Subhorizontal Fault Research Articles

Geology of the Kokchetav UHP‐HP metamorphic belt, Northern Kazakhstan

Abstract Ultrahigh‐pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh‐pressure–high‐pressure (UHP–HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich‐like; that is, the UHP–HP units are separated from underlying low‐P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top‐to‐the‐south sense of shear along the top, and top‐to‐the‐north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.

Rapid deformation of Kilauea Volcano: Global Positioning System measurements between 1990 and 1996

Campaign Global Positioning System (GPS) measurements from 1990 to 1996 are used to calculate surface displacement rates on Kilauea Volcano, Hawaii. The GPS data show that the south flank of the volcano, which has generated several large earthquakes in the past 3 decades, is displacing at up to ∼8 cm/yr to the south‐southeast. The summit and rift zones are subsiding, with maximum subsidence rates of ∼8 cm/yr observed a few kilometers south of the summit caldera. Elastic dislocation modeling of the GPS data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone, fault slip along a subhorizontal fault near the base of the volcano, and deflation near the summit caldera. A nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry. There is a broad range of model geometries that fit the data reasonably well. However, certain models can be ruled out, including those that have shallow rift opening or shallow fault slip. Some offshore, aseismic slip on a fault plane that dips between 25° northnorthwest and 8° south‐southeast is required. Best fitting slip and rift opening rates are 23–28 cm/yr, although rates as low as 10 cm/yr are permitted by the data.

Regional north-south Terengganu faults: Besut, Kampung Buluh and Ping-Teris

Three major, roughly north-striking fault zones in Terengganu outcrop as multiple, each tens to hundreds of metres wide intervals of mylonite and phyllonite amoilg unfaulted or less faulted rock sequences. Drag features and subhorizontal fault striations establish that these regional fractures: Besut fault zone, Kampung Buluh fault belt, and Ping-Teris fault zone are strike-slip faults. Lateral displacements on the first two mentioned had been left lateral, while the Ping-Teris fauit zone moved in right-lateral sense. The amounts of displacement are not known. Along their strikes, the three fault zones appear to continue as the wide belt of north to north-northwest striking faults in the north-western end of the Malay basin. Total traceable map-lengths of the fault zones reach 300 km each. In the basin, these faults transect upper Miocene and older rocks, but onshore Terengganu the three fault zones are only known to be of post-granitoid age, that is post-Carnian. There is no evidence that the onshore faults were active during the Cenozoic. There is indication along the Ping-Teris fault zone that its pre-Tertiary slip sense was right-lateral, while during the Cenozoic its subsea extension in the Malay basin moved left-laterally.

Geology of the Upheaval Dome impact structure, southeast Utah

Two vastly different phenomena, impact and salt diapirism, have been proposed for the origin of Upheaval Dome, a spectacular scenic feature in southeast Utah. Detailed geologic mapping and seismic refraction data indicate that the dome originated by collapse of a transient cavity formed by impact. Evidence is as follows: (1) sedimentary strata in the center of the structure are pervasively imbricated by top‐toward‐the‐center thrust faulting and are complexly folded as well; (2) top‐toward‐the‐center normal faults are found at the perimeter of the structure; (3) clastic dikes are widespread; (4) the top of the underlying salt horizon is at least 500 m below the surface at the center of the dome, and there are no exposures of salt or associated rocks of the Paradox Formation in the dome to support the possibility that a salt diapir has ascended through it; and (5) planar microstructures in quartz grains, fantailed fracture surfaces (shatter surfaces), and rare shatter cones are present near the center of the structure. We show that the dome formed mainly by centerward motion of rock units along listric faults. Outcrop‐scale folding and upturning of beds, especially common in the center, are largely a consequence of this motion. We have also detected some centerward motion of fault‐bounded wedges resulting from displacements on subhorizontal faults that conjoin and die out within horizontal bedding near the perimeter of the structure. The observed deformation corresponds to the central uplift and the encircling ring structural depression seen in complex impact craters.

Higher-degree moment tensor inversion using far-field broad-band recordings: theory and evaluation of the method with application to the 1994 Bolivia deep earthquake

We present a method to estimate parameters of the extended earthquake source using higher-degree moment tensors at 27 centroid locations. We show that a Taylor series expansion of Green’s functions around a single centroid is not accurate enough when working with seismic wave periods and wavelengths in the range of the rupture duration and spatial extent of the fault, respectively. Introducing a grid of 27 centroid locations on the fault and using higher-degree moment tensors we are able to model adequately body and surface waves with periods and wavelengths smaller than the rupture duration and fault dimensions. Under simplifying assumptions an iterative inversion scheme is coded to estimate parameters of planar, Haskell-type faults. Realistic inversion examples for deep and shallow earthquakes show that uni- and bidirectional rupture models, rupture direction, fault and auxiliary plane and kinematic source dimensions and × can be constrained with teleseismic body and/or surface waves. The application to the deep Bolivia event indicates a subhorizontal fault plane. Unidirectional rupture to the north is slightly preferred. The rupture duration of 25s and fault dimensions of 47 × 25km agree well with the estimates for the main pulse moment release given in other studies.

On the interpretation of subevents in teleseismic waveforms: The 1994 Bolivia deep earthquake revisited

We evaluate the methodologies that aim at resolving the space‐time slip distribution during large earthquakes. In one common approach, the source process of an event is assumed to be composed of a series of subevents. Critical points in teleseismic body waves, such as peaks and coherent arrivals, are viewed as discrete points in the spatiotemporal history of an earthquake. We show that these features can be interpreted as the instantaneous centroid locations of the moment release history. We demonstrate this result with an analysis of the Mw=8.2 Bolivia 1994 deep event. We reconstruct the slip distribution in two contrasting ways. First, broadband P waves are interpreted as being composed of subevents. Five peaks in the moment release rate are located relative to the onset of the rupture using the azimuthal variations of onset‐to‐peak times. Second, we invert broadband P waves for the slip distribution on a subhorizontal fault. We use a nonlinear tomographic imaging technique called simulated annealing and an L1 misfit norm. Rupture velocities appear to be <2 km s−1, and largest slip values are found to the east and north of the onset in a 40 by 50 km region. The instantaneous centroids of the moment release history correspond closely to the locations found using the subevent analysis. However, especially near the middle and the end of the rupture, the instantaneous centroid locations are in regions of little moment release. We discuss how the subevent analysis and the instantaneous centroid approach lead to dissimilar interpretations of rupture processes, especially for events with length to width ratios near unity. Another result of this study is that the rupture of the 1994 Bolivia event appears to end 50 km to the northwest of the onset, implying that the slip area is larger that previously thought. Comparing the slip distribution with the aftershock distribution suggests that more than one physical mechanism is necessary to account for the rupture process of this great deep event.

Finite fault source study of the Great 1994 Deep Bolivia Earthquake

We apply an empirical Green's function method to recordings of teleseismic body waves from the June 9, 1994 deep‐focus Bolivia earthquake. The far‐field source time functions reveal that rupture occurred on a sub‐horizontal fault plane. The main rupture began some 25 km to the east of the hypocenter, consists of three large subevents, and covers a total area of about 60×40 km². The static stress drop is quite large, more than 150 MPa, and the obtained rupture velocity is less than 2 km/sec. While the total dimensions of the earthquake are large compared to estimates of the width of a metastable olivine wedge at this depth, the moment release is confined largely to a narrow band along an EW axis, and may be compatible with phase transitions within a thickened slab core.

Seismic reflections from the near‐vertical San Andreas Fault

Major near‐vertical strike‐slip faults should reflect seismic energy due to velocity contrasts within and across the fault zone, analogous to sub‐horizontal faults commonly imaged by both multichannel and wide‐angle reflection surveys. An increase of velocity with depth in the upper crust allows such reflections to be detected in surface seismic surveys. We have observed fault reflection events from both sides of the San Andreas Fault in long‐offset common‐receiver gathers acquired near San Francisco. The events can be traced to the known surface position of the fault and provide constraints on the subsurface position to 3–7 km depth. Such reflections can be used to find the geometry of major near‐vertical faults, even in areas without active seismicity, providing information crucial to the understanding of the tectonics and seismic hazards of strike‐slip faulting.

Palaeomagnetic evidence for large-magnitude, low-angle normal faulting in a metamorphic core complex

CONTROVERSY exists over whether large-magnitude extensional deformation can occur along low-angle, master detachment (normal) faults1–16. Large amounts of Cenozoic extension occurred in the Basin and Range province of the western United States, and master detachment faults related to this extension are currently exposed as subhorizontal structures along crustal arches known as metamorphic core complexes (MCCs)10,11,17. One set of models suggests that MCCs represent crustal-scale blocks bounded by originally high-angle normal faults which have tilted to sub-horizontal attitudes1–6. More dynamic models propose isostatically induced flexural tilting of an initial subhorizontal fault, to an active high-angle fault, to final abandonment as a subhorizontal structure7–9; these models require MCC detachment faults and their footwalls to have been tilted by 30–60°. Alternatively, MCC detachment faults may have originated as subhorizontal or low-angle structures10–16. Here we test these models with palaeomag-netic data from undeformed portions of the South Mountains, a typical Cenozoic MCC in the southern Basin and Range province18–24. Comparison with time-averaged expected directions of the geomagnetic field25–27 yields no evidence for tilting of the footwall. We conclude that the South Mountains master detachment fault was active as a low-angle extensional structure, with a dip of ∼10°.

Uplift in the Nepal Himalaya revealed by Spirit leveling

A 250‐km‐long first‐order leveling line linking the geodetic surveys of Tibet and India through the Kingdom of Nepal has been measured twice in the past 15 years. Changes in relative bench mark elevation projected along a 200 km line orthogonal to the mean structural trend in the Himalaya indicate a pattern of uplift with wavelengths of 30–50 km not related to earthquake occurrence. Relative vertical velocities of bench marks of up to 4 mm/year show no evident correlation with level line slope. An absence of microearthquake data prevents localized uplift from being attributed to slip on mapped or buried faults, although circumstantial evidence suggests this is likely. In the North, in line with the high Himalayan peaks, relative uplift rates of 4 ± 1 mm/yr and a wavelength of 50 km are coincident with a steepening in the Main Boundary Thrust and the Moho proposed by Lyon‐Caen and Molnar [1985]. In the South, uplift rates of 2 ± 0.3 mm/yr with a wavelength of 30 km correspond to active folding of an anticline on the southern flank of the Lesser Himalaya. Observed tilt rates of 0.06 μrad/year are similar to the long term tilt rates needed to produce the 40–70° flanks of the anticline. A range of detachment geometries and slip velocities can generate the observed deformation field, but geologically realistic models fall into a group of horizontal to shallow dipping, sub‐horizontal faults between 12 and 6 km in depth. No unique solution exists, although the wavelength of the observed deformation field permits models with significant slip closer than 4 km to the surface to be excluded.

Structural geology of the Lardeau Group near Trout Lake, British Columbia: implications for the structural evolution of the Kootenay Arc

The Lardeau Group is a heterogeneous assemblage of lower Paleozoic eugeoclinal strata present in the Kootenay Arc in southeastern British Columbia. It is in fault contact with lower Paleozoic miogeoclinal strata for all or some of its length along a structure termed the Lardeau shear zone. The Lardeau Group was deformed prior to mid-Mississippian time, as manifested by layer-parallel faults, folds, and evidence for early greenschist-facies metamorphism. Regional constraints indicate probable Devono-Mississippian timing of orogeny, and possible juxtaposition of the Lardeau Group over miogeoclinal strata along the Lardeau shear zone at this time. Further ductile deformation during the Middle Jurassic Columbian orogeny produced large folds with subhorizontal axes, northwest-striking foliation and faults, and orogen-parallel stretching lineations. This deformation was apparently not everywhere synchronous, and may have continued through Late Jurassic time northeast of Trout Lake. This was followed by Cretaceous(?) dextral strike-slip and normal movement on the Lardeau shear zone and other parallel faults. While apparently the locus of several episodes of faulting, the Lardeau shear zone does not record the accretion of far-travelled tectonic fragments, as sedimentological evidence ties the Lardeau Group and other outboard units to the craton.

Seismic quiescence at Parkfield due to detachment faulting

ON the San Andreas fault near Parkfield, California, rates of seismicity1,2 and geodetic line shortening3,4have been lower since 1986 than before. Wyss et al.1,3interpret the rate decreases as precursors to an imminent moderate earthquake (magnitude M = 5.5−6) and estimate the earthquake time to be March 1991 ± 1 yr. The earthquake was previously predicted to occur in 1988 ± 5 yr based on five of six similar earthquakes which occurred at intervals ∼22 yr, the last one in 19665. Here I present a model that attributes the rate changes to the initiation of slip on a subhorizontal detachment fault under the pending rupture area. In this model, the slip starts late in the seismic cycle, and diminishes the loading rate on the nearby San Andreas, thus acting as a buffer.

Active normal faulting and crustal extension

Summary A world-wide review of fault-plane solutions and focal depths for large normal-faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6–15 km on faults dipping between 30 and 60°. In the few cases where levelling or seismic data are good enough, these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust. There is evidence that, in some cases, as a result of the enormous transitory increase in strain rate during large earthquakes, rupture continues into the normally ‘ductile’ lower crust on surfaces with substantially gentler dips. These low-angle surfaces may be analogous to some of the ‘detachments’ seen in metamorphic core complexes of the western USA, but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the ‘detachments’ and the blocks on either side. Such contrasts, however, are unlikely to introduce substantial curvature to an originally planar shear zone. If the observed spread in active normal-fault dips is caused by rotation of the faults and the blocks they bound during extension, then a maximum β value of 1.7 can be accommodated by seismic activity on a single generation of normal faults. With continued extension, either a new generation of steeper faults, cutting the rotated first faults, is likely to form, or the deformation will continue aseismically on faults dipping at less than 30°. Large earthquakes have not been observed to nucleate on very low-angle (<20°) normal faults within the continental crust anywhere in the world. Such faults can of course move aseismically, but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active. Thus extensional models that require concentrated simple shear on large sub-horizontal faults within the brittle upper crust will also require a spatial separation between aseismic, very low-angle faulting and seismic high-angle faulting. Since seismogenic high-angle faults dominate the topography of extending regions, upper-crustal very-low-angle faulting, if it occurs on a large scale, presumably does so in areas that are relatively flat.

Geologic and gravity evidence for thrust emplacement of the Smartville ophiolite

Detailed structural relationships at the margins of ophiolites can provide important information about the mode of emplacement of ophiolitic terranes. In this study we use geologic mapping and gravity data to determine the structural and tectonic relations between a Middle to Late Jurassic volcanic arc complex (the Smartville Ophiolite block) and a pre‐Late Jurassic melange terrane in the northern Sierra Nevada foothills. The volcanic arc complex overlies the melange along a complex fault zone. Although this fault zone is steeply dipping wherever it is observed, map patterns suggest that the Smartville Upper Volcanic unit was originally emplaced along a subhorizontal fault that has been subsequently deformed. Two‐dimensional interactive computer analysis of gravity data from over 200 stations shows that the Smartville Upper Volcanic unit is a relatively shallow feature with a maximum thickness ranging from 0.5 to 3 km and that it was thrust as a sheet over the previously deformed melange. Both units were subsequently folded, giving rise to the steeply dipping faults observed at the surface. Recognition of low‐angle thrusting along the northern margin of the Smartville Ophiolite block has important implications for the mode of emplacement of the block. The most plausible mechanism for the thrusting of this particular oceanic island arc sequence onto the continental margin involves a subduction zone dipping westward beneath the Smartville Ophiolite block, similar to the present‐day situation in the Molucca Sea collision zone.

The upper Batinah Complex, Oman: allochthonous sediment sheets above the Semail ophiolite

The upper Batinah Complex consists of allochthonous sedimentary sheets that structurally overlie both the Semail ophiolite and its cover of mélange (lower Batinah Complex). Sequences represent a former sedimentary transition across the outer part of a passive Mesozoic continental margin. From proximal to distal, units include redeposited quartzitic sandstones and shallow-water limestones, large carbonate buildups floored by Triassic mafic lavas, and pelagic radiolarites or hemi-pelagic calcilutites also underlain by Triassic mafic lavas. Sheets of these upper Batinah Complex sequences were emplaced onto the upper surface of the Semail ophiolite in Late Cretaceous time, during or soon after the advance of the Semail Nappe onto the Arabian margin. Emplacement was along subhorizontal fault zones, with sheets of distal facies followed by successively more proximal sheets. Structural style suggests a low overburden during deformation, with ductile flow of argillaceous material but more brittle behavior of other lithologies. Large recumbent folds are present. Fold orientations imply sheet transport from between northeast and southeast.We conclude that the Batinah sheets represent part of the passive margin to a continental mass originally northeast of the main Arabian margin, but which is no longer present. We favour high-level gravity emplacement of Batinah sheets, and argue against trench accretion as an emplacement mechanism for either the sheets or the Semail Nappe itself.