Distributed Shear Zone Research Articles

Landslide hypothesis for the origin of Haleakala volcano's crater and great valleys, Hawaii

Abstract Active Haleakala volcano on the island of Maui is the second largest volcano in the Hawaiian Island chain. Prominently incised in Haleakala's slopes are four large (great) valleys. Haleakala Crater, a prominent summit depression, formed by coalescence of two of the great valleys. The great valleys and summit crater have long been attributed solely to fluvial erosion, but two significant enigmas exist in the theory. First, the great valleys of upper Keanae/Koolau Gap, Haleakala Crater, and Kaupo Gap are located in areas of relatively low annual rainfall. Second, the axes of some valley segments are oblique for long distances across the volcanic slopes. This study tested the prevailing erosional theory by reconstructing the volcano's topography just prior to valley incision. The reconstruction produces a belt along the volcano's east rift zone with a morphology that is inconsistent with volcanic aggradation alone, but it is readily explained if it is assumed the surface was displaced along scarps formed by a giant landslide on Haleakala's northeastern flank. Although the landslide head location is well defined, topographic evidence is lacking for the toe and lateral margins. Consequently, the slope failure is interpreted as a sackung-style landslide with a zone of deep-seated distributed shear and broad surface warping downslope of the failure head. Maximum downslope displacement was likely in the range of 400–800 m. Capture of runoff at the headscarps formed atypically large streams that carved Haleakala's great valleys and explains their existence in low-rainfall areas and their slope-oblique orientations. Sackung-style landslides may be more prevalent on Hawaiian volcanoes than previously recognized.

Thermo-kinematic modeling of detachment-dominated extension, northeastern Death Valley area, USA: Implications for mid-crustal thermal-rheological evolution

The deep structure of continental detachment faults remains debated. Thermo-mechanical models generate detachments that either transect the lithosphere or become distributed shear zones in the mid-lower crust, depending on prescribed thermo-rheological conditions. However, these geometries and prescribed conditions remain little constrained by geology-based reconstructions. We present stepwise, balanced reconstructions of a 160 km-long cross-section through two detachment faults in the southwest USA. Reconstructions form the basis of iteratively improved 2D forward thermo-kinematic numerical simulations of detachment fault slip, footwall exhumation, heat advection, and footwall zircon (U-Th)/He cooling ages. Thermo-kinematic model solutions are calibrated iteratively against surface heat flow, pre- and post-extensional geotherms, inferred Moho temperatures, and thermochronometric data from one detachment footwall. Best-fit models predict the thermal and geometric evolution of the crust and detachments, respectively, during extension. The detachment initially rooted into a mid-crustal shear zone (~7.5–12 km depth) and was probably delocalized in the deep middle crust (>12–15 km). The maximum principal stress was likely non-vertical in the middle crust at detachment initiation, possibly due to mantle upwelling. Our reconstructions suggest that the upper crust and lower crust-mantle lithosphere were decoupled by a weak, mid-crustal layer during early detachment faulting. The weak layer was thinned, cooled, partially embrittled, and therefore strengthened by continued detachment slip. This increased lithospheric mechanical coupling and caused the locus of upper-crustal extension to shift. Thinning of a weak mid-crustal layer, as is thought to precede coupled hyperextension and mantle exhumation during rifting, was mostly complete in our study area by ~7–6 Ma.

Length scales and types of heterogeneities along the deep subduction interface: Insights from exhumed rocks on Syros Island, Greece

Abstract We use structural and microstructural observations from exhumed subduction-related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that represent different oceanic protoliths and deformation conditions within a subduction interface shear zone, including: (1) prograde subduction of oceanic crust to eclogite facies; (2) exhumation of oceanic crust from eclogite through blueschist-greenschist facies; and (3) exhumation of mixed mafic crust and sediments from eclogite through blueschist-greenschist facies. All three localities preserve rheological heterogeneities that reflect metamorphism of primary lithological, geochemical, and/or textural variations in the subducted protoliths and that take the form of brittle pods and lenses within a viscous matrix. Microstructural observations indicate that the matrix lithologies (blueschists and quartz-rich metasediments) deformed by distributed power-law viscous flow accommodated by dislocation creep in multiple mineral phases. We estimate bulk shear zone viscosities ranging from ∼1018 to 1020 Pa-s, depending on the relative proportion of sediments to (partially eclogitized) oceanic crust. Eclogite and coarse-grained blueschist heterogeneities within the matrix preserve multiple generations of dilational shear fractures and veins formed under high-pressure conditions. The veins commonly show coeval or overprinting viscous shear, suggesting repeated cycles of frictional and viscous strain. These geologic observations are consistent with a mechanical model of episodic tremor and slow slip (ETS), in which the deep subduction interface is a rheologically heterogeneous distributed shear zone comprising transiently brittle (potentially tremor-genic) sub-patches within a larger, viscously creeping interface patch. Based on our observations of outcrop and map areas of heterogeneous patches and the sizes, distributions, and amounts of brittle offset recorded by heterogeneities, we estimate that simultaneous brittle failure of heterogeneities could produce tremor bursts with equivalent seismic moments of 4.5 × 109–4.7 × 1014 N m, consistent with seismic moments estimated from geophysical data at active subduction zones.

Tectonic evolution of the middle crust in southern Tibet from structural and kinematic studies in the Lhagoi Kangri gneiss dome

Field, structural, kinematic, and deformation temperature analyses were conducted on rocks from the Lhagoi Kangri gneiss dome (southern Tibet) in order to establish the geologic history of the dome, identify major phases of deformation within the dome, and to relate these phases of deformation to the tectonic evolution of the Himalayan middle crust. The Lhagoi Kangri dome, one of a series of gneiss-cored domes in southern Tibet, records stratigraphy and structural features similar to previously studied north Himalayan gneiss domes. Field mapping reveals a sequence of rocks that comprise a cover of unmetamorphosed to amphibolite-grade siliciclastic and minor carbonate rocks overlying a core predominantly composed of foliated and lineated orthogneiss intruded by relatively undeformed granite, which also intrudes the cover rocks both concordantly and discordantly. Field observations and microstructural analyses suggest that the contact between the core and cover rocks was originally a nonconformity, but we do not rule out the possibility of subsequent slip along the surface, as has been reported for correlative structures in other domes. Lhagoi Kangri rocks were pervasively deformed during at least two major tectonic phases. The earliest deformation event (D1) resulted in shortening and thickening of crust, the record of which is largely eliminated, particularly in lower structural levels, by transposition and recrystallization during the second phase of deformation (D2). Ductile deformation during D2 is characterized at higher structural levels by crenulation cleavage that tightens with depth, while at lower structural levels D2 manifests as a distributed shear zone that records some evidence of both plane strain and coaxial flattening, possibly indicating overall heterogeneous general shear. The shear zone is ∼3 km thick and contains rocks with mostly symmetrical top-to-north and top-to-south shear sense indicators with a dominant top-to-north component at lower structural levels. Microstructural analyses and quartz c-axis fabrics indicate a range of D2 shear zone temperatures from 200 to 300 °C at the upper boundary to ≥630 °C at the lowest structural levels sampled with minimal evidence of lower temperature overprinting. The interpreted temperatures define a wide range in thermal field gradients (18–90 °C/km) that suggest that temperature indicators were locked in at relatively late stages of D2. The structural framework and kinematic history of the Lhagoi Kangri dome are similar to previously studied north Himalayan gneiss domes as well as to transects through the South Tibetan detachment system, which supports previous interpretations of structural continuity between the north Himalayan gneiss domes and other middle crustal exposures in the Himalaya. The Lhagoi Kangri distributed shear zone, in particular, may represent a deeper ductile manifestation of the South Tibetan detachment system.

Polyphase deformation, dynamic metamorphism, and metasomatism of Mount Everest’s summit limestone, east central Himalaya, Nepal/Tibet

New samples collected from a transect across the summit limestone of Mount Everest (Qomolangma Formation) show that multiple distinct deformational events are discretely partitioned across this formation. Samples from the highest exposures of the Qomolangma Formation (Everest summit) preserve a well-developed mylonitic foliation and microstructures consistent with deformation temperatures of ≥250 °C. Thermochronologic and microstructural results indicate these fabrics were ingrained during initial contractile phases of Himalayan orogenesis, when crustal thickening was accommodated by folding and thrusting of the Tethyan Sedimentary Sequence. In contrast, samples from near the base of the Qomolangma Formation (South Summit) preserve extensional shear deformation, indicate metasomatism at temperatures of ∼500 °C, and contain a synkinematic secondary mineral assemblage of muscovite + chlorite + biotite + tourmaline + rutile. Shear fabrics preserved in South Summit samples are associated with activity on the Qomolangma detachment, while the crystallization of secondary phases was the result of reactions between the limestone protolith and a volatile, boron-rich fluid that infiltrated the base of the Qomolangma Formation, resulting in metasomatism. The 40 Ar/ 39 Ar dating of synkinematic muscovite indicates the secondary assemblage crystallized at ca. 28 Ma and that shear fabrics were ingrained at ≥18 Ma. This paper presents the first evidence that Everest’s summit limestone records multiple phases of deformation associated with discrete stages in Himalayan orogenesis, and that the structurally highest strand of the South Tibetan detachment on Everest was initially active as a distributed shear zone before it manifested as a discrete brittle detachment at the base of the Qomolangma Formation.

Imaging P and S Attenuation in the Sacramento-San Joaquin Delta Region, Northern California

We obtain 3D Q P and Q S models for the Delta region of the Sacramento and San Joaquin rivers, a large fluvial–agricultural portion of the Great Valley located between the Sierra Nevada batholith and the San Francisco Bay–Coast Ranges region of active faulting. Path attenuation t * values have been obtained for P and S data from 124 distributed earthquakes, with a longer variable window for S based on the energy integral. We use frequency dependence with an exponent of 0.5, consistent with other studies and weakly favored by the t * S data. A regional initial model was obtained by solving for Q as a function of seismic velocity. In the final model, the Great Valley basin has low Q , with very low Q (<50) for the shallowest portion of the Delta. There is an underlying strong Q contrast to the ophiolite basement, which is thickest with highest Q under the Sacramento basin, and a change in structure is apparent across the Suisun Bay as a transition to thinner ophiolite. Moderately low Q is found in the upper crust west of the Delta region along the faults in the eastern North Bay area, whereas moderately high Q is found south of the Delta, implying potentially stronger ground motion for earthquake sources to the south. Very low Q values in the shallow crust along parts of the major fault zones may relate to sediment and abundant microfractures. In the lower crust below the San Andreas and Calaveras–Hayward–Rodgers Creek fault zones, the observed low Q is consistent with grain‐size reduction in ductile shear zones and is lowest under the San Andreas, which has large cumulative strain. Similarly, moderately low Q in the ductile lower crust of the Bay area block between the major fault zones implies a broad distributed shear zone. Online Material: Distribution of earthquake magnitude, figures of t * values, 3D initial regional velocity and Q models, station terms, and Q P inversion with 1D initial Q P model.

Till‐forming processes beneath parts of the Cordilleran Ice Sheet, British Columbia, Canada: macroscale and microscale evidence and a new statistical technique for analysing microstructure data

This paper presents the first integrated macroscale and microscale examination of subglacial till associated with the last‐glacial (Fraser Glaciation) Cordilleran Ice Sheet (CIS). A new statistical approach to quantifying till micromorphology (multivariate hierarchical cluster analysis for compositional data) is also described and implemented. Till macrostructures, macrofabrics and microstructures support previous assertions that primary till in this region formed through a combination of lodgement and deformation processes in a temperate subglacial environment. Macroscale observations suggest that subglacial environments below the CIS were probably influenced by topography, whereby poor drainage of the substrate in topographically constricted areas, or on slopes adverse to the ice‐flow direction at glacial maximum, facilitated ductile deformation of the glacier bed. Microscale observations suggest that subglacial till below the CIS experienced both ductile and brittle deformation, including grain rotation and squeeze flow of sediment between grains under moist conditions, and microshearing, grain stacking and grain fracturing under well‐drained conditions. Macroscale observations suggest that ductile deformation events were probably followed by brittle deformation events as the substrate subsequently drained. The prevalence of ductile‐type microstructures in most till exposures investigated in this study suggests that ductile deformation signatures can be preserved at the microscale after brittle deformation events that result in larger‐scale fractures and shear structures. It is likely that microscale ductile deformation can also occur within distributed shear zones during lodgement processes. Cluster analysis of microstructure data and qualitative observations made from thin sections suggest that the relative frequency of countable microstructures in this till is influenced by topography in relation to ice‐flow direction (bed drainage conditions) as well as by the frequency and distribution of voids in the till matrix and skeletal grain shapes.

Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology

The kinematics of the collisional crustal wedge in the external Western Alps is discussed in the light of new cross sections of the whole external zone at the latitude of the Belledonne–Grandes Rousses–Oisans massifs (External Crystalline Massifs, ECM), as well as a detailed analysis of the deformation of their basement and cover. The cross sections were built from new field data and published geological maps, and were restored through time (Oligo-Miocene times) to unravel the successive stages of the Dauphinois margin contractional deformation. During Oligocene times, the Variscan basement was deformed essentially by greenschist facies Alpine distributed shear zones with no significant reactivation of the inherited Jurassic normal faults or the Variscan foliation. However, the inherited syn-rift basins localized the deformation characterized by a thick-skinned style and a cover disharmonically folded over basement shear zones, with no major décollement in between (shortening of 11.5km, 20%). Those early deformations progressively localized on the frontal crustal ramp and caused shortening in the folded belt (16.5km of shortening, 23%). A similar sequence of deformation has been inferred in other Alpine ECMs. This strongly suggests that the crust of Dauphinois proximal passive margin was thin enough, and its buoyancy low enough (as a result of the Liassic rifting), to experience significant tectonic burial during the Tertiary collision. As a consequence of a weakening by both the presence of inherited basins (and their weak syn-rift sedimentary rocks) and the P–T conditions (greenschist facies), the crust was shortened and the structural style was thick-skinned.

Precise hypocenter distribution of deep low-frequency earthquakes and its relationship to the local geometry of the subducting plate in the Nankai subduction zone, Japan

[1] We determine the precise hypocenter distribution of deep low-frequency earthquakes (LFEs) in the Nankai subduction zone and compare it with the local geometry of the subducting Philippine Sea plate. We apply a new hypocenter determination method utilizing the summed cross correlation coefficient over many stations, termed a network correlation coefficient (NCC), to 112 LFEs in the western Shikoku and 1566 LFEs in the whole Nankai subduction zone. While the catalog depths are widely distributed in some regions, the relocated hypocenters in every region construct a plane surface several km above the oceanic Moho interface and quite consistent with the geometry of the oceanic Moho. This result strongly supports the hypothesis that LFEs in the Nankai subduction zone occur on the subducting plate boundary and are directly generated by shear slips. If LFEs are indeed direct indicators of the locations of the plate interface, they might be useful to investigate the minute structure of the plate interface. The thin distributions of LFEs indicate that the interface between the subducting and the overriding plates is a distinct very thin boundary, and not a distributed shear zone.

Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California

A new fault map of the shelf offshore of San Francisco, California shows that faulting occurs as a distributed shear zone that involves many fault strands with the principal displacement taken up by the San Andreas fault and the eastern strand of the San Gregorio fault zone. Structures associated with the offshore faulting show compressive deformation near where the San Andreas fault goes offshore, but deformation becomes extensional several km to the north off of the Golden Gate. Our new fault map serves as the basis for a 3-D finite element model that shows that the block between the San Andreas and San Gregorio fault zone is subsiding at a long-term rate of about 0.2–0.3 mm/yr, with the maximum subsidence occurring northwest of the Golden Gate in the area of a mapped transtensional basin. Although the long-term rates of vertical displacement primarily show subsidence, the model of coseismic deformation associated with the 1906 San Francisco earthquake indicates that uplift on the order of 10–15 cm occurred in the block northeast of the San Andreas fault. Since 1906, 5–6 cm of regional subsidence has occurred in that block. One implication of our model is that the transfer of slip from the San Andreas fault to a fault 5 km to the east, the Golden Gate fault, is not required for the area offshore of San Francisco to be in extension. This has implications for both the deposition of thick Pliocene–Pleistocene sediments (the Merced Formation) observed east of the San Andreas fault, and the age of the Peninsula segment of the San Andreas fault.

Uplift Mechanisms of Pipes Buried in Sand

Reliable design against upheaval buckling of offshore pipelines requires the uplift response to be predicted. This paper describes a model-scale investigation into the mechanisms by which uplift resistance is mobilized in silica sand, and illustrates how the observed mechanisms are captured in prediction models. A novel image-based deformation measurement technique has been used. The results show that peak uplift resistance is mobilized through the formation of an inverted trapezoidal block, bounded by a pair of distributed shear zones. The inclination of the shear zone is dependent on the soil density, and therefore dilatancy. After peak resistance, shear bands form and softening behavior is observed. At large pipe displacements, either a combination of a vertical sliding block mechanism and a flow-around mechanism near the pipe or a localized flow-around mechanism without surface heave is observed, depending on the soil density and particle size.

Experiments on strike-slip faulting and block rotation

Analogue model experiments investigating strike-slip faulting in zones of distributed shear deformation show that fault orientations and fault evolution indicate modifications of the stress field with increasing bulk shear strain. Dextral bulk shear deformation at low strain is accommodated dominantly by synthetic strike-slip faults (Riedel shears). At higher strain, secondary antithetic and synthetic faults develop mostly between earlier formed major Riedel shears. Closely spaced parallel antithetic faults (cross faults) delimit domains that rotate about vertical axes with continuing shear deformation. Rotation of fault-bounded domains results in sigmoidal antithetic faults that have a dip-slip component and a dip direction that changes along strike. There is good agreement between models and natural examples of distributed shear zones where block rotations about vertical axes have been documented.

Microearthquake seismicity and fault plane solutions around the Nékor strike-slip fault, Morocco

Three hundred and twenty eight earthquakes recorded in the course of 5 weeks in 1989 in the central Rif of northern Morocco provided no evidence for activity on the Nékor fault (the principal geological structure in the region) itself; most of the seismicity is located northwest of the Nékor fault. Most of the 41 fault-plane solutions show mainly strike-slip motion (left-lateral on N-S striking planes) with P axes trending N145°. This deformation is consistent with the N-S trending normal faults caused by the E-W Quaternary extension and with the present relative NW-SE convergence between Africa and Europe. The active deformation around the eastern Rif is better represented by a distributed shear zone than by a strike-slip fault.

Variscan transpressive inversion in the northwestern central rhenohercynian belt of western Germany

Important orogen-parallel strike-slip components during the convergence of the Variscan orogen of Central Europe have been increasingly recognized. These oblique-slip faults often develop from pre-existing tectonic boundaries and are influenced by pre-existing structural trends in the basement. In this paper evidence is presented for a 3–6 km wide distributed shear zone (here named the Monschau shear zone) in epizone metamorphic Cambro-Ordovician and Lower Devonian clastic rocks of the northwestern Rhenohercynian belt. Kinematic indicators including field observations of anomalous mullion and fold axes patterns, a set of two cleavages, stretching lineations, and shear and kink bands, microscopical observations of stretching-parallel asymmetric pressure shadows of quartz on prekinematic, epizone metamorphic pyrite porphyroblasts, and microfabrics of phyllosilicates by X-ray texture goniometry point to a sinistral tranpressive shear zone with emplacement of the southeast hangingwall under retrograde conditions. Coincidence of the anchizone-epizone metamorphic boundary with the change along strike from penetrative shearing to distributed thrusting suggests a ductile-brittle transition. The Monschau shear zone is situated on the backlimb of an allochthonous anticlinorium located above a basement ramp that corresponds roughly to a pre-orogenic synsedimentary normal fault. Its continuation shows up clearly in a DEKORP seismic reflection profile to a depth of 12–15 km.

Tertiary paleomagnetism of regions around the South China Sea

Paleomagnetic data from the Philippine Sea Plate (PSP) reveal a history of plate-wide clockwise (CW) rotation and northerly translation since the late Eocene about a nearby pole to the east. The motion has generated left lateral oblique convergence between the Philippine Sea Plate and S.E. Asia. Paleomagnetic data from Luzon in the northern Philippines show early Miocene CCW rotation followed by late Miocene CW rotation. In contrast, the Southern and Central Philippines display early Miocene CW rotation and unrotated late Miocene directions. These results define two different paleomagnetic domains with distinct post early Miocene histories. Pre-Miocene CCW rotation is suggested by data from Zambales, the Visayas and the Celebes Sea. In Borneo, a history of Tertiary CCW rotation has been found in Sarawak, and Sabah. Conflicting results have been reported from Kalimantan, some show no rotation with respect to Eurasia, while others give CCW rotations. In the Malaysian peninsula, the Segamat basalts and Kuantan dykes, of probable late Cretaceous early Tertiary age show CCW rotations similar to those seen in Sarawak. To the north peninsular Thailand, CW rotations have been found in two Miocene non-marine basins. Late Tertiary basalts from Northern and Central Thailand yield similar CW rotations while coeval flows on the Khorat plateau in Eastern Thailand are unrotated. The tectonic implications of the results remain problematical. In particular, the relative importance of true plate rotations and localized rotation of upper crustal blocks in distributed shear zones is unclear. The substantial region of CCW rotation in Borneo, the Celebes Sea and the Philippines is consistent with the broad features of the Holloway model, although the timing of the rotations precludes simple coherent rotation. The model must also be modified to include the effect of the left lateral oblique convergence between the PSP and Eurasia. The CW rotations seen in peninsular Thailand and Malaysia are consistent with the propagating extrusion tectonic model, but we cannot yet preclude a local transtensional origin for these rotations. The lack of Tertiary rocks in the peninsula has hampered attempts to relate these late Tertiary CW rotations to the late Mesozoic and Tertiary CCW rotations seen in Malaysia and Borneo.

Internal Deformation of Thrust Sheets, Sawtooth Range, Northwest Montana

A popular conception in the geological community is that sedimentary rocks of the Cordilleran foreland fold-and-thrust belt are unmetamorphosed and lack significant internal deformation. This idea has been incorporated by many workers into regionally balanced cross sections and palinspastic restorations that yield approximately 50% shortening. This amount of shortening is based entirely on plane strain, rigid body translations, and rotations of strata above a regional decollement that becomes structurally and stratigraphically deeper to the west. Detailed field studies in the Sawtooth Range (Disturbed belt) of northwestern Montana challenge the popular model of thrust belt internal deformation and shortening. A variety of deformation fabrics exist at the microscopic and mesoscopic scales, including pervasive axial-planar pressure-solution cleavage, boudinage, pencil structures, deformed oolitic and pelletal carbonate grains, distributed shear zones, and a host of extension and shear fractures. These features demonstrate that these rocks have experiences substantial internal finite strains. The internal strains are primarily the result of layer-parallel longitudinal shortening, coupled with pure and simple shear. These observations are important for the following reasons. (1) The amount of total shortening across the fold-and-thrust belt has been traditionally underestimated, producing errors in palinspastic facies maps. (2) Internal deformational fabrics are important controls onmore » reservoir heterogeneity and permeability. (3) Recognition of internal deformation requires the reevaluation of dynamic and thermochemical models for foreland thrust belt formation.« less