top to bottom, that extend horizontally outward for as much as 120 km. The dykes are contained largely within the volcanic edifices, and, because of such shallow emplacement, the direction of dyke propagation is interpreted to be strongly influenced by the gravitational stresses within these edifices. Simple isolated shields, such as Kauai and West Molokai, had nearly symmetrical stress fields, influenced only slightly by regional stresses, and the dykes injected into these volcanoes had little or no tendency to cluster into well-defined rifts. Other volcanoes, such as Koolau and Kilauea, pierced the thick, sloping apron of pre-existing neigh­bour volcanoes. The dykes that propagated from these centres were strongly influenced by the gravitational stress fields of the sloping aprons in which they grew. Accordingly, they clustered into well-defined rifts oriented roughly perpendicular to the downslope direction of these aprons. With only minor exceptions, 8 of the 14 volcanoes forming the southeast part of the Hawaiian chain conformed to this pattern of growth; the influence of regional Pacific structure on rift orientation is suspected in only 6 volcanoes that grew as simple, iso­lated shields, away from the influence of gravitational stresses of any neighbour volcano.

Orientation and Growth of Hawaiian Volcanic Rifts: The Effect of Regional Structure and Gravitational Stresses

Origin of the Differentiated and Hybrid Lavas of Kilauea Volcano, Hawaii

http://science.sciencemag.org/content/sci/165/3891/341.full.pdf

Inflation of Kilauea Volcano Prior to Its 1967-1968 Eruption

Submarine equivalents of ash flows in the Tokiwa Formation, Japan

We report the results of modeling the three-dimensional internal structure of Kilauea's magmatic passageways The approach uses a clear plexiglass model containing equally-spaced levels upon which well-located seismic hypocenters are plotted Application of constraining geologic and geophysical criteria to this distributed volume of earthquakes permits the interpretation of seismic structures produced by fracturing in response to locally high fluid pressures Four magma transport and storage structures have been identified within and beneath Kilauea: (1) Primary conduit The conduit transporting magma into Kilauea's summit storage reservoir rises from the model base (146 km) to the 65 km depth level It is a zone of intense fracturing and inferred intrusion, whose horizontal sections are elliptical in planform Over its height, the average major axis of the component horizontal sections is 33 km, with an average minor axis of 17 km This yields an aspect ratio of ξ = 052 At the 146 km level, the strike of the major axis is N67°E During passage from the upper mantle through the oceanic crust, this axis rotates in a right-handed sense, until the strike is N41°W at the 65 km level (2) Magma chamber complex floor The interval from 65 to 57 km, immediately over the primary conduit, is aseismic This suggests differentially high fluid-to-rock ratios, and relatively weak pathways for further vertical transport into higher levels of the storage complex, as well as lateral leakage eastward into the Mauna Ulu staging area—for later vertical ascent beneath the upper east rift zone Seismicity within the immediately subjacent rocks that form the top of the primary conduit (at 65 km) suggests that this inferred magma-rich horizon forms the effective floor of the summit storage complex (3) Magma chamber crown Intense seismicity over the 11–19 km depth interval defines an elliptical region in plan view The top of this region has a broad apex with an average major axis of 15 km, and an average minor axis of 11 km producing an aspect ratio of ξ = 073 This region coincides with the epicentral position of summit vertical displacement maxima during inflationary deformation and is interpreted as a region of intense diking, crowning the summit storage complex (4) Upper east rift zone pipe From a depth of 57 km, a cylindrical region of seismicity rises beneath the upper east rift zone, to the 19 km level, where it merges with the subhorizontal upper east rift zone duct This pipe-like zone has a mean diameter of 14 km and a vertical axis that pierces the surface of the volcano at the intersection of the Koae fracture system and the upper east rift zone We suggest this pipe forms a principal connection between the base of the summit storage reservoir, and near-surface storage compartments and ducts in the upper east rift zone In this role, the pipe is suggested to have contributed heavily to Mauna Ulu's magma supply during 1969–1974 The model of magma transport and storage thus constructed provides a context in which other types of data (eg, ground deformation and electrical conductivity) may be interpreted as well as a means of refining petrological inferences based on lava chemistry

Richard S. Fiske

Papers

Orientation and Growth of Hawaiian Volcanic Rifts: The Effect of Regional Structure and Gravitational Stresses

Origin of the Differentiated and Hybrid Lavas of Kilauea Volcano, Hawaii

Inflation of Kilauea Volcano Prior to Its 1967-1968 Eruption

Submarine equivalents of ash flows in the Tokiwa Formation, Japan

Modeling the three-dimensional structure of macroscopic magma transport systems: Application to Kilauea Volcano, Hawaii