Crustal-scale Normal Fault Research Articles

The distinct morphologic signature of underplating and seamounts in accretionary prisms, insights from thermomechanical modeling applied to Coastal Iranian Makran

The active structures of the Iranian Makran, especially the presence of normal faults, vary laterally in the upper plate of the subduction zone, and their relationship with the deep duplexes and seamounts at depth remains unknown due to poor coverage of data onshore. In this paper, we investigate the relationship between deep structures and the topographic slope using thermo-mechanical simulations. The initial and boundary conditions of the models, including basal heat flux and convergence rate, are calibrated using the depth of the seafloor, bottom seafloor reflectors and the few available well temperature. We specifically test the influence of seamount subduction and thermally controlled changes in rock strength and décollement on the relationship between topography and deep tectonic structures. The inclusion of the brittle-ductile transition and dehydration reactions like the smectite-illite transition produce the three slope segments observed in the Makran accretionary prism. The three segments correspond to a regular accretionary prism, a flat segment that marks the smectite-illite transition, and a rise in topography located above the zone where the tegument reaches its temperature-controlled brittle-ductile transition causing underplating. Nevertheless, the models show that neither the decrease in friction associated with dehydration nor the onset of underplating is sufficient for the observed normal faults to arise in a self-consistent manner from the simulations. Crustal-scale normal faults only emerge in simulations that include subduction of a large seamount. These simulations also produce a large-thrust-slice that is the scar of former subducting seamount and serves as a buttress for the formation of a new imbricated zone. Using offshore seismic reflection data, the published onshore tomographic profiles, and our thermo-mechanical simulations, we propose two onshore-offshore cross-sections of the Iranian Makran accretionary prism.

The boundary between the North China Craton and the Central Asian Orogenic Belt in NE China: Seismic evidence from receiver function imaging

• 38 broadband seismic stations (profile NCISP-10, 400-km-long) were deployed across the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) in Northeast China. • Receiver function CCP images reveal a southward thrust nappe structure at the boundary of the NCC and the CAOB. • Yilan-Yitong fault (YYF) cuts through the Moho and has a vertical offset of 2–3 km indicating that it represents an ultra-crust normal fault. This study imaged the crustal structure and tectonic state of the interacting North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using receiver function imaging. The seismic profile derived from the NCISP-10 broadband seismic array spanning a northeasterly area of the NCC and a southeasterly area of the CAOB. The profile shows a flat Moho with localized anomalies but generally resting at an average depth of 32 km. The average Vp/Vs ratio was about 1.75 but the crust around the Songliao Basin exhibits a relatively high Vp/Vs ratio and a shallow Moho. An obvious velocity discontinuity appears within the crust beneath the Songliao Basin as evidenced by theoretical model tests which managed effects from multiple waves generated by sedimentary layers in the basin. This discontinuity inclines gently and is interpreted to represent the deep extension of the Chifeng-Kaiyuan fault (CKF), which itself serves as the surface boundary between the NCC and CAOB. The NCC feature appears to the south of this boundary, while a nappe structure formed during subduction-collision and composed primarily of a subduction-accretionary complex lies to the north. The Tanlu Fault consists of two branches in the study area, the Yilan-Yitong fault (YYF) and the Dunhua-Mishan fault (DMF). The YYF cuts through the Moho and has a vertical offset of 2–3 km indicating that it represents a crustal-scale normal fault. The DMF gives no obvious indication of vertical offset in the images but may be mainly characterized by sinistral strike slip movement.

Spatial-Temporal Evolution of Extensional Faulting and Fluid Circulation in the Amatrice Basin (Central Apennines, Italy) During the Pleistocene

In extensional continental settings, crustal-scale normal faults can accommodate deformation and subsidence at their hanging wall via activation and deactivation of subsidiary tectonic structures. Geological data obtained from subsidiary structures are required to infer the position of the tectonic deformation during the spatial-temporal evolution of the growth-fault system, with significant implications for structures belonging to seismogenic settings. Here, we describe a subsidiary tectonic structure (the Amatrice Fault System) accommodating Quaternary extensional deformation in the Amatrice Basin (central Apennines, Italy), which is an intermountain morpho‐structural depression involved by the 2016–2017 seismic sequence. Structurally, the Amatrice Fault System defines a ~10 km-long tectonic feature running through the Amatrice Basin, and consists of NNW-SSE-striking and E-W-striking fault segments that interact and link over time. Cross-cutting fault relationships are used to reconstruct a kinematic scenario of fault growth and propagation under an ENE-WSW-directed crustal stretching, consistent with the paleostress regime governing the Quaternary activity of the central Apennines. The analysis of stable carbon and oxygen isotopes on syn-kinematic carbonate mineralizations (calcite veins and calcite fibers on fault surfaces) indicates a meteoric water circulation during the development of the growing fault structure, characterized by variable contributions of organic carbon (soil CO2), and suggesting surface rupture and hydrodynamic interconnection with the vadose zone during faulting. Geochronological U-Th dating on the same mineralizations indicates Middle-Late Pleistocene ages for tectonic activity of the Amatrice Fault System, with the younger age being 108±10 ka. Our results shed light on the long-term tectonics in the Amatrice Basin, in which the Amatrice Fault System records fault growth, hydrodynamic regime and structural permeability network developed under possible coseismic conditions during the Pleistocene. Activation and deactivation of minor tectonic structures, such as the Amatrice Fault System, can provide insights on the localization of tectonic deformation at the hanging wall of a master fault, with implication on the releasing seismogenic potential in active tectonic domains similar to the central Apennines.

Distributed normal faulting in the tip zone of the South Alkyonides Fault System, Gulf of Corinth, constrained using 36Cl exposure dating of late-Quaternary wave-cut platforms

The geometry, rates and kinematics of active faulting in the region close to the tip of a major crustal-scale normal fault in the Gulf of Corinth, Greece, are investigated using detailed fault mapping and new absolute dating. Fault offsets have been dated using a combination of 234U/230Th coral dates and in situ36Cl cosmogenic exposure ages for sediments and wave-cut platforms deformed by the faults. Our results show that deformation in the tip zone is distributed across as many as eight faults arranged within ~700 m across strike, each of which deforms deposits and landforms associated with the 125 ka marine terrace of Marine Isotope Stage 5e. Summed throw-rates across strike achieve values as high as 0.3–1.6 mm/yr, values that are comparable to those at the centre of the crustal-scale fault (2–3 mm/yr from Holocene palaeoseismology and 3–4 mm/yr from GPS geodesy). The relatively high deformation rate and distributed deformation in the tip zone are discussed in terms of stress enhancement from rupture of neighbouring crustal-scale faults and in terms of how this should be considered during fault-based seismic hazard assessment.

Fast cooling of normal-fault footwalls: Rapid fault slip or thermal relaxation?

Abstract Rapid rock exhumation in mountain belts is commonly associated with crustal-scale normal faulting during late-orogenic extension. The process of normal faulting advects hot footwall rocks toward Earth’s surface, which shifts isotherms upwards and increases the geothermal gradient. When faulting stops, this process is reversed and isotherms move downwards during thermal relaxation. Owing to these temporal changes of the geothermal gradient, it is not straightforward to derive the history of faulting from mineral cooling ages. Here, we combine thermochronological data with thermokinematic modeling to illustrate the importance of syntectonic heat advection and posttectonic thermal relaxation for a crustal-scale normal fault in the European Alps. The north-south–trending Brenner fault defines the western margin of the Tauern window (Austria) and caused the exhumation of medium-grade metamorphic rocks during Miocene orogen-parallel extension of the Alps. We analyzed samples from a 2-km-thick crustal section, including a 1000-m-long drill core. Zircon and apatite (U-Th)/He ages along this transect increase with elevation from ca. 8 to ca. 10 Ma and from ca. 7 to ca. 9 Ma, respectively, but differ by only ∼1 m.y. in individual samples. Thermokinematic modeling of the ages indicates that the Brenner fault became active at 19 ± 2 Ma and caused 35 ± 10 km of crustal extension, which is consistent with independent geological constraints. The model results further suggest that the fault slipped at a total rate of 4.2 ± 0.9 km/m.y. and became inactive at 8.8 ± 0.4 Ma. Our findings demonstrate that both syntectonic heat advection and posttectonic thermal relaxation are responsible for the cooling pattern observed in the footwall of the Brenner normal fault.

Tsunamigenic structures in a creeping section of the Alaska subduction zone

Segments of subduction zones that are capable of generating tsunamigenic earthquakes appear to have characteristic structural configurations. These structures include heterogeneous plate interfaces, a small wedge of deformed sediment at the toe of the overriding plate (the frontal prism), and splay faults in the crust of the overriding plate that root within the plate boundary megathrust. Here we use seismic reflection imaging to show that these features also exist within a creeping segment of the Alaska subduction zone, the Shumagin Gap. We identify an active crustal-scale normal fault system that dips landward and resembles that involved in the 2011 Tohoku-oki earthquake in Japan. We also find that the Shumagin Gap has a small frontal prism, a deep-water splay fault, and that the plate interface here is rough and thinly sedimented. We propose that lateral propagation of rupture from a neighbouring segment into the Shumagin Gap may explain a tsunamigenic earthquake that occurred there in 1788 and that tsunamigenic potential should be considered in hazard assessments for the region. Our results demonstrate that structural configurations similar to those in Tohoku may exist in other subduction zones, including within creeping segments or segments with no record of historical megathrust earthquakes, but are under-recognized. Identifying similar configurations globally may improve our ability to anticipate regions capable of generating large tsunamis. Creeping subduction zones are unlikely to generate tsunamigenic earthquakes. Analysis of a creeping part of the Alaskan subduction zone reveals fault structures similar to those in Tohoku, suggesting it may host large earthquakes and tsunamis.

Meteoric fluid infiltration in crustal-scale normal fault systems as indicated by δ18O and δ2H geochemistry and40Ar/39Ar dating of neoformed clays in brittle fault rocks

Both the sources and pathways of fluid circulation are key factors to understanding the evolution of low-angle normal fault (LANF) systems and the distribution of mineral deposits in the upper crust. In recent years, several reports have shown the presence of meteoric waters in mylonitic LANF systems at mid-crustal conditions. However, a mechanism for meteoric water infiltration to these mid-crustal depths is not well understood. Here we report paired δ 18 O and δ 2 H isotopic values from dated, neoformed clays in fault gouge in major detachments of the southwest United States. These isotopic values demonstrate that brittle fault rocks formed from exchange with pristine to weakly evolved meteoric waters at multiple depths along the detachment. 40 Ar/ 39 Ar dating of these same neoformed clays constrains the Pliocene ages of fault-gouge formation in the Death Valley area. The infiltration of ancient meteoric fluids to multiple depths in LANFs indicates that crustal-scale normal fault systems are highly permeable on geologic timescales and that they are conduits for efficient, coupled flow of surface fluids to depths of the brittle-plastic transition.

Segmentation of the 2010 Maule Chile earthquake rupture from a joint analysis of uplifted marine terraces and seismic-cycle deformation patterns

The segmentation of major fault systems in subduction zones controls earthquake magnitude and location, but the causes for the existence of segment boundaries and the relationships between long-term deformation and the extent of earthquake rupture, are poorly understood. We compare permanent and seismic-cycle deformation patterns along the rupture zone of the 2010 Maule earthquake (M8.8), which ruptured 500 km of the Chile subduction margin. We analyzed the morphology of MIS-5 marine terraces using LiDAR topography and established their chronology and coeval origin with twelve luminescence ages, stratigraphy and geomorphic correlation, obtaining a virtually continuous distribution of uplift rates along the entire rupture zone. The mean uplift rate for these terraces is 0.5 m/ka. This value is exceeded in three areas, which have experienced rapid emergence of up to 1.6 m/ka; they are located at the northern, central, and southern sectors of the rupture zone, referred to as Topocalma, Carranza and Arauco, respectively. The three sectors correlate with boundaries of eight great earthquakes dating back to 1730. The Topocalma and Arauco sectors, located at the boundaries of the 2010 rupture, consist of broad zones of crustal warping with wavelengths of 60 and 90 km, respectively. These two regions coincide with the axes of oroclinal bending of the entire Andean margin and correlate with changes in curvature of the plate interface. Rapid uplift at Carranza, in turn, is of shorter wavelength and associated with footwall flexure of three crustal-scale normal faults. The uplift rate at Carranza is inversely correlated with plate coupling as well as with coseismic slip, suggesting permanent deformation may accumulate interseismically. We propose that the zones of upwarping at Arauco and Topocalma reflect changes in frictional properties of the megathrust resulting in barriers to the propagation of great earthquakes. Slip during the 1960 (M9.5) and 2010 events overlapped with the ∼90-km-long zone of rapid uplift at Arauco; similarly, slip in 2010 and 1906 extended across the ∼60-km-long section of the megathrust at Topocalma, but this area was completely breached by the 1730 (M∼9) event, which propagated southward until Carranza. Both Arauco and Topocalma show evidence of sustained rapid uplift since at least the middle Pleistocene. These two sectors might thus constitute discrete seismotectonic boundaries restraining most, but not all great earthquake ruptures. Based on our observations, such barriers might be breached during multi-segment super-cycle events.

Titanium-in-quartz thermometry on synkinematic quartz veins in a retrograde crustal-scale normal fault zone

Abstract Previous studies have suggested that estimation of deformation temperatures in quartz mylonites by titanium-in-quartz geothermometry is only possible at temperatures > 500 °C, above which efficient Ti-exchange is achieved via grain boundary migration recrystallization. Based on quartz mylonite samples collected across the Simplon Fault Zone (SFZ) we demonstrate that deformation temperatures of dynamic recrystallization can be obtained down to ~ 350 °C. A prerequisite for such temperature estimates at the low temperature end of ductile deformation of quartz is the formation of synkinematic quartz veins and their immediate overprint either by subgrain rotation (SGR) or bulging recrystallization (BLG). It is the slow growth of the synkinematically precipitating vein quartz that allows for equilibration of Ti in the vein quartz. This Ti-concentration may only slightly be modified during SGR; hence, Ti-in-qtz thermometry provides a close approach to the vein formation temperature. Ti-concentrations are partially reset during BLG, and resulting temperatures are thus maximum temperatures of quartz recrystallization. Importantly, undeformed vein quartz always yield vein formation temperatures. Investigation of the dynamic recrystallization processes overprinting synkinematic quartz veins thus allows for a critical, independent evaluation of the Ti-in-quartz temperatures obtained. For the SFZ, there is a decrease in recrystallized grain sizes towards the fault plane and a change in the dominant recrystallization process associated with a narrowing of the shear zone. As indicated by the Ti-in-quartz temperature estimates, this strain localization correlates with cooling from ~ 560 °C in the oldest microstructures at the periphery of the shear zone down to ~ 350 °C in the youngest microstructures of the footwall near the hanging wall contact. A great benefit of the approach presented here is that intermediate to low temperature plastic deformation in quartz can now also be assessed. Such novel temperature constraints on quartz crystallization are essential for better constraining deformation and rheology in the upper Earth's crust.

Deep crustal melting in the Peruvian Andes: Felsic magma generation during delamination and uplift

The Miocene-aged Yungay and Fortaleza ignimbrites (YFI), 9° S, Cordillera Blanca, Peru, share geochemical affinities typical of Phanerozoic adakite-like rocks and Archaean tonalite–trondhjemite–granodiorite (TTG) suites. In this contribution, we will investigate the melt source region(s) of the adakitic ignimbrites and their relationship to the dynamic tectonic regime in Peru at the time of eruption. The geochemistry of the YFI differs significantly from spatially related lavas in western Peru, which are characterised by classic calc-alkaline arc signatures. The YFI were erupted along crustal-scale normal faults at the culmination of major tectonic changes affecting the western Peruvian margin, where they represent the last volcanic activity recorded in the area. These regional changes included crustal thickening, shallowing of the Nazca Plate from c. 30° to c. 5°, the arrival and subduction of the Nazca Ridge and rapid crustal uplift and exhumation. The previously unstudied adakitic YFI are examined, then a series of high-pressure piston-cylinder experiments are detailed. Synplutonic mafic-intermediate dyke material is used as an analogous source, and direct experiments are performed on ignimbrite samples. These show that the geochemical signature and mineralogy of the YFI can be produced at pressures >2.2GPa and temperatures >1025°C. These data support an origin for both ignimbrites as partial melts of juvenile, garnet-bearing, hydrous, mafic lower crust (granulite to eclogite), modified by ~20% contamination by igneous crust. A slab source is considered highly improbable. Eclogite and granulite at >2GPa (>65km depth) are absent within the modern lithospheric architecture of the Peruvian margin, requiring removal of this source region syn- or post-eruption. A likely mechanism for removal is crustal delamination in an east-relative motion, associated with shortening caused by progressive shallowing of the Nazca Plate subducting slab. Due to the restricted mantle flow through the thinned mantle wedge, there have been no further magmatic episodes since the eruption of the Yungay and Fortaleza ignimbrites.

Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway

The Nordfjord-Sogn Detachment Zone of western Norway represents an archetype for crustal-scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure (UHP) terranes. In this paper, we investigate the role of normal-sense shear zones with respect to UHP exhumation using structural geology, thermobarometry, and geochronology of the Hornelen segment of the Nordfjord-Sogn Detachment Zone. The Hornelen segment of the zone is a 2–6 km thick, top-W shear zone, primarily developed within amphibolite-grade allochthonous rocks, that juxtaposes the UHP rocks of the Western Gneiss Complex in its footwall with lower-grade allochthons and Carboniferous-Devonian Basins in its hanging wall. New thermobarometry and Sm/Nd garnet geochronology show that these top-W fabrics were initiated at lower crustal depths of 30–40 km between 410 Ma and 400 Ma. Structural geology and quartz petrofabrics indicate that top-W shear was initially relatively evenly distributed across the shear zone, and then overprinted by discrete ductile-brittle detachment faults at slower strain rates during progressive deformation and exhumation. These results require a three-stage model for UHP exhumation in which normal-sense shear zones exhumed UHP rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile-brittle structures.

Rift-initiation development of normal fault blocks: insights from the Hammam Faraun fault block, Suez Rift, Egypt

An integrated structural and stratigraphic study of the Hammam Faraun fault block, Suez Rift, Egypt, provides insights into the rift-initiation tectonostratigraphic evolution of the crustal-scale normal fault blocks. The shallow marine to offshore Tayiba Formation (Lower Oligocene) represents the youngest preserved pre-rift unit, and key stratal surface development indicates that relative sea-level variations exerted a marked control on its stratigraphic evolution. A major sea-level fall, which may have been a regional (i.e. eustatic) event, occurred during the mid-Oligocene and was synchronous with the onset of rifting. A major erosional unconformity (the base synrift unconformity) formed in response to the sea-level fall and defines a series of NNE–SSW- to NE–SW-trending palaeovalleys up to 40 m deep by 500 m wide, which are infilled by continental deposits and volcanic rocks of the Abu Zenima Formation (Upper Oligocene–Lower Miocene). During the rift initiation, palaeovalleys controlled depositional patterns and the evolving fault-controlled topography was insufficient to modify drainage patterns. Through time, however, surface-breaking faults began to exert a marked control on deposition. This study indicates the complexity that can occur during the rift-initiation phase caused by extrabasinal factors such as eustatic sea-level variations and antecedent drainage.

Normal fault growth, displacement localisation and the evolution of normal fault populations: the Hammam Faraun fault block, Suez rift, Egypt

Fault segment linkage, migration of the locus of fault activity, and displacement localisation were important processes controlling the late Oligocene–Recent evolution of the normal fault population of the Hammam Faraun fault block, Suez rift. Initial fault activity was distributed across the fault block on fault segments that had attained their final length within 1–2 My of rifting. These initial segments then either grew by increasing displacement and linked to form longer segmented fault zones or died, during a rift initiation phase that lasted 6–8 My. Following this rift initiation phase, displacement became localised onto >25-km-long border fault zones bounding the fault block and many of the early high-displacement intra-block fault zones died. Following displacement localisation onto the major faults bounding the fault block, the locus of maximum displacement continued to migrate, with post-Middle Miocene displacement focused on the western margin of the fault block. This migration of fault activity between major crustal-scale normal faults can be viewed in terms of strain localisation at the rift scale. The results from this study question conventional fault growth models based on final displacement distributions, and highlight the sequential nature of faulting on major normal faults bounding domino-style tilted fault blocks.

Late Quaternary slip rates on the Acireale-Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily)

Mt. Etna is located along the east coast of Sicily, near the boundary between the continental crust of the Hyblean Plateau and the Mesozoic oceanic crust of the Ionian basin. The main active faults near Mt. Etna cut the base of its eastern flank, forming a 30 km long system of NNE- and NNW-trending, ene´chelon fault segments (the Acireale-Piedimonte system), showing dip-slip and oblique (right-lateral) motion. Most segments are associated with shallow-depth seismicity and all have Late Pleistocene to Holocene vertical slip rates ranging between 1 and 2 mm/yr, typical of major active normal faults worldwide. Eruptive fissures, arranged in NNE- to NE-trending zones, cut the highest slopes of the volcano, on the footwall of the normal fault system. Structural analysis suggests that current motions along both the active faults and eruptive fissures are kinematically compatible and simply linked with ongoing, WNW-ESE-directed regional extension. Such extension characterizes, at a greater scale, the active tectonics of southern Italy, where all large, shallow, historical earthquakes have remained confined within a narrow normal fault belt or rift zone stretching from the northern Calabrian Arc to southeastern Sicily. The southernmost, west-dipping, faults of that rift zone in Calabria (Aspromonte) cut across the Calabro-Peloritan thrust belt to join the mostly east-dipping, Acireale-Piedimonte faults, along the western boundary of Ionian oceanic lithosphere, which continues southwards as the Malta escarpment. We thus relate magmatism at Etna to dilational strain on the footwall of an east-facing, crustal-scale normal fault at the bend where the Siculo-Calabrian rift zone veers ≈ 60° eastwards as it begins to cross the Calabrian Arc, leaving the east-facing margin of the Ionian Sea, to catch up with the west-facing margin of the Tyrrhenian Sea farther north.

Mechanisms generating normal fault curvature: a review illustrated by physical models

Abstract This paper briefly reviews the mechanisms generating fault curvature during extension, illustrates them using physical models, and discusses their applicability at different scales and for specific extensional styles. These mechanisms are in two groups. In the first, initially curved fault planes are essentially of mechanical origin. They commonly result from marked vertical changes in stress orientation and rheology within the faulted layer. Experiments suggest that such mechanisms are more likely to influence secondary faults that form where deformation and displacements are severely constrained. The second group includes faults that become curved after they form, either by volume loss due to compaction, by internal deformation of fault blocks, or by interacting sedimentation and faulting. Fault curvature due to compaction is mainly restricted to growth-faulted areas. Internal deformation of blocks affects fault shapes at the transition zone between continuous and discontinuous deformation. Block rotation during rapid deposition can lead to strongly curved fault profiles. Blocks or faults are distorted during block rotation to maintain geometric compatibility within and between the blocks. This review suggests that normal growth faults in deltaic areas are more likely to be listric than crustal-scale normal faults.

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

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