Basement Fault Zone Research Articles

Structural inheritance in Carolinia, external to the excised Rheic Suture: Savannah River Site, South Carolina, USA.

Abstract Phanerozoic reactivations of basement fault zones are documented in 5000 m of basement core recovered from beneath the updip Atlantic Coastal Plain underlying the US Department of Energy Savannah River Site (SRS) in South Carolina. These basement fault zones are adjacent to the excised Rheic Ocean suture. Meta-intrusive rocks from c. 620 and 625 Ma contain a mylonitic fabric and intrude foliated mafic metavolcanic rocks. At c. 305 Ma, granulite facies orthogneisses were thrust over amphibolite facies meta-igneous rocks in the transpressive Tinker Creek Nappe. The overturned limb of the nappe localizes the Triassic Dunbarton Basin Border Fault. The border fault acted as a conduit for fluids in the Mesozoic and Cenozoic. At c. 220 ± 5 Ma, a potassium and silica metasomatic event affected the SRS basement. A propylitic event flushed reducing fluids through rocks as young as the Santonian. The remains of a Triassic sub-basin were identified in the northwesten part of the site. A Cretaceous and younger vein paragenesis overprints the previous events. More than 30 pseudotachylytes are found in the SRS basement and are preferentially localized on metasomatized Alleghanian chloritic fractures. Pseudotachylyte post-dates mineralized fractures. The Pen Branch Fault offsets the basement–Cretaceous unconformity and is present in c. 242 m of core between PBF-7-419 m and PBF-7-660.8 m. The Pen Branch Fault cross-cuts mineralized fractures and must post-date strike-normal zeolites.

Crustal study based on integrated geophysical techniques in the Northwestern Himalayas, Pakistan

The present research aims to explicate the tectonics of the northwest Himalayas, Pakistan, based on integrated gravity, magnetic and earthquake data. The raw gravity and magnetic data were processed, and seven isoanomaly maps were prepared for qualitative interpretation, whereas a gravity model was prepared along the selected profile for quantitative interpretation. The local earthquake data have also been used to identify the various seismogenic layers in the subsurface and to understand how seismicity relates to regional structural features. Three northwest‐trending thick‐skinned blind basement faults are identified from the present study, namely the Indus Kohistan Seismic Zone (IKSZ), Bagh Basement Fault and Hazara Lower Seismic Zone. The IKSZ is the tectonically most active among them and responsible for the 2005 Kashmir earthquake (Mw = 7.6) and 1974 Pattan earthquake (Mw = 6.2) in the region. The present study also delineated the prominent location of the Main Central Thrust in which the high‐grade metamorphic rocks of Higher Himalayan Crystalline thrust over low‐grade metamorphic rocks of Lesser Himalayas along this thick‐skinned fault. The field evidence such as ductile deformation, zone of inverted metamorphism, and brittle deformation also confirm this location. The sharp bending of the crystalline crust, as well as deep basement just south of Kohistan Island Arc (KIA), indicate that the high density and susceptibility rocks of this arc thrust on the leading margin of the Indian Plate along the Main Mantle Thrust. This thrust fault is a major suture zone separating the KIA in the north from the Indian Plate in the south. The other faults in the region exist in the sedimentary/meta‐sedimentary wedge and are thin‐skinned. It is inferred from the current investigation that the study area exhibits a composite style of deformation. Moreover, the gravity model combined with earthquake data suggested the existence of multiple décollement zones below the Hazara‐Kashmir Syntaxis. The model envisaged an increase in the thickness of the Indian Plate continental crust in the northeast direction.

Cenozoic porphyry and other hydrothermal ore deposits along the South Caucasus-West Iranian tectono-magmatic belt: A critical reappraisal of the controlling factors

In this contribution, we investigate the space and time distribution of the Cenozoic magmatism and the associated porphyry Cu-Mo ± Au and other hydrothermal (mostly epithermal) ore deposits along the south Caucasus-west Iranian tectono-magmatic belt. We use a comprehensive georeferenced database containing whole-rock and geochronological analyses of 3150 samples from the igneous rocks distributed in the study area. Overall, the timing of the ore deposits overlaps with the timing of the Eocene to Miocene diachronous collision between the Arabia and Eurasia plates. This is associated with a change in the geochemical fingerprints of the magmas during and after collisional thickening, with a marked increase in Sr/Y values associated with a general transition towards adakitic signatures. This is interpreted with a change towards an amphibole- (±garnet) rich residue at the arc roots. Clusters of ore mineralization are observed within areas of long-lived magmatism (lasting >10 Myr), usually located at the intersections of basement fault zones, parallel and orthogonal to the collisional boundaries and likely formed along inherited zones of crustal/lithospheric weakness. The isotopic data of the ore-forming magmas show a pronounced correlation with young (<0.7 Ga) Nd depleted mantle model ages (TDM Nd ) and ( 87 Sr/ 86 Sr) i isotopic ratios ranging from ∼0.704 to ∼0.706. This evidence attests to post-Cadomian metal- and volatile-rich sources at crust roots, during incremental magmatic underplating. We suggest that optimum conditions to form clusters of ore deposits along the collisional zone are achieved when passive asthenosphere upwelling along slab windows (or induced by lithosphere delamination and/or slab break-off) causes the partial melting of the fertile arc roots in the amphibole ± garnet stability field. The end of mineralization along the west Iran magmatic belt is associated with crust overthickening (as evidenced by the inferred presence of a garnet-dominated residue), either as a consequence of consumption of the fertile metal-enriched sources or after the total removal/foundering of the arc roots. • We present a database of magmatism and linked deposits for Armenia and west Iran. • Ore deposits mainly occur in areas of long-lived and stationary magmatism. • The diachronous collision-related crustal thickening controls timing of deposits. • Productive melts are derived from partial melting of the fertile lower crust roots. • Inherited block boundaries host prolonged magmatism, leading to ore deposition.

Analysis of a basement fault zone with geothermal potential in the Southern North Sea

Extraordinary 3D seismic data from the Central Offshore Platform (Southern North Sea), complemented by information from 38 boreholes, reveal a 10 km-wide basement fault zone above which fluid anomalies emanate from sub-salt reservoirs to terminate in lower Cretaceous strata. Fluid blow-out pipes, chimneys and low-amplitude trails were mostly sourced from the region where NW-striking syn-rift faults intersect the N-striking basement fault zone. As a result, 73% of the mapped fluid-flow anomalies (94 out of 129) occur within the basement fault zone of interest or follow a N-S strike along its shoulders. We postulate a strong control of the basement fault zone on past fluid and heat flow, as basin models confirm that fluid and heat were mostly produced during the Cretaceous. Bottom-hole data record temperatures of ∼140 °C at present, highlighting the geothermal potential of the study area. These temperatures nevertheless contrast with the relatively constant gradient of ∼ 32 °C/km occurring both in and outside the basement fault zone. This work is important as its shows that past fluid and heat flow over a basement fault zone does not necessarily correlate with the existence of an enhanced hydrothermal system at present. However, as bottom-hole temperatures are within the benchmark values considered across Europe, it also stresses the importance of basement fault zones as key structures to find, and assess, as potential geothermal sites.

Spatio-Temporal Evolution of Intermediate-Depth Seismicity Beneath the Himalayas: Implications for Metamorphism and Tectonics

Intermediate-depth earthquakes (&amp;gt;40 km) have been observed beneath the central Himalayas over decades, with little known about their nature and characteristics. Here, we apply a state-of-the-art systematic processing routine, starting from continuous waveform data, to obtain the most comprehensive high-quality earthquake catalog with a focus on the intermediate-depth seismicity beneath the central Himalayas. We construct a catalog containing 414 robust earthquake locations with depths ranging from 40 to 110 km spanning from late 2001 till mid-2003. We calculate earthquake magnitudes in a consistent way and obtain values ranging between ML0.8 and 4.5 with a magnitude of completeness of Mc2.4. This information allows us to study the spatiotemporal characteristics of the seismicity in great detail. Earthquakes mainly take place in a cluster, consisting of two linear segments at ca. 35° azimuth difference, situated beneath the high Himalayas in NE Nepal and adjacent S. Tibet. Seismicity there does not feature any mainshock-aftershock patterns but presents a few sequences with potential seismicity migration rates compatible with linear or diffusive migration. This result, along with previous studies in the lower Indian crust, allows interpreting these events as related to metamorphic reactions involving dehydration processes. However, given the geodynamic context, a tectonic interpretation with a dextral basement fault zone propagating beneath the Himalaya and continuing as a westward propagating tear fault would also be possible. This represents a continuous fault zone from the deep crust in S. Tibet, across the Himalaya along the Dhubri-Chungthang fault zone (DCFZ) to the Shillong plateau, which could be an inherited tectonic feature.

Structure of Masuleh Shear Zone: Evidence for Early–Middle Jurassic Dextral Shear Along Paleo-Tethys Suture Zone in the Western Alborz, NW Iran

The Paleo-Tethys suture zone in northern Iran was formed when the Paleo-Tethys Ocean, (between Gonwana-derived Alborz Microcontinent and the Turan Plate), closed during the Eocimmerian orogeny and after they collided together in the Mid-Late Triassic. The NW-striking Boghrov-Dagh basement Fault Zone that lies in the vicinity of Masuleh village and the southern boundary of Gasht Metamorphic Complex is a part of the Eocimmerian suture zone in the Western Alborz. Along this part of the suture zone, tourmaline leucogranites intruded in metamorphic rocks. We recognize three distinct deformation stages (D1 to D3) in the study area especially in the Masuleh Shear Zone. D1 which was synchronous with formation of the main metamorphic minerals, such as sillimanite and staurolite under medium- to high-grade metamorphic conditions probably during the Hercynian event and a NE-directed shortening. The slaty cleavage in metamorphosed Upper Paleozoic rocks and crenulation cleavage and folds in the older rocks were produced due to D2 deformation during the Eocimmerian event under greenschist facies conditions. The Masuleh Shear Zone formed as a result of a ductile strike-slip shear during the Early-Middle Jurassic Mid-Cimmerian D3 event with a pure dextral to transtension shear sense at low to locally medium-grade conditions. All of the D3 structural features agree with a NNW-directed compression and an ENE-directed extension caused by overall dextral shear parallel to the Masuleh shear zone and the Boghrov-Dagh Fault Zone. Based on the available evidence, especially cross-cutting relationships between structural fabrics and rock units, emplacement of the Gasht-Masuleh leucogranites occurred after the D2 collisional event coeval to the possible slab break-off and before the D3 event, between Eocimmerian and Mid-Cimmerian movements.

Eustatic and far-field tectonic control on the development of an intra-platform carbonate-shoal complex: upper tongue of the Tanglewood Member, Upper Ordovician Lexington Limestone, central Kentucky, U.S.A.

The Lexington or Trenton Limestone is an Upper Ordovician (Chatfieldian–Edenian; upper Sandbian–lower Katian), temperate-water unit, averaging about 60-m thick, that was deposited in relatively shallow waters across the Lexington Platform in east-central United States during the Taconian Orogeny. Lexington/Trenton shallow-water deposition ended across most of the platform in late Chatfieldian time and from that point deepened upward into the more shale-rich Clays Ferry, Point Pleasant and Kope formations due to apparent sea-level rise. In central Kentucky, however, deposition of the Lexington Limestone continued into early Edenian time and includes up to 50m of additional coarse calcarenites and calcirudites at the top, which form the Tanglewood buildup and reflect locally regressive conditions, apparently related to local structural uplift. Consequently, in central Kentucky, the Lexington is more than 100-m thick, and Lexington deposition on the buildup continued into early Edenian time as an intra-platform shoal complex that tongues out into deeper-water units in all directions. In an attempt to understand how this shoal complex developed, we examined the last major body of coarse skeletal sands in the central Kentucky Lexington Limestone, the upper tongue of the Tanglewood Member, a 12-m-thick succession of fossiliferous calcarenite and calcirudite that occurs across an area of 5200km2 near the center of the Lexington Platform. Although relatively homogeneous, the upper Tanglewood is divisible into five, small-scale, fining-upward, sequence-like cycles, which contain prominent, widespread deformed horizons. Facies analysis indicates that four lithofacies, which reflect distinct depositional environments, comprise the sequences across the shoal complex. Lithofacies were correlated across the shoal complex by integrating cyclicity and widespread deformed horizons in order to delineate the locations of major depositional environments. Facies analysis shows that the thickest and coarsest parts of each sequence, and the shallowest depositional environments, coincide with basement fault blocks, which are known to have experienced uplift during earlier Lexington Limestone deposition. The occurrence of thick, coarse facies on the same blocks suggests that the blocks continued to experience uplift into shallow water, where tides and waves redistributed sediments during upper Tanglewood deposition. Although eustasy apparently controlled cyclicity, Taconian far-field forces generated by orogeny in the east seem to have influenced facies distribution in each cycle through reactivation of basement fault zones as synsedimentary growth faults. The example of the upper Tanglewood Member shows that tectonic far-field forces can exert important influences on the development of carbonate depositional environments, even in distal intracratonic settings like the Lexington Platform.

Seismicity on Basement Faults Induced by Simultaneous Fluid Injection–Extraction

Large-scale carbon dioxide (CO2) injection into geological formations increases pore pressure, potentially inducing seismicity on critically stressed faults by reducing the effective normal stress. In addition, poroelastic expansion of the reservoir alters stresses, both within and around the formation, which may trigger earthquakes without direct pore-pressure diffusion. One possible solution to mitigate injection-induced earthquakes is to simultaneously extract pre-existing pore fluids from the target reservoir. To examine the feasibility of the injection–extraction strategy, we compute the spatiotemporal change in Coulomb stress on basement normal faults, including: (1) the change in poroelastic stresses $$\Delta \tau _s+f\Delta \sigma _n$$ , where $$\Delta \tau _s$$ and $$\Delta \sigma _n$$ are changes in shear and normal stress. respectively, and (2) the change in pore-pressure $$f\Delta p$$ . Using the model of (J. Geophys. Res. Solid Earth 99(B2):2601–2618, 1994), we estimate the seismicity rate on basement fault zones. Fluid extraction reduces direct pore-pressure diffusion into conductive faults, generally reducing the risk of induced seismicity. Limited diffusion into/from sealing faults results in negligible pore pressure changes within them. However, fluid extraction can cause enhanced seismicity rates on deep normal faults near the injector as well as shallow normal faults near the producer by poroelastic stressing. Changes in seismicity rate driven by poroelastic response to fluid injection–extraction depends on fault geometry, well operations, and the background stressing rate.

Injection‐induced seismicity on basement faults including poroelastic stressing

AbstractMost significant induced earthquakes occur on faults within the basement beneath sedimentary cover. In this two‐dimensional plane strain numerical study, we examine the full poroelastic response of basement faults to fluid injection into overlying strata, considering both (1) the permeability of the fault zone and (2) the hydraulic connectivity of the faults to the target horizon. Given hydraulic and mechanical properties, we compute the spatiotemporal change in Coulomb stress, which we separate into (1) the change in poroelastic stresses Δτs+fΔσn, where Δτs and Δσn are changes in shear and normal stress (Δτs&gt;0 and Δσn&gt;0 both favor slip), and (2) the change in pore pressure fΔp. Pore pressure diffusion into hydraulically connected, permeable faults dominates their mechanical stability. For hydraulically isolated or low‐permeability faults, however, poroelastic stresses transmitted to deeper basement levels can trigger slip, even without elevated pore pressure. The seismicity rate on basement fault zones is predicted using the model of Dieterich (1994). High seismicity rates can occur on permeable, hydraulically connected faults due to direct pore pressure diffusion. Lower rates are predicted on isolated steeply dipping normal faults, caused solely by poroelastic stressing. In contrast, seismicity on similarly oriented reverse faults is inhibited.

Kinematics and surface fracture pattern of the Anaran basement fault zone in NW of the Zagros fold–thrust belt

The preexisting north-south trending basement faults and their reactivation played an important role during the evolution of the Zagros fold-thrust belt. The Anaran basement fault in the Lurestan r ...

Initial rifting process and dynamics mechanism of Huaguang Sag: Evidence from a numerical modeling method

Huaguang Sag is located in the deep seawater area of Qiongdongnan Basin, and its tectonic position belongs to the intersection of NE-trending, SN-trending and NW-trending tectonic systems in the continental margin of the Northwest South China Sea. To investigate the initial rifting process and further more the dynamics mechanism of Huaguang Sag, this paper sets up the structure model of basement which mainly makes up with several depression-controlling faults, and simulates the initial rifting process of Huaguang Sag by the FLAC software. The simulation results show that only affected by the S-N trending extensional stress, the rifting center appears in northern boundary basement faults (two NEE-trending and NWW-trending faults) of Huaguang Sag while does not take place at the NNE-trending and NE-trending basement fault zone in the middle sag, and doesn’t match the current pattern that the basement fault plays a main role in controlling the sediment. In the other case, affected by the S-N trending and E-W trending extensional stress at the same time, the areas of the northern boundary faults zone and internal NNE-trending basement faults zone come to be rifting center quickly, the sedimentary is controlled by the main basement faults to different degrees, and is consistent with the tectonic-sedimentary framework of Huaguang Sag which obtained by the data of geophysical interpretation. In combination with the analysis of regional tectonic background, the paper proposes that two remote tectonic effects occurred by the collision of India-Eurasian Plate: One remote effect was the rotational extrusion of IndoChina Block, which led to form a series of NE-trending and NNE-trending basement faults, as well as the E-W trending tensile stress field in Huaguang Sag. The other remote effect was that the deep mantle material of South China Block flowed southward, which resulted in the S-N trending extensional rifting of the lithosphere in northern South China Sea, and finally formed a series of EW-trending and NEE-trending basement faults and the S-N trending tensile stress field in Huaguang Sag. Affected by the above tensile stress fields and the basement faults, the initial rifting occurred in E-W and nearly S-N directions along the pre-existed basement faults (the weak structural zones) in Huaguang Sag.

Prediction of the Toxic and Harmful Gas along the Sichuan-Tibet Railway

The geological setting of Sichuan-Tibet Railway is complicated, while magmatic and metamorphic rocks are widely distributed. Reference to the other engineering experience of underground works under similar geological conditions at home and abroad, the analysis shows that it exists the hazardous possibility that the inorganic gas are toxic and harmful in the tunnels of Sichuan-Tibet Railway. Combining the conditions that inorganic gas produced and the geological conditions along the Sichuan-Tibet Railway, it analyzes and predicates that CH4, CO2, H2S, SO2, CO, NO2, NH3 and H2 are the main types of poisonous gas along the Sichuan-Tibet Railway. The poisonous gas is mainly distributed in the three suture zones, three crustal fault zones and more than three basement fault zones and mafic and ultrabasic rock zone, granites zone, volcanic zone and contact metamorphic zone. All these zones are the favorable migration point to area and gathering area to the toxic and harmful gas. It should be highly mentioned on the construction of the tunnels.

Nature and tectonic significance of co-seismic structures associated with the Mw 8.8 Maule earthquake, central-southern Chile forearc

The Mw 8.8 Maule earthquake on February 27, 2010 affected the central-southern Chilean forearc of the Central Andes. Here we show the results of field investigations of surface deformation associated with this major earthquake. Observations were carried out within three weeks after the seismic event, mostly in the central and northern part of the forearc overlying the rupture zone. We provide a detailed field record of co-seismic surface deformation and examine its implications on active Andean tectonics. Surface rupture consisted primarily of extensional cracks, push-up structures, fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems. A major group of NW-WNW striking fractures representing co-seismic extensional deformation is found at all localities. These appear to be spatially correlated to long-lived basement fault zones. The NW-striking normal focal mechanism of the Mw 6.9 aftershock occurred on March 11 demonstrates that the basement faults were reactivated by the Mw 8.8 Maule earthquake. The co-seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement-involved structures. We propose that co-seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes. The fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone.

Ouachita, Appalachian, and Ancestral Rockies deformations recorded in mesoscale structures on the foreland Ozark plateaus

Mesoscale structures in Paleozoic rocks of the Ozark plateaus reveal four Pennsylvanian deformation episodes in midcontinent North America. The two earliest episodes can be assigned to progressive northwestward docking of the Ouachita terrane with North America. Early extensional structures (Event 1) indicate a northwest/southeast maximum horizontal stress (Hmax) during Early Pennsylvanian Ouachita terrane advance. Event 2 extensional and strike-slip structures indicate Hmax across the Ozark plateaus that varies systematically from north-northwest/south-southeast in the south to northeast/southwest in the north. This suggests development of a slip-line deformation field in response to minor northeastward lateral escape of lithospheric blocks away from the northwestward-moving Ouachita terrane's leading edge, which acted as an indenter in western Arkansas, southeastern Oklahoma, and Texas. Younger contractional and strike-slip structures of Event 3 indicate northeast/southwest Hmax across the entire Ozark plateaus, and deformation orientation and intensity are not readily assigned to Ouachita foreland deformation and may be related to Middle Pennsylvanian Ancestral Rockies contractional deformation. Finally, Event 4 contractional structures indicate northwest/southeast Hmax consistent with southern Appalachian late stage convergence. Deformation episodes are localized along basement fault zones, particularly at major bends, suggesting minor restraining-bend uplifts along strike-slip faults. Geometries of conjugate normal fault and hybrid shear joint arrays indicate localized areas of high differential stress consistent with basement block uplift at these bends. High-angle faults reactivated in a reverse sense and bedding-parallel veins suggest tensile minimum stresses and pore fluid pressures exceeding lithostatic stress, consistent with brine pulses driven into the midcontinent during Late Paleozoic orogeny (as proposed by other authors).

Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins

Several selected seismic lines are used to show and compare the modes of Late-Cretaceous–Early Tertiary inversion within the North German and Polish basins. These seismic data illustrate an important difference in the allocation of major zones of basement (thick-skinned) deformation and maximum uplift within both basins. The most important inversion-related uplift of the Polish Basin was localised in its axial part, the Mid-Polish Trough, whereas the basement in the axial part of the North German Basin remained virtually flat. The latter was uplifted along the SW and to a smaller degree the NE margins of the North German Basin, presently defined by the Elbe Fault System and the Grimmen High, respectively. The different location of the basement inversion and uplift within the North German and Polish basins is interpreted to reflect the position of major zones of crustal weakness represented by the WNW-ESE trending Elbe Fault System and by the NW-SE striking Teisseyre-Tornquist Zone, the latter underlying the Mid-Polish Trough. Therefore, the inversion of the Polish and North German basins demonstrates the significance of an inherited basement structure regardless of its relationship to the position of the basin axis. The inversion of the Mid-Polish Trough was connected with the reactivation of normal basement fault zones responsible for its Permo-Mesozoic subsidence. These faults zones, inverted as reverse faults, facilitated the uplift of the Mid-Polish Trough in the order of 1–3 km. In contrast, inversion of the North German Basin rarely re-used structures active during its subsidence. Basement inversion and uplift, in the range of 3–4 km, was focused at the Elbe Fault System which has remained quiescent in the Triassic and Jurassic but reproduced the direction of an earlier Variscan structural grain. In contrast, N-S oriented Mesozoic grabens and troughs in the central part of the North German Basin avoided significant inversion as they were oriented parallel to the direction of the inferred Late Cretaceous–Early Tertiary compression. The comparison of the North German and Polish basins shows that inversion structures can follow an earlier subsidence pattern only under a favourable orientation of the stress field. A thick Zechstein salt layer in the central parts of the North German Basin and the Mid-Polish Trough caused mechanical decoupling between the sub-salt basement and the supra-salt sedimentary cover. Resultant thin-skinned inversion was manifested by the formation of various structures developed entirely in the supra-salt Mesozoic–Cenozoic succession. The Zechstein salt provided a mechanical buffer accommodating compressional stress and responding to the inversion through salt mobilisation and redistribution. Only in parts of the NGB and MPT characterised by either thin or missing Zechstein evaporites, thick-skinned inversion directly controlled inversion-related deformations of the sedimentary cover. Inversion of the Permo-Mesozoic fill within the Mid-Polish Trough was achieved by a regional elevation above uplifted basement blocks. Conversely, in the North German Basin, horizontal stress must have been transferred into the salt cover across the basin from its SW margin towards the basins centre. This must be the case since compressional deformations are concentrated mostly above the salt and no significant inversion-related basement faults are seismically detected apart from the basin margins. This strain decoupling in the interior of the North German Basin was enhanced by the presence of the Elbe Fault System which allowed strain localization in the basin floor due to its orientation perpendicular to the inferred Late Cretaceous–Early Tertiary far-field compression.

Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation

Research Article| May 01, 2001 Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation Randel Tom Cox; Randel Tom Cox 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Search for other works by this author on: GSW Google Scholar Roy B. Van Arsdale; Roy B. Van Arsdale 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Search for other works by this author on: GSW Google Scholar James B. Harris; James B. Harris 2Department of Geology, Millsaps College, Jackson, Mississippi 39210, USA Search for other works by this author on: GSW Google Scholar Daniel Larsen Daniel Larsen 3Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Randel Tom Cox 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Roy B. Van Arsdale 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA James B. Harris 2Department of Geology, Millsaps College, Jackson, Mississippi 39210, USA Daniel Larsen 3Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Publisher: Geological Society of America Received: 26 Sep 2000 Revision Received: 23 Jan 2001 Accepted: 29 Jan 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2001) 29 (5): 419–422. https://doi.org/10.1130/0091-7613(2001)029<0419:NOTSRR>2.0.CO;2 Article history Received: 26 Sep 2000 Revision Received: 23 Jan 2001 Accepted: 29 Jan 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Randel Tom Cox, Roy B. Van Arsdale, James B. Harris, Daniel Larsen; Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation. Geology 2001;; 29 (5): 419–422. doi: https://doi.org/10.1130/0091-7613(2001)029<0419:NOTSRR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The northeast-striking New Madrid fault system of central North America has been described as a right-lateral strike-slip system with a left-stepping restraining-bend thrust. The fault system has one of the highest rates of seismic energy release in an intraplate setting, and it has been regarded as a zone of significant earthquake hazard. The New Madrid fault system is the central part of the wider Reelfoot rift fault system, a northeast- striking basement fault zone. Although component faults of the New Madrid system are well defined by microseismicity, recent geodetic surveys suggest that little if any strain is accumulating on the principal (southern) strike-slip arm of the fault system. Seismicity and geologic data show that the “restraining-bend” thrust continues past the southern strike-slip arm to the southeastern Reelfoot rift margin. Thus, we suggest that earthquakes defining the southern arm of the New Madrid fault system are primarily aftershocks of an earthquake on that arm during the last sequence of great earthquakes (A.D. 1811– 1812), and it is the southeastern rift-margin fault system that is currently accommodating right-lateral strain along the boundary of the thrust block. This interpretation is consistent with recent geodetic results. The southeastern rift margin coincides with a 150-km-long linear topographic scarp from near Memphis to the Tennessee-Kentucky line, and S-wave reflection profiles, auger data, and a trench excavation reveal late Wisconsin–early Holocene surface faulting and late Holocene liquefaction associated with this fault-line scarp. Variation in sense of throw along strike and flower-structure geometry suggest that this is a strike-slip fault. Recognition of this rift margin as an important element of active tectonism in the Mississippi embayment has broad implications for assessment of the seismic hazard of this and similar intraplate settings. Temporal shifts in strain accommodation may give rise to short-term seismicity patterns and/or geodetic velocities that do not reveal long-term tectonic patterns. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Quaternary faulting in the southern Mississippi embayment and implications for tectonics and seismicity in an intraplate setting

The recently recognized Saline River fault zone in the southwestern Mississippi embayment (strike = 135°) is characterized by moderate historic seismicity. We document paleoseismicity (possibly strong) on the Saline River fault zone that is outside the known region of neotectonism in the northern embayment. Six surface faults were excavated within the Saline River fault zone, and all faults displace marine Eocene units and fluvial Pliocene–Pleistocene units; five show post-Wisconsin loess movement (thermoluminescence [TL] age = 23.6 ± 3 ka), and three deform middle to late Holocene eolian silt (TL age = 5.1 ± 0.6 ka and 3.6 ± 0.5 ka) as fault-tip flexures. Along the principal excavated fault, fault-plane braiding and plunging drag folding suggest a strong component of left slip, and subsidiary faults show both normal and reverse displacement (<1m) of Pliocene−Pleistocene, Wisconsin, and Holocene units. We interpret these faults as components of a left-slip flower structure. A shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows Eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m, supporting a strike-slip interpretation. In addition, the only focal plane mechanism available in the area indicates left slip with a small reverse component. We conclude that the Saline River fault zone is a Paleozoic–Mesozoic basement fault zone reactivated as a left-slip system in a Quaternary east-west compressive stress field. Field evidence suggests that earthquakes with several meters of strike slip may be characteristic, and the width of a liquefaction field on the Saline River fault zone suggests an event of magnitude 5.5–6. Considering a similarity of strike and sense of Holocene movements of the Meers fault of southern Oklahoma and of the Saline River fault zone, there may be additional active fault zones that pose seismic hazards concealed by sediments in southern North America.

Quantifying orthoclase and albite muscovitisation sequences in fault zones

A new graphical method is described for assessing the degrees of alteration of orthoclase and albite feldspars to muscovite. The method uses major element data from whole rock samples, and given a range of samples of varying degrees of alteration, pathways of the alteration sequences may be determined. The method may be used quantitatively if likely starting compositions are known and aluminium immobility is assumed. Molar proportions of K, Na and Al are plotted in the form of an Na/(Na+K) versus (Na+K)/Al graph. Modelled plots of muscovitisation of an orthoclase/albite system are presented, for comparison with actual data, so that actual compositions may be assessed in terms of feldspar muscovitisation. An example of the application of this technique to muscovitised granitic fault rocks in the external western Alps is presented, illustrating how the method is best used in conjunction with microstructural and field constraints on the fault rock evolution. This yields an evolution pathway for muscovitisation of the feldspars during fault zone deformation, and links specific alteration steps with particular microstructural changes within the fault zone. We also apply this technique to a previously published data set on basement shear zone geochemical changes from the Pyrenees where orthoclase albitisation provides an additional interest. Given that these reactions are fluid-induced, this work provides further information on the effects of fluid flow in such fault zones in terms of basement fault zone rheologies.

Conjugate Riedel deformation band shear zones

Our investigations have disclosed that individual Riedel shear zones may organize themselves into broadly distributed though rigorously oriented intraformational conjugate systems which may form without relationship to, or dependence upon, an underlying basement fault zone. The Riedel shear zones we mapped are zones of deformation bands, which developed as the preferred deformation mechanism in porous Navajo Sandstone (Jurassic). In the Cottonwood area, located at the northern end of the Kaibab Uplift, a conjugate normal Riedel deformation band shear system developed during the Laramide in the uppermost Navajo Sandstone on the outer arc of the upper hinge zone of the East Kaibab monocline. In the Sheets Gulch area, located at the northern end of the Waterpocket Fold, a conjugate strike-slip Riedel deformation band shear system developed during the Laramide in upper Navajo Sandstone within an imperfect transfer zone between the northeast-vergent Circle Cliffs Uplift and the southwest-vergent Miners Mountain Uplift. Within both the Cottonwood and Sheets Gulch areas there are tens to hundreds of Riedel shear zones, the largest of which are up to hundreds of meters in trace length. In classic Riedel fashion, the synthetic R-shears within each Riedel shear zone depart by ∼15° from the zone as a whole and are arranged in an en échelon, overstepping geometry. The antithetic R′-shears depart by ∼75° from the Riedel shear zones of which they are a part, and are especially abundant in transfer zones where they create hard linkages between overstepping R-shears. At both localities the Riedel shear zones occur in two sets that intersect at ∼60°. The Riedel shear geometry is self-similar from the scale of hand samples (and thin sections) where offsets are measured in centimeters (or millimeters), to the map scale where displacements are measured in meters. Because of the small amount of deformation which had to be accommodated in each of the two study areas, and the limits imposed by the strain-hardening nature of deformation banding, we may be seeing a rare snapshot that records an image of early, arrested fault-system development in relatively homogeneous, porous sandstone. The literature on classic Riedel shear zones postulates that displacement and shear along Riedel shears brings about a localized reorientation of stress. This interpretation can be tested and confirmed, using the geometry and kinematics of conjugate Riedel systems. Detailed understanding of the total nested geometric characteristics of the conjugate Riedel deformation band shear zone systems also provides insight regarding controls on reservoir-scale fluid flow. The low permeability of the deformation band shear zones tends to compartmentalize the Navajo Sandstone into chambers along which fluid flow is channeled. The geometry and spacing of the deformation band patterns controls shapes and sizes of the compartments, which in these examples tend to be long, polyhedral, porous chambers marked by either diamond- or rhombic-shaped cross-sections.

Monocline development by oblique-slip fault-propagation folding: the East Kaibab monocline, Colorado Plateau, Utah

Fault relationships along a 50-km stretch of the East Kaibab monocline in southern Utah suggest that Late Cretaceous/early Tertiary development of the structure involved a significant component of right-lateral strike-slip displacement, accommodated by basement-rooted faulting and fault-propagation folding. Evidence of oblique slip is provided mainly by pervasive map-scale and outcrop-scale faults that define a shear zone occupying the steep east-dipping limb of the monocline for at least its northernmost 50 km. Dominant fault orientations are synthetic and antithetic to the shear zone, and accommodate reverse-right-lateral and reverse-left-lateral slip, respectively. Structural style within the shear zone changes character and increases in intensity with progressively lower structural and stratigraphic levels in the fold, suggesting that the shear zone propagated upward from a basement-rooted fault during monocline formation. We conclude that horizontal, ENE-directed, Laramide compression drove reverse-right-lateral slip on the basement fault zone beneath the developing East Kaibab monocline. The resulting transpressional fault-propagation fold is marked in southern Utah by 1600 m of reverse displacement and possibly 8000 m of right-lateral displacement across the shear zone and associated monoclinal flexure.