Late Cenozoic Deformation Research Articles

Late Cenozoic deformation of the Gavrovo and Ionian zones in NW Peloponnesos (Western Greece)

The structural deformation of Mesozoic-Tertiary sediments of the Ionian and Gavrovo zones in NW Peloponnesos is related to the propagation of a fold-thrust system during the Cenozoic. The sediments of the Gavrovo zone have been deformed by high angle reverse faulting generating an imbricate fan. Skolis mountain represents the Gavrovo thrust front. The detachment occurred in the underlying flysch of the Ionian zone. The Ionian zone has also been affected by shortening above a detachment horizon situated in the lower horizons of Triassic evaporites. The main compressional structure of the Ionian zone is a broad anticline revealed by a seismic survey west of Skolis mountain. The Gavrovo-sheet emplacement caused the downthrow and bending of the eastern part of the Ionian zone followed by halokinesis of Triassic evaporites to the west. Post-compressional normal faulting has predominated since the Pliocene, resulting in the formation of the Kato Achaia and Simopoulo basins in the peripheral area of Skolis mountain. Diapirs of Triassic evaporites occur in the mentioned basins that complicate the tectonic pattern in front of the Skolis thrust.

Late Cenozoic deformation of the Kura fold-thrust belt, southern Greater Caucasus

Although the geometry and kinematics of the first-order structures accommodating Arabia-Eurasia convergence are relatively well known in Turkey and Iran, major shortening structures remain poorly understood within the central portion of the collision zone, in eastern Anatolia and the Caucasus. New remotely sensed neotectonic mapping, synthesis of regional geologic and stratigraphic data, and balanced cross sections suggest that the Kura fold-thrust belt has accommodated the majority of Arabia-Eurasia convergence since the early Pliocene between the longitudes of ∼45°E and ∼49°E. This belt lies southeast of the N80°W-striking Greater Caucasus Mountains and forms an eastward-narrowing band of elevated topography that roughly parallels the range front for ∼400 km along strike. The belt is separated from the Greater Caucasus to the north by the 10- to 25-km-wide Alazani Basin and comprises a series of predominantly south-verging folds deforming Eocene–Quaternary flysch and molasse. To document structural geometries within the Kura fold-thrust belt, we have used the Real-time Interactive Mapping System (RIMS) software to analyze Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), visible to near-infrared (VNIR), and digital elevation model (DEM) data. This neotectonic mapping indicates an along-strike, eastward decrease in both structural complexity and the degree to which deformed geomorphic surfaces are dissected. Existing geologic maps indicate a corresponding eastward decrease in the depth of exposure. By integrating the structural geometries determined in our analysis of remote-sensing data with existing geologic data, we have constructed two balanced cross sections, which suggest these systematic along-strike variations result from a west-to-east decrease in total shortening within the Kura fold-thrust belt. We interpret this variable shortening to stem from eastward propagation of the Kura fold-thrust belt. Comparison of our preliminary total shortening estimates with those predicted by current plate motions suggest that the Kura fold-thrust belt has accommodated ∼30%–40% (∼25 km) of total Arabia-Eurasia convergence since 5 Ma, and thus forms a first-order structural system within the central portion of the collision zone.

Reconstructing late Cenozoic deformation in central Panamint Valley, California: Evolution of slip partitioning in the Walker Lane

Geosphere, June 2009, v. 5, p. 172-198, doi:10.1130/GES00178.1, Supplemental Table - Analytical data for 40Ar/39 ages.

Reconstructing late Cenozoic deformation in central Panamint Valley, California: Evolution of slip partitioning in the Walker Lane

New geologic mapping and Ar-Ar geochronology of the late Cenozoic volcanic-sedimentary units in central and southern Panamint Valley, California, provide the first known Miocene palinspastic reconstruction vectors for Panamint Valley. Panamint Valley contains active faulting and potentially accommodates a significant percentage of the slip of the Walker Lane at this latitude. Volcanism in Panamint Valley occurred during two time intervals, one ca. 15–13.5 Ma ago and a second ca. 4.5–4 Ma ago. The reconstruction vectors are based on unique relationships of sedimentary source areas and the only known Miocene intrusive zones to determine the displacement across Panamint Valley since ca. 15 Ma ago. The Argus Range was displaced ~17 km to the west-northwest, and the southern Slate Range was displaced 10.5 km to the north-northwest relative to the Panamint Range. Our displacement vector for reconstructing the past ~15 Ma of slip across Panamint Valley is 14 km shorter than previously published reconstruction models. We interpret this smaller slip value to be a function of the previous studies using displacement vectors that included a component of pre–15 Ma ago slip. The Harrisburg fault of the Tucki Mountain detachment system is a likely candidate for an earlier slip, possibly during the regionally observed extension during Late Cretaceous and Eocene. We created a model of the ca. 0–15 Ma ago displacement history of Panamint Valley using our new slip vectors and the slip vector for the Hunter Mountain fault. The Miocene extension begins with or slightly before ca. 15 Ma ago volcanism and may have continued to <~13.5 Ma ago. We interpreted the slip during Miocene extension to have occurred on one master detachment fault. Pliocene and younger extension is oblique to the Miocene extension, and the detachment fault was then cut up into discrete segments, the Emigrant, Panamint, and Slate Range detachment faults. The Panamint detachment was reactivated in an oblique normal sense, while slip on the other two detachment faults ceased; slip now occurs on nearby steeper normal faults. The Panamint detachment ends to the north and south in triple junctions: at the north end, slip is partitioned onto the Hunter Mountain and Towne Pass faults, and at the south end, slip is partitioned onto the Manly Pass and Southern Panamint Valley faults. The southern triple junction has an unstable geometry and it must migrate northward, lengthening the Southern Panamint Valley fault at the expense of the Panamint detachment. The continued slip on the unfavorably oriented low-angle Panamint detachment may be explained by the presence of weak fault gouge along it or by a regional pattern of slip partitioning. Major regional strike-slip faults, the Northern Death Valley and Garlock faults, are proximal to the northern and southern triple junctions. These two large faults may drag the two ends of the Panamint detachment with them, creating the triple junctions. The modern complex geometries and kinematics of Panamint Valley may therefore be a function of older structures being reactivated and interference with nearby faults.

Temporal constraints and pulsed Late Cenozoic deformation during the structural disruption of the active Kashi foreland, northwest China

In response to the ongoing Indo‐Asian collision, structural deformation has encroached into the Tian Shan foreland in western China since the early Miocene. In order to reconstruct a detailed history of foreland deformation along the southern margin of the Tian Shan, we have synthesized extensive mapping, analysis of seismic sections, magnetostratigraphy of the foreland fill and associated growth strata, apatite fission track dating, changes in sediment accumulation rates, and geodetic surveys of deformed fluvial terraces. Across an area exceeding 6500 km2 that spans the Kashi foreland in the northwestern Tarim Basin, we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation. Stage 1 involved initial uplift of hinterland structures beginning at 20–25 Ma. Stage 2 began at ∼16.3 Ma when the basement‐involved deformation front stepped south to the Kashi Basin Thrust that bounds the foredeep strata. Stage 3 occurred from 13.5 to 4 Ma, as the deformation front migrated south in episodic steps above thin‐skinned detachment and ramp‐flat folding. Finally, stage 4 began after 4 Ma with growth of the Keketamu Anticline followed by rapid southward migration of the deformation front to the Atushi and then Kashi detachment anticlines. On the basis of the deformed Tertiary strata and available seismic data, calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early Miocene, of which 7–12 km occurred since ∼4 Ma. Although average shortening rates since 16.3 Ma were 0.8–1.3 mm/a, rates were faster from 16.3 to 13.5 Ma (1.1–3.2 mm/a), after which they decreased to 0.5 mm/a between 13.5 and 4 Ma before increasing to 2.2–2.7 mm/a after 4 Ma. Similarly, the rate of southward propagation of the deformation front was initially 3–7 mm/a for the first 10 Ma of basin development, slowed to 1.5 mm/a between 13.5 and 4 Ma, and increased to more than 10 mm/a since 4 Ma. These changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the Neogene. Surveyed stream terraces suggest that, although most current deformation is located along the southern front of the foreland, modest deformation and shortening continue in the hinterland. Structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin, to ramp‐flat faults and detachment folds along the southern margin. Where the basement rocks are previously deformed, such as in the hinterland of the Kashi foreland, thick‐skinned fault ramps and related fault propagation folds dominate the structural style. As syntectonic sedimentation within the foreland produced thick, subhorizontal stratigraphy, the structural style evolved to detachment and fault bend folding above these footwall flats within the Neogene strata.

Late Cenozoic deformation along the northwestern continuation of the Xianshuihe fault system, Eastern Tibetan Plateau

The Xianshuihe fault system is a highly active, left-lateral slip fault that has played an important role in accommodating late Cenozoic crustal deformation of the Eastern Tibetan Plateau. The left-lateral displacement on the western portion of the fault system, the Ganzi fault, is ∼80 km, based on geologic and geomorphic markers. Left-lateral movement along the fault may have initiated in middle Miocene time (12 Ma), followed by the intensive, brittle, left-lateral movement in late Cenozoic time (2–4 Ma). Lying within the headwater regions of the Jinsha (Yangtze) River in the Yushu region, the northwestern end of the Xianshuihe fault system consists of four branches. These are, from north to south—the Dangjiang fault, the Yushu fault, the Batang fault, and the Xialaxiu fault. Both the Yushu and Dangjiang faults display clear geologic and geomorphic evidence for active, left-lateral slip movement. Several offset markers of Triassic age along the Dangjiang fault suggest ∼39 km of left-lateral slip, but the geomorphic evidence along the Dangjiang and Yushu faults suggests only ∼16 km and ∼25 km of left-lateral slip, respectively. Both the Batang and Xialaxiu faults demonstrate clear evidence for normal dip slip, by an associated series of E-W–trending grabens and horsts that absorb ∼9 km of left-lateral movement in early to middle Pleistocene time. About 32 km of left-lateral movement on the fault system was estimated to be absorbed by NE-SW shortening along the rugged North Yushu block in late Neogene time. This is interpreted to have formed as a rhomb-shaped restraining bend, along which the left-lateral movement along the northwestern end of the fault system was partially transferred into crustal shortening. The westward continuation of the Dangjiang fault is marked by an E-W–trending fault, which can be traced westward for 150 km to central Tibet, where it joins the Fenghuo Shan thrust belt. This thrust belt is interpreted to represent the northwestern continuation of the Xianshuihe fault system, along which 39 km of the left-lateral movement was entirely transferred into the N-S shortening. It indicates that the left-lateral movement within the southeastern part of the Tibetan Plateau along the fault system, either caused by lateral extrusion or by clockwise rotation, is largely restricted to the northwestern end of the fault system. Quaternary extension occurred along the northwestern continuation of the Xianshuihe-Xiaojiang fault was coeval with extension along its southern end, indicating that the southeastern margin of the plateau underwent clockwise rotation.

Development of the southern Eastern Cordillera, NW Argentina, constrained by apatite fission track thermochronology: From early Cretaceous extension to middle Miocene shortening

For the Puna Plateau and Eastern Cordillera of NW Argentina, the temporal and spatial pattern of deformation and surface uplift remain poorly constrained. Analysis of completely and partially reset apatite fission track samples collected from vertical profiles along an ESE trending transect extending from the plateau interior across the southern Eastern Cordillera at ∼25°S reveals important constraints on the deformation and exhumation history of this part of the Andes. The data constrain the Neogene Andean development of the Eastern Cordillera as well as rift‐related exhumation for some of the sampled locations in the Late Jurassic/Early Cretaceous. An intervening Eocene‐Oligocene exhumation episode in the southern Eastern Cordillera was probably related to crustal shortening. Subsequent reburial of the area by Andean foreland basin strata commenced between 30 and 25 Myr. Magnitude and duration of sedimentation, revealed by thermal modeling, differ between the sample locations, pointing to an eastward propagating basin system. In the southern Eastern Cordillera, Andean deformation commenced at 22.5–21 Myr, predating both the inferred formation of significant topography by 5–7.5 Myr and preservation of sediments in the adjacent Cenozoic basins by 6.5–8 Myr. Comparing the calculated structural depth of partially reset samples suggests that newly formed west dipping reverse faults along the former Salta Rift margin accommodated most of the Neogene tectonic movement. Late Cenozoic deformation at the southern Eastern Cordillera began earlier in the west and subsequently propagated eastward. The lateral growth of the orogen is coupled with a foreland basin system developing in front of the range and then becomes subsequently compartmentalized by later emergent topography.

Paleomagnetic evidence for a mid-Miocene clockwise rotation of about 25° of the Guide Basin area in NE Tibet

Abstract Ten sections of Neogene molasse-type sediments were sampled in the Guide Basin of northeastern Tibet for magnetostratigraphy [X.M. Fang, M.D. Yan, R. Van der Voo, D.R., Rea, C. Song, J.M. Pares, J. Gao, J. Nie, S. Dai, Late Cenozoic deformation and uplift of the NE Tibetan plateau: evidence from high resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China, Geol. Soc. America Bull. 107 (2005) 1208–1225 [1]], but they also yield seven well-dated formation-mean directions that reveal changing declinations as rotations occurred in response to crustal deformation north of the India–Asia collision zone. Three formations are of early Miocene and Oligocene age, as indicated by fossils and magnetic reversal records, whereas four younger formations yield late Miocene and Pliocene ages. The dual-polarity magnetizations are typically antipodal, but reveal inclinations that are too shallow, most likely because of post-depositional inclination flattening. The late Miocene and younger directions show formation-mean declinations between 354° and 7°, whereas three early Miocene and late Oligocene mean declinations range from 31° to 44°. This indicates that a clockwise rotation of 25.1 ± 4.6° took place during the middle part of the Miocene (best estimate 11–17 Ma). No rotations appear to have occurred, during that time, in the Xining, Lanzhou, Linxia and Jingning basins (Longzhong Basin) to the northeast and east of the Guide Basin; however, a rotation of similar magnitude has been documented by Dupont-Nivet and colleagues for pre-Miocene (> 29 Ma) time in these areas. Collectively, these results show that the basins in NE Tibet have had independently evolving structural histories.

Late Cenozoic deformation and uplift of the western flank of the Altiplano: Evidence from the depositional, tectonic, and geomorphologic evolution and shallow seismic activity (northern Chile at 19°30′S)

We analyze the west vergent thrust system (WTS) along the western flank of the Altiplano in northern Chile (18°S–21°S). In our study area (19°20′S–19°50′S), the WTS consists of three thrust propagation monocline folds (flexures) developing growth strata. The relative uplift accommodated by the flexures is rapid between 26 and 8 Ma (0.1 mm/yr), diminishing to 0.02 mm/yr after 8 Ma. Approximately 2000 m of relative surface uplift was accommodated by the flexures since the late Oligocene. Sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 Ma, coeval with the shifting of deformation from the Altiplano to the sub‐Andean zone, where the underthrusting of the Brazilian Craton would have resulted in crustal thickening, surface uplift in the orogen, and westward ductile subcrustal flow. Forearc tilting is accommodated by east vergent thrusts (ETS) issued from the Benioff zone beneath the Central Depression emerging into the Western Cordillera, contributing 500–1400 m of surface uplift. The WTS connects the ETS in the brittle‐ductile crustal transition (∼25 km depth), continuing farther east as the Altiplano low‐velocity zone, configuring the western Altiplano as a crustal‐scale fault bend fold. Forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ETS. Meanwhile, between 19°S and 21°S, the WTS accommodates dextral strike slip, and ∼3 km of N‐S shortening occurred in the Coastal Cordillera. Transcurrence and strain partitioning are probably the result of slight plate convergence obliquity, strong coupling within the interplate zone, westward continental concavity, and high elevation opposing horizontal contraction.

Late Cenozoic deformation and uplift of the NE Tibetan Plateau: Evidence from high-resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China

The Cenozoic intramontane Gonghe–Guide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strike-slip framework of the Kunlun and Altyn Tagh–South Qilian faults in the northeastern Tibetan Plateau. The basin is filled with thick Cenozoic clastic sedimentary formations, which provide important evidence of the deformation of this part of the plateau, although they have long lacked good age constraints. Detailed magnetostratigraphic and paleontologic investigations of five sections in the Guide Basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit (the Guide Group) into six formations (where ages are now magnetostratigraphically well established, they are given in parentheses): the Amigang (1.8–2.6 Ma), Ganjia (2.6–3.6 Ma), and Herjia formations (3.6 to ca. 7.0–7.8 Ma), and the older Miocene Ashigong, Garang, and Guidemen formations. These rocks document a generally upward coarsening sequence, characterized by increasing accumulation rates. Increasing gravel content and sizes of its components, changes of bedding dips and source rock types, and marginal growth faults collectively reflect accelerated deformation and uplift of the NE Tibetan Plateau after 8 Ma, punctuated by a sharp increase in sedimentation rate at ca. 3.2 Ma that reflects the boulder conglomerates of the Ganjia formation. Interestingly, much of the vergence of the compressional deformation in the basin is to the south, accommodated by a sequence of six thrusts (F1–F6), which become active one by one progressively later toward the south, undoubtedly contributing to the uplift of this part of the plateau. F1 likely initiated the Guide Basin due to crustal flexure in the Oligocene, F2 was active in the early Miocene, F4 and F5 at ca. 3.6 Ma, and F6 was active in the early Pleistocene. The detailed late Miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and, hence, uplift of northeastern Tibet, which, when compared with ages for events on other parts of the plateau, provides important boundary conditions for the geodynamical evolution of Tibet.

Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone

Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north–south Arabia–Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike–slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia–Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In the future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.

Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

A 65 km seismic reflection transect was shot in 1998 across Mackenzie Basin to Mount Cook Village, South Island, New Zealand, to provide a detailed image of the crustal structure in the central Southern Alps. The first 5 s (two‐way time (TWT)) of data were processed separately, with a maximum offset of 14 km for each shot, to image the upper 12–15 km of the crust. Data were processed as one continuous section, although the line was physically segmented due to an area of relatively steep topography with no vehicular access. No major, continuous regional‐scale features > 10 km are present in the data, but numerous 2–3 km scale reflections and discontinuities occur which are consistent with the known geology of monotonous greywacke sequences overlying schist. Strong, well‐defined reflections mark the active Irishman Creek Fault and confirm it to be a southeasterly dipping reverse fault with c. 1300–1700 m of Late Cretaceous‐Pleistocene sediments preserved in the footwall and an uplifted greywacke basement “high” in the hanging wall. Some evidence exists for active faults beneath latest Quaternary gravels at the Jollie valley and Tekapo River. Oppositely dipping reflections and discontinuities define a large, c. 15 km wavelength antiform beneath Tasman valley and Mount Cook that is imaged to 10 ± 2 km depths (3.5 s TWT). Two “end‐member” interpretations are consistent with the seismic data observations, velocity models, and constraining features of exposed geology, and extend existing geological cross‐sections to 10–15 km depth. One interpretation assumes imaged structures are primarily backthrusts developed in response to distributed Cenozoic deformation southeast of the Alpine Fault plate boundary, incorporating features observed in contemporary geodetic strain and numerical plate boundary models. The second interpretation assumes structures are mostly Mesozoic, either reactivated or preserved by late Cenozoic deformation. The main difference between the interpretative cross‐sections is the degree to which active structures link into basal detachment and high‐strain zones at depth.

Movement of Ordos block and alternation of activity along its boundaries

The GPS data in and around the Ordos block area indicate that the left-lateral slip rate along the northern or southern margin of the Ordos block is about twice or three times as fast as the right-lateral slip rate along the eastern or western margin of the block. However, many researchers stressed the dextral-slip of the eastern or western boundaries of the Ordos block, and suggested that the block as a whole rotated counterclockwise based on the available geological data. Focusing on the inconsistency, we reexamine the late Cenozoic deformation pattern in the Ordos region based on seismicity data and geodesy data (GPS and leveling) around it. The results indicate that the rigid block-like motion appears to be the basic characteristic of the kinematics of the Ordos region, and this motion is absorbed by the displacement of the faults around the block. When the faults along the northern and southern boundaries of the Ordos block are active, its eastern boundary is inactive. However, if the faults along the eastern boundary are active, the northern and southern are inactive. In recent years, the northern and southern boundaries of the Ordos block are in active. But in the long term, the Ordos block is moving southeastward relative to the Alxa and Yinshan blocks because of the strong pushing of the Tibetan Plateau on its southwestern side, and this deformation is accommodated by the counterclockwise rotation of the block itself.

Late Cenozoic deformation subsequence in northeastern margin of Tibet —Detrital AFT records from Linxia Basin

Two events of Tibet uplifting are revealed by detrital apatite fission track (AFT) age data from Linxia Basin. They occurred at about 14 and 5.4-8.0 MaBP respectively. We interpret the first one to be related to the uplifting of the northern Tibet, which might have resulted from convectively removing the thickened lower lithosphere. The second one is a result of Laji Mountain uplifting. Numerous studies of the Tibetan Plateau suggest that the onset time of the deformation in the northeastern margin of Tibetan Plateau and the time of Tibet attaining to its present elevation is about 8 MaBP. They are approximately coincident with the uplift of Lajishan Mountain. It suggests that the northeastern margin of Tibet propagated northeastwardly to its present site in about 8 MaBP for accommodating the sustained convergence between India-Eurasia plate and for keeping its high elevation. The active block pattern dominating the strong earthquake distribution of Chinese continent probably formed at about 8.0-5.4 MaBP.

Active intraplate strike-slip faulting and transpressional uplift in the Mongolian Altai

Abstract The Mongolian Altai is a Late Cenozoic intraplate strike-slip deformation belt which formed as a distant strain response to the Indo-Eurasian collision over 2000 km to the south. We report results from 5 weeks of detailed fieldwork carried out during summer 2000 in northwestern Mongolia investigating the crustal architecture of the Altai at latitude 48°N. The region can be divided into discrete Cenozoic structural domains each dominated by a major dextral strike-slip fault system or range-bounding thrust fault. Gentle bends along the major strike-slip faults are marked by transpressional uplifts including asymmetric thrust ridges, restraining bends, and triangular thrust-bounded massifs. These transpressional uplifts (Tsambagarav Massif, Altun Huhey Uul, Sair Uul, Hoh Serhiyn Nuruu, Omno Hayrhan Uula, Mengildyk Nuruu) comprise the highest mountains in the Mongolian Altai and are structural and metamorphic culminations exposing polydeformed greenschist-amphibolite grade basement recording at least two phases of Palaeozoic ductile deformation overprinted by Cenozoic brittle structures. Cenozoic thrust faults with the greatest amounts of displacement bound the W and SW sides of ranges throughout the region and consistently verge WSW. Each major range is essentially a NE-tilted block and this is reflected by asymmetric internal drainage patterns. Many faults are considered active because they deform surficial deposits, form prominent scarps, and define range fronts with low sinuosity where active alluvial fan deposition takes place. Reactivation of the prevailing NW-striking, NE-dipping Palaeozoic basement anisotropy is a regionally important control on the orientation and kinematics of Cenozoic faults. At first order, the Altai is spatially partitioned into a low-angle thrust belt that overthrusts the Junggar Basin on the Chinese side and a high-angle SW-vergent dextral transpressional belt on the Mongolian side. The mechanically rigid Hangay craton and Junggar basement block which bound the Altai on either side have played a major role in focusing Late Cenozoic deformation along their boundaries and within the Altai. The geometric relationship between rigid block boundaries, Palaeozoic basement structural anisotropy, and the dominantly NE SHmax (derived from India’s continued NE indentation) has dictated the kinematics of Late Cenozoic deformation in the Altai, Gobi Altai, and Sayan regions.

Structural and Sedimentologic Evolution of a Miocene Supradetachment Basin, Silver Peak Range and Adjacent Areas, West-Central Nevada

The Silver Peak region of west-central Nevada provides a unique opportunity to study the history of a supradetachment basin, its deformed basement, and the detachment fault itself. The region experienced Late Cretaceous to Paleogene plutonism and stretching of a Neoproterozoic through Ordovician miogeoclinal section. Upper-crustal thinning above the Mineral Ridge pluton was accommodated by development of gently dipping schistosity in Neoproterozoic strata, and a mosaic of low-angle attenuation faults in overlying Cambrian strata. Middle and Late Miocene detachment faulting accommodated approximately 8-15 km of northwest transport of hanging-wall strata. The Mineral Ridge detachment fault apparently reactivated the basal décollement of the Late Cretaceous to Paleogene attenuation faults. The Esmeralda basin evolved in the hanging wall of the detachmentfault system, and represents subsidence over a significantly broader region than modern extensional basins. The supradetachment basin fill consists of three interfingering lithofacies assemblages: (1) an east-derived, conglomeratic, sedimentaclastic, alluvial-fan assemblage deposited adjacent to the breakaway; (2) mudstone and coal deposited in lacustrine and paludal environments; and (3) a westderived, sandy, volcaniclastic assemblage deposited in braidplain/braiddelta environments. Geometric relations indicate that the detachment fault dipped gently while active, and was the sole fault for a stack of listric-normal growth faults. Late Miocene arc volcanism within the Silver Peak region postdated cessation of detachment faulting. Present physiography is controlled by Pliocene to Holocene normal and strike-slip faulting associated with volumetrically minor basaltic volcanism. This complex history of late Mesozoic and Cenozoic deformation, magmatism and sedimentation in the Silver Peak region may be analogous to other parts of the Basin and Range Province, suggesting broad applicability of the model developed herein.

Paleomagnetic data bearing on style of Miocene deformation in the Lake Mead area, southern Nevada

Paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late Miocene Hamblin–Cleopatra Volcano, which was structurally dismembered during the development of the Lake Mead Fault System (LMFS), provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the Basin and Range province. Sinistral separation along the fault system dissected the volcano into three major blocks. The eastern, Cleopatra Lobe of the volcano is structurally the most intact section of the volcano. Normal and reverse polarity data from paleomagnetic sites collected along traverses in the Cleopatra Lobe yield an in situ grand mean of Declination (D)=339°, Inclination (I)=+54°, α95=3.1°, k=27.2, N=81 sites. The rocks of the central core of the volcano yield an in situ grand mean of D=3°, I=+59°, α95=6.8°, k=42.5, N=11 sites (six normal, five reverse polarity). Sites collected within the western Hamblin Lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of D=168°, I=−58°, α95=6.5°, k=28.9, N=18 sites. Interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area, considers the possibility of two different types of structural corrections. A stratigraphic tilt correction involves restoring flows to the horizontal using the present strike. This correction assumes no initial, possibly radial, dip of flows of the volcano and is considered invalid. A structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of D=9°, I=+53°, α95=3.1°, k=27.2, and D=58°, I=+78°, α95=6.8°, k=42.5, for the Cleopatra Lobe and the central intrusive core, respectively. The data from the Cleopatra Lobe are slightly discordant, in a clockwise sense, from expected middle- to late-Miocene field directions. The data from the volcano are not consistent with a proposed structural model of uniform, moderate magnitude, statistically significant, counter-clockwise vertical axis rotation of fault-bounded blocks during overall sinsitral displacement along the LMFS. We also analyzed dikes of the northernmost part of the Miocene Wilson Ridge hypabyssal igneous complex, strata of the Triassic Chinle Formation, and basalt flows of the Miocene West End Wash/Callville Mesa volcanic centers. Dikes in the Wilson Ridge pluton and the Triassic strata yield magnetizations with directions suggestive of statistically significant, clockwise, vertical-axis rotations consistent with local, large-magnitude shear of crustal fragments near some of the faults of the LMFS. Late Cenozoic deformation of the Hamblin–Cleopatra volcano area appears to have been non-uniform in scale and magnitude and no single structural model, involving strictly strike-slip faulting, can account for the observed paleomagnetic data.

Cenozoic normal faulting and regional doming in the southern Hangay region, Central Mongolia: implications for the origin of the Baikal rift province

The Hangay Dome in central Mongolia is a mountainous region covering more than 200,000km2 with numerous flat-topped peaks over 3000m that preserve a Late Cretaceous–Paleogene erosion surface. Doming began in the middle Oligocene producing more than 2000m of regional topographic uplift. The range represents an important kinematic link between the Baikal rift province to the north and Altai transpressional ranges to the south and west. Structural field investigations of major faults visible on satellite imagery indicate that the southern Hangay Dome region is dominated by Late Cenozoic normal faults that bound small half-graben. Most faults are NE-striking and faults at the highest elevations are the most recently active. Most graben appear to be isolated systems that constitute small sediment sinks perched on the flanks and crests of the dome. The first-order feature is the dome itself and most sediment eroded off of the dome is deposited in the Mongolian Valley of Lakes or is carried northwards by the Selenga river and its extensive tributaries.The basement of the dome is a Precambrian craton although the shape and dimensions of the craton are poorly constrained to the north and east. Late Cenozoic uplift of the southern dome region appears to be confined to the area underlain by cratonic basement whereas the Altai region to the south and west is underlain by mechanically weaker Palaeozoic arc and accretionary belts. With respect to the regional northeast directed SHmax, the Hangay craton appears to have acted as a rigid passive indentor focusing Late Cenozoic transpressional deformation around its west and southern margins. Models invoking a Late Cenozoic plume as the driving force for doming and widespread alkaline volcanism on the dome are not strongly supported by geochemical and isotopic data on Neogene–Recent volcanics and the spatial correlation between areas that are domed and older cratonic crust appears too coincidental to be ignored. Convective removal of an overthickened lithospheric root leading to adiabatically decompressed asthenosphere could explain regional doming and volcanism, however major crustal thickening last occurred in the Permian in the southern Hangay region and the time lag between thickening and postulated root removal and plateau uplift (>200Myr) appears too long. An alternative model is explored based on speculated lithospheric mantle flow patterns driven by India's continued northeastward indentation. It is suggested that lithospheric mantle flow diversion around the overthickened Hangay craton crustal keel could cause lithospheric thinning beneath the craton and passive asthenospheric upwarp leading to regional topographic uplift and decompression melting/alkali volcanism. In general, the angular relationship between Precambrian craton boundaries and the prevailing northeasterly SHmax appears to control the kinematics of late Cenozoic deformation throughout the Hangay, Altai, Sayan and Baikal regions.

Dome and basin refolding and transpressive inversion along the Karatau Fault System, southern Kazakstan

The Karatau Fault System of southern Kazakstan forms a crustal-scale zone of strike-slip dominated transpressional tectonics which has undergone multiple phases and styles of deformation during a protracted history of reactivation from the Neoproterozoic to the Cenozoic. Ductile fabrics associated with dextral kinematic indicators are present in amphibolites along the Main Karatau Fault. The ages of the (possibly ophiolitic) protoliths and the ductile fabrics are not well constrained, but they are plausibly pre-late Riphean. Late Ordovician granites stitch thrusts and folds which deform late Riphean to Llanvirn clastics, preserved beneath a regional Late Devonian angular unconformity. This deformation may be related to a contemporary continental collision NE of the Karatau. Two phases of late Palaeozoic deformation affected the Upper Devonian and Carboniferous carbonate succession. The first phase is related to sinistral transpression along the Karatau Fault System, the second to dextral transpressional reactivation. The combination of these events produced a regional-scale dome-and-basin fold interference pattern. Similar polyphase deformation appears to have affected large areas of Central Asia and was possibly caused by the late Palaeozoic orogenies at Asia’s margins, such as the accretion of Tarim and the East European Craton to Asia, and the closure of the Kazakstan Orocline. Normal faults in the Karatau are related to the formation of the Leontiev Graben, South Turgay Basin and the Yarkand–Fergana Basin in the Early to Mid-Jurassic, during renewed dextral slip along the Karatau/Talas–Fergana Fault. Late Cenozoic deformation is minor, and resulted in the uplift and incision of a Cretaceous–Palaeogene peneplain without a major tectonic overprint. Reversals in the sense of strike-slip dominated transpressive deformation across major fault systems results in transpressive inversion . This represents an ideal process with which to generate overprinting orthogonal fold systems, resulting in classic dome and basin interference patterns on a regional scale.

Mobility of continental mantle: Evidence from postseismic geodetic observations following the 1992 Landers earthquake

The crust around the rupture zone of the 1992 Landers earthquake has continued to deform in the years following the earthquake at rates ∼3 times greater than pre‐earthquake rates. We use a combination of Global Positioning System (GPS) and synthetic aperture radar (InSAR) data collected during a ∼3‐year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities. In order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation. The combination of GPS and InSAR data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models. The data particularly require the presence of a highly ductile uppermost mantle beneath the central Mojave Domain, with temperature between the wet and dry basalt solidus. This is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle. Further consideration of seismic velocity variations in conjunction with faulting patterns within the Mojave Desert suggests that the primary faulting characteristics of the Mojave Desert, namely, the pervasive late Cenozoic deformation within the Eastern California Shear Zone versus the near absence of faults in the Western Mojave Domain, are controlled by the rheology of the uppermost mantle.