Weakness and mechanical anisotropy of phyllosilicate-rich cataclasites developed after mylonites of a low-angle normal fault (Simplon Line,Western Alps)

The Simplon Fault Zone is a late-collisional low-angle normal fault (LANF) of the Western Alps. The hanging wall shows evidence of brittle deformation only, while the footwall is characterized by a c. 1 km-thick shear zone (the Simplon Fault Zone), which continuously evolved, during exhumation and cooling, from amphibolite facies conditions to brittle-cataclastic deformations. Due to progressive localization of the active section of the shear zone, the thermal-rheological evolution of the footwall resulted in a layered structure, with higher temperature mylonites preserved at the periphery of the shear zone, and cataclasites occurring at the core (indicated as the Simplon Line). In order to investigate the weakness of the Simplon Line, we studied the evolution of brittle/cataclastic fault rocks, from nucleation to the most mature ones. Cataclasites are superposed on greenschist facies mylonites, and their nucleation can be studied at the periphery of the brittle fault zone. This is characterized by fractures, micro-faults and foliated ultracataclasite seams that develop along the mylonitic SCC′ fabric, exploiting the weak phases mainly represented by muscovite and chlorite. Approaching the fault core, both the thickness and frequency of cataclasite horizons increase, and, as their thickness increases, they become less and less foliated. The fault core itself is represented by a thicker non-foliated cataclasite horizon. No Andersonian faults or fractures can be found in the footwall damage zone and core zone, whilst they are present in the hanging wall and in the footwall further from the fault. Applying a stress model based on slip tendency, we have been able to calculate that the friction coefficient of the Simplon Line cataclasites was <0.25, hence this fault zone is absolutely weak. In contrast with other fault zones, the weakening effect of fluids was of secondary importance, since they accessed the fault zone only after an interconnected fracture network developed exploiting the cataclasite network.     

Fluid overpressure along an Oligocene out-of-sequence thrust in the Shimanto Belt,SW Japan

Out-of-sequence thrusts (OSTs) exposed in ancient accretionary prisms are considered as fossil analogs of present-day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity. In modern as well as in ancient accretionary prisms, first-order megasplay faults or OSTs truncate or merge with faults of lesser importance called second-order OSTs. Structural analysis of the Makinokuchi fault, a branch of an Oligocene to lower Miocene second-order OST in the Tertiary Shimanto Belt of central Kyushu, SW Japan, brings information about the conditions of deformation at the time of thrusting. The studied exposure shows that the fault footwall and, to a much lesser extent, the fault hanging-wall, consist of quartz-cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component. The footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias. The continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault. Fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions (245–285 °C and 5–8 km depth). These similarities suggest that the fluids responsible for syn-tectonic hydraulic brecciation were collected from the footwall through the conduits. The fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone. Our analysis shows that fluid overpressures can play a key role in the growth and activity of second-order OSTs in accretionary prisms and suggests that fluids collected along second-order OSTs or splay faults may flow upward along first-order OSTs or megasplay faults.     

Structure of a normal seismogenic fault zone in carbonates: The Vado di Corno Fault,Campo Imperatore,Central Apennines (Italy)

The Vado di Corno Fault Zone (VCFZ) is an active extensional fault cutting through carbonates in the Italian Central Apennines. The fault zone was exhumed from ∼2 km depth and accommodated a normal throw of ∼2 km since Early-Pleistocene. In the studied area, the master fault of the VCFZ dips N210/54° and juxtaposes Quaternary colluvial deposits in the hangingwall with cataclastic dolostones in the footwall. Detailed mapping of the fault zone rocks within the ∼300 m thick footwall-block evidenced the presence of five main structural units (Low Strain Damage Zone, High Strain Damage Zone, Breccia Unit, Cataclastic Unit 1 and Cataclastic Unit 2). The Breccia Unit results from the Pleistocene extensional reactivation of a pre-existing Pliocene thrust. The Cataclastic Unit 1 forms a ∼40 m thick band lining the master fault and recording in-situ shattering due to the propagation of multiple seismic ruptures. Seismic faulting is suggested also by the occurrence of mirror-like slip surfaces, highly localized sheared calcite-bearing veins and fluidized cataclasites. The VCFZ architecture compares well with seismological studies of the L'Aquila 2009 seismic sequence (mainshock M_W 6.1), which imaged the reactivation of shallow-seated low-angle normal faults (Breccia Unit) cut by major high-angle normal faults (Cataclastic Units).     

The Rila-Pastra Normal Fault and multi-stage extensional unroofing in the Rila Mountains (SW Bulgaria)

The Rhodope Metamorphic Province represents the core of an Alpine orogen affected by strong syn- and postorogenic extension. We report evidence for multiple phases of extensional unroofing from the western border of the Rila Mountains in the lower Rila valley, SW Bulgaria. The most prominent structure is the Rila-Pastra Normal Fault (RPNF), a major extensional fault and shear zone of Eocene to Early Oligocene age. The fault zone includes, from base to top, mylonites, ultramylonites and cataclasites, indicating deformation under progressively decreasing temperature, from amphibolite-facies to low-temperature brittle deformation. It strikes E–W with a top-to-the-N-to NW-directed sense of shear. Basement rocks in the hanging wall and footwall both display amphibolite-facies conditions. The foliation of the hanging-wall gneisses, however, is discordantly cut by the fault, while the foliation of the footwall gneisses is seen to curve into parallelism with the fault when approaching it. Two ductile splays of the RPNF occur in the footwall, which are subparallel to the foliation of the surrounding gneisses and merge laterally into the mylonites of the main fault zone. The concordance between the foliation in the footwall and the RPNF suggests that deformation and cooling in the footwall occurred simultaneously with extensional shearing, while the hanging-wall gneisses had already been exhumed previously. The RPNF is associated with thick deposits of an Early Oligocene, syntectonic breccia on top of its hanging wall. Integrating our results with previous studies, we distinguish the following stages of extensional faulting: (1) Late Cretaceous NW–SE extension (Gabrov Dol Detachment), exhumation of the present day hanging wall of the RPNF; (2) Eocene to Early Oligocene NW–SE to N–S extension (RPNF); (3) Miocene to Pliocene E–W extension (Western Border Fault), formation of the Djerman Graben; (4) Holocene to recent N–S to NW–SE extension (Stob Fault), reactivating the SW part of the Western Border Fault.     

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone,New Hampshire,USA

We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na‐poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric‐forming Na‐rich muscovite is also progressively replaced by fabric‐forming Na‐poor muscovite. The mineralogy of the new phyllonitic fault‐rock produced is dominated by Na‐poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric‐forming Na‐rich muscovite is selectively replaced at high‐strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high‐grade porphyroblasts by weaker Na‐poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na‐rich muscovite‐defined foliation destroys not only the metastable high‐grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.     

Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps)

The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9–11 km in depth and 250–300 °C. In the study area the fault accommodates 1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons (faults_1–2–3). Fault displacement is in the range of few centimeters (faults₃) to a maximum of a few tens of meters in major faults₁. Faults_1–2 nucleated on pre-existing joints, whereas faults₃ are newly generated fractures produced during slip along faults_1–2. Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe³⁺ compared to host rock.We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid–rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios.     

Shear localization,velocity weakening behavior,and development of cataclastic foliation in experimental granite gouge

Microstructural aspects of room-temperature deformation in experimental Westerly granite gouge were studied by a set of velocity stepping rotary-shear experiments at 25 MPa normal stress. The experiments were terminated at: (a) 44 mm, (b) 79 mm, and (c) 387 mm of sliding, all involving variable-amplitude fluctuations in friction. Microstructural attributes of the gouge were studied using scanning (SEM) and scanning transmission electron microscopy (STEM), image processing, and energy dispersive X-ray (EDX) analyses. The gouge was velocity weakening at sliding distances >10 mm as a core of cataclasites along a through-going shear zone developed within a mantle of less deformed gouge in all experiments. Unlike in experiment (a), the cataclasites in experiments (b) and (c) progressively developed a foliation defined by stacks of shear bands. The individual bands showed an asymmetric particle-size grading normal to shearing direction. These microstructures were subsequently disrupted and reworked by high-angle Riedel shears. While the microstructural evolution affected the effective thickness and frictional strength of the gouge, it did not affect its overall velocity dependence behavior. We suggest that the foliation resulted from competing shear localization and frictional slip hardening and that the velocity dependence of natural fault gouge depends upon compositional as well as microstructural evolution of the gouge.     

PETROLOGIC AND GEOCHRONOLOGIC STUDIES OF THE KONGUR SHAN DETACHMENT SYSTEM, CENTRAL PAMIR

PETROLOGIC AND GEOCHRONOLOGIC STUDIES     

Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.     

The Yaean Metamorphic Core Complex and Extensional Detachment Fault in Inner Mangolia, China

The article describes the characteristics of the Yagan metamorphic core complex, especially the associated detachment fault and various extensional structures in its footwall. The age of the complex is discussed in some detail as well. The basic features of the Yagan metamorphic complex (Jurassic in age) are similar to those of the metamorphic core complex (Tertiary in age) in the Cordilleran area; they are as follows: (a) mylonitic gneisses in the footwall, (b) chloritized sheared mylonitic rocks, (c) pseudotachylites and flinty cataclasites or microbreccias, (d) unmetamorphosed or epimetamorphic rocks in the hanging wall with a layer of fault gouges or incohesive fault breccia next to the detachment fault. In contrast to its Cordilleran counterpart, however, there are many extensional faults with different styles (from dactile low-angle normal faults through brittle-ductile to brittle high-angle normal faults)in the footwall.     

Exhumation of the Dayman dome metamorphic core complex, eastern Papua New Guinea

The ～750 km² Dayman dome of the Late Cretaceous Suckling‐Dayman massif, eastern Papua New Guinea, is a domed landform that rises to an elevation of 2850 m. The northern edge of the dome is a fault scarp >1000 m high that is now part of an active microplate boundary separating continental crust of the New Guinea highlands from continental and oceanic crust of the Woodlark microplate. Previous work has shown that a parallel belt of eclogite‐bearing core complexes north‐east of the Dayman dome were exhumed from up to 24–28 kbar in the last few millions of years. The remarkably fresh and lightly eroded scarp of the Dayman dome exposes shallowly‐dipping mylonitic (S1) metabasite rocks (500 m thick) on the northern flank of Mount Dayman. Field relationships near the base of this scarp show a cross cutting suite of ductile and brittle meso‐structures that includes: (i) rare ductile S2 folia with a shallowly ESE‐plunging mineral elongation lineation defined by sodic‐calcic blue amphibole; (ii) narrow steeply‐dipping ductile D2 shear zones; and (iii) semi‐brittle to brittle fault zones. Pumpellyite‐actinolite facies assemblages reported by previous workers to contain local aragonite, lawsonite and/or glaucophane are found in the core of the complex at elevations greater than 2000 m. These assemblages indicate peak metamorphic pressures of 6–9.5 kbar, demonstrating exhumation of the core of the Dayman dome from depths of 20–30 km. The S1 metamorphic mineral assemblage in metabasite includes actinolite‐chlorite‐epidote‐albite‐quartz‐calcite‐titanite, indicative of greenschist facies conditions for the main deformation. New mineral equilibria modelling suggests that this S1 assemblage evolved at 5.9–7.2 kbar at ～425 °C. Modelling variable Fe³⁺ indicates that the sodic‐calcic blue amphibole (D2) formed under a higher oxidation state compared with the S1 assemblage, probably at <4.5 kbar. A SE‐dipping, Mio‐Pliocene sedimentary sequence (Gwoira Conglomerate) forms a hangingwall block juxtaposed by low‐angle fault contact with the metabasite footwall. Prehnite‐bearing D3 brittle fault zones separate the two blocks and likely accommodated the final exhumation of the S1 greenschist facies assemblage in the footwall. These results indicate that the extensive Mt Dayman fault surface coincides with a domed S1 greenschist facies foliation that was last active at >20 km depth. Exhumation of this foliation must therefore be controlled by brittle faults of the active microplate boundary that are largely not observed in the study area. The structural record of the final exhumation of the Dayman dome to the surface was likely lost as a result of erosion, poor exposure or wide spacing of semi‐brittle to brittle fault zones.     

Gold mineralization without quartz veins in a ductile-brittle shear zone, Macraes Mine, Otago Schist, New Zealand

Greenschist facies schist which hosts the Macraes Mine in East Otago, New Zealand has been pervasively altered by post-metamorphic (lower greenschist facies) fluids over a 120 m thick section perpendicular to foliation. Metamorphic titanite has been replaced by rutile, and epidote has been replaced by a variety of metamorphic minerals including siderite, chlorite, muscovite and calcite. The early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding. The cleavage was widely overprinted by a subparallel set of spaced (mm scale) microshears which are locally enriched in rutile and hydrothermal graphite. Strain was then concentrated into narrow (m scale) zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced (100 μm scale) microshears. The highly strained rocks show a combination of mylonitic and cataclastic microstructures, including crystal-plastic grain size reduction and recrystallization of micas to form a new foliation. Muscovite has grown at the expense of albite in the mylonitic cataclasites. Hydrothermal alteration was accompanied by addition of pyrite, arsenopyrite and gold without development of quartz veins. Gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite. The whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold, scheelite, rutile, pyrite and arsenopyrite. This deposit displays a continuum of post-metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt. Received: 4 December 1997 / Accepted: 21 September 1998     

Late orogenic crustal extension in the northern Menderes massif (western Turkey): evidence for metamorphic core complex formation

The Simav metamorphic core complex of the northern Menderes massif, western Turkey, consists of a plutonic (Tertiary) and metamorphic (Precambrian) core (footwall) separated from an allochthonous cover sequence (hanging wall) by a low-angle, ductile-to-brittle, extensional fault zone (i.e. detachment fault). The core rocks below the detachment fault are converted into mylonites with a thickness of a few hundred metres. Two main deformation events have affected the core rocks. The first deformational event (D₁) was developed within the Precambrian metamorphic rocks. The second event (D₂), associated with the Tertiary crustal extension, includes two distinct stages. Stage one is the formation of a variably developed ductile (mylonitic) deformation (D_2d) in metamorphic and granitic core rocks under greenschist facies conditions. The majority of the mylonites in the study area have foliations that strike NNW to NNE and dip SW to SE. Stretched quartz and feldspar grains define the mineral lineation trending SW-NE direction and plunging gently to SW. The kinematic indicators indicate a top-to-NE sense of shear. Stage two formation of brittle deformation (D_2b) that affected all core and cover rocks. D_2b involves the development of cataclasites and high-angle normal faults. An overall top towards the north sense of shear for the ductile (mylonitic) fabrics in the core rocks is consistent with the N-S regional extension in western Turkey.     

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome,Bohemian Massif

Eclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Mi?dzygórze in the Orlica–?nie?nik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high‐pressure and medium‐pressure rocks. The structural succession in all lithologies shows an early shallow‐dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow‐dipping axial‐plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet‐rich (grs = 22–29 mol.%) and omphacite‐rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ～19–22 kbar and ～700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ～9 kbar and ～730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K‐feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ～5 kbar and ～520 °C to ～7 kbar and ～610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ～7.5 and ～630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.     

Structural and geochronological constraints on the tectonic evolution of the Dulong-Song Chay tectonic dome in Yunnan province, SW China

The Dulong-Song Chay tectonic dome lies on the border of China (SE Yunnan Province) and northern Vietnam, and consists of two tectonic and lithologic units: a core complex and a cover sequence, separated by an extensional detachment fault. These two units are overlain unconformably by Late Triassic strata. The core complex is composed of gneiss, schist and amphibolite. SHRIMP zircon U–Pb dating results for the orthogneiss yield an age of 799±10 Ma, which is considered to be the crystallization age of its igneous protolith formed in an arc-related environment. A granitic intrusion within the core complex occurred with an age of 436–402 Ma, which probably formed during partial closure of Paleotethys. Within the core complex, metamorphic grades change sharply from upper greenschist-low amphibolite facies in the core to low greenschist facies in the cover sequence. There are two arrays of foliation within the core complex, detachment fault and the cover sequence: S₁ and S₂. The pervasive S₁ is the axial plane of intrafolial S₀ folds. D₁ deformation related to this foliation is characterized by extensional structures. The strata were structurally thinned or selectively removed along the detachment faults, indicating exhumation of the Dulong-Song Chay tectonic dome. The major extension occurred at 237 Ma, determined by SHRIMP zircon U–Pb and ³⁹Ar/⁴⁰Ar isotopic dating techniques. Regionally, simultaneous tectonic extension was associated with pre-Indosinian collision between the South China and Indochina Blocks. The S₂ foliation appears as the axial plane of NW-striking S₁ buckling folds formed during a compressional regime of D₂. D₂ is associated with collision between the South China and Indochina Blocks along the Jinshajiang-Ailao Shan suture zone, and represents the Indosinian deformation. The Dulong granites intruded the Dulong-Song Chay dome at 144±2, 140±2 and 116±10 Ma based on ³⁹Ar/⁴⁰Ar measurement on muscovite and biotite. The dome was later overprinted by a conjugate strike-slip fault and related thrust fault, which formed a vortex structure, contemporaneously with late Cenozoic sinistral movement on the Ailao Shan-Red River fault.     

Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault,Wollaston Forland,NE Greenland)

Structurally controlled, syn-rift, clastic depocentres are of economic interest as hydrocarbon reservoirs; understanding the structure of their bounding faults is of great relevance, e.g. in the assessment of fault-controlled hydrocarbon retention potential. Here we investigate the structure of the Dombjerg Fault Zone (Wollaston Forland, NE Greenland), a syn-rift border fault that juxtaposes syn-rift deep-water hanging-wall clastics against a footwall of crystalline basement. A series of discrete fault strands characterize the central fault zone, where discrete slip surfaces, fault rock assemblages and extreme fracturing are common. A chemical alteration zone (CAZ) of fault-related calcite cementation envelops the fault and places strong controls on the style of deformation, particularly in the hanging-wall. The hanging-wall damage zone includes faults, joints, veins and, outside the CAZ, disaggregation deformation bands. Footwall deformation includes faults, joints and veins. Our observations suggest that the CAZ formed during early-stage fault slip and imparted a mechanical control on later fault-related deformation. This study thus gives new insights to the structure of an exposed basin-bounding fault and highlights a spatiotemporal interplay between fault damage and chemical alteration, the latter of which is often underreported in fault studies. To better elucidate the structure, evolution and flow properties of faults (outcrop or subsurface), both fault damage and fault-related chemical alteration must be considered.     

Fault geometry and mechanics of marly carbonate multilayers: An integrated field and laboratory study from the Northern Apennines,Italy

Sealing layers are often represented by sedimentary sequences characterized by alternating strong and weak lithologies. When involved in faulting processes, these mechanically heterogeneous multilayers develop complex fault geometries. Here we investigate fault initiation and evolution within a mechanical multilayer by integrating field observations and rock deformation experiments. Faults initiate with a staircase trajectory that partially reflects the mechanical properties of the involved lithologies, as suggested by our deformation experiments. However, some faults initiating at low angles in calcite-rich layers (θ_i = 5°–20°) and at high angles in clay-rich layers (θ_i = 45°–86°) indicate the important role of structural inheritance at the onset of faulting. With increasing displacement, faults develop well-organized fault cores characterized by a marly, foliated matrix embedding fragments of limestone. The angles of fault reactivation, which concentrate between 30° and 60°, are consistent with the low friction coefficient measured during our experiments on marls (μ_s = 0.39), indicating that clay minerals exert a main control on fault mechanics. Moreover, our integrated analysis suggests that fracturing and faulting are the main mechanisms allowing fluid circulation within the low-permeability multilayer, and that its sealing integrity can be compromised only by the activity of larger faults cutting across its entire thickness.     

40Ar/39Ar geochronological constraints on the formation of the Dayingezhuang gold deposit: New implications for timing and duration of hydrothermal activity in the Jiaodong gold province,China

China's largest gold resource is located in the highly endowed northwestern part of the Jiaodong gold province. Most gold deposits in this area are associated with the NE- to NNE-trending shear zones on the margins of the 130–126 Ma Guojialing granite. These deposits collectively formed at ca. 120 ± 5 Ma during rapid uplift of the granite. The Dayingezhuang deposit is a large (> 120 t Au) orogenic gold deposit in the same area, but located along the eastern margin of the Late Jurassic Linglong Metamorphic Core Complex. New ⁴⁰Ar/³⁹Ar geochronology on hydrothermal sericite and muscovite from the Dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130 ± 4 Ma and is the earliest important gold event that is well-documented in the province. The Dayingezhuang deposit occurs along the Linglong detachment fault, which defines the eastern edge of the ca. 160–150 Ma Linglong granite–granodiorite massif. The anatectic rocks of the massif were rapidly uplifted, at rates of at least 1 km/m.y. from depths of 25–30 km, to form the metamorphic core complex. The detachment fault, with Precambrian metamorphic basement rocks in the hangingwall and the Linglong granitoids and migmatites in the footwall, is characterized by early mylonitization and a local brittle overprinting in the footwall. Gold is associated with quartz–sericite–pyrite–K-feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault. Our results indicate that there were two successive, yet distinct gold-forming tectonic episodes in northwestern Jiaodong. One event first reactivated the detachment fault along the edge of the Linglong massif between 134 and 126 Ma, and then a second reactivated the shears along the margins of the Guojialing granite. Both events may relate to a component of northwest compression after a middle Early Cretaceous shift from regional NW–SE extension to a NE–SW extensional regime.     

Formation of chlorite during thrust fault reactivation. Record of fluid origin and P–T conditions in the Monte Perdido thrust fault (southern Pyrenees)

The chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low-grade metamorphic conditions, an approach that is also used in the present study of the Monte Perdido thrust fault from the South Pyrenean fold-and-thrust belt. The Monte Perdido thrust fault is a shallow thrust juxtaposing upper Cretaceous–Paleocene platform carbonates and Lower Eocene marls and turbidites from the Jaca basin. The core zone of the fault, about 6 m thick, consists of intensely deformed clay-bearing rocks bounded by major shear surfaces. Illite and chlorite are the main hydrous minerals in the fault zone. Illite is oriented along cleavage planes while chlorite formed along shear veins (<50 μm in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. δ¹⁸O and δD values of newly formed chlorite support equilibration with sedimentary interstitial water, directly derived from the local hanging wall and footwall during deformation. Given the absence of large-scale fluid flow, the mineralization observed in the thrust faults records the P–T conditions of thrust activity. Temperatures of chlorite formation of about 240°C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7 km are determined for the Monte Perdido thrust reactivation, coupling calculated temperature and fluid inclusion isochores. The present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low-grade metamorphic conditions.     

Geology and wall rock alteration at the Hercynian Draa Sfar Zn–Pb–Cu massive sulphide deposit, Morocco

Draa Sfar is a siliciclastic–felsic, volcanogenic massive sulphide (VMS) Zn–Pb–Cu deposit located 15 km north of Marrakesh within the Jebilet massif of the western Moroccan Meseta. The Draa Sfar deposit occurs within the Sarhlef series, a volcano-sedimentary succession that hosts other massive sulphide deposits (e.g., Hajar, Kettara) within the dominantly siliciclastic sedimentary succession of the lower Central Jebilet. At Draa Sfar, the footwall lithofacies are dominated by grey to black argillite, carbonaceous argillite and intercalated siltstone with localized rhyodacitic flows and domes, associated in situ and transported autoclastic deposits, and lesser dykes of aphanitic basalt and gabbro. Thin- to thick-bedded, black carbonaceous argillite, minor intercalated siltstone, and a large gabbro sill dominate the hanging wall lithofacies. The main lithologies strike NNE–SSW, parallel to a pronounced S1 foliation, and have a low-grade, chlorite–muscovite–quartz–albite–oligoclase metamorphic assemblage. The Draa Sfar deposit consists of two stratabound sulphide orebodies, Tazakourt to the south and Sidi M'Barek to the north. Both orebodies are hosted by argillite in the upper part of the lower volcano-sedimentary unit. The Tazakourt and Sidi M'Barek orebodies are highly deformed, sheet-like bodies of massive pyrrhotite (up to 95% pyrrhotite) with lesser sphalerite, galena, chalcopyrite, and pyrite. The Draa Sfar deposit formed within a restricted, sediment-starved, fault-controlled, anoxic, volcano-sedimentary rift basin. The deposit formed at and below the seafloor within anoxic, pelagic muds.The argillaceous sedimentary rocks that surround the Draa Sfar orebodies are characterized by a pronounced zonation of alteration assemblages and geochemical patterns. In the more proximal volcanic area to the south, the abundance of medium to dark green chlorite progressively increases within the argillite toward the base of the Tazakourt orebody. Chlorite alteration is manifested by the replacement of feldspar and a decrease in muscovite abundance related to a net addition of Fe and Mg and a loss of K and Na. In the volcanically distal and northern Sidi M'Barek orebody alteration within the footwall argillite is characterized by a modal increase of sericite relative to chlorite. A calcite–quartz–muscovite assemblage and a pronounced decrease in chlorite characterize argillite within the immediate hanging wall to the entire Draa Sfar deposit. The sympathetic lateral change from predominantly sericite to chlorite alteration within the footwall argillite with increasing volcanic proximity suggests that the higher temperature part of the hydrothermal system is coincident with a volcanic vent defined by localized rhyodacitic flow/domes within the footwall succession.