Regional Strain Field Research Articles

Reinterpretation of the active faulting in central Mongolia

Research Article| August 01, 2007 Reinterpretation of the active faulting in central Mongolia R.T. Walker; R.T. Walker 1Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), Department of Earth Science, University of Oxford, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar E. Nissen; E. Nissen 1Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), Department of Earth Science, University of Oxford, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar E. Molor; E. Molor 2Mongolian University of Science and Technology, Ulaanbaatar, Mongolia Search for other works by this author on: GSW Google Scholar A. Bayasgalan A. Bayasgalan 2Mongolian University of Science and Technology, Ulaanbaatar, Mongolia Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (8): 759–762. https://doi.org/10.1130/G23716A.1 Article history received: 23 Jan 2007 rev-recd: 21 Mar 2007 accepted: 04 Apr 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R.T. Walker, E. Nissen, E. Molor, A. Bayasgalan; Reinterpretation of the active faulting in central Mongolia. Geology 2007;; 35 (8): 759–762. doi: https://doi.org/10.1130/G23716A.1 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 We present remote-sensing and field observations of an ∼350-km-long east-west left-lateral strike-slip fault (the South Hangay fault) in the Hangay Mountains of central Mongolia, an area previously believed to be deforming solely by slip on scattered, and randomly oriented, normal faults. The known dip-slip faults are shown to be short segments introduced at bends in the much longer strike-slip fault. Our observations show that the active faulting in the Hangay Mountains is consistent with the regional strain field of Mongolia and does not require, as suggested in other studies, that the faults result from stresses introduced by the locally elevated topography. Our observations help to define the active tectonics of this important part of the India-Eurasia collision. The South Hangay strike-slip fault is a potential source of large-magnitude earthquakes and constitutes a previously unrecognized hazard in this part of Mongolia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Source characteristics of the 6 June 2000 Orta–Çankırı (central Turkey) earthquake: a synthesis of seismological, geological and geodetic (InSAR) observations, and internal deformation of the Anatolian plate

Abstract This paper is concerned with the seismotectonics of the North Anatolian Fault in the vicinity of the Orta–Çankırı region, and consists of a study of a moderate-sized (Mw=6. 0) earthquake that occurred on 6 June 2000. The instrumental epicentre of this earthquake is far from the North Anatolian Fault Zone (NAFZ), and rapid focal mechanism solutions of USGS–NEIC and Harvard-CMT also demonstrate that this earthquake is not directly related to the right-lateral movement of the North Anatolian Fault. This earthquake is the only instrumentally recorded event of magnitude (Mw) >5.5 since 1900 between Ankara and Çankırı, and therefore provides valuable data to improve our understanding of the neotectonic framework of NW central Anatolia. Field observations carried out in the vicinity of Orta town and neighbouring villages immediately after the earthquake indicated no apparent surface rupture, but the reported damage was most intense in the villages to the SW of Orta. We used teleseismic long-period P- and SH-body waveforms and first-motion polarities of P-waves, broadband P-waves, and InSAR data to determine the source parameters of the 6 June 2000 (Orta–Çankırı, t o =02:41:53.2, Mw=6. 0) earthquake. We compared the shapes and amplitudes of long-period P- and SH-waveforms recorded by GDSN stations in the distance range 30–90°, for which signal amplitudes were large enough, with synthetic waveforms. The best-fitting fault-plane solution of the Orta–Çankırı earthquake shows normal faulting with a left-lateral component with no apparent surface rupture in the vicinity of the epicentre. The source parameters and uncertainties of this earthquake were: Nodal Plane 1: strike 2°±5°, dip 46°±5°, rake –29°±5°; Nodal Plane 2: strike 113°, dip 70°, rake –132°; principal axes: P=338° (48°), T=232° (14°), B=131° (39°); focal depth 8±2 km (though this does not include uncertainty related to velocity structure), and seismic moment M o =(140–185)×10 16 N m. Furthermore, analysis of a coseismic interferogram also allows the source mechanism and location of the earthquake to be determined. The InSAR data suggest that the north–south fault plane (Nodal Plane 1 above) was the one that ruptured during the earthquake. The InSAR mechanism is in good agreement with the minimum misfit solution of P- and SH-waveforms. Although the magnitude of slip was poorly constrained, trade-off with the depth range of faulting accurred such that solutions with a large depth range had small values of slip and vice versa. The misfit was small and the geodetic moment constant for fault slips greater than c . 1 m. The 6 June 2000 Orta–Çankırı earthquake occurred close to a restraining bend in the east–west-striking rightlateral strike-slip fault that moved in the much larger earthquake of 13 August 1951 (Ms=6.7). The faulting in this anomalous earthquake could be related to the local geometry of the main strike-slip system, and may not be a reliable guide to the regional strain field in NW central Turkey. We tentatively suggest that one possible explanation for the occurrence of the 6 June 2000 Orta–Çankırı earthquake could be localized clockwise rotations as a result of shear of the lower crust and lithosphere.

Fracture-driven intrusion and upwelling of a mid-crustal pluton fed from a transpressive shear zone--The Rieserferner Pluton (Eastern Alps)

The Rieserferner Pluton was emplaced at a depth of 12–15 km into steeply dipping, greenschist-facies mylonitic rocks of the Austroalpine basement, just south of the Tauern Window (Eastern Alps). Intrusion occurred during north-south–directed shortening and east-west horizontal extension in front of the rigid Southern Alpine Indenter. The regional strain field is transpressive, with a strong coaxial component, and is characterized by east-west stretching. Tonalitic melt ascended through a feeder channel preserved in the steep southern part of the Rieserferner Pluton, within and adjacent to the steep mylonitic foliation of a major shear zone, the Defereggen-Antholz-Vals (DAV) Line. Melt ascent was perpendicular to the north-south shortening direction in the country rocks. Magma emplacement involved melt-induced hydro-fracturing to form a subhorizontal, tabular pluton that protruded northward from the DAV Line into the previously folded country rocks. During the late stage of emplacement, buoyant upwelling of the partly recrystallized magma induced doming of the pluton roof as well as vertical ductile shortening of the directly overlying country rocks. Magma pressure therefore locally exceeded the lithostatic pressure. Thermal modeling constrains the maximum time for doming and solidification of the pluton to have been 32,000 yr. The wavelength of the domes in the pluton roof indicates the viscosity contrast between country rock and partly crystallized tonalite in the pluton to have been at least 100:1.

Emplacement and deformation of the Cachoeirinha pluton (Borborema province, NE Brazil) inferred through petrostructural studies: Constraints on regional strain fields

The East Pernambuco shear zone system (EPSZ) conventionally has been taken as the limit between the central and southern domains of the oriental sector of the Neoproterozoic Borborema province (NE Brazil). To constrain the tectonic evolution of the southern domain during the Brasiliano orogeny, the authors undertake a structural and anisotropy of magnetic susceptibility study of the 580 Ma Cachoeirinha pluton, located 10 km south of the EPSZ. The results suggest a model for its emplacement and deformation in which (1) successive magma batches accumulated at the gently dipping interface between orthogneisses and overlying schists and paragneisses and (2) homogeneous NNE–SSW stretching of the pluton was followed by strain localization along conjugate subvertical corridors of noncoaxial shear during the last stages of crystallization. Outcrop-scale, magmatic to solid-state dextral and sinistral shear zones that affect the pluton have similar orientations to the large transcurrent mylonitic belts that characterize the central domain of the Borborema province. Thus, at the time of intrusion of the Cachoeirinha pluton, the central and southern domains of the Borborema province were undergoing deformation related to the same far-field stresses, and the EPSZ is not a major suture zone between domains with distinct geological histories. Instead, the present-day separation of the eastern Borborema province into central and southern domains resulted from progressive deformation that led to the development of the EPSZ. The absence of large shear zones in the southern domain can be explained by the faster regional cooling than in the central domain, where the longer magmatism duration resulted in persistent elevated crustal temperatures, which in turn allowed a greater amount of finite strain to accumulate.

Characterization of the time‐dependent strain field at seismogenic depths using first‐motion focal mechanisms: Observations of large‐scale decadal variations in stress along the San Andreas Fault system

We present a method for summing moment tensors derived from first‐motion focal mechanisms to study temporal dependence in features of the subsurface regional strain field. Time‐dependent processes are inferred by comparing mechanisms summed over differing time periods. We apply this methodology to seismogenic zones in central and southern California using focal mechanisms produced by the Northern and Southern California Seismograph Networks for events during 1980–1999. We find a consistent pattern in both the style of deformation (strike‐slip versus compressional) and seismicity rate across the entire region. If these temporal variations are causally related, it suggests a temporal change in the regional‐scale stress field. One change consistent with the observations is a rotation in the regional maximum horizontal compressive stress direction, followed by a reversal to the original direction. Depending upon the dominant style of deformation locally, this change in orientation of the regional stress will tend to either enhance or hinder deformation. The mode of enhanced deformation can range from increased microseismicity and creep to major earthquakes. We hypothesize that these temporal changes in the regional stress field are the result of subtle changes in apparent relative plate motion between the Pacific and North American plates, perhaps due to long‐range postseismic stress diffusion. Others have hypothesized that small changes in plate motion over thousands of years, and/or over decades, are responsible for changes in the style of deformation in southern California. We propose that such changes, over the course of just a few years, also affect the style of deformation.

Fold axis-parallel rotation within the Laramide Derby Dome Fold, Wind River Basin, Wyoming, USA

Derby Dome, a doubly plunging anticline (7×3 km) on the eastern flank of the Wind River Range, Wyoming, trends NW–SE in response to the regional NE–SW directed shortening of the Cretaceous–Eocene Laramide orogeny. Mesozoic sediments are exposed around the fold hinge above an east-dipping thrust fault that offsets Archean crystalline rocks at depth. Stress and strain ellipsoidal data were determined through the measurement of mechanically twinned calcite in limestones (Triassic Alcova through J–K Morrison Formation rocks; 13 samples), calcite cements (5 samples), and synfolding calcite veins (16 samples) around the northern half of the fold. On the outer limbs of the fold the maximum shortening strain axis (−3.5%, 15% NEVs) in the limestones and cements is sub-horizontal, layer-parallel and normal (NE–SW) to the fold hinge reflecting regional Sevier–Laramide shortening. This regional layer-parallel strain fabric is rotated into a fold axis-parallel orientation (NW–SE) near the fold hinge indicating that significant rotations occurred during folding. Synfolding calcite veins, of varying orientations, also preserve a local sub-horizontal, hinge-parallel shortening strain (−4.0%, 17% NEVs), suggesting that the regional Laramide stress and strain field was locally rotated into parallelism with the fold during shortening and displacement on the underlying thrust fault. In both the country rock, cement and vein data sets, the strain overprint noise (NEVs) increases toward the fold hinge. Inferred differential stress magnitudes are also higher for the vein calcite than for the country rock limestones or cements, and there is no interpretable pattern around the fold (avg.=560 bars, range of 240–2000 bars). Fracture measurements ( n=74) in different lithologies have different orientations on each side of the adjacent Dallas Dome Fold suggesting layer-parallel rotation during folding, or active fracturing occurred uniquely on each fold limb.

Salton Trough Regional Deformation Estimated from Combined Trilateration and Survey-Mode GPS Data

The Salton Trough in southeastern California, United States, has one of the highest seismicity and deformation rates in southern California, including 20 earthquakes M 6 or larger since 1892. From 1972 through 1987, the U.S. Geological Survey (USGS) measured a 41-station trilateration network in this region. We re-measured 37 of the USGS baselines using survey-mode Global Positioning System methods from 1995 through 1999. We estimate the Salton Trough deformation field over a nearly 30-year period through combined analysis of baseline length time series from these two datasets. Our primary result is that strain accumulation has been steady over our observation span, at a resolution of about 0.05 μstrain/yr at 95% confidence, with no evidence for significant long-term strain transients despite the occurrence of seven large regional earthquakes during our observation period. Similar to earlier studies, we find that the regional strain field is consistent with 0.5 ± 0.03 μstrain/yr total engineering shear strain along an axis oriented 311.6° ± 23° east of north, approximately parallel to the strike of the major regional faults, the San Andreas and San Jacinto (all uncertainties in the text and tables are standard deviations unless otherwise noted). We also find that (1) the shear strain rate near the San Jacinto fault is at least as high as it is near the San Andreas fault, (2) the areal dilatation near the southeastern Salton Sea is significant, and (3) one station near the southeastern Salton Sea moved anomalously during the period 1987.95-1995.11. Online material: Tables of stations used in this study and of estimated strain fields. Manuscript received 8 January 2003.

Granite fabrics and regional‐scale strain partitioning in the Seridó belt (Borborema Province, NE Brazil)

Fabrics of the Brasiliano‐age granitoid plutons of the Seridó belt (northeastern Brazil) were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover. Nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility. These plutons, emplaced between 580 and 575 Ma, are mainly potassic calk‐alcalic magnetite‐bearing monzogranites. The presence of primary coarse‐grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy, magnetic susceptibilities, hysteresis ratios, and thermomagnetic curves. The plutons show a strong internal coherence of magnetic fabric, with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation. The magnetic fabric was used to trace the strain partitioning along the belt. In the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching. On the other hand, in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional/extensional deformation. Lineations in these granites trend in a NE direction, which is parallel to the transport, inferred from kinematic indicators along the shear zones. A crustal‐scale east trending dextral shear zone connects both the transtensional and transpressional domains.

The middle Devonian basins of western Norway: sedimentary response to large-scale transtensional tectonics?

The Devonian basins of western Norway were formed during late- to post-orogenic extension of overthickened Caledonian crust. The basins are situated in the hanging wall of the extensional Nordfjord–Sogn Detachment Zone (NSDZ) and display extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies configurations, paleocurrent patterns and intrabasinal structures, we infer an anticlockwise rotation of the syndepositional extension direction from NW–SE in the south (Solund basin) to WSW–ENE in the north (Hornelen basin). The axes of folds that are roughly parallel to the local extension direction are rotated correspondingly. The Kvamshesten basin is located between the Solund and Hornelen basins. Sedimentological and structural data show evidence of an early, southeastwards tilt direction followed by a more eastwards tilt and associated E–W flowing paleodrainage. Correspondingly, NW–SE trending folds and reverse faults are superposed by E–W trending ones at low to intermediate stratigraphic levels. The variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strain field. The above observations and inferences indicate that the Devonian basins in western Norway formed in a strain field dominated by regional transtension, accommodated by extension along the NSDZ and sinistral strike–slip along orogen-parallel shear zones and faults to the north of the basins; alternatively, NW-directed extension preceded the introduction of a sinistral strike–slip component. The models are in accordance with recent work carried out in the footwall of the NSDZ and illustrates the tectono-sedimentary response to a complex interplay between extension and strike–slip that appears to have been fundamental in the late-stage disintegration of the Caledonian orogen.

Measurement of local stress and estimation of regional stress associated with stability assessment of an open-pit rock slope

This paper discusses the concept of a new methodology for rock slope stability assessment. Then, results on rock stress measurement using the compact conical-ended borehole overcoring (CCBO) technique at Torigata limestone mine in Japan are presented. A procedure for back analysis of the regional strain and stress field with the 3-D finite element method , using the measured local stress, is suggested and demonstrated successfully in relation to Torigata limestone mine. Finally, to estimate the state of stress at the mine excavation level, 3-D finite element analyses were performed using boundary conditions from the analyzed regional strain and stress field. It is shown that the horizontal stress at the present excavation level is not reduced, and that the horizontal stress component cannot be disregarded in estimating the stability of rock slopes at this location, even though the mine is located near the top of a mountain.

Exhumation of the ultrahigh‐pressure continental crust in east central China: Cretaceous and Cenozoic unroofing and the Tan‐Lu fault

The orogenic architecture of the world's largest ultrahigh‐pressure exposure, the Hong'an‐Dabie Mountains of the Triassic Qinling‐Dabie orogenic belt, is dominated by Cretaceous and Cenozoic structures that contributed to its exhumation from ≤30 km depth. Cretaceous magmatic crustal recycling (≥50% for the entire Dabie) and heating (>250° to >700°C) were most prominent in Dabie, and exhumation, magmatism, and cooling were all controlled by Cretaceous transtension. Exhumation was accomplished principally by an asymmetric Cordilleran‐type extensional complex in the northern Dabie (Northern Orthogneiss unit) between 140 and 120 Ma, at rates as fast as 2 mm/yr and average horizontal stretching rates of up to 6 mm/yr. Cretaceous reactivation occurred within a regional transtensional strain field as a result of far‐field collisions and Pacific subduction. The onset of crustal extension was preceded and possibly facilitated by a reheating of the Hong'an‐Dabie crust (∼140 Ma) coeval with the onset of voluminous magmatism in eastern China (∼145 Ma), which resulted from a change in Pacific subduction from highly oblique to orthogonal. The Tan‐Lu continental‐scale fault was a normal fault zone in the mid‐Cretaceous (∼110‐90 Ma) and underwent ≥5.4 km dip slip and ≥4 km throw in the Cenozoic. During the India‐Asia collision the Qinling‐Dabie belt acted as the structural discontinuity between the strike‐slip‐dominated escape tectonics south of the Qilian‐Qinling‐Dabie belt and the rifting‐dominated tectonism north of it. The most prominent Cretaceous and Cenozoic structures of the Hong'an‐Dabie, the Xiaotian‐Mozitang and the Jinzhai fault zones, respectively, reactivated major lithospheric structures of the Triassic orogen, i.e., the Huwan detachment zone and the suture.

Emplacement and deformation of granites during transpression: magnetic fabrics of the Archean Sparrow pluton, Slave Province, Canada

The Sparrow pluton is part of the Prosperous Suite of two-mica granites that crop out within amphibolite grade meta-greywackes of the southern Yellowknife Domain, in the Slave Province of the Canadian Shield. The magnetic susceptibility ( K) and the anisotropy of magnetic susceptibility (AMS) were used to systematically map the structural patterns in the pluton and to establish the relationship between plutonism and Late Archean tectonics in the region. Paramagnetic Fe-phyllosilicates (biotite, chlorite) and very fine-grained magnetite contribute to K and to the AMS. The magnetic foliation and the magnetic lineation are predominantly controlled by the biotite fabrics and their orientations are consistent with the regional D 2 strain field. The horizontal magnetic lineations in the Sparrow pluton suggest a horizontal stretching component associated with the regional D 2 event. The zonation defined by K values is compatible with a fold pattern trending parallel to the regional F 2 folds and S 2 foliation and to the magnetic fabric trends in the pluton. The intensities and symmetries of the AMS also define map patterns that are consistent with D 2 deformation. Microstructural study indicates the pluton recorded D 2 strain as it crystallized and cooled from the solidus, demonstrating syn- D 2 emplacement. The results indicate the pervasive structural patterns in the Sparrow pluton are an integral part of the regional strain field, and that they are kinematically consistent with a transpressive D 2 strain regime. Mapping the fabric patterns within syntectonic plutons provides a useful approach to the kinematic analysis of synemplacement deformation events in multiply deformed metamorphic terranes.

Local and regional components of western Aegean deformation extracted from 100 years of geodetic displacement measurements

SUMMARY Our objectives are as follows. First, we wish to develop a methodology to recover the long-term component of deformation from any set of distributed, time-averaged geodetic strain measurements that were subject to seismic disturbance, given a catalogue of local seismicity that occurred during the measurement period. Second, using seismic and geodetic data sets that span approximately 100 years, we apply this technique in the western Aegean to assess the role of local seismicity in regional deformation. The methodology is developed using a model for crustal deformation constructed from a long-term, smooth regional strain field combined with instantaneous, local perturbations from upper-crustal earthquakes approximated by static elastic dislocations. By inverting geodetic displacements for the smooth field while simultaneously floating influential but uncertain earthquake source parameters, an estimate of the regional component of deformation that is approximately independent of the seismicity can be made. In the western Aegean we find that the horizontal component of regional deformation can be described with minor inaccuracy by a quadratic relative displacement field. The principal horizontal extensional axes calculated from the regionally smooth displacement field agree in orientation with the T-axes of earthquakes in the region. These observations indicate that the instantaneous elastic strain of the 10 km thick seismogenic layer is driven by a stress field that is smooth on the scale of the geodetic network as a whole, 200-300 km.

Pluton emplacement processes and tectonic setting of the 1.42 Ga Signal batholith, SW USA: important role of crustal anisotropy during regional shortening

Plutons in the 1.42 Ga Signal batholith provide an important case study for understanding the regional tectonic setting of Mesoproterozoic magmatism in the south-western United States and processes of emplacement for plutons far from active plate margins. Mesoproterozoic plutons in the Signal batholith were emplaced at ∼3 kbar and have a well-developed magmatic fabric which is subparallel to variably oriented Paleoproterozoic country rock foliations and shear zones, indicating that pluton emplacement was strongly influenced by crustal anisotropy. Solid-state deformation of Mesoproterozoic granites is best developed in shallowly dipping roof domains (40–60°), whereas in steep to vertical fabric domains, granites are generally undeformed and locally cross-cut older fabrics. This suggests that roof domains remained hotter and experienced greater thermal softening and subsequent deformation. Structural and kinematic study of three separate plutons in the Signal batholith indicate that pluton emplacement occurred in a regional contractional strain field, although local complexities suggest possible stress-field modification by pluton-related stresses. Evidence to support regional contractional strain during emplacement includes: (1) consistent dyke orientations throughout the batholith indicating a NNE extension and SE-NW shortening; (2) magmatic to solid-state fabric transition in thrust-sense deformation along the 40–60° dipping south-east margin of the Signal granite, including melt-filled shear zones and asymmetric K-feldspar porphyroclasts; and (3) syn-tectonic textures in contact metamorphic andalusite prophyroblasts. Regional variation in shortening direction from SE-NW to ∼NS during batholith emplacement may be related to pluton-emplacement stresses locally dominating over the relatively low deviatoric stresses of regional deformation. The main controlling factor for the geometry of pluton emplacement in the Signal batholith was Paleoproterozoic foliations and shear zones, which provided conduits for movement of magma into the middle crust. The flow of magma parallel to older structures may have been accommodated by a reduction in effective normal stresses by magma pressure and separation along the structural anisotropies. This model for anisotropy-influenced emplacement during weak regional contraction may be applicable to other ∼1.4 Ga plutons across SW Laurentia.

Syncontractional Crustal Anatexis and Deformation during Emplacement of ~1435 Ma Plutons, Western Needle Mountains, Colorado

Mesoproterozoic deformation and biomodal magmatism in southwest Colorado offer important insight into the nature and tectonic regime of 1.4 Ga magmatism in the western United States. We document syncontractional emplacement of coeval gabbroic and granitic plutons in the middle crust at ~1435 Ma in the western Needle Mountains. This is incompatible with extension-dominated "anorogenic" models proposed for the generation of 1.4 Ga plutonic rocks in the Southwest but lends further support for synkinematic intracontinental magmatism in response to regional tectonic strains created by transpressive or compressive plate margin interactions between 1500-1300 Ma. Intrustion of the ~1435 Ma Electra Lake Gabbro and Eolus Granite into Paleoproterozoic rocks caused local crustal anatexis and extensive thermal metamorphism in the Needle Mountains. Deformation of Paleoproterozoic country rocks at ~1435 Ma is widespread but is concentrated in pluton aureoles where thermally softened crust accommodated the strain. We interpret the kinematic framework of structural elements in the deformed pluton aureoles to indicate a regional synmagmatic strain field involving subhorizontal N-S contraction and E-W extension. This is reflected by brittle fracture arrays and conjugate ductile shear bands filled with remelted trondhjemitic gneiss, transposed amphibolite layers and ~1435 Ma granite dikes, local solid-state ductile deformation in ~1435 Ma intrusive rocks, broad-scale folding of migmatites, and trends of ~1435 Ma granitic and gabbroic dikes. Aureole metamorphism and kinematics of deformation recorded in the Needle Mountains at ~1435 Ma are similar to those that are documented in other 1.4 Ga plutonic complexes throughout Colorado, Arizona, and New Mexico. This regional record argues for a widespread episode of synorogenic magmatism in the southwestern United States at 1.4 Ga.

Tectonic setting of the Sandia pluton: An orogenic 1.4 Ga granite in New Mexico

Structural studies of the circa 1.42 Ga Sandia pluton and its aureole document significant deformation synchronous with pluton emplacement and call into question the “anorogenic” label associated with this and other 1.4 Ga granites in the southwestern United States. The SE margin of the pluton is a 1‐ to 2‐km‐wide NW dipping ductile shear zone. Field and microstructural observations (melt‐filled shear bands, high‐temperature dynamic recrystallization of K‐feldspar megacrysts, and crosscutting pegmatite dikes) indicate that top‐to‐the‐NW (normal) movement in the shear zone took place in the presence of melt. Subparallel magmatic fabrics north of and structurally above the shear zone contain kinematic indicators consistent with top‐to‐the‐NW shear sense, suggesting that over large regions of the pluton, magmatic flow mimicked solid‐state strain. In the northern aureole, contact metamorphic aluminosilicate porphyroblasts grew during the formation of a NE striking crenulation cleavage (S3) and related folds of late‐stage pegmatite dikes. These features document the synchroneity of magma emplacement, shortening, and metamorphism and indicate that the Sandia pluton is syntectonic, not anorogenic. We interpret the kinematic consistency of structural elements from the base of the pluton, the interior of the pluton, and the northern aureole to reflect a regional (larger than the pluton) strain field and suggest that the “orogeny” recorded in and around the Sandia pluton involved a three dimensional strain field with subhorizontal extension (N–S) and contraction (E–W) directions. N–S extension is documented by the orientation of mineral lineations and movement directions in the basal shear zone and in high‐strain zones in the northern aureole and by the orientations of tabular pegmatite and aplite dikes in the pluton and aureole. East to SE shortening is documented in the northern aureole by orientations of folded pegmatite dikes and associated S3 crenulation cleavage, and east to SE shortening (or least extension) directions in the pluton proper are documented by the intersections of orthogonal dikes. Thus emplacement of the Sandia pluton is interpreted to record a snapshot of regional strains inboard of an active plate margin, rather than local strains generated by emplacement.

Tectonic deformation in the New Madrid Seismic Zone: Inferences from map view and cross‐sectional boundary element models

The lack of instrumental recordings and obvious fault scarps associated with the 1811–1812 New Madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events. Morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting (strike‐ or dip‐slip), number, strike, length, and width of the major faults in the New Madrid seismic zone (NMSZ). This is accomplished through two‐dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains. Tectonic deformation is reflected in (1) the seismicity, (2) the Lake County uplift, (3) Reelfoot Lake, (4) the fractured rocks of the Blytheville arch, and (5) the St. Francis Sunk Lands. Many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left‐stepping right‐lateral strike‐slip faults that are coincident with the northeast trending zones of seismicity and the Blytheville arch. The morphology appears to be, at least in part, a consequence of major earthquakes that rupture these faults. The locations of the 1811–1812 and largest post‐1812 earthquakes and the models are consistent with a process in which the 1811–1812 earthquakes relieved accumulated regional shear strain causing the greatest post‐1812 shear strains to exist at the ends of the fault zone. Modeling results also suggest that the numerous small earthquakes in the NMSZ are not aftershocks of the 1811–1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field. The Bootheel lineament does not appear to be significant in shaping the morphology, geologic structure, and pattern of seismicity in the NMSZ. The absence of fault scarps and coherence of shallow reflectors overlying the Blytheville arch suggests that the major NMSZ faults may not have ruptured to the surface during the 1811–1812 earthquakes. The inferred lengths of the 1811–1812 earthquake ruptures also suggest that their sizes may have been overestimated. Model‐predicted subsidence within the St. Francis Sunk Lands indicates that tectonic deformation also may have influenced alluvial processes in the NMSZ.

Interference between pluton expansion and non-coaxial tectonic deformation: three-dimensional computer model and field implications

Pluton emplacement contributes to the complexity of structures in orogenic belts because the geometry of these structures is often controlled by interactions between regional tectonic-related and local pluton-related strain fields. Many questions remain about the genesis of these structures. For example: (1) to what extent does pluton-related strain contribute to the formation of these structures? (2) what is the three-dimensional geometry of these structures? and (3) how does this geometry change with progressive deformation? The present simulation models in three dimensions the position, type and kinematic evolution of structures around plutons that expand in a non-coaxial tectonic environment. The simulation generates patterns of four parameters: strain ellipsoid shapes and magnitudes, and orientation of foliation and mineral stretching lineations. Concave, lens-shaped, regions of high strain form on the sides of the pluton, whereas ellipsoidal regions of low strain form at the ends. In addition, foliation, stretching lineations, and constrictional regions form irregular three-dimensional rings around the pluton. Higher tectonic-related strains and strain rates favor rings that migrate towards the pluton, whereas higher expansion-related strains and strain rates favor ring migration away from the pluton. Furthermore, ring migration rates are much slower than tectonic deformation rates or expansion rates. Although analyses of natural strain fields should always be carried out in conjunction with radiometric age determinations, structural and microstructural analysis and strain measurements, the simulated strain patterns offer practical guidelines for field work. Specifically, these patterns can be used to evaluate: (1) the origin, geometry and evolution of structures around plutons; (2) the three-dimensional sense of movement and orientations of ductile shear zones; and (3) the contribution of pluton-related strains to deformation of country rocks in orogenic belts.

Deformation resulting from regional extension during pluton ascent and emplacement, central Sierra Nevada, California

Solid-state foliation, lineation, small-scale folds and domainal shear zones have developed in preexisting granitic and minor metasedimentary wallrock during a combined deformation involving a regional extensional strain and ascent and emplacement of the Mt Givens pluton (MGP). Mylonite is common throughout much of the ∼1–2 km wide, 10 km long shear zone, with ultramylonite best developed near the contact with the MGP, which itself lacks significant solid-state deformation. Migmatization accompanies ultramylonite formation in the northern half of the zone, but both these features are poorly developed or absent in its southern half where the shear zone is distributed over a wider area. Strain estimates across the shear zone using microgranitic enclaves as markers show a positive gradient and an increasing ratio of simple shear/pure shear towards the MGP. Microprobe analyses on hornblende and plagioclase yield pressure and temperature estimates of ∼3.5 kb and ∼680°C respectively, during shear zone formation, at least at its late stages of development. Zircon Pb/U and 40Ar/ 39Ar ages constrain timing of the high-temperature movement on the shear zone to ∼90 Ma, essentially the age of the MGP, although movement immediately prior to that time appears likely. We speculate that a regional extensional shear zone was developing prior to the emplacement of the MGP, which, as it ascended, heated the wallrock facilitating both further strain in the zone as well as buoyant rise of the pluton along the zone. The MGP was near its critical melt fraction during the last several kilometers (?) of its ascent, and could have possessed sufficient viscosity (strength) to impose a weak shear strain on the shear zone rocks, although most of the foliation and extensional features in the zone are probably related to the regional strain field. Late-stage folding of the foliation is attributed to shouldering aside of the wallrock by the MGP during the last increment of its ascent and final emplacement.

Tectonic Deformation in the New Madrid Seismic Zone: Inferences from Boundary Element Modeling

Abstract The lack of instrumental recordings and of obvious fault scarps associated with the 1811–1812 New Madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting responsible for these events. Morphologic and geologic features and the distribution of modern seismicity are used to infer the number, strike, length, width, type of faulting (strike- or dip-slip), and spatial variability of slip for the major faults in the New Madrid Seismic Zone (NMSZ). This is accomplished through two-dimensional boundary-element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains. Tectonic deformation is reflected in the seismicity and in morphologic and geologic features including (1) the Lake County uplift, (2) Reelfoot Lake, (3) the deformed rocks of the Blytheville arch, and (4) the St. Francis Sunk Lands. Many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left-stepping right-lateral strike-slip faults that are coincident with the northeast-trending zones of seismicity and the Blytheville arch. The morphology appears to be, at least in part, a consequence of major earthquakes that rupture these faults. The locations of the 1811–1812 and largest post-1812 earthquakes and the models are consistent with a process in which the 1811–1812 earthquakes relieved accumulated regional shear strain causing the greatest post-1812 shear strains to exist at the ends of the fault zone. Modeling results also suggest that the numerous small earthquakes in the NMSZ are not aftershocks of the 1811–1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field. The Bootheel lineament does not appear to be significant in the shaping the morphology, geologic structure, and pattern of seismicity of the NMSZ. The inferred length of the 1811–1812 earthquake ruptures suggest that their sizes may have been overestimated. Model predicted subsidence within the St. Francis Sunk Lands suggests that tectonic deformation may also influence alluvial processes in the NMSZ.