Outcrop-scale Faults Research Articles

Structural geology of the Mount Meager Volcanic Complex, BC, Canada: implications for geothermal energy and geohazards

The Mount Meager Volcanic Complex ( Qwe̓ lqwe̓ lústen or Mt. Meager) coincides tectonically with the intra-arc to back-arc transition zone and exhibits the loci of strain partitioning in response to a rapid change in orientation of the Pemberton and Garibaldi Arc segments that are coeval with a shift in Pacific plate motion after 5 Ma. This strain partition is manifested through development of a transpressional deformation from 5 to 1.9 Ma at the latitude of Mt. Meager. Mt. Meager is an active volcanic system with at least two explosive eruptions in the last 25 000 years, the most recent occurring around 2360 BP. Additionally, it is the site of the largest landslide in Canadian history, which occurred during the summer of 2010, originating from the southeastern side the massif. During early exploration at Mt. Meager, geothermal boreholes drilled to 3 km reached 270 °C but did not find sufficient permeability to sustain self-flowing conditions. To understand the geological challenges in Mt. Meager’s geothermal exploration, we analyzed outcrop-scale faults and folds, incorporating structural mapping, volcanic rock paleomagnetism, and radiometric dating to establish kinematic history and kinematic compatibility of structural geology features, including faults and folds. Our findings suggest that stress partitioning during the last 5 Ma resulted in the formation of a transpressional structure exhibited as an elongate and rhomboidal structure at Mt. Meager with anomalously high topographic elevations which led to east northeast–west southwest crustal shortening and exhumation of crystalline basement. This new structural geology model improves our understanding of the geothermal reservoir and potentially significant geohazards.

Synorogenic extension and extrusion in southern Taiwan

Paleostress inversion of 141 outcrop-scale faults across the eastern flank of the southern Central Range of Taiwan where leveling and GPS data suggest a steep gradient in rock uplift rates yields two main kinematic phases of deformation. Phase 1 consists of 93 normal faults that generally dip moderately northeast whereas phase 2 consists of 48 strike-slip faults that generally dip steeply west-northwest. Both phases record NE-trending sub-horizontal extension but different orientations of principal shortening; in phase 1, the principal shortening axis is nearly vertical whereas in phase 2 it plunges gently to moderately southeast. The northeast extension is consistent with extension directions obtained from GPS and earthquake focal mechanisms in the central part of the southern Central Range. However, these indicators of contemporary deformation also reveal more complicated states of stress along the eastern and western flanks of the range and in the deep crust southwest of the range. We interpret these more complicated stress states as reflecting “forceful extrusion” of the southern Central Range where the lower crust is being pinched between more rigid crustal blocks represented by the Peikang High and the Luzon Arc. In this context, the temporal progress from strike-slip to normal faulting observed in outcrops may reflect the advection of the rocks from lower to higher structural levels. The northeast extension normal faults can be interpreted that accommodating the lateral and vertical movement of the crust in the southern Central Range. Based on thermochronological data and the onset of extrusion in southwest Taiwan in the late Pleistocene, we infer that this SW extrusion process may be younger the 0.5 Ma.

Structural analysis of the Planchón-Peteroa Volcanic Complex: Insights for the geothermal system

The Planchón-Peteroa Volcanic Complex (PPVC) is located in the Transitional Southern Volcanic Zone of the Andean Ridge. Structural control of the main NNE-striking El Fierro fault system (EFFS) over the volcanic activity has been commonly assumed, although a paucity of evidence remains. The aim of this paper is to explore the relationship between the local stress field, the superficial structural setting and the geothermal fluid flow paths related to the volcanic complex. To conduct the structural analysis, this work combines remote sensing determination of lineaments, recognition of fracture patterns, with the inversion of kinematic indicators on outcrop scale faults, to finally evaluate the role of the local stress field over the 2D slip and dilation tendencies of the recognized structures. Mesoscale morphostructural lineaments present three main directions: ~E-W, ~NE-SW, and ~NW-SE. Major lineaments develop inflections, giving rise to left bends between the NW-SE to E-W along the strike, compartmentalized by ~ NNE trending lineaments. Strike-slip fault solutions with an ENE-WSW to NE-SW oriented shortening axis were obtained from fault-slip data analysis. The inversion of fault kinematic indicators constrains a Quaternary to recent strike-slip regime, with a ~ENE-WSW trending σ1, and a subvertical σ2. Under the defined local stress field, ~NE-SW and ~WNW-ESE oriented structures have high slip tendency, while those spanning from NE-SW to E-W are prone to dilate. Our analysis suggests that these transverse structures exert first-order controls on the location of Vergara Pass Hill, and Peñón River and Azufre River Valleys hydrothermal manifestations. It seems that intersection zones with submeridian structures, as EFFS, increase structural damage and facilitate hot fluid migration. Circulation would profit the most from ~ENE-WSW and ~NW-SE striking outcrop-scale faults and fractures associated with the damage zones.

Evolution of the Late Cretaceous Dadaepo Basin, SE Korea, in response to oblique subduction of the proto-Pacific (Izanagi/Kula) or Pacific plate

The Dadaepo Basin is a small Late Cretaceous sedimentary basin in SE Korea, located on the eastern margin of Asia. The basin is an isolated extensional basin situated between the NNE-striking Yangsan and Dongnae faults. The basin-fill sediments, named the Dadaepo Formation, consist of channelized conglomerates and sandstones intercalated with dominantly purple mudstones in the lower part. The upper part is dominated by fine- to coarse-grained tuffaceous sandstones and olive to dark gray mudstones with abundant volcanic interbeds. The formation unconformably overlies dacitic rocks dated at ca. 94Ma and is overlain by basaltic andesite dated at ca. 69Ma (Ar–Ar ages). The overall configuration of the strata of the Dadaepo Formation indicates syndepositional tilting of the basin floor to the north-northeast. A number of outcrop-scale faults are observed in the basin-fill sediments, of which the majority are NW-striking normal faults, including syndepositional growth faults. The orientations of mafic (magmatic) and clastic dikes, interpreted as being approximately contemporaneous with the deposition of the Dadaepo Formation, are also nearly parallel to the strikes of outcrop-scale normal faults. All these extensional structures consistently indicate NE–SW extension of the basin and obliquely intersect the basin-bounding Yangsan and Dongnae faults at angles of 40°–60°. It is thus concluded that the Dadaepo Formation was deposited in a pull-apart basin that subsided as a result of NNE-striking sinistral strike–slip faulting in the southeastern part of the Korean Peninsula during the Campanian (Late Cretaceous). This strike–slip faulting was related to north-northwestward oblique subduction of the proto-Pacific (Izanagi/Kula) or Pacific plate under the eastern margin of the Eurasian plate.

Interactions and growth of faults in an outcrop-scale system

Abstract Fault growth could be achieved by (1) synchronous increases in displacement and length or (2) rapid fault propagation succeeded by displacement-dominated growth. The second of these growth models (here referred to as the constant length model) is rarely applied to small outcrop-scale faults, yet it can account for many of the geometric and kinematic attributes of these faults. The constant length growth model is supported here using displacement profiles, displacement–length relationships and tip geometries for a system of small strike-slip faults (lengths of 1–200 m and maximum displacements of 0.001–3 m) exposed in a coastal platform in New Zealand. Displacement profiles have variable shapes that mainly reflect varying degrees of fault interaction. Increasing average displacement gradients with increasing fault size (maximum displacement and length) may indicate that the degree of interaction increases with fault size. Horsetail and synthetic splays confined to fault-tip regions are compatible with little fault propagation during much of the growth history. Fault displacements and tip geometries are consistent with a two-stage growth process initially dominated by propagation followed by displacement accumulation on faults with near-constant lengths. Retardation of propagation may arise due to fault interactions and associated reduction of tip stresses, with the early transition from propagation- to displacement-dominated growth stages produced by fault-system saturation (i.e. the onset of interactions between all faults). The constant length growth model accounts for different fault types over a range of scales and may have wide application.

Geology of the late Miocene south-eastern Volterra Basin (Northern Apennines, Italy)

We present a 1:10,000 scale geological map for the south-eastern sector of the Volterra Basin (Northern Apennines, Italy), together with supporting stratigraphic-structural data. The Volterra Basin consists of a major structural depression within the Northern Apennines hinterland, NNW-SSE-oriented and filled with more than 2000 m of late Miocene-Quaternary deposits. Its south-eastern sector is classically considered as a type area for late Tortonian non-marine strata, here mapped and refined in terms of internal stratigraphy adopting a scheme of depositional and lithostratigraphic units. Stratigraphic assessments helped in redefining the character of the lower boundary of the non-marine succession, as well as in mapping a newly recognized angular unconformity. Deformation structures affecting the basin fill include blind normal faults rooted to a deep detachment, outcrop-scale transtensional faults and clusters of gentle folds. Faults and folds appear to be kinematically linked. Our structural observations largely agree with those present in the literature, supporting a model of post-orogenic crustal stretching.

The evolution of the Quaternary Eupcheon fault, SE Korea

The Dadaepo Basin is a small Late Cretaceous sedimentary basin in SE Korea, located on the eastern margin of Asia. The basin is an isolated extensional basin situated between the NNE-striking Yangsan and Dongnae faults. The basin-fill sediments, named the Dadaepo Formation, consist of channelized conglomerates and sandstones intercalated with dominantly purple mudstones in the lower part. The upper part is dominated by fine- to coarse-grained tuffaceous sandstones and olive to dark gray mudstones with abundant volcanic interbeds. The formation unconformably overlies dacitic rocks dated at ca. 94 Ma and is overlain by basaltic andesite dated at ca. 69 Ma (Ar–Ar ages). The overall configuration of the strata of the Dadaepo Formation indicates syndepositional tilting of the basin floor to the north-northeast. A number of outcrop-scale faults are observed in the basin-fill sediments, of which the majority are NW-striking normal faults, including syndepositional growth faults. The orientations of mafic (magmatic) and clastic dikes, interpreted as being approximately contemporaneous with the deposition of the Dadaepo Formation, are also nearly parallel to the strikes of outcrop-scale normal faults. All these extensional structures consistently indicate NE–SW extension of the basin and obliquely intersect the basin-bounding Yangsan and Dongnae faults at angles of 40°–60°. It is thus concluded that the Dadaepo Formation was deposited in a pull-apart basin that subsided as a result of NNE-striking sinistral strike–slip faulting in the southeastern part of the Korean Peninsula during the Campanian (Late Cretaceous). This strike–slip faulting was related to north-northwestward oblique subduction of the proto-Pacific (Izanagi/Kula) or Pacific plate under the eastern margin of the Eurasian plate.

Neotectonics, drainage pattern and geomorphology of the orogen-parallel Upper Enns Valley (Eastern Alps)

Neotectonics, drainage pattern and geomorphology of the orogen-parallel Upper Enns Valley (Eastern Alps)The geomorphology and neotectonics of the Upper Enns Valley (Austria) in the Eastern Alps reveal the formation of a fault-controlled orogen-parallel valley. In the study area, the Eastern Alps have been under surface uplift since Early Miocene times. Quaternary processes such as uplift and cyclic glaciations likely interfere with neotectonic activity as the Upper Enns Valley follows the Salzach-Enns-Mariazell-Puchberg (SEMP) fault. The geomorphologically different landscapes comprise three main tectonic units: (1) the Austroalpine crystalline basement exposed in the Niedere Tauern, (2) the Austroalpine Paleozoic units (Greywacke Zone) and (3) the Dachstein Plateau dominated by Triassic carbonate successions. The Upper Pleistocene Ramsau Conglomerate overlying the Greywacke Zone on the northern slope of the Upper Enns Valley is a crucial element to reconstruct the evolution of the valley. A new14C date (uncalibrated) indicates an age older than 53,300 years, outside of the analytical limit of the methods. Provenance analysis of the Ramsau Conglomerate shows the Niedere Tauern as a source region and consequently a post-early Late Pleistocene dissection of the landscape by the Upper Enns Valley. Paleosurfaces at elevations of about 1100 m on the northern and southern slopes of the Upper Enns Valley allow us to estimate surface uplift/incision of about 2.5 mm/yr. Regularly oriented outcrop-scale faults and joints of the Ramsau Conglomerate document Pleistocene to Holocene tectonic deformation, which is consistent with ongoing seismicity. Paleostress tensors deduced from slickensides and striae of pre-Cenozoic basement rocks indicate two stages of Late Cretaceous to Paleogene deformation independent of the SEMP fault; the Oligocene-Neogene evolution comprises NW-SE strike-slip compression followed by E-W compression and Late Pleistocene ca. E-W extension, the latter recorded in the Ramsau Conglomerate.

Palaeostress perturbations near the El Castillo de las Guardas fault (SW Iberian Massif)

Use of stress inversion methods on faults measured at 33 sites located at the northwestern part of the South Portuguese Zone (Variscan Iberian Massif), and analysis of the basic dyke attitude at this same region, has revealed a prominent perturbation of the stress trajectories around some large, crustal-scale faults, like the El Castillo de las Guardas fault. The results are compared with the predictions of theoretical models of palaeostress deviations near master faults. According to this comparison, the El Castillo de las Guardas fault, an old structure that probably reversed several times its slip sense, can be considered as a sinistral strike-slip fault during the Moscovian. These results also point out the main shortcomings that still hinder a rigorous quantitative use of the theoretical models of stress perturbations around major faults: the spatial variation in the parameters governing the brittle behaviour of the continental crust, and the possibility of oblique slip along outcrop-scale faults in regions subjected to general, non-plane strain.

Kinematic analysis of a transtensional fault system: The Atuel depocenter of the Neuquén basin, southern Central Andes, Argentina

Abstract The Atuel depocenter of the Neuquen basin originated as an Upper Triassic to Lower Jurassic rift system, later inverted during the Andean contractional deformation. In order to study the extensional architecture and the kinematic evolution of this depocenter, we collected a large amount of field and sub-surface data, consisting of slip data from outcrop-scale normal faults, thickness and facies distribution within the synrift deposits, and structural data from angular and progressive unconformities. The Atuel depocenter has a NNW trend, showing a bimodal distribution of NNW and WNW major faults (first and second order faults). On the other hand, from kinematic indicators measured on outcrop-scale faults (third and fourth order faults), we found a mean NE internal extension direction, which is oblique to the general trend of the sub-basin. Taking these particular characteristics into account, we interpreted the Atuel depocenter as an oblique rift system. We evaluated two mechanisms in order to explain the development of this transtensional system: 1) reactivation of upper-crustal NNW-oriented Paleozoic shear zones, and 2) oblique stretching of a previous NNW-oriented lithospheric weakness zone.

Paleostress field reconstruction in the Oslo region

The geodynamic history of a region is archived in its geologic record which, in turn, may reflect deformation patterns that causally can be related to certain configurations of paleostresses. In the Oslo Region, the exposed geological record ranges from Precambrian high-grade metamorphic rocks through Cambro-Silurian sedimentary rocks to Permo-Carboniferous sedimentary and magmatic rocks, the latter being related to the development of the Oslo rift system. We investigate the kinematics of outcrop-scale faults to derive the diversity of paleostress states responsible for the observed strain. For this purpose, we combine different graphical and numerical approaches to separate heterogeneous fault-slip data sets and estimate the associated reduced stress tensors. A reduced stress tensor consists of the directions of the three principal stress axes with σ 1 ≥ σ 2 ≥ σ 3 and the ratio of principal stress differences, R = ( σ 2 − σ 3)/( σ 1 − σ 3). The present study gives evidence for three major regional paleostress fields that affected the Oslo Region: A compressional stress field with a NW–SE-directed maximum compression ( σ 1) has been identified as a Caledonian imprint. The most prominent regional stress field, however, is tensional, characterised by a WNW–ESE directed minimum compression ( σ 3), and related to the Permo-Carboniferous rifting. Later, the area was affected by a wrench regime with a roughly N–S directed σ 1 and a maximum age of Permian, whereas the absolute timing of this stress field is unclear due to the lack of exposed rocks younger than Permian. For a large number of estimated stress states, none of the principal axes are sub-vertical. These “oblique” paleostresses cannot be integrated into a common regional stress field but rather correspond to local effects of intense magmatic activity during the Permo-Carboniferous phase of rifting. The present study tends to show that the Oslo Region remained unaffected by major tectonic activity for much of the Mesozoic and Cenozoic – which is when the Central European Basin System to the south experienced several phases of intense deformation.

A plea for more caution in fault–slip analysis

The analysis of fault–slip data is a widely used tool in structural geology. This method consists of three major steps: (a) data acquisition by measurements in outcrops, (b) data separation into homogeneous subsets and calculation of the principal stress or strain axes with one of several available methods, and (c) data interpretation in a regional tectonic context, which includes the assignment of ages and the derivation of regional patterns. Each one of these three steps may introduce artefacts and misinterpretations. The major problem during step (a) is sampling bias due to unfavourable outcrop geometry or due to neglecting faults with subtle slip–sense indicators. Step (b) contains numerous possibilities to influence the calculation results, e.g. by arbitrary separation of data into subsets, choice of the analysis method, and uncritical use of software. Age assignment in step (c) can in most cases be made only indirectly, e.g. by the assumption that measured outcrop-scale faults have the same age as parallel map-scale faults or by the inference that subsets from different sites with parallel-oriented principal axes are syngenetic. Neither assumption is necessarily valid. With this paper we want to sharpen researchers' awareness of the problems related to fault–slip analysis.

Fine-scale partitioning of contemporary strain in the southern Walker Lane: Implications for accommodating divergent strike-slip motion

Treating the crust as a micropolar continuum and inverse modeling a high-quality catalog of earthquake focal mechanisms reveals that >km-scale heterogeneous strain is partitioned at ∼km-scales into pairs of plane strains with sub-perpendicular principal axes. These earthquakes occurred in a shallow crustal volume centered on the western Indian Wells Valley of eastern California, the site of divergent, right-lateral strike-slip motion along the boundary between the Sierran microplate and the southern Basin and Range province. The seismic events defining the plane strains occur in a diffuse seismic source zone and cannot be additionally subdivided on the basis of the type of strain they accommodate. The results indicate that seismic moment release for background earthquakes is partitioned into events associated with either fault-zone-perpendicular extension by normal faults or horizontal shearing by conjugate strike-slip faults. The spatial scales over which this partitioning occurs suggest that outcrop-scale brittle faults may in some cases record nearly perpendicular plane strains related to a single continuous deformation.

Damage zone characteristics of outcrop-scale NW-striking faults and their relation to the Red Sea rifting, Duwi area, Egypt

Damage zone characteristics of outcrop-scale NW-striking faults and their relation to the Red Sea rifting, Duwi area, Egypt

Fault and fracture systems in a fold and thrust belt: An example from Bolivia

This paper presents an outcrop-analog study of fractures in low-porosity sandstones in the Subandean thrust belt. We analyze the evolution of fault and joint systems in these sandstones, quantify their density along the structural trend, and identify the main factors controlling their variability.We show that faults and joints occur at different scales in a hierarchical fashion, as a consequence of progressive shearing. The first generation is an orthogonal set of joints, one parallel and the other perpendicular to the bedding azimuth. Shearing along these joints transformed them into small faults and created new sets of fractures, oblique to the bedding attitude. Linkage of these small faults facilitated the formation of larger faults with significant strike-slip offset. Shearing along bedding planes created subvertical splay joints that induced the formation of conjugate normal faults. In this thrust belt, subordinate strike-slip and normal faults are concomitant products of compressive deformation.This study documents a hierarchical correspondence between spacing of structural heterogeneities and stratigraphic architecture. We measured spacings of joints and outcrop-scale faults along the backlimb of the Abra del Condor anticline. We subdivided the structural discontinuities into four main groups: joints, small faults, intermediate faults, and fault zones. Spacing of joints, small faults, and intermediate faults has a lognormal distribution, whereas spacing of fault zones shows a normal distribution. The mean of these distributions is about the same as the thickness of the confining stratigraphic intervals. Therefore, spacing and dimensions of joints and faults have a first-order relationship to the thickness of the confining stratigraphic sequences.

Monocline development by oblique-slip fault-propagation folding: the East Kaibab monocline, Colorado Plateau, Utah

Fault relationships along a 50-km stretch of the East Kaibab monocline in southern Utah suggest that Late Cretaceous/early Tertiary development of the structure involved a significant component of right-lateral strike-slip displacement, accommodated by basement-rooted faulting and fault-propagation folding. Evidence of oblique slip is provided mainly by pervasive map-scale and outcrop-scale faults that define a shear zone occupying the steep east-dipping limb of the monocline for at least its northernmost 50 km. Dominant fault orientations are synthetic and antithetic to the shear zone, and accommodate reverse-right-lateral and reverse-left-lateral slip, respectively. Structural style within the shear zone changes character and increases in intensity with progressively lower structural and stratigraphic levels in the fold, suggesting that the shear zone propagated upward from a basement-rooted fault during monocline formation. We conclude that horizontal, ENE-directed, Laramide compression drove reverse-right-lateral slip on the basement fault zone beneath the developing East Kaibab monocline. The resulting transpressional fault-propagation fold is marked in southern Utah by 1600 m of reverse displacement and possibly 8000 m of right-lateral displacement across the shear zone and associated monoclinal flexure.

Flexure and faulting of sedimentary host rocks during growth of igneous domes, Henry Mountains, Utah

A sequence of sedimentary rocks about 4 km thick was bent, stretched and uplifted during the growth of three igneous domes in the southern Henry Mountains. Mount Holmes, Mount Ellsworth and Mount Hillers are all about 12 km in diameter, but the amplitudes of their domes are about 1.2, 1.85 and 3.0 km, respectively. These mountains record successive stages in the inflation of near-surface diorite intrusions that are probably laccolithic in origin. The host rocks deformed along networks of outcrop-scale faults, or deformation bands, marked by crushed grains, consolidation of the porous sandstone and small displacements of sedimentary beds. Zones of deformation bands oriented parallel to the beds and formation contacts subdivided the overburden into thin mechanical layers that slipped over one another during doming. Measurements of outcrop-scale fault populations at the three mountains reveal a network of faults that strikes at high angles to sedimentary beds which themselves strike tangentially about the domes. These faults have normal and reverse components of slip that accommodated bending and stretching strains within the strata. An early stage of this deformation is displayed at Mount Holmes, where states of stress computed from three fault samples correlate with the theoretical distribution of stresses resulting from bending of thin, circular, elastic plates. Field observations and analysis of frictional driving stresses acting on horizontal planes above an opening-mode dislocation, as well as the paleostress analysis of faulting, indicate that bedding-plane slip and layer flexure were important components of the early deformation. As the amplitude of doming increased, radial and circumferential stretching of the strata and rotation of the older faults in the steepening limbs of the domes increased the complexity of the fault patterns. Steeply-dipping, map-scale faults with dip-slip displacements indicate a late-stage jostling of major blocks over the central magma chamber. Radial dikes pierced the dome and accommodated some of the circumferential stretching.