Tracking Deep Meteoric Fluid Circulation Along a Regional Detachment Fault Using Field Infrared Spectroscopy

The Ada Tepe gold deposit, 230 km SE of Sofia, formed in the eastern part of the Rhodope Mountains that underwent extension and metamorphic core complex formation, followed by normal faulting, basin subsidence, and silicic to mafic magmatism during the Maastrichtian–Oligocene. The region comprises numerous volcanic-hosted epithermal and base-metal vein deposits spatially and temporally associated with the Oligocene magmatism. Ada Tepe is a typical low-sulfidation epithermal gold deposit, unusual in that it is older than adjacent magmatic-related deposits and is hosted in Maastrichtian–Paleocene sedimentary rocks above a detachment fault contact with underlying Paleozoic metamorphic rocks. Gold mineralization is located in: (1) a massive, tabular ore body above the detachment fault; and (2) open spacefilling ores along predominantly east–west oriented listric faults. The ores are zones of intensive silicification and brecciation synchronous with detachment faulting. Brittle deformation opened spaces in which bands of opaline silica and electrum, quartz, pyrite, massive and bladed carbonates were deposited. Mineralization is exclusively a Au system with Au/Ag ~3, trace As and no base metals. Alteration consists of quartz, adularia, chlorite, sericite, calcite, pyrite and clay minerals. Adularia and abundant bladed carbonates indicate boiling within the entire span of the deposit, whereas bands of opaline silica with dendritic gold suggest that silica and gold were transported as colloids. The physical setting of formation of the Ada Tepe deposit was very shallow and low temperature. The Sr and Pb isotope ratios of carbonates and pyrite reflect hydrothermal fluid signatures derived predominantly from the metamorphic rocks. The age of mineralization and association with the detachment fault suggest that gold mineralization at Ada Tepe is more closely linked to the Kessebir metamorphic core complex rather than to local magmatism.

The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension, thus explaining the spatial relationship between structures.

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Combined with field studies, microscopic observations, and EBSD fabric analysis, we defined a possible Early Cretaceous metamorphic core complex (MCC) in the Wulian (五莲) area along the Sulu (苏鲁) orogenic belt in eastern China. The MCC is of typical Cordilleran type with five elements: (1) a master detachment fault and sheared rocks beneath it, a lower plate of crystalline rocks with (2) middle crust metamorphic rocks, (3) syn-kinematic plutons, (4) an upper plate of weakly deformed Proterozoic metamorphic rocks, and (5) Cretaceous volcanic-sedimentary rocks in the supradetachment basin. Some postkinematic incursions cut across the master detachment fault zone and two plates. In the upper plate, Zhucheng (诸城) Basin basement consists of the Proterozoic Fenzishan (粉子山) Group, Jinning period granite (762–834 Ma). The supradetachment basin above the Proterozoic rocks is filled with the Early Cretaceous Laiyang (莱阳) (∼135–125 Ma) and Qingshan (青山) groups (120–105 Ma), as well as the Late Cretaceous Wangshi (王氏) Group (85–65 Ma). The detachment fault zone is developed at the base and margin of the superposed basin. Pseudotachylite and micro breccia layers located at the top of the detachment fault. Stretching lineation and foliation are well developed in the ductile shear belt in the detachment faults. The stretching lineation indicates a transport direction of nearly east to west on the whole, while the foliations trend WNW, WSW, and SE. Protomylonite, mylonite, and ultramylonite are universally developed in the faults, transitioning to mylonitic gneiss, and finally to gneiss downward. Microstructure and quartz preferred orientation show that the mylonites formed at high greenschist facies to low greenschist facies as a whole. The footwall metamorphic rock series of the Wulian MCC are chiefly UHP (ultrahigh pressure) metamorphic rocks. Syntectonic rocks developed simultaneously with the Wulian MCC detachment and extension. Geological research has demonstrated that the MCC is associated with small-scale intrusive rocks developing in the vicinity of the detachment faults, for instance, dike. Geochronology results indicate that the denudation of the Wulian MCC occurred at about 135–122 Ma. Its development and exhumation was irrelevant to the Sulu UHP metamorphism zone rapid exhumation during Triassic Period but resulted from the crustal extension of North China Craton and adjacent area.

We present thermochronologic and geochronologic data that constrain the slip history of the Buckskin‐Rawhide detachment fault in west central Arizona, one of the largest extensional fault systems in the North American Cordillera. (U‐Th)/He zircon and apatite thermochronology, integrated with 40Ar/39Ar geochronology of postdetachment volcanic rocks, indicate that large‐magnitude extension associated with the detachment fault initiated at ~21–20 Ma and continued until ~12–11 Ma in the southwestern portion of the Buckskin‐Rawhide metamorphic core complex. (U‐Th)/He footwall cooling ages from the breakaway zone in the western Bouse Hills to upper greenschist‐facies mylonites in the southern Buckskin Mountains indicate that the slip rate on the detachment fault was 3 + 1.5/−1 km/Myr during the early Miocene. Space‐time patterns of hanging wall tilting suggest that at 17–16 Ma, a secondary detachment fault breakaway developed ~12 km northeast of the primary detachment fault breakaway. Proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6–11 km northeast in the middle Miocene by the Buckskin‐Rawhide detachment fault at a slip rate of 1.2–2.7 km/Myr. The total displacement across the detachment fault in the southwestern portion of the core complex is 24 ± 10 km, well short of the previous estimate of 66 ± 8 km across the entire core complex. Based on these data and new observations, we propose that total displacement on the Buckskin‐Rawhide detachment fault system increases in the slip direction to ~40–50 km at the northeastern end of the exposed footwall, corresponding to time‐averaged slip rates that ranged from ~2 km/Myr to ≤6 km/Myr across the entire core complex.

The Liaodong Peninsula Early Cretaceous extension province (LEP), located in the eastern part of the North China Craton (NCC), was highly extended in early Cretaceous. The Liaonan and the Wanfu metamorphic core complexes (MCCs) are two most important elements in the province. Both are typical Cordilleran‐type core complexes that are composed of a general three‐layered structure but show various differences. Shearing along the Jinzhou detachment fault zone in the Liaonan MCC generated ductile to brittle tectonites, various deformation microstructures, and several types of quartz c‐axis fabrics, implying that the MCC has a complicated progressive exhumation history. However, medium‐temperature microstructures and fabrics are superimposed by low‐temperature features along the Wanfu detachment fault zone suggesting a relatively short and simple exhumation history of the Wanfu MCC. Magmatic zircon U‐Pb geochronological dating reveals that the early Cretaceous extension of the LEP began with the initiation of shearing along the Jinzhou detachment fault zone and exhumation of the Liaonan MCC before ca. 134 Ma. Subsequently, relatively slow cooling and exhumation of the lower plate accompanied a giant magmatic event from 130 Ma to 120 Ma. Exhumation of a new MCC, i.e., the Wanfu MCC, was triggered by progressive extension after ca. 120 Ma. Rapid cooling and exhumation of the metamorphic lower plate of the two MCCs are attributed to coeval detachment faulting along both the Jinzhou and Wanfu faults from 120 Ma to 113 Ma. The extension ended at ca. 107 Ma. These data provide reliable evidence that the two MCCs were exhumed progressively and sequentially during two stages of crustal extension of the LEP. Our results indicates that heterogeneous lithosphere extension and upper‐middle crust/upper mantle detachment, possibly due to the interaction between the Paleo‐Pacific and Eurasian plates, are among the most important processes during the early Cretaceous thinning of the lithosphere. Tectonic extension may have led to the detachment in East Asia, which contributed to the thinning of the lithosphere and the destruction of the NCC. A phase of uniform and consistent WNW‐ESE extension was responsible for the generation of the extensional structures in the eastern Asia continent and the destruction of the NCC.

The Buckskin-Rawhide and northern Plomosa Mountains in west-central Arizona are metamorphic core complexes that record NE-directed, large-magnitude extension in the early to middle Miocene. Both core complexes consist of mylonites exposed in the footwall of corrugated, low-angle detachment faults. The Late Cretaceous Orocopia Schist and early Miocene intrusions dominate the mylonitic footwall of the northern Plomosa Mountains. The Orocopia Schist was emplaced during low-angle subduction of the Farallon plate and exhibits the hallmarks of the underplated Laramide subduction complexes, including blocks of metasomatized peridotite encased in quartzofeldspathic schist. In the Buckskin-Rawhide Mountains, carbonate-rich metasedimentary rocks that were buried to midcrustal depths by Mesozoic thrust faults preferentially absorbed Miocene footwall strain and localized the Buckskin detachment fault. A correlation between distinct granodiorite in the footwall of the Buckskin detachment fault in the eastern Bouse Hills and a hanging-wall conglomerate sourced from this granodiorite provides constraints on middle Miocene displacement across the detachment fault and indicates that displacement increased northeastward in the slip direction. The termination of slip on the Buckskin detachment fault also younged northeastward, largely ending by ca. 19 Ma in the western Bouse Hills and ca. 17 Ma in the westernmost Buckskin Mountains, but continuing to <16 Ma in the southern Buckskin Mountains and to ca. 12 Ma in the eastern Buckskin Mountains. Late stages of slip on the detachment fault record minor NW-SE shortening during amplification of corrugation folds. Postdetachment faulting across the region was dominated by dextral and oblique-dextral slip on NW-striking faults influenced by the Pacific–North America plate boundary. Locations visited on this three-day field trip highlight the structural evolution of these metamorphic core complexes.

The origin of low‐angle normal faults (LANFs) and shear zones is subject to debate, and the Linglong metamorphic core complex (MCC) in eastern China provides good examples of how they form. Here we present structural and geochronological data that shed light on the geometry, kinematics, and evolution of this MCC. The MCC is bound by the Zhaoping and Jiaojia detachments with an opposite vergence and appears as a bivergent MCC. Both detachments consist of low‐angle ductile shear zones and parallel normal faults. A series of NNE‐SSW‐striking low‐angle normal‐sense shear zones, including ones that are subhorizontal, occur within the northern part of the MCC, and they show opposite tectonic vergence on either side of the MCC. Microstructures in the shear zones suggest that temperatures during deformation were mainly 300–400°C, corresponding to the brittle‐ductile transition. These shear zones are typically overprinted by brittle deformation structures represented by foliation‐parallel normal faults, local LANFs, and widespread high‐angle normal faults. Zircon U‐Pb dating of intrusive rocks within these shear zones constrains the formation of the MCC to 137–108 Ma. Regional NW‐SE extension was responsible for the development of the MCC. The two low‐angle detachments were initiated along the boundary of the preexisting Linglong batholith. The low‐angle shear zones within the MCC formed due to synchronous extension and arching, and their orientations were affected by synkinematic intrusion. Later LANFs developed mainly along the previous low‐angle shear zones due to their high reactivation angles and the arching‐induced flexural stresses.

Lower Permian volcanic and sedimentary rocks of the Collio Formation in the Orobic Anticline do not rest with a depositional contact on the Variscan basement but are separated from it by the subhorizontal Grassi Detachment Fault, consisting of a cataclasite layer underlain by mylonite. Field relations indicate that both the cataclasite and the mylonite are Early Permian in age. The mylonite formed in a continuous process before, during, and after the intrusion of the Val Biandino Quartz Diorite in the footwall of the detachment fault. Microstructure and quartz texture of the mylonite indicate top-to-the-southeast displacement. Quartz textures of mylonite close to the intrusive bodies are characterized by c-axis single maxima near the Y-direction of the finite strain, indicating prism <a>glide as the dominant gliding system and hence high temperatures (above c. 500 °C) during mylonitization. This is explained by heat advection through the rising quartz diorite melt. During detachment faulting, the footwall of the Grassi Detachment Fault was bowed up to form a metamorphic core complex. The Ponteranica Conglomerate was deposited as a proximal, syntectonic fan-delta on the southeast side of the metamorphic core complex late in its evolution. The unconformity of the Verrucano Lombardo over the Collio Formation and the basement results from erosion of the topography created by detachment faulting, core complex updoming, and block tilting. These results indicate dramatic SE–NW stretching (in present-day coordinates) of the South-Alpine crust during the Early Permian. The return from the thickened, orogenic crust at the end of the Hercynian orogeny to the normal crustal thickness ( c. 30 km) of Late Permian and Early Triassic times was accommodated to a large extent by crustal extension, at least in this part of the southern Alps.

Reactivation of a collisional suture by Miocene transpressional domes associated with the Red River and Song Chay detachment faults, northern Vietnam

In terranes that have undergone substantial extension, three sets of faults dominate: ( a ) shallow- to steep-dipping, commonly rotational normal faults; ( b ) a really extensive, shallow-dipping, normal detachment faults; and ( c ) steep-dipping transfer faults that strike at high angles to the normal faults. These fault systems may extend through a large fraction of the crust. Reactivation of these fault systems will depend primarily on the relative strengths of the faults (shear zones) and their host rock, and their orientation in the prevailing stress field. It is concluded that reactivation is generally mechanically favoured, but that it will probably only take place when the fault-shear zones are in near-ideal orientations. Consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse (thrust) reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles. Examples of these styles are described from the Bass Strait Basins of southeastern Australia. Because extended terranes commonly underlie sedimentary basins (for example, on passive continental margins), reactivation of extensional faults may be a key control on the tectonic evolution of such basins (i.e. basin inversion).

This paper summarizes the results of completed and ongoing research in three areas of the Basin and Range Province of the western United States that casts doubt on the interpretation of specific regional detachment faults and the large extensional strains with which such faults are commonly associated. Given that these examples were influential in the development of ideas about low-angle normal faults, and particularly in making the case for frictional slip at dips of appreciably less than the 30° lock-up angle for μ ≈ 0.6 (where μ is the coefficient of friction), we advocate a critical re-examination of interpreted detachments elsewhere in the Basin and Range Province and in other extensional and passive margin settings. The Sevier Desert ‘detachment’ of west-central Utah is reinterpreted as a Palaeogene unconformity that has been traced to depth west of the northern Sevier Desert basin along an unrelated seismic reflection (most probably a splay of the Cretaceous-age Pavant thrust). The absence of evidence in well cuttings and cores for either brittle deformation (above) or ductile deformation (below) is inconsistent with the existence of a fault with as much as 40 km of displacement. The Pavant thrust and the structurally higher Canyon Range thrust are erosionally truncated at the western margin of the southern Sevier Desert basin, and are not offset by the ‘detachment’ in the manner assumed by those inferring large extension across the basin. The Mormon Peak detachment of SE Nevada is reinterpreted as a series of slide blocks on the basis of detachment characteristics and spatially variable kinematic indicators that are more closely aligned with the modern dip direction than the inferred regional extension direction. A particularly distinctive feature of the detachment is a basal layer of up to several tens of centimetres of polymictic conglomerate that was demonstrably involved in the deformation, with clastic dykes of the same material extending for several metres into overlying rocks in a manner remarkably similar to that observed at rapidly emplaced slide blocks. The Castle Cliff detachment in the nearby Beaver Dam Mountains of SW Utah is similarly regarded as a surficial feature, as originally interpreted, and consistent with its conspicuous absence in seismic reflection profiles from the adjacent sedimentary basin. The middle Miocene Eagle Mountain Formation of eastern California, interpreted on the basis of facies evidence and distinctive clast provenance to have been moved tectonically more than 80 km ESE from a location close to the Jurassic-age Hunter Mountain batholith of the Cottonwood Mountains, is reinterpreted as having accumulated in a fluvial–lacustrine rather than alluvial fan–lacustrine setting, with no bearing on either the amount or direction of tectonic transport. The conglomeratic rocks upon which the provenance argument was based are pervasively channelized, with erosional relief of less than 1 m to as much as 15 m, fining-upwards successions at the same scale and abundant trough cross-stratification – all characteristic features of fluvial sedimentation and not of alluvial fans. The interpretation of the Eagle Mountain Formation as having been deposited within a few kilometres of the Hunter Mountain batholith, which depends strongly on assumptions about the dimensions of alluvial fans, is therefore not required. The result is important because the Eagle Mountain offset has been viewed as representing the strongest evidence for extreme extension in this part of California, and for the existence of detachment faults of regional dimensions.

Metamorphic core complex (MCC) is characterized by the exhumation of lower crust over a large-scale detachment fault, providing natural records for tectonic extension. MCCs are widely identified in the North China Craton (NCC), which have been intensively studied on their structural and geological characteristics. Yet, the condition for the formation of MCCs and their link with NCC destruction are still in debate. In this study, we perform numerical simulations to investigate MCC formation under extension, with a focus on the effect of crustal rheologies. Results indicate that three end-member modes of deformation may occur: the metamorphic core complex mode, the detachment fault-uplifting mode and the pure shear mode. Weaker lower crust and stronger upper crust may promote the formation of MCC. In contrast, stronger lower crust (>1.3×1021 Pa s) may prohibit the exhumation of lower crust (detachment fault-uplifting mode), while weaker upper crust (<7.8×1021 Pa s) may fail to develop detachment faults (pure shear mode). Given that cratons typically have a strong crust, we suggest that the lower crust of NCC was weakened prior to extension, which promoted the formation of MCC in a later stage under the back-arc extension.

Two models, 'model-NF' without detachment fault (contains three layers; upper crust, lower crust, and mantle) and 'model-F' with detachment fault (contains four layers; upper crust, lower crust, mantle, and detachment fault zone) are simulated to determine the effect of detachment fault on the distribution of normal faults in the incipient Red Sea area under elastic plane strain condition. The influence of rock layer properties on fault development are compared in the two models. Three types of displacement boundary condition (extensional, spreading, and spreading plus upward) are used in both models to examine the results and to search the best fitted boundary condition. Results show that (1) detachment fault controls the normal fault system in the incipient Red Sea area, (2) cohesion and friction angle are sensitive to fault development, (3) spreading and spreading plus upward displacement boundary conditions realize the reasonable results which support the active rifting hypothesis in the development of the Red Sea rift system.

Role of heat and detachment in continental extension as viewed from the eastern basin and range province in Arizona