Analysis Of Fault-slip Data Research Articles

Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)

New regional seismological data acquired in Colombia during 1993 to 1996 and tectonic field data from the Eastern Cordillera (EC) permit a reexamination of the complex geodynamics of northwestern South America. The effect of the accretion of the Baudó‐Panama oceanic arc, which began 12 Myr ago, is highlighted in connection with mountain building in the EC. The Istmina and Ibagué faults in the south and the Santa Marta‐Bucaramanga fault to the northeast limit an E‐SE moving continental wedge. Progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the Cordilleras (northward of 5°N) and transpressive deformation in the Santander Massif. Crustal seismicity in Colombia is accurately correlated with active faults showing neotectonic morphological evidences. Intermediate seismicity allows to identify a N‐NE trending subduction segment beneath the EC, which plunges toward the E‐SE. This subduction is interpreted as a remnant of the paleo‐Caribbean plateau (PCP) as suggested by geological and tomographic profiles. The PCP shows a low‐angle subduction northward of 5.2°N and is limited southward by a major E‐W transpressive shear zone. Normal oceanic subduction of the Nazca plate (NP) ends abruptly at the southern limit of the Baudó Range. Northward, the NP subducts beneath the Chocó block, overlapping the southern part of the PCP. Cenozoic shortening in the EC estimated from a balanced section is ∼120 km. Stress analysis of fault slip data in the EC (northward of 4°N), indicates an ∼E‐SE orientation of σ1 in agreement with the PCP subduction direction. Northward, near Bucaramanga, two stress solutions were observed: (1) a late Andean N80°E compression and (2) an early Andean NW‐SE compression.

Tectonic significance of fault-slip data

The statistical analysis of populations of minor faults is commonly used by structural geologists working in areas of brittle rock. It is based on measurements of fault attitude, direction and sense of slip. At individual sampling localities, results are classically interpreted as indicators of stress or strain rate fields, assuming a homogeneous stress or strain rate tensor, respectively. However, fault patterns are expected to vary with time because of displacements, rigid rotations, and internal strains, which generally occur along boundaries of fault-bounded blocks as deformation proceeds. Thus, in the general situation where early faults accumulate displacements and rigid rotations, and where new faults develop during progressive deformation, fault-slip data can be rather complex and variable in space, and reflect neither local stress or strain rate tensors, nor finite strains and finite rotations in a simple way. We use two examples of faulted regions to illustrate the spatial variability of fault-slip data. This can be due to local complications at the edges of fault blocks, or to complex kinematic conditions at regional boundaries. Such complications may make it difficult to deduce a consistent and simple pattern of either stresses or strain rates. Instead, our results suggest that information contained in fault-slip data can be pertinent to finite deformation. In particular, the principal axes that we have deduced from the analysis of fault-slip data are consistent with the finite displacements at block boundaries that we have calculated by numerical restoration.

Compressional- and transpressional-stress pattern for Pliocene and Quaternary brittle deformation in fore arc and intra-arc zones (Andes of Central and Southern Chile)

Kinematic analysis of fault slip data for stress determination was carried out on Late Miocene to Quaternary rocks from the fore arc and intra-arc regions of the Chilean Andes, between 33° and 46° south latitudes. Studies of Neogene and Quaternary infilling (the Central Depression), as well as plutonic rocks of the North Patagonian Batholith along the Liquiñe–Ofqui Fault Zone, have revealed various compressional and/or transpressional states of stress. In the Pliocene, the maximum compressional stress ( σ 1) was generally oriented east–west. During the Quaternary, the deformation was partitioned into two coeval distinctive states of stress. In the fore arc zone, the state of stress was compressional, with σ 1 oriented in a N–S to NNE–SSW direction. In the intra-arc zone the state of stress was transpressional with σ 1 striking NE–SW. Along the coast, in one site (37°30′S) the Quaternary strain deformation is extensional, with an E–W direction, which can be explained by a co-seismic crustal bending readjustment.

Quaternary transfer faulting and belt front deformation at Pakuashan (western Taiwan)

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along‐strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along‐strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left‐lateral tear fault or transfer fault that offsets two distinct frontal thrust‐and‐fold sheets.

Kinematics of Late Pliocene-Quaternary Normal Faulting in the Southeastern End of the Gediz Graben, Western Anatolia, Turkey

The kinematic analysis of fault-slip data obtained from lower Pliocene and Pleistocene deposits indicates two successive extensional events in the southeastern end of the Gediz graben. The late Pliocene N-S extensional phase was followed by a NNE-SSW extension in the Pleistocene. This change in extension direction from N-S to NNE-SSW is indicated by slip vectors on active fault planes and historic fault offsets. This younger extensional event is still active, as suggested by recent seismic activity and focal mechanisms of earthquakes in the region. The slip regime has important implications for the Neogene tectonic evolution of western Anatolia.

The active Nea Anchialos Fault System (Central Greece): comparison of geological, morphotectonic, archaeological and seismological data

The Nea Anchialos Fault System has been studied integrating geological, morphological, structural, archaeological and seismic data. This fault system forms the northern boundary of the Almyros Basin which is one of the Neogene-Quaternary tectonic basins of Thessaly. Specific structural and geomorphological mapping were carried out and fault-slip data analysis allowed the Late Quaternary palaeo-stress field to be estimated. The resulting N-S trending purely extensional regime is consistent with the direction of the T-axes computed from the focal mechanisms of the summer 1980, Volos seismic sequence and the April 30, 1985 Almyros earthquake. A minor set of structural data indicates a WNW-ESE extension which has been interpreted as due to a local and second order stress field occurring during the N-S regional extension. Furthermore, new archaeological data, discovered by the author, have improved morphology and tectonics of the area also allowing a tentative estimate of the historic (III-IV century AD. to Present) fault slip rate. Several topographic profiles across the major E- W topographic escarpment as well as along the streams, have emphasised scarps and knick-points, further supporting the occurrence of very recent morphogenic activity. In the last section, the structural, morphological and archaeological data are compared with the already existing seismological data and their integrated analysis indicates that the Nea Anchialos Fault System has been active since Lower(?)-Middle Pleistocene.

Active tectonism in the intracontinental Middle Atlas Mountains of Morocco: synchronous crustal shortening and extension

Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE–SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE–SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa.

Kinematics of Tertiary deformation in the basement-involved fold-thrust complex, western Nordenskiøld Land, Svalbard: tectonic implications based on fault-slip data analysis

Kinematic analysis based on the interpretation of small-scale fault and fold data supports a kinematic evolution history involving heterogeneous crustal shortening and uplift, and subsequent extension (collapse) for the major, west coast fold-thrust complex in western Nordenskiøld Land, Svalbard. This deformation can be divided into three main kinematic events. Stage 1 represents an early, distinctive, NNE-SSW-oriented contractional episode that generated layer parallel and low-angle thrusts and internal folds. Stage 1 structures (population 1) are arranged in an apparent en-échelon geometry and oblique direction relative to the major NNE-SSW-trending fold-thrust complex of stage 2 affinity. Stage 2 structures include ENE-verging chevron folds and steeply WSW-dipping thrusts and duplexes, that evolved during a progressive WSW-ENE-oriented contractional episode. Initial (pre-fold) stratal shortening (stage 2a) was followed by a continuous buildup of the fold-thrust complex and general crustal thickening (syn-fold, stage 2b), until a supercritical height/thickness was reached (post-fold, stage 2c). At this stage a change in the stress field caused failure of the fold-thrust complex, and continued NE-SW-directed shortening was accommodated as vertical strike-slip faults (population 2c). Late W-E-to WSW-ENE-directed extension, the stage 3 episode, was probably related to collapse of the overthickened stage 2 fold-thrust complex. The detailed reconstruction of kinematic events in western Nordenskiøld Land supports initial breakup of the Greenland-Svalbard area in Early Paleocene (?) times during dextral, NNE-SSW-directed transpression (stage 1). The major deformational episode (stage 2), probably of mid-Paleocene to Eocene age, was characterized by progressive WSW-ENE-directed shortening and was related to decoupled deformation where broad zones of convergent strain were linked to narrow strike-slip zones. This strain partitioning is considered to have begun when the Hornsund Fault Zone widened into and reactivated the basin-bounding faults of the Carboniferous St. Jonsfjorden Trough. The latest kinematic episode (stage 3) is likely of Eocene age, and may be ascribed to extensional collapse in the hinterland of the western Spitsbergen orogenic wedge within a regional, dextral transpressive setting.

Palaeostress analysis and geodynamical implications of Cretaceous-Tertiary faulting in Kent and the Boulonnais

Detailed studies of fracture patterns and palaeostress reconstruction based on fault slip data analysis allow identification of five main palaeostress fields in the Cretaceous formations of the Boulonnais and Kent areas. These stress patterns correspond to six successive tectonic events: (1) a N–S compression/E–W extension of early Cenomanian age, (2) an E-W extension of late Cenoma-nian age, and (3) four post-Santonian events with N–S, E–W, NW–SE and NE–SW extensions, respectively. The first event, interpreted from conjugate strike-slip faults is related to the beginning of the Sub-Hercynian phase and linked to a right-lateral motion of the Nord-Artois Shear Zone. The first E-W extensional event and the N-S extensional event could be related to the last rifting episodes of the North Sea graben and English Channel (Wessex basin). The others post-Santonian extensional events are related to Late Cretaceous, Tertiary and Recent tectonic episodes.

The influence of dikes on auriferous shear zone development within granitoid intrusions: the Bourlamaque pluton, Val-d'Or district, Abitibi greenstone belt

Shear-zone-related gold–quartz veins in granitoid intrusions are commonly intimately associated with mafic dikes, which may have a profound influence on the localization, orientation, and kinematics of auriferous shear zones. The Bourlamaque pluton of the Val-d'Or district contains several economic auriferous shear zones, most of which follow and overprint diorite dikes. Mineralization in all deposits consists of quartz–tourmaline–pyrite veins in reverse- oblique orientation with a significant range of strike, dip, and slip direction. The geometry and kinematics of shear zone and vein array within the pluton is more complex than the simple conjugate pattern predicted for a deforming homogeneous intrusion. The stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly-directed compression recorded by similar shear zones outside the pluton. This indicates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes, which acted as weak layers that were activated during subsequent deformation, showing the importance of layer anisotropy in auriferous shear zone development.The plunges of orebodies bear simple geometric relationships to the slip direction along a host shear zone: these are generally perpendicular to, or in some cases parallel to, the slip direction. Knowledge of the slip directions along activated dikes would therefore allow prediction of the possible plunge(s) of orebodies at early stages of exploration programs. Slip direction along an activated layer is controlled by the orientation of the layer with respect to the stress field and by the relative magnitudes of the three principal stresses. Using techniques developed for analysis of fault slip data, both parameters can be determined, provided there is a sufficient database, and slip direction can be predicted for activated layers of any orientations.

Rotations in the northeastern collision belt of Taiwan: Preliminary results from paleomagnetism

We present the results of determinations of the ellipsoids of magnetic susceptibility anisotropy and of the paleomagnetic directions for 17 sedimentary rock sites of the northeastern collision belt of Taiwan. The results of magnetic fabric determinations indicate that magnetic lineations have developed in most sites. Two major groups of lineations have been reconstructed: the trends are distributed around the NNE-SSW direction for the first one and around the ENE-WSW direction for the second one. Comparing these directions with the results of tectonic analysis of fault-slip data in the same area (Angelier et al., 1990), the first group of magnetic lineation is found to be perpendicular to the maximum compression axis σ 1 of the compressional event number 3 (direction 113). This direction of major compression corresponds to the present-day convergent direction of the Philippine Sea plate relative to the Eurasian plate. The second group of magnetic lineations is perpendicular to another axis σ 1 of a major compressional event number 4 (direction 150). Because no relationship between magnetic lineations and paleocurrent distributions has been found, we consider that the magnetic fabric in our samples principally results from the influence of tectonic paleostress. Our paleomagnetic results indicated reversed polarities in sampling sites. The declinations range from 190° to 220°, indicating that a clockwise rotation of at least 15° had occurred in the northeastern part of Taiwan. This supports previous results principally based on microtectonic analyses that led Angelier et al. (1986, 1990) to propose that a clockwise rotation of approximately 20° had occurred in the northern Taiwan belt. This clockwise rotation is probably related to the N-S opening of the Okinawa Trough as well as to the SE-NW compression induced by the collision between the northwestern tip of the Philippine Sea plate and the Eurasian plate in northern Taiwan.

Some constraints on the kinematics of the Tertiary Piemonte Basin (northwest Italy)

An analysis of fault-slip data in Oligocene and Miocene sediments of the Piemonte Basin (northwest Italy) shows that a nearly continous sweep of shortening directions exists between the Northern Apennines and the Maritime Alps: transport directions change from N-S to NW-SE near the western termination of the Northern Apennines, through W-E in the Ligurian Alps, to SW-NE in the Maritime Alps. (N)NW directed thrusting along the front of the Monferrato culmination fits within this general kinematic framework. In addition, basement thrusts and transform faults were not sealed by an Oligocene transgression but remained active throughout the Oligocene and Miocene. These data provide a distinct constraint on the Tertiary kinematics and dynamics of the Alps-Apennine system.

Paleostress analysis as a key to margin extension: The Penghu Islands, South China Sea

The Penghu Islands belong to the Eurasian passive margin, west of the collision zone of Taiwan. From south to north, the South China Sea basins become narrower and disappear in the Taiwan Strait area. The presence of the Penghu Islands allow description of the tectonic evolution of the margin through the reconstruction of tectonic paleostress orientation based on analysis of fault slip data collected in the field. Brittle structures have been studied on four islands in Neogene basalt flows with thin interbedded sediments. Sets of minor striated faults, tension gashes, dikes and joints enabled us to reconstruct the orientation of late Cenozoic stresses. Two main extensional tectonic events were thus identified: a N-S extension dominated during the Middle-Late Miocene (especially in the northern Penghu Islands), and a more recent NW-SE extension prevailed during the Late Miocene (especially in the southernmost island). The latter extension induced complicated patterns of perpendicular normal fault systems and strike-slip conjugate and transfer fault systems, through permutations between principal stress axes ( σ 1/ σ 2 and σ 2/ σ 3 modes). The ages of these tectonic events are ascertained by the existence of syndepositional normal faults, dike injections and superposed brittle structures. A third, less important compressional tectonic event, is correlated with the Plio-Quaternary collision in adjacent Taiwan (ENE-WSW compression). The comparison between the results of marine studies in the South China Sea basins and our paleostress reconstruction in the Penghu Islands suggests that repeated changes between N-S and NW-SE directions of extension occurred in the past. The counterclockwise change from N-S to NW-SE recorded in the Penghu Islands took place between about 12 and 8 Ma ago, indicating that significant extensional phenomena along the South China margin may postdate the seafloor spreading activity in the central ridge.

Kinematic analysis of fault-slip data

An array of graphical and numerical techniques facilitate qualitative and quantitative kinematic analysis of fault-slip data. Graphical contouring and Bingham statistics of the shortening and extension axes for kinematically scale-invariant faults characterize the distributions and orientations of the principal axes of average incremental strain. Numerical analysis by means of moment tensor summation yields the orientations and magnitudes of the principal strain axes as well as rotational information. Field data can be weighted for moment tensor summation using measurements of fault gouge thickness and/or fault plane width, from which average displacement and fault area can be estimated. The greatest uncertainties of kinematic analysis derive from assumptions about the weighting of the data, the effects of post-faulting rotation on the data, the degree to which sampling is representative of the entire fault population, and the spatial homogeneity of strain. These assumptions can be evaluated for a specific data set. Geometric criteria can distinguish the kinematic heterogeneities produced by triaxial deformation, anisotropy reactivation, strain compatibility constraints and/or multiple deformations. Strain compatibility, material anisotropy and heterogeneity may be characterized by integrating the results of kinematic and dynamic fault-slip analyses.

Tectonic analysis of fault slip data sets

Using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations, it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio ϕ = (σ2−σ3)/(σ1−σ3) between principal stress values (σ1≥σ2≥σ3, compression being positive). No assumption is made concerning the orientation of fault planes relative to stress axes, so that reactived faults are taken into account as well as newly created ones. Qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years. The practical limitation of the methods and the necessity for critical field observations are emphasized. These methods can be applied to focal mechanisms of earthquakes. A more complex analysis of heterogeneous data sets, involving an iterative separation of different stress systems, is also discussed. This analysis enables one to distinguish successive faulting events. Careful qualitative study in the field is in all cases essential.