Detrital zircon and sandstone provenance analysis from Permian and Lower Cretaceous sedimentary units to constrain total and incremental left-lateral offset along the East Gobi Fault Zone, southeastern Mongolia
This study presents initial U-Pb detrital zircon geochronology results coupled with sandstone provenance point-counting to constrain sedimentary basin evolution along the East Gobi Fault Zone (EGFZ), southeastern Mongolia. These results fingerprint potential piercing points in sedimentary units argued here to have once been part of the same depositional basins, now offset by the strike-slip fault zone. Detrital zircon samples were collected and analyzed using LA-ICPMS from two sets of proposed offset-basin systems: Permian fluvial and marine units at Bulgan Uul and Nomgon, and Cretaceous fluvial units at Tavan Har and Ulgay Khid (11 sandstone samples total). The age probability plots for Bulgan Uul and Nomgon, as well as Tavan Har and Ulgay Khid, show very convincing correlations with significant implications. The Late Permian Lugyn Gol units at Nomgon are currently ~270 km northeast of those Late Permian Onch Uul samples at Bulgan Uul, and provide similar age probability plots, with most significant peaks at ~280 Ma and ~440 Ma, for zircons of igneous origins based on U/Th ratios. Lower Cretaceous strata from Tavan Har and Ulgay Khid are currently located ~90 km left-laterally off-set from each other, and also show strong correlation with significant age probability peaks centered at ~280 Ma and ~120 Ma, also with igneous origins. As the detrital zircon data are not intended to stand alone, 23 samples including all the zircon samples were point-counted for establishing provenance. QFL ternary plots indicate semi-mature, recycled orogen and arc sources for samples collected at Bulgan Uul, Nomgon, and Ulgay Khid. Tavan Har, however, primarily plotted in the dissected arc field. The results presented here argue for the following three main findings: First, detrital zircon analysis shows a clear connection of Bulgan Uul and Nomgon, as well as Ulgay Khid and Tavan Har. These cross-fault stratigraphic connections are further supported in most cases by the provenance point-counting results as well. Second, the ability to connect these cross-fault sedimentary units establishes piercing points thereby providing a means to calculated total and incremental offset along the EGFZ. Roughly 270 km of total sinistral offset has occurred along the EGFZ since the Late Permian, approximately 90 km of which has occurred since the mid Cretaceous. Therefore ~180 km of left-lateral offset along the EGFZ has occurred in the Triassic and Jurassic periods of deformation previously identified by other investigators. Paleo-reconstructions using these results and proprietary total magnetic survey data (TMI), suggest Bulgan Uul would have been located northwest of Nomgon prior to the onset of initial deformation. Finally, the presence of 910 Ma zircons in the detrital record for samples collected at Bulgan Uul and rarely at Nomgon supports the indenter model proposed by Johnson et al. (2008) for closure of the Paleoasian Ocean in the Late Permian, with the North China block being emplaced by this time.
Exhumation history of the Sierra Azul Block of the Santa Cruz Mountains Revealed Using Low Temperature Thermochronology
Restraining bends along strike-slip faults are important features for transforming lateral plate motion into vertical rock uplift. One of the most archetypal of these bends is the Santa Cruz Mountains restraining bend in the San Andreas Fault in the southern Bay Area, California. The Sierra Azul structural block lies within this restraining bend, and was the site of coseismic and postseismic deformation caused by the M=6.9 1989 Loma Prieta Earthquake. Previous geomorphic studies and Apatite Fission Track (AFT) thermochronology indicate that this restraining bend is fixed to the northeastern side of the fault, and so deformation accrues within the Sierra Azul while crust to the southwest is uplifted as it is translated through the restraining bend by lateral movement along the SAF. In this view, rapid and recent deformation has continuously accrued within the Sierra Azul since 6–8 Ma, and young AFT ages within the core of the range support this general idea. In this contribution, we present the results of additional AFT, as well as new (U-Th)/He ages from the Sierra Azul block. Structurally, the range is composed of a generally east-northeast-vergent fold-and-thrust belt. The deepest exposures found in the range are found adjacent to the SAF, and here, AFT data have been reset within latest Miocene-Pliocene time. However, within the shallower exposures to the east, AFT samples are only partially reset. In contrast, (U-Th)/He samples within rocks throughout the range yield ages between 5—7 Ma, suggesting that exhumation of the entire range occurred rapidly between 5—7 Ma and that limited exhumation has occurred since. Thus, the details of the tectonic development of this restraining bend is apparently more complicated than expected based on previous models of this restraining bend. In addition, despite the spatially uniform exhumation within the range, basin and channel relief varies significantly and systematically between different thrust sheets in the block. Thus, differential erosion of different rock types exposed within the range apparently exerts an important role in controlling the topographic structure of this range that may obscure the imprint of tectonic rates on the landscape if this differential erosion is not considered.
Kinematics of a Strike-Slip Fault Segment at Various Time Scales, Determined From GPS and Geomorphic Measurements: the Example of the Wadi Araba Fault, Dead Sea Transform.
This work investigates slip rate evolution over time along one large strike-slip fault, the Dead Sea Transform (DST), which is the 1000-km long plate boundary between the Arabia plate and the Sinai sub-plate. We focus on the Wadi Araba fault, the southernmost segment of the DST. No agreement has been reached yet about the slip rate of the DST. Proposed values vary from 2 to 10 mm/yr. Here, we present results from GPS profiles and measurements of offset geomorphologic features, which ages are comprised between 10 ka and ~300 ka. We installed 17 campaign-style GPS sites distributed along three profiles perpendicular to the fault, with far-field points up to 90 km away from the fault. The sites have been measured twice, in 1999 and 2005, during 48h-long sessions. Campaign data are complemented by data from permanent stations in Israel. Using a locked-fault model, we estimate the present-day slip rate to be 4.9 ± 1.4 mm/yr over 6 years. To estimate the slip rate over longer periods of time, we targeted abandoned alluvial fans offset by the fault at four sites. We mapped and sampled these sites for 10Be cosmogenic dating. At one site, best offset of 48 ± 7 m of a surface dated at 12.1 ± 3.6 ka yields a slip rate of 4.6 ± 2 mm/yr, in very good agreement with the present-day slip rate. Moreover, our morphologic analysis at this site invalidates previous study that suggested a value of 10 mm/yr. At a second site, an offset of 137 ± 7 m of a surface younger than ~50 ka provides a minimum slip rate of 2.6 mm/yr and a larger offset of 598 ± 30 of a surface interpreted to be 93.7 ± 35.4 ka old leads to a slip rate of 7.4 ± 3 mm/yr. The offsets determined at the two other sites, where we obtained ages comprised between 50 to ~300 ka, turned out not to be precise enough to bring new constraints on a Middle to Late Pleistocene time scale. Yet, these results are not inconsistent with previous intervals. Although variations in fault slip rate at the time scale of a few ten thousand years cannot be excluded, the slip rates presented here, complemented by previous work that include geological time-scale studies, agree on a constant slip rate of 5 ± 2 mm/yr and suggest that the Wadi Araba fault produces large earthquakes (M>6.5) quasi-periodically.
Quantifying Strain Across a Paleotransform Fault in the Mantle Section of an Ophiolite, New Caledonia
The Massif du Sud in New Caledonia exposes the mantle section of a Cretaceous-age ophiolite generated by spreading in a small basin northeast of the island. Most of the ophiolite has shallow SW-dipping foliations and subhorizontal NS lineations interpreted as reflecting fabric formed at a mid-ocean spreading ridge. Fabric intensity increases and foliation steepens to NS-striking and subvertical in the Bogota Peninsula shear zone while lineations remain shallowly S-plunging. The rotation of field foliation and pyroxenite dike orientations across a 50-km wide region is symmetric about a central high-strain zone and shear sense indicators suggest that relative displacement was dextral. The symmetry of fabrics is consistent with formation in the mantle section of a true paleotransform fault zone as opposed to a fracture zone, where fabric rotation would only be expected on one side of the shear zone. Thus, the New Caledonia ophiolite includes rocks that record both ridge- and transform-related deformation. To better understand the evolution of the transform fault, the fabrics within the Bogota Peninsula shear zone were subdivided into three regions (farfield, nearfield, and central high strain zone) of approximately homogeneous deformation. We used fabrics from these regions together with those from the Massif du Sud to model how kinematics change from ridge-dominated deformation outside the shear zone to transform- dominated inside the shear zone. Shear zone localization was modeled as superimposed increments of deformation using a grid search method that varied parameters including the orientation of the shear plane, shear sense, and transpressional versus transtensional kinematics. The best models were determined by comparing (1) the orientation of the strain ellipse with field foliation and lineation, and (2) the predicted and observed rotations of pyroxenite dikes across the shear zone. This approach allows quantitative estimation of the total deformation accumulated across this ridge-transform system.
Transformations in shallow fault zones; evidence from fault rocks in young strike-slip systems.
Shallow fault rocks are typically interpreted in terms of brittle deformation features, such as fracture patterns, processes like cataclasis, and frictional properties from laboratory experiments. There is growing evidence from observations in natural rocks, however, that chemical and state transformations play an important, perhaps even a key role in shallow fault processes. Sheared mudrocks from a recent, active part of the San Andreas Fault (SAFOD) drillhole (3-3.3 km depth) show abundant, hydrous mixed-layer clay mineral phases. These hydrous phases formed during enhanced circulation of aqueous fluids along permeable fractures by low-temperature dissolution-precipitation reactions. Of particular significance is their occurrence as thin, nm- thick clay coatings on polished and striated fracture surfaces, similar in appearance to, but much smaller than slickensided surfaces commonly found in exhumed brittle fault rocks. These clay precipitates on secondary surfaces may be key to understanding creep and weak fault behavior, as they are restricted to displacement surfaces. Their occurrence also explains the low degree of preferred orientation, measured by X-ray texture goniometry, which is typical for clay gouges. Another area where transformations influence fault behavior at shallow crustal levels is by friction melting and associated neocrystallization. At seismic slip conditions, the formation of friction melts has been proposed from calculations and laboratory experiments. Few, unaltered natural laboratories are available, but the Alpine Fault of New Zealand provides opportunity for study in recent strike-slip activity. A suite of samples collected near a type locality show that brief melt generation occurred during a single period (with several pulses?) of displacement. Dating of these samples, in conjunction with thermal modeling, shows that pseudotachylyte formed at 3.5-5 km depth, which is just below SAFOD drilling depth. A general picture is emerging where cataclasis creates nucleation sites for neomineralization or produces localized melting in upper-crustal strike-slip fault systems (< 5 km depth), which is coupled to slip rate. Transformations in shallow fault rock involve the localization of clay neomineralization along slip surfaces in creeping segments, controlling strength in the shallowest segment of fault zones. During sudden, large displacements, the energy can be sufficient to produce friction melts that are similarly generated at small slip surfaces. Thus, shallow faults rocks preserve mineral and state transformations during faulting, and provide information on the history of fluid activity, mass transport and mechanical behavior. Additionally, dating of neocrystallized mineral phases by radiogenic techniques provides the age of faulting and can constrain rates of crustal deformation.
Characteristics of Middle and Deep Crustal Expression of an Arc – Forearc Boundary Strike-Slip Fault System
Strike-slip faults below the seismogenic zone are commonly assumed to widen with depth into broad region of distributed strain or flatten into subhorizontal shear zones within the middle crust. While this may occur in some continental strike-slip systems, we propose that strike-slip faults at a major rheologic boundary, such as an arc-forearc transition, remain relatively narrow at depth, with localized high strain zones separating discrete packages of less-deformed metamorphic rock. Strain localization allows for greater displacements and explains the juxtaposition of significantly different crustal levels exposed in such strike-slip systems. We present metamorphic and geochronologic evidence for the initiation of one such strike slip system in western Argentina. The Valle Fertil, Desaguadero-Bermejo lineament is a prominent high angle lineament which currently accommodates significant shortening in the western Sierra Pampeanas of Argentina. The lineament is characterized geophysically as a high-angle to steeply east-dipping boundary with denser and more magnetic rocks on the east. The fault zone is bounded by the Cambrian-Ordovician Famatina arc, an intermediate composition batholith, to the east and an arc-forearc package of predominantly metasedimentary rocks intruded by Ordovician mafic to intermediate composition plutonic rocks to the west. The two packages currently expose markedly different crustal levels; those to the east expose rocks metamorphosed at 2-8 kbar, while those to the west expose rocks metamorphosed 11-14 kbar. Both units experienced high-grade metamorphism and granulite facies migmatization between ~470-450 Ma. Separate isolated packages within the fault/ shear zone record separate histories from those exposed to the east and west of the lineament. Low grade-limestone as well as 1.1 Ga and 845 Ma granitoids are overprinted by low-grade shear zones and show no significant thermal effect of the Ordovician magmatism and metamorphism. Regional metamorphic and structural studies indicate that this fault zone initiated a significant component of strike-slip no later than 415 Ma. Zircon U-Pb and garnet Lu-Hf ages indicate the Famatina arc and forearc system evolved from a convergent margin to collisional orogen with accretion of continental block to west between 480 and 450 Ma. Exhumation and cooling occurred from 440 to 415 Ma as recorded by muscovite and hornblende Ar/Ar ages. In contrast, metasedimentary rocks within the Valle Fertil lineament record a single metamorphic event from 415 to 407 Ma followed by cooling from 405 to 401 Ma. Mylonite and ultramylonite bounding this package record sinistral transtension at that time. No rocks west of the lineament record evidence of the Devonian metamorphism and deformation. We propose that the thermochronologic and structural history record focused tectonic burial and exhumation from shallow to deep and back to middle crustal levels over a time span of 15 Ma during a regional sinistral slip event.
GPS constraints on slip rate of the Hunter Mountain-Panamint Valley Fault Zone, Eastern California Shear Zone
The Eastern California Shear Zone (ECSZ) is a relatively young (<10 Ma) system of sub-parallel strike-slip faults. We present a new, detailed Global Positioning System (GPS) velocity field that spans the ECSZ north of the Garlock fault and south of Saline Valley. We apply different modeling approaches, including elastic block models and viscoelastic coupling models, to estimate the long term slip rate of the three major strike slip fault systems here, the White Mountain-Owens Valley fault zone to the west, the central Saline Valley- Hunter Mountain-Panamint Valley fault zone, and Fish Lake-Furnace Creek-Death Valley fault zone to the east. We find that the maximum velocity gradient for the central fault zone occurs on the Ash Hill Fault, a near-vertical, dominantly strike slip fault west of the Panamint Valley Fault. This system may constitute a paired strike slip - normal fault system, with the strike slip fault being the dominant fault. The estimated geodetic rate of the central fault system is considerably faster than previous geologic estimates. We interpret these data with a model involving simplification of the ECSZ with time, combined with progressive westward migration of deformation.
Fault Zone Healing and Structural Evolution of Fault Systems - Observations and Numerical Simulations
We study the evolving geometrical and material properties of large strike-slip fault zones and associated deformation fields using numerical simulations applying a continuum damage rheology model. We present results that demonstrate the important role material healing has in the structural evolution of fault systems. Geophysical observations of healing within the Eastern California Shear Zone are used as constraints for the damage rheology healing parameters and they manifest the natural variability in healing processes within such large fault systems. Our 3D simulations of fault evolution in a layered crust underlain by a visco-elastic upper mantle indicate that fault zone structures vary significantly in time and space within realistic fault systems. Rapid and (almost) complete fault zone healing result in a wide damage zone with several parallel active faults and relatively distributed strain patterns. However, where healing is less significant faults are relatively week during the entire seismic cycle and therefore the damage zone is narrow and strain is highly localized. The healing processes therefore have direct implications on the fault zone structure, fault system evolution and on seismicity patterns. Our simulations also indicate that fault zones initially form as complex segmented structures and evolve overall with continuing deformation toward contiguous, simpler structures. Along fault segments, the models produce a broad damage zone in the top few kilometers of the crust and highly localized damage at depth. These "flower structures" form during an early evolutionary stage of the fault system (before a total offset of about 0.05 to 0.1 km has accumulated), and persist as continued deformation localizes further along narrow slip zones. The models produce releasing stepovers between fault zone segments that are locations of ongoing interseismic deformation. Material within the fault stepovers remains damaged during the entire earthquake cycle (with significantly reduced rigidity and shear wave velocity) to depths of 10 to 15 km. These persistent damage zones should be detectable by geophysical imaging studies and could have important implications for earthquake dynamics and seismic hazard.