Step-over Zone Research Articles

The structural evolution of dilational stepovers in regional transtensional zones

Abstract We propose a theoretical model, supported by a field study, to describe the patterns of fault/fracture meshes formed within dilational stepovers developed along faults accommodating regional scale wrench-dominated transtension. The geometry and kinematics of the faulting in the dilational stepovers is related to the angle of divergence (α), and differs from the patterns traditionally predicted in dilation zones associated with boundary faults accommodating strike-slip displacements (where α = 0°). For low values of oblique divergence (α<30°) and low strain, the fault–fracture mesh comprises interlinked tensile fractures and shear-extensional planes, consistent with wrench-dominated transtension. At higher values of strain, a switch occurs from wrench- to extension-dominated transtension, leading to the reactivation and/or disruption of the early-formed structures. These structural processes lead to the development of a geometrically complex and kinematically heterogeneous fault pattern, which may affect and/or perturb the development of a through-going fault linking and facilitating the slip transfer between the two overlapping fault segments. As a result, dilational stepover zones will tend to form long-lived sites of localized extension and subsidence in regional transtensional tectonic settings. Cyclic increases/decreases of structural permeability will be related to slip on the major boundary faults that control the distribution of fluid-flow paths and, consequently, the long- and short-term structural evolution of these sites. Our model also predicts complex and more realistic subsurface fluid migration pathways relevant to our current understanding of hydrothermal ore deposits and hydrocarbon migration and storage.

The relationship between length and width of plutons within the crustal-scale Cobequid Shear Zone, northern Appalachians, Canada

The lengths and widths have been measured for 69 component bodies of composite plutons along the Cobequid Shear Zone. Plutons on major fault strands, those with mylonite zones >0.1 km wide, exhibit evidence of multiple intrusion of magma batches. Small plutons along short faults in stepover zones appear related to rapid emplacement of magma in bodies 1.5–4 km long by 0.1–2 km wide. Such small plutons show low enrichment in incompatible elements in older component bodies, but increasing amounts in younger bodies as a result of progressive magma expulsion from crystal mush during crystallization and shear-enhanced compaction in fault zones. Wider plutons generally occur along longer fault strands accommodating more strain and penetrating deeper into the crust and show enrichment in incompatible elements. The width of the mylonitic fault zone is about 15% of the width of these plutons. The length-to-width ratio of component bodies and composite plutons varies between 2 and 11. The best-fit line describing these data has a slope of 1.056, which implies scaling behavior between plutonism and tectonic processes. Scalar properties of plutonic bodies are similar to those of faults, but scalar relationships observed in component bodies do not apply to composite plutons.

Late Paleozoic strike-slip faults and related vein arrays of Cape Elizabeth, Maine

Strike-slip faults and related quartz vein arrays of Late Paleozoic-age cut gently-dipping metasedimentary rocks at Cape Elizabeth in southern coastal Maine and formed in response to regional dextral shearing along the Norumbega fault system. Vertical quartz veins up to 20 m wide and 10s of meters long were emplaced orthogonal to the local shear zone-parallel elongation fabric, reflecting strain partitioning during transpression. Earlier veins were reoriented by clockwise rotation toward this NE-trending regional shear direction. The later brittle strike-slip faults are oblique to the regional shear direction and interpreted as a 10-km-scale R-shear array on the southeast flank of the Norumbega fault system. These left-stepping en échelon fault zones consist of the three Two Lights fault zones (∼200 m lengths and up to ∼5 m displacements) and the Richmond Island fault zone (∼1.6 km length and ∼40 m displacement). Displacements on these fault zones have developed fine-grained silicified, obliquely-foliated and laminated cataclasites and locally, millimeter-thin pseudotachylyte fault and injection veins. Individual fault core zones are up to 10s of centimeters thick as part of several complex anastamosing zones of faulting 10s of meters wide. Initial segments within each fault zone are typically terminated with oblique extension fractures in horsetail configurations. The left-stepping en échelon relationships between these segments led to dominantly contractional step-over zones where P-shear linkages created a through-going fault that truncated the ends of the earlier-formed terminated segments. This linkage-growth model for fault zone evolution works toward larger scales and longer fault lengths as displacement accumulates, within a limiting maximum displacement/length ratio characteristic of the host lithologies. Length–frequency data for fault segments within these zones suggest a transition to linkage-dominated growth once fault segments were longer than ∼15 m. Continued displacement was accommodated along the P-shear linked en échelon faults through imbrication, contractional duplexing and adhesive wear on the outcrop-scale. Core zone processes on the micro-scale reflect cataclasis and frictional sliding during coseismic slip as well as cataclastic flow and pressure solution during post-seismic creep. The development of foliated-to-laminated cataclasite was accompanied by pore volume collapse, pressure solution and fluid expulsion that, in turn, triggered the development of the late fault-related quartz vein arrays.

Determination of horizontal extension from fissure-ridge travertines: a case study from the Denizli Basin, southwestern Turkey

Travertine deposits reflect some aspects of the regional tectonics because of the close association between travertine deposits and active fractures, that later of which provide conduits along which travertine-depositing waters may rise. Fissure-ridge travertines form above extensional fissures which are located in the hanging walls of normal faults, in step-over zones between fault segments, or in active or recently active) volcanic provinces. Numerous active and inactive fissure-ridge travertines are located in the hanging walls of normal faults in the Denizli Basin. A typical fissure-ridge comprises a central fissure along its long axis and flanking bedded travertines dipping away from the fissure. Central fissures of travertine ridges have been dilating since the initiation of the fissures. Samples from both the margins and centres of banded travertine deposits were dated by Th/U methods in order to determine dilation rates. Individual fissures have been dilating at average rates of between 0.008 and 0.1 mm yr–1 during travertine deposition, and ~ 0.001 and 0.007 mm yr–1 after cessation of travertine deposition. There is a noticable decrease in dilation rate from west to east in the Denizli Basin, and this decrease in dilation rate may be related to decrease in overall extension in southwest Turkey, which decreases eastward.

Uplift and contractional deformation along a segmented strike-slip fault system: the Gargano Promontory, southern Italy

The Gargano Promontory in southern Italy is a region of localized uplift and contractional deformation within the Apulian foreland province. Our field mapping has defined two main fault systems that controlled the uplift of the Gargano block. E–W-trending strike-slip faults bound the northern and southern margins of the uplifted block, defining a compressive stepover zone between the two fault zones. The second system strikes NW–SE with primarily reverse to oblique-reverse displacements. Uplift was accommodated by a broad antiformal fold and reverse motion on the NW–SE fault system. This deformation is attributed to localized high compressive stress within the stepover region, driven by slip on the bounding strike-slip faults. Three-dimensional numerical models incorporating the observed fault geometries and slip sense support our interpretation. Results show that significantly higher compressive stresses occur within the fault overstep region, and computed stress trajectories agree closely with the orientations of contractional features observed in the field. Although the role of strike-slip faulting in the deformation of the Gargano Promontory has been proposed previously, this mechanism provides a new model for the localized uplift and related deformation. We also cite similar occurrences of this mechanism in the complexly deformed interior of the Apennine fold–thrust belt.

Crustal architecture and active growth of the Sutai Range, western Mongolia: a major intracontinental, intraplate restraining bend

The Sutai Range is a structural and topographic culmination at the southeastern end of the Mongolian Altai and a world-class example of an actively forming restraining bend. The range occurs at a major stepover zone along the Tonhil dextral strike-slip fault within a wider region dominated by late Cenozoic transpressional deformation. Analysis of satellite imagery and the results of field investigations reveal that the range is structurally asymmetric with an overall NE tilt due to several major SE-directed thrusts within the core of the range and along its SW margin. The distribution of alluvial sediments shed from the range and stream length asymmetries also indicate a regional NE tilt for the range. Faults that splay off of the main Tonhil Fault bound discrete uplifted blocks that define an oblique-slip duplex at the surface and asymmetric flower structure in cross-section. Outward growth of the range is partly accommodated by growth of foreberg thrust ridges within the adjacent Dariv Basin. Thrust blocks within the centre of the range expose basement schists and mylonitic granite suggesting that the greatest uplift and crustal exhumation has occurred within the core of the restraining bend, although much of the exhumation is likely to be due to older Palaeozoic structural events. The southeast and northwest ends of the range are characterized by smooth unbroken surface ramps (“gangplanks”) that are upwarped towards the centre of the range where maximum Cenozoic uplift has occurred. The geometry and evolution of Sutai Uul and other intracontinental and intraplate restraining bends is fundamentally influenced by the initial width of the stepover zone, the attitude of regional basement structures and extent of brittle reactivation, the direction of SHmax relative to basement structures, and progressive fault and block rotation which may change the kinematics along faults or lead to their abandonment.

Dextral strike-slip faulting in the Cariboo Mountains, British Columbia: a natural example of wrench tectonics in relation to Cordilleran tectonics

The Cariboo Mountains, British Columbia, contain an intracontinental dextral strike-slip fault system that crosscuts the regional fold structures. This fault system accounts for a minimum of 120 km and a maximum of 200 km of dextral strike-slip displacement. This probably accommodates some of the motion associated with the southern termination of the Northern Rocky Mountain Trench Fault and is part of a step-over zone between the Northern Rocky Mountain Trench Fault and the Fraser River – Straight Creek fault systems. The Isaac Lake Synclinorium is a kilometre-scale Jurassic fold structure that is bounded by the dextral oblique Isaac Lake and Winder strike-slip faults. These faults are part of the regional strike-slip fault system that is found throughout the Cariboo Mountains. Deformation associated with the strike-slip faults is complex and is partitioned into motion along the faults and into the formation of kilometre-scale folds that are found in areas between the faults. The angular relationship between the strike-slip faults and folds conforms to models developed for dextral strike-slip fault systems with drag on high-friction faults. We interpreted these structures to have formed during a continuous deformation event. Timing constraints indicate that faulting started by the Late Cretaceous and may have had a long and protracted history into the Tertiary.

Structure and tectonic development of the Ghab basin and the Dead Sea fault system, Syria

We examine the structure and evolution of the Ghab basin that formed on the active, yet poorly understood, northern segment of the Dead Sea transform fault system. The basin formed in Plio-Quaternary time at a complex step-over zone on the fault. Subsidence occurred along cross-basin and transform-parallel faults in two asymmetric depocentres. The larger depocentre in the south of the basin is asymmetric towards the east, the margin along which most active transform displacement apparently occurs. The Syrian Coastal Ranges, located directly west of the Ghab basin, are a consequence of Late Cretaceous and Cenozoic regional compression, heavily modified by the Dead Sea fault system and Ghab basin formation. We prefer a model whereby the Dead Sea fault system in northwest Syria developed in Plio-Quaternary time, consistent with previously proposed models of two-phase Dead Sea fault system movement and Red Sea spreading.

The geometry, kinematics and rates of deformation within an en échelon normal fault segment boundary, central Italy

The geometry, kinematics and rates of deformation have been investigated within an en échelon fault segment boundary between two left-stepping major normal faults in the central Apennines, Italy. Examination of faulted post-glacial sediments and geomorphic features attributed to glacial retreat (18 ka) reveals that the two major faults dominate the recent slip, with rates that are 5–10 times greater than the other faults. At the centres of the major faults, slip is parallel to the regional extension direction (NE–SW) defined by focal mechanisms and borehole breakouts. Slip-vector azimuths defined by striations on faults in the stepover zone indicate NE–SW extension on NW–SE faults, sub-parallel to the regional extension direction, together with significant and contemporaneous NW–SE extension, along the strike of the Apennines, on ESE–WNW and ENE–WSW faults. Thus, distributed three-dimensional strain occurs in the stepover between the two major bounding faults. Extension rates summed across the stepover are 0.68 mm/y parallel to the regional extension, compared to ∼2 mm/y at the centre of one of the major faults. Extension rates along-strike of the fault zone are 0.14 mm/y within the stepover and zero at the centres of the two major bounding faults. The above information is used to discuss how extension is partitioned between different structures during the growth and linkage of normal faults.

Structure of the Lake County uplift: New Madrid seismic zone

Abstract The central segment of the New Madrid seismic zone lies within a left step-over zone between two northeast-striking, right-lateral, strike-slip fault systems. Within this compressional step-over zone is the topographically and structurally high Lake County uplift, which includes the Tiptonville dome and Ridgely ridge. We believe these structures are a consequence of deformation in the hanging wall above the northwest-striking, southwest-dipping Reelfoot reverse fault. Reelfoot fault dips 73° from the surface to the top of the Precambrian at a depth of approximately 4 km. From 4 to 12 km depth, the fault dips 32° and is seismically active. Based on a fault-bend fold model, we believe that the Reelfoot fault becomes horizontal and aseismic at the top of the quartz brittle-ductile transition zone, at approximately 12 km depth. Our data indicate that the western margin of the Tiptonville dome-Ridgely ridge and the western margin of the Lake County uplift are bounded by east-dipping kink bands (backthrusts). Recent work suggests that the Reelfoot fault is responsible for the 7 February 1812, M 8 New Madrid earthquake. However, the Reelfoot fault has a surface area that is less than that necessary for an M 8 earthquake. A possible solution to this discrepancy between magnitude and fault plane area is that the associated backthrusts are seismogenic.

Near-surface structural model for deformation associated with the February 7, 1812, New Madrid, Missouri, earthquake

The February 7, 1812, New Madrid, Missouri, earthquake (M [moment magnitude] 8) was the third and final large-magnitude event to rock the northern Mississippi Embayment during the winter of 1811–1812. Although ground shaking was so strong that it rang church bells, stopped clocks, buckled pavement, and rocked buildings up and down the eastern seaboard, little coseismic surface deformation exists today in the New Madrid area. The fault(s) that ruptured during this event have remained enigmatic. We have integrated geomorphic data documenting differential surficial deformation (supplemented by historical accounts of surficial deformation and earthquake-induced Mississippi River waterfalls and rapids) with the interpretation of existing and recently acquired seismic reflection data, to develop a tectonic model of the near-surface structures in the New Madrid, Missouri, area. This model consists of two primary components: a north-northwest–trending thrust fault and a series of northeast-trending, strike-slip, tear faults. We conclude that the Reelfoot fault is a thrust fault that is at least 30 km long. We also infer that tear faults in the near surface partitioned the hanging wall into subparallel blocks that have undergone differential displacement during episodes of faulting. The northeast-trending tear faults bound an area documented to have been uplifted at least 0.5 m during the February 7, 1812, earthquake. These faults also appear to bound changes in the surface density of epicenters that are within the modern seismicity, which is occurring in the stepover zone of the left-stepping right-lateral strike-slip fault system of the modern New Madrid seismic zone.

Geometry and style of partitioned deformation within a late Cenozoic transpressional zone in the eastern Gobi Altai Mountains, Mongolia

The Gobi Altai is the easternmost extension of the Mongolian Altai and consists of topographically discontinuous E-W-trending ranges with peaks averaging 2000–3000 m in elevation. The region is seismically active and characterized by prominent E-W left-lateral strike-slip faults that localize transpressional deformation and uplift along their lengths and at stepover zones. This report summarizes structural field investigations made in the easternmost Gobi Altai to document the structural geometry and style of late Cenozoic transpressional deformation in the region in order to better understand processes of intracontinental mountain building and the distant intracontinental strain response to the Indo-Eurasian collision. The Artsa Bogd range marks the northeastern terminus of the Gobi Altai and is topographically asymmetric with a high northern margin marked by N-vergent thrust faults and left-lateral oblique-slip faults. The northern side of the range is also bounded by a foreland basin that contains N-vergent thrust faults and folds that deform Quaternary sediments. The southern margin of Artsa Bogd appears tectonically inactive but contains S-vergent thrust faults and left-lateral wrench zones. The range appears to have a flower structure cross-sectional geometry that may reflect transpressional inversion of a Mesozoic basin. The isolated, high and narrow Tsost Uul range south of Artsa Bogd occupies a restraining bend position along the left-lateral Tsost Uul strike-slip fault system. Major faults within the range define a half-flower structure cross-sectional geometry. To the south of the Tsost Uul range, the Gobi Bulag left-lateral strike-slip fault system is marked by small push-up ridges and one major restraining bend mountain where the fault steps to the right near its western end. Throughout the region, Late Cretaceous-Tertiary basalts and Tertiary and Quaternary sediments are deformed by the major fault systems indicating late Cenozoic fault activity. These fault systems and the ranges formed along them occur at fairly regular intervals (approximately 20 km) between the North Gobi Altai fault system and the Gobi Tien Shan fault system, two major left-lateral strike-slip faults that cut across southern Mongolia. Together the faults define a parallel array of discrete linear belts of Cenozoic E-W left-lateral transpressional deformation south of the Hangay Dome. The regular spacing of the fault systems may suggest more uniform distributed left-lateral flow at depth. Eastward-directed lower crustal and lithospheric mantle flow is suggested by existing seismic anisotropy data for the eastern Gobi Altai and is believed to be the driving force for the upper crustal deformation.

Surface breakage of the 1992 Landers earthquake and its effects on structures

AbstractThe Landers, California, earthquake (Mw = 7.3) provides an exceptional opportunity to study surface rupture of an earthquake fault. Detailed maps of the lateral distribution of fracturing adjacent to main traces show that rupture patterns are much more complex than documented in past studies of surface ruptures. The rupture occurs in tabular zones, up to hundreds of meters wide. A main trace within each rupture zone accommodates much of the shear deformation, but considerable fracturing occurs throughout the tabular zone. The en-echelon pattern of fracturing in step-over zones between main traces is typically even more complex than those along major fault zones. Inspection of several on-grade concrete slabs indicates that unreinforced concrete foundations generally crack when subjected to distinct ground ruptures beneath them or when they are twisted because of differential ground movements across broad zones. Methods of mitigating the potential hazards associated with earthquake fault rupture are presented.

A mid-crustal contractional stepover zone in a major strike-slip system, North Cascades, Washington

The segmentation of strike-slip faults at the Earth's surface is well known, but the geometry of these faults in the mid-crust is poorly understood, as are the intervening stepover zones. A mid-crustal example of a segmented strike-slip fault is present within the > 500-km-long Ross Lake fault system where the NW-striking Twisp River fault zone and Gabriel Peak tectonic belt occur as overlapping, left-stepping, dextral strike-slip shear zones. The contractional stepover between these structures is occupied by the Black Peak batholith, which is riddled by ductile shear zones that are typically 0.5–1.5-m thick and are restricted to the stepover. Gently to moderately NW-dipping, reverse-slip ductile shear zones in the center of the stepover strike at a high angle to the Twisp River fault zone and Gabriel Peak tectonic belt and lie in an appropriate position to transfer most of the displacement between the strike-slip segments.The orientation of the reverse-slip ductile shear zones relative to the strike-slip segments is broadly similar to that of folds and thrusts in upper-crustal stepovers. The stepover width is relatively large, however, in comparison to many active stepovers. Emplacement of the Black Peak batholith across an originally narrower stepover or bend in the fault system may have created a rigid buttress and subsequent deformation jumped to the mechanically weak southwest margin of the body. Because plutons are commonly intruded into strike-slip zones, such partitioning of later displacements may be widespread.

Kinematic analysis of the San Juan thrust system, Washington

Shear-related fabric in the middle to Late Cretaceous San Juan thrust system, Washington, is observed in imbricated blocks and in fault zones a few millimeters to tens of meters thick. Foliation and lineation are defined in tectonic blocks by alignment of (1) newly grown muscovite and chlorite and (2) flattened and stretched radiolaria, mineral porphyroclasts, and rock fragments. Thin argillaceous shear zones bear a pronounced lineation defined by lithologic streaks and slickenside corrugations. The slickenside and stretching lineations show a well-defined preferred orientation in a horizontal northwest-southeast direction. Widespread S-C fabrics, asymmetric pressure shadows, fault drag features, and asymmetric folds in the X-Z plane yield a sense of tectonic transport at most localities of upper plate to the northwest. Aragonite, prehnite, lawsonite, pumpellyite, and rare Na-amphibolite and Na-pyroxene characterize the metamorphism during the thrusting. The findings of this study indicate north-west, orogen-parallel, transport of the San Juan thrust system. We suggest that this structure is a large step-over zone transferring dextral strike slip from the western Cascades to the oceanward side of Wrangellia.

The Marinduque intra-arc basin, Philippines: Basin genesis and in situ ophiolite development in a strike-slip setting

The Marinduque basin is a marine intra-arc basin in the north-central Philippine volcanic arc system. A suite of marine geophysical observations show the basin as a whole to be rhombic in shape, with its long axis trending north-northwest. A conspicuous volcanic ridge trends east-northeast across the basin, dividing it into two smaller active depocenters. Symmetric linear magnetic anomalies, striking parallel to, and centered over, the central volcanic ridge indicate that it formed by extension in a north-south direction by a process analogous to sea-floor spreading. Geometric and temporal relations between the central volcanic ridge and several en echelon sets of north-trending, left-stepping faults indicate that sea-floor spreading occurred in the extensional stepover zone of a left-lateral, strike-slip fault system, and that the Marinduque basin as a whole is a composite pull-apart basin whose floor is in part composed of oceanic-type crust. The evolution of the central volcanic ridge presents an actualistic model for in situ development and emplacement of ophiolites in an island-arc setting. The presently active, left-lateral Philippine fault zone truncated the eastern flank of the basin as well as the central volcanic ridge, following their initial formation. Shortening associated with a probable restraining bend in the Philippine fault has resulted in uplift of basin flank sediments, which have been thrust back toward the basin interior. The Philippine fault has also displaced fragments of the ridge and basin floor toward the north. The overall history of the Marinduque basin further suggests that strike-slip processes may play an important role in the origin and growth of intra-arc basins. Such basins may ultimately become isolated in a forearc or backarc setting.

Scales of structural heterogeneity within neotectonic normal fault zones in the Aegean region

Structural heterogeneities within neotectonic normal fault zones in the Aegean region are important controls on the geomorphic expression of their uplifted footwall blocks. Contrasting fault rocks and slip-plane phenomena (corrugations, comb fractures, slip-parallel fractures and pluck holes) are responsible for mesoscopic inhomogeneities within fault scarps. Along-strike disruptions in the continuity of fault scarps (step-over zones, step-up zones, step-down zones, footwall cross-faults and hangingwall cross-faults) are products of perturbations in slip-plane geometry or intersections with transverse faults. Large-scale variations in range-front morphology reflect contrasting patterns of deformation within fault zones. Some major fault zones, and many minor fault zones, are characterized by a migration of the active slip plane towards the hangingwall, resulting in a distributed network of high-angle faults and a range front which is stepped in profile. The presence of a localized mass of uneroded bedrock (hangingwall salients) protruding from the hangingwall of some major fault zones, results in intense deformation along a principal slip plane, and a range front which is ramp-like in profile. As a consequence of structural heterogeneities within neotectonic normal fault zones, fault scarps and range fronts in the Aegean region are subject to a temporally and spatially variable pattern of degradation.