Contractional Faults Research Articles

The TR method: the use of slip preference to separate heterogeneous fault-slip data in compressional stress regimes. The surface rupture of the 1999 Chi-Chi Taiwan earthquake as a case study

Abstract Synthetic contractional fault-slip data have been considered in order to examine the validity of widely applied criteria such as the slip preference, slip tendency, kinematic (P and T) axes, transport orientation and strain compatibility in different Andersonian compressional stress regimes. Radial compression (RC), radial–pure compression (RC–PC), pure compression (PC), pure compression–transpression (PC–TRP), and transpression (TRP) are examined with the aid of the Win-Tensor stress inversion software. Furthermore, the validity of the recently proposed graphical TR method, which uses the concept of slip preference for the separation of heterogeneous fault-slip data, is also examined for compressional stress regimes. In these regimes only contractional faults can be activated, and their slip preferences imply the distinction between “real”, i.e., RC, RC–PC and PC, and “hybrid”, i.e., PC–TRP and TRP stress regimes. For slip tendency values larger than 0.6, the activated faults dip at angles from 10° to 50°, but in the “hybrid” regimes faults can dip with even higher angles. The application of the TR method is here refined by introducing two controlling parameters, the coefficient of determination (R2) of the Final Tensor Ratio Line (FTRL) and the “normal” or “inverse” distribution of the faults plotted within the Final Tensor Ratio Belt (FTRB). The application of the TR method on fault-slip data of the 1999 Chi-Chi earthquake, Taiwan, allowed the meaningful separation of complex heterogeneous contractional fault-slip data into homogeneous groups. In turn, this allowed the identification of different compressional stress regimes and the determination of local stress perturbations of the regional or far-stress field generated by the 1999 Chi-Chi earthquake. This includes clear examples of “stress permutation” and “stress partitioning” caused by pre-existing fault structures, such as the N–S trending Chelungpu thrust and the NE–SW trending Shihkang–Shangchi fault zone.

Faulting and folding beneath the Canterbury Plains identified prior to the 2010 emergence of the Greendale Fault

Prior to the 2010–2011 earthquake sequence, several fault and fold structures were mapped beneath the Canterbury Plains using seismic reflection surveys and surface observations and depicted on the Christchurch and Aoraki 1:250,000 scale geological maps. Localised grabens associated with east-southeast-striking normal faults formed largely during the Late Cretaceous. South of Rakaia River, some graben-bounding faults show minor normal offset extending into the late Cenozoic. Near Ashley River, proximity to a Late Cretaceous–Paleogene graben suggests that the active, predominantly contractional, east-striking Ashley Fault is at least in part a rejuvenated pre-existing normal fault. The easterly strike of the previously unknown Greendale Fault implies that it too may be a reactivated Late Cretaceous fault. Northeast-striking, southeast-facing reverse faults and fault-propagation folds beneath the western and northern parts of the plains are primarily late Cenozoic features. Variation in the distributions of Miocene sedimentary strata strongly suggests that contractional faulting was initiated as early as the Miocene. The overall late Cenozoic tectonic pattern is extension beneath the southern Canterbury Plains and contraction farther north.

Physical modeling of deformation patterns in monoclines above oblique-slip faults

Many basement-involved fault-related folds are thought to form by reactivation of pre-existing faults. Although oblique slip is expected on such structures it is often difficult to identify because the underlying faults are blind, paleomagnetic records are ambiguous, and mesoscopic deformation patterns in the associated monoclines are often incompletely exposed or difficult to interpret. This study seeks to establish a relationship between oblique slip on a basement reverse fault and the deformation patterns in the overlying folded rock layers. Using scaled physical models of wet clay overlying a rigid basement, we create a suite of Laramide-style folds over basement faults with variable obliquity. Precise displacements and strains on the surface of the clay are recorded using close-range photogrammetry. We use these measurements to predict deformation patterns that we would expect to find in analogous natural structures. The results of our models show that there are three general strain zones that form in monoclines that form above oblique-slip faults. The upper-hinge region of the monocline is dominated by extension, the lower-hinge region by contraction, and the middle of the fold limb is dominated by shear strains. The boundaries of these three zones, as well as the magnitude of strain in each of the zones vary with the amount of oblique slip and fault throw. Deformation should be dominated by joints and extensional faults in the extensional zone, by contractional faults and cleavage in the contractional zone, and may contain extensional, contractional, or strike-slip faults along with joints and cleavage arranged in Riedel style geometries in the shear strain dominated zone. At fault obliquities above 45° there is significant overlap of the different strain zones and deformation patterns in these regions can consequently be very complex, involving reactivation or overprinting of earlier structures, or the formation of mixed-mode structures. Displacements on the surface of our model monoclines suggest that previous paleomagnetic interpretations of vertical axis rotations on natural monoclines may need to be re-evaluated. Although such rotations increase with increasing fault obliquity, the magnitude of these rotations will vary significantly with position across a monocline.

Crustal extensional faulting triggered by the 2010 Chilean earthquake: The Pichilemu Seismic Sequence

We report a sequence of crustal quakes that began after the Mw = 8.8 thrust‐subduction Maule earthquake that affected the Central Chile margin on 27 February 2010. This activity lasted by several months, having the most important events on 11 March 2010 (Mw = 6.9 and Mw = 7.0) with normal focal mechanisms. Seismicity shows a rupture oriented along a NW‐striking and SW‐dipping normal fault from the surface down to the interplate contact. Seismicity can be correlated with neotectonics extensional structures similarly oriented in the region, which have coexisted with NNE‐SSW reverse faults since the late Pliocene, even though both have older periods of activity since the Paleozoic. This crustal rupture would have been triggered by the high Coulomb stress change produced by the Maule earthquake, enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high Vp/Vs ratio. The occurrence of relevant neotectonic activity in coincidence with short‐term deformation suggests a relationship with long‐term tectonic features of this region, which would have been acting as a barrier during the interseismic period, increasing the strain accumulation and triggering contractional faulting in the crust, as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting. Crustal faulting in Pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the Maule earthquake.

The 11 May 2011 earthquake at Lorca (SE Spain) viewed in a structural-tectonic context

Abstract. The Lorca earthquake of 11 May 2011 in the Betic Cordillera of SE Spain occurred almost exactly on the Alhama de Murcia fault, a marked fault that forms part of a NE-SW trending belt of faults and thrusts. The fault belt is reminiscent of a strike-slip corridor, but recent structural studies have provided clear evidence for reverse motions on these faults. Focal mechanisms of the main earthquake, but also of a foreshock, are strikingly consistent with structural observations on the Alhama de Murcia fault. This strengthens the conclusion that, rather than a strike-slip fault, the fault is at present a contractional fault with an oblique reverse sense of motion, presumably in response to the NW-directed motion of Africa with respect to Europe.

Late Pleistocene slip rate of the Höh Serh-Tsagaan Salaa fault system, Mongolian Altai and intracontinental deformation in central Asia

SUMMARY The Mongolian Altai is an intracontinental oblique contractional orogen related to the far-field effects of the Indo-Asian collision. Global Positioning System (GPS) data suggest that ∼10–15 per cent of total Indo-Asia convergence is accommodated across this orogen. The Hoh Serh–Tsagaan Salaa fault system is one of several NNW–SSE-trending oblique contractional faults acting to partition strain and accommodate shortening and dextral shear in the Mongolian Altai. This fault zone displaces late Pleistocene alluvium along the southwest piedmont of the Hoh Serh range in western Mongolia. Along the central third of the fault zone, strain is partitioned onto two separate strands, one that accommodates nearly pure dextral shear and one that accommodates thrust motion. We determined late Pleistocene rates of deformation along each of the Hoh Serh–Tsagaan Salaa fault strands based on differential GPS surveys and cosmogenic nuclide 10Be geochronology. Combining the measured offsets and 10Be dates yields a minimum right-lateral slip rate of 0.9 +0.2/−0.1 mm a−1; the minimum shortening rate is 0.3 ± 0.1 mm a−1, with uplift of at least 0.1 ± 0.1 mm a−1. Resolving the shortening and dextral components of deformation yields a slip vector of 0.8 +0.2/−0.1 mm a−1 toward 336°. This long-term deformation vector is consistent with the short-term strain field determined by GPS in the region and indicates that ∼20 per cent of Indo-Asian deformation in the Mongolian Altai (∼2 per cent of the total Indo-Asia strain accumulation) occurs along the Hoh Serh–Tsagaan Salaa fault zone. Although rate data for other active faults in the Mongolian Altai are sparse, our results suggest that strain may be accommodated almost exclusively on discrete structures in this intraplate tectonic setting.

Evolution of faulting and volcanism in a back-arc basin and its implications for subduction processes

[1] The formation of the Taranaki Basin, an active volcanic back-arc rift situated in the continental Australian Plate, is related to subduction of the Pacific Plate along the Hikurangi margin. The Taranaki Basin contains an almost complete Miocene-Recent sedimentary record of the evolution of faulting and submarine andesitic volcanoes in the back-arc. Detailed study of extensive regional seismic reflection and coastal outcrop data sets yields valuable information about the extent to which back-arc rifting and reverse faulting have been controlled by the evolution of the Hikurangi margin subduction. Normal faulting and andesitic volcanism commenced in the northern part of the basin at ∼12 and ∼16 Ma, respectively, and were synchronous with contraction in the southern part of the basin. The rift, contractional faults and folds, and volcanism migrated southward during the last 12 Ma. Southward migration of faulting was episodic and geologically instantaneous with 100–150 km increases in the length of the rift at ∼12–8 and ∼4 Ma. From ∼4 Ma, displacement rates in the northern basin slowed and ceased at ∼2 Ma. The death of normal faults in the northern Taranaki Basin together with sympathetic variations in the timing of faulting and the overlapping rift geometries between the Taranaki Basin and the Central Volcanic Region are attributed to displacement transfer between the two rift systems. Southward migration of andesitic volcanism, rifting, and contractional deformation are consistent with clockwise rotation of the subduction margin associated with slab rollback coupled with southward motion of the southern termination of subduction and mantle corner flow.

Rough faults, distributed weakening, and off‐fault deformation

We report systematic spatial variations in fault rocks along nonplanar strike‐slip faults cross‐cutting the Lake Edison Granodiorite, Sierra Nevada, California (Sierran wavy fault) and Lobbia outcrops of the Adamello Batholith in the Italian Alps (Lobbia wavy fault). In the case of the Sierran fault, pseudotachylyte formed at contractional fault bends, where it is found as thin (1–2 mm) fault‐parallel veins. Epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends. We argue that the presence of fluids, as illustrated by this example, does not necessarily preclude the development of frictional melt. In the case of the Lobbia fault, pseudotachylyte thickness varies along the length of the fault, but the pseudotachylyte veins thicken and pool in extensional bends. We conduct a quantitative analysis of fault roughness, microcrack distribution, stress, and friction along the Lobbia fault. Numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness, resulting in an overall weaker fault than suggested by the corresponding static friction coefficient. The models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks, indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry. Together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip, rather than being confined to the region close to the dynamically propagating crack tip.

Structural evolution of bode Saadu area, Southwestern Nigeria

The Bode Saadu area comprises metasedimentary and metaigneous rocks which have been subjected to polyphase deformation and have subsequently been intruded by post-tectonic granitic rocks of probably Pan-African (600 ± 150Ma)age. Five phases of post-sedimentary tectonic deformation have been recognized in the rocks of this area. The first phase was associated with the development of the regional foliation, S1, and tight to isoclinal minor folds. The second phase involved heterogeneous deformation which gave rise to ductile shear zones, extensional and contractional faults. Second phase structures also include minor asymmetrical folds which deform S1 and S0. The third phase produced the dominant major folding on approximately N-S axis including the major Bode Saadu antiform. The fourth phase gave rise to open folds and crenulation of the earlier structures. Late brittle structures include transcurrent faults, both dextral and sinistral, which occasionally occurred in conjugate sets under generally N-S trending, maximum conpressive stress.

Alluvial fan development and morpho‐tectonic evolution in response to contractional fault reactivation (Late Cretaceous–Palaeocene), Provence, France

ABSTRACTAlong‐strike variability within a Late Cretaceous to early Palaeocene contractional growth structure and associated alluvial fan deposits is documented at the northern margin of the Arc Basin (Provence, SE France). This contribution shows that alluvial fans can be used as high‐resolution proxies to reconstruct structural segmentation and palaeo‐geomorphological evolution of a source/basin margin system. Facies‐based reconstruction allows the spatial and temporal distribution of alluvial fan bodies to be mapped. Relationships between fan area and catchment size from modern alluvial fan systems were used to estimate palaeo‐catchment size. Combining alluvial fan morphologies with catchment area, pebble provenance analysis and growth structure reconstruction, we show that: (1) fan distribution and related depositional processes were strongly influenced by intrinsic parameters such as drainage basin evolution, local structural inheritance and lateral facies changes in source area lithologies; (2) Inherited structures trending N100 effectively controlled the first‐order location of the fold and thrust structures (Montagne Sainte‐Victoire Range) and adjacent depositional areas (Arc Basin); (3) Syn‐sedimentary faults trending N010‐030 influenced the source/basin margin development and interacted with developing growth structures; (4) Facies changes in Jurassic carbonates controlled fold development and consequently the structural evolution of the source area; and (5) the N010‐030 faults and along‐strike variability of the source/basin margin system were ultimately controlled by basement structures that controlled where Late Cretaceous deformation nucleated. The overall architecture of the source/basin margin system reflects segmentation and strain partitioning along strike, as demonstrated by diachronous alluvial fan distribution.

Watch Faults Grow Before Your Very Eyes in a Deformational Sandbox

In this paper, we provide both a step-by-step description of how to build an apparatus for investigating extensional and contractional faulting and we provide lab procedures that teachers at both university and high-school levels can use to enhance students knowledge of the fundamentals of faulting mechanisms. The experiments are based on the analog modeling principle, where large geologic processes (both in terms of time and space) are scaled down to be observed in real time within a classroom. The technique is relatively simple, low-cost and reproducible in a short time span. The experiments offer students both a hands-on experience that is a proxy for more complicated laboratory techniques used by researchers and the opportunity for students to see faults grow before their very eyes. We followed this approach with student groups at different training levels (university and high-school) and found that the experiments boost understanding of the processes of faulting. We highly encourage university level teachers to use this technique both within their own curriculum and within programs that transfer knowledge of geologic concepts to high school teachers and students.

Mercury, Up-Close Again

![Figure][1] CREDITS: NASA/JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/ARIZONA STATE UNIVERSITY/CARNEGIE INSTITUTION OF WASHINGTON Planetary science is much about comparisons. Studying other bodies in our solar system with different sizes and compositions provides essential context for understanding Earth's formation and evolution. Three planets—Mercury, Venus, and Mars—and the Moon, are most like Earth in their initial composition and relative size, but their differences are enlightening. The Moon, Venus, and especially Mars have all been visited and probed recently by spacecraft. Now it is Mercury's turn. The MESSENGER[*][2] spacecraft flew by the planet and observed it in January. The papers in this Special Issue feature these observations. After two more passes, it will settle into orbit around Mercury in 2011. Mercury was visited by one earlier spacecraft, Mariner 10, in the mid-1970s. Its observations, and some difficult ground-based studies, provided most of our information on the planet, but raised many enigmatic questions. Despite being the smallest planet, Mercury has an actively generated magnetic field like that on Earth, but unlike that on the Moon, Mars, and Venus, and a huge iron-rich inner and molten outer core. Images of half of the surface revealed abundant scarps thought to indicate contractional faults, implying that Mercury was originally a bit larger early in its history. The extent to which its surface was shaped by impacts or volcanism was uncertain and debated. Its proximity to the Sun means that it has intense interactions with the solar wind, which, despite the magnetic field, impacts the surface of the planet, altering it and excavating some ions. Ground-based radar data hinted that ice may be present in shadowed regions of the poles. The first MESSENGER observations provide some important early answers and a wealth of data for further study. Observations of the surface by several instruments reveal that Mercury does have a volcanic history. More contractional faults are seen in areas observed by Mariner and in an additional ∼20% of the planet seen up close for the first time. A variety of impact craters help reveal relative ages of surface units and enrich the role of impact processes in shaping the planet. Impacts and solar bombardment have greatly weathered the surface; most of the iron is not in silicate minerals but apparently in nanoscale metal or oxide grains. In addition to sodium, MESSENGER detected ablation of calcium from the planet and in its magnetotail and measured in greater detail the magnetic field and its effect on its space environment. This first pass did not cover the polar regions, so confirmation of the presence of ice will have to await future observations, which will also image other parts of the planet and come as the Sun's activity increases. But already MESSENGER has helped fill in Mercury's history and environment, allowing a better understanding of all the terrestrial planets, including Earth. [1]: pending:yes [2]: #fn-1

Nature, origin and tectonic significance of anomalous transverse structures, southeastern Skeena Fold Belt, British Columbia

Abstract An anomalous domain of complex structure occurs in the eastern Skeena Fold Belt at Mosque Mountain, at the northwestern end of a regional belt of Cretaceous Sustut Group exposure. Folds and contractional faults with northwest- and northeast- to east-trends, that are parallel and at a high angle to the regional structural trend respectively, deform Jurassic Hazelton and Bowser Lake groups and unconformably overlying Sustut Group strata in this area. Anomalous structural trends at Mosque Mountain appear to reflect a history of superposed deformation. The older deformation occurred prior to and during deposition of the Middle Albian or younger basal Sustut Group. It caused the formation of a northwest-trending basement uplift of Hazelton and Bowser rocks whose abrupt northwest termination directly underlies the Mosque Mountain area. The main post late Campanian to early Maastrichtian contractional deformation formed northwest- and northeast- to east-trending structures in the Mosque Mountain area. Structural geometries and cross-cutting relationships suggest these nearly orthogonal contractional structures formed alternately. These relationships are most reasonably attributed to deformation on alternately active detachments or (blind) thrusts. Northwest-trending structures conform to regional strain patterns. Northeast- to east-trending structures were most likely caused by detachment deflection at the transverse, north end of the basement uplift; by movement up an oblique ramp formed by the geometric inversion of the north end of the basement uplift along a blind thrust, and/or reactivation of the north margin of the basement uplift. Geometric inversion of the early formed basement uplift on a footwall flat probably produced the south-facing monocline that localized the northwestern limit of preserved Sustut strata to the Mosque Mountain area. Late dextral oblique-slip and extensional faults cross-cut Skeena Fold Belt structures and indicate that transtensional deformation, common in the Central Interior of British Columbia, extended northward into the Mosque Mountain area.

Tectonic Framework and Evolution of the Gawler Craton, Southern Australia

The Gawler craton is the major crustal province in the southern Australian Proterozoic and is pivotal in models seeking to describe the evolution of Proterozoic Australia. The craton is host to the Olympic Dam iron oxide Cu-Au-U-REE deposit, as well as a number of other iron oxide copper-gold (IOCG), Au, and iron ore deposits. The evolution of the Gawler craton is dominated by two major phases of tectonic activity, Late Archean and late Paleoproterozoic-early Mesoproterozoic, in total spanning ca. 1 billion years. The Late Archean (2560–2500 Ma) basin development was coeval with arclike felsic magmatism and mafic-ultramafic magmas, including komatiites. Collisional deformation between 2480 and 2420 Ma led to ca. 400 m.y. of tectonic quiescence, conceivably within an early Paleoproterozoic continental interior. The second major phase of tectonic activity was in the middle to late Paleoproterozoic and early Mesoproterozoic (2000–1450 Ma). Paleoproterozoic tectonism was initially dominated by rift-related events, which produced a series of basins over the interval ca. 1900 to 1730 Ma, some of which contain significant iron ore reserves. Transient contractional deformation at ca. 1850 Ma led to batholith-scale granitic magmatism, in part derived from melting of the late Archean basement. Major basin development was finally terminated by the 1730 to 1690 Ma Kimban orogeny, the effects of which are widespread and include the formation of crustal-scale shear systems, granitic magmatism, and low- to high-grade metamorphism. The Kimban orogeny was followed by a renewed period of extension between ca. 1680 and 1640 Ma, leading to local magmatism and sedimentation which appears in part to have been coeval with the high-grade ca. 1650 Ma Ooldean event in the western Gawler craton. At ca. 1620 to 1615 Ma, the generation of the arc-related St. Peter Suite in the southern Gawler craton implies that craton components were located at an active plate margin. The arc environment was superseded by a transition to a continental interior regime, which coincided with the voluminous Gawler Range Volcanics (1595–1590 Ma) and Hiltaba Suite granitoids (1595–1575 Ma). The Hiltaba Suite is dominated by high T fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement. Emplacement of the Hiltaba Suite coincided with the major mineralizing interval in the Gawler craton. Regionally, two distinct mineral systems have been recognized: the Olympic IOCG province in the eastern part of the craton, and the gold-dominated systems within the central Gawler gold province. The spatial distribution of IOCG versus Au-dominated mineral systems appears to reflect regional variations in crustal composition and Hiltaba Suite petrogeneses. Hiltaba-aged granites in the Olympic IOCG province are isotopically more evolved, richer in U and Th, and oxidized compared to similar aged granites in the Au-dominated province. Modern-day heat flow in the IOCG province is significantly higher (90 ± 10 mWm−2) compared to the Au-dominated province (54 ± 5 mWm−2), suggesting there are important lithospheric compositional differences between the two metallogenic provinces, which may reflect an older phase of craton assembly. Widespread northwest-southeast contractional deformation coeval with the emplacement of the Hiltaba Suite is expressed by the formation and/or reactivation of shear zones ranging up to crustal scale. Within this regime, northwest-trending structures such as those in the vicinity of the Olympic Dam deposit are likely to have accommodated dilation associated with strike-slip movements. Intersection between these structures and northeast-trending contractional faults may have formed suitable structural traps for 1590 to 1580 Ma mineralization. The timing of syn-Hiltaba deformation overlaps with transpression and medium to high-grade metamorphism associated with the ca. 1570 to 1540 Ma Kararan orogeny which dominates the geophysical architecture of the central northern part of the craton. The youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca. 1470 and 1450 Ma and regional cooling.

The Geology of Mercury: The View Prior to the MESSENGER Mission

Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface, a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current image coverage of Mercury is comparable to that of telescopic photographs of the Earth’s Moon prior to the launch of Sputnik in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor (~1 km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history and internally generated volcanic resurfacing may have ceased at ~3.8 Ga; (5) a planet where impact craters can be used to disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry; (6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route to Mercury. The MESSENGER mission will address key questions about the geologic evolution of Mercury; the depth and breadth of the MESSENGER data will permit the confident reconstruction of the geological history and thermal evolution of Mercury using new imaging, topography, chemistry, mineralogy, gravity, magnetic, and environmental data.

Miocene extensional basin development in the Betic Cordillera, SE Spain revealed through analysis of the Alhama de Murcia and Crevillente Faults

ABSTRACTThe Alhama de Murcia and Crevillente faults in the Betic Cordillera of southeast Spain form part of a network of prominent faults, bounding several of the late Tertiary and Quaternary intermontane basins. Current tectonic interpretations of these basins vary from late‐orogenic extensional structures to a pull‐apart origin associated with strike–slip movements along these prominent faults. A strike–slip origin of the basins, however, seems at variance both with recent structural studies of the underlying Betic basement and with the overall basin and fault geometry. We studied the structure and kinematics of the Alhama de Murcia and Crevillente faults as well as the internal structure of the late Miocene basin sediments, to elucidate possible relationships between the prominent faults and the adjacent basins. The structural data lead to the inevitable conclusion that the late Miocene basins developed as genuinely extensional basins, presumably associated with the thinning and exhumation of the underlying basement at that time. During the late Miocene, neither the Crevillente fault nor the Alhama de Murcia fault acted as strike–slip faults controlling basin development. Instead, parts of the Alhama de Murcia fault initiated as extensional normal faults, and reactivated as contraction faults during the latest Miocene–early Pliocene in response to continued African–European plate convergence. Both prominent faults presently act as reverse faults with a movement sense towards the southeast, which is clearly at variance with the commonly inferred dextral or sinistral strike–slip motions on these faults. We argue that the prominent faults form part of a larger scale zone of post‐Messinian shortening made up of SSE‐ and NNW‐directed reverse faults and NE to ENE‐trending folds including thrust‐related fault‐bend folds and fault‐propagation folds, transected and displaced by, respectively, WNW‐ and NNE‐trending, dextral and sinistral strike–slip (tear or transfer) faults.

The role of inversion tectonics in the structure of the Cordillera Oriental (NW Argentinean Andes)

The Salta Basin of Cretaceous age in the Andes of NW Argentina was inverted during the development of the Cordillera Oriental between the Neogene and Recent. The different orientations of the extensional faults of the Salta Rift System and the subsequent W-E shortening resulted in a great variety of inversion structures. Extensional faults dipping NE to ESE were preferentially reactivated as contractional faults. The initial stepped geometry of these faults gave rise to a distinctive sigmoidal pattern of basement-involved thrusts and related folds. These contractional structures tended to run parallel to the extensional faults and acquired a N-S trend across the previous accommodation zones, resulting in folding interference patterns. Other interference patterns are attributed to the superposition of contractional folds on previous rollovers and to the constrictional deformation imposed by the reactivation of adjacent extensional faults with different orientations. The NNW to WSW dipping extensional faults show no evidence of reactivation despite the fact that some of them are folded. The inversion at crustal scale is limited owing to the presence of the Cretaceous Salta Rift across the different Andean structural units and foreland.

Progressive fault triggering and fluid flow in aftershock domains: Examples from mineralized Archaean fault systems

In strike-slip fault systems, Coulomb failure stress changes due to mainshocks can trigger large aftershocks or further earthquakes. The combination of static stress changes from mainshocks and large aftershocks potentially has a profound influence on the final distribution of aftershocks and crustal-scale fluid redistribution. Because mineralization acts as a high fluid flux indicator the interaction of static stress changes, fault triggering and fluid flow can be studied from mineralized fossil fault systems. Two examples are presented from separate fault systems in the Kalgoorlie greenstone terrane, Western Australia (the Black Flag and Boulder-Lefroy Fault systems). Using mapped fault geometries, slip directions and the known distribution of fault-hosted gold mineralization we show that the repeated arrest of mainshock ruptures, at both dilational and contractional fault step-overs, controlled aftershock-related fluid flow. Importantly, the largest aftershocks or subsequent triggered earthquakes exerted a very strong control on where the highest fluid fluxes occurred through small-event aftershock fault networks (at distances up to ∼ 15 km away from the step-overs). Fluid flow through mid-crustal fault systems in crystalline rock is spatially localised in regions where repeated clusters of aftershocks cause permeability enhancement. It is dependent on the seismogenic behaviour of the system, rather than a passive exploitation of the internal structure and fabrics developed by faults or damage zones. Field evidence implies that high pore fluid factors were repeatedly attained in the aftershock-related mineralized faults and that the fluids were derived from deep-level, overpressured reservoirs, rather than local wall rock porosity. It is apparent that high-pressure fluids, possibly released in a pulse after a mainshock, contribute to the rupture of structures already promoted towards failure from static stress changes.

Salt tectonics in two convergent-margin basins of the Cyprus arc, Northeastern Mediterranean

Seismic reflection profiles from the Adana–Cilicia and Iskenderun–Latakia Basin complexes in the NE Mediterranean Sea show the presence of linked extensional–contractional fault systems, detached at the base of the Messinian evaporites. The extensional domain is developed within the thick proximal part of the Plio-Quaternary delta succession in the inner part of each basin. It is characterized by imbricate fans of listric normal faults, which delineate turtle anticlines and their intervening syncline. The faults sole into the gently southwest-dipping detachment surface. In the inner basins, salt is largely evacuated toward the outer basins, but remnants occur as rollers in the footwalls of the listric extensional faults and small subdued pillows beneath turtle anticlines. The narrow synclines between the two turtle back growth anticlines record a more complicated history of rise and fall of the salt related to switch in vergence of the extensional fault fan. A large salt wall is developed at the toe of the delta complex in both basins. Two wide peripheral sinks are present within the Plio-Quaternary units on both flanks of the salt wall. Locally small segments of salt welds are present near the base of the salt wall. The contractional domain is characterized by a relatively thin Plio-Quaternary cover overlying a relatively uniform, 500–800 ms-thick salt substrate. A series of salt-cored growth folds are commonly associated with thrusts of variable vergence. The halokinetic structures in the Cilicia and Latakia Basins display a remarkable resemblance to detached and linked systems of extensional and contractional fault fans documented in classical salt provinces on passive margins. The study area also resembles scaled experiments which document synchronous development of extensional and contractional fault systems detached in a ductile substrate. However, a number of differences also exist, which may arise from several factors, including the nature of ductility contrast between the ductile portion of the evaporite layer and the supra-salt succession, the thickness ratio of overburden to evaporite unit, and the rate of progradation and the nature of sediment distribution during the evolution of the delta complex. The special aspect of these associations of linked gravity-driven extensional and contractional salt structures is that they are developed as a self-contained system wholly situated within a Plio-Quaternary transtensional basin.

Dynamics and inversion of the Mesozoic Basin of the Weald–Boulonnais area: role of basement reactivation

The geometry and dynamics of the Mesozoic basins of the Weald–Boulonnais area have been controlled by the distribution of preexisting Variscan structures. The emergent Variscan frontal thrust faults are predominantly E–W oriented in southern England while in northern France they have a largely NW–SE orientation. Extension related to Tethyan and Atlantic opening has reactivated these faults and generated new faults that, together, have conditioned the resultant Mesozoic basin geometries. Jurassic to Cretaceous N–S extension gave the Weald–Boulonnais basin an asymmetric geometry with the greatest subsidence located along its NW margin. Late Cretaceous–Palaeogene N–S oriented Alpine (s.l.) compression inverted the basin and produced an E–W symmetrical anticline associated with many subsidiary anticlines or monoclines and reverse faults. In the Boulonnais extensional and contractional faults that controlled sedimentation and inversion of the Mesozoic basin are examined in the light of new field and reprocessed gravity data to establish possible controls exerted by preexisting Variscan structures.