Late Cenozoic Faults Research Articles

Late Cenozoic Faults and Shallow Oil Accumulation in the Nanpu Sag

The Nanpu Sag, located in the north part of Huanghua Depression of Bohai Bay Basin, is a Cenozoic petroliferous extensional sag, the shallow oil reservoirs have great exploration potential. Former exploration suggested that there is complex relationship between Late cenozoic faults and hydrocarbon accumulation, but there is no detailed discussion. Based on the structural interpretation of 3D seismic data, the Late Cenozoic fault characters including typical fault configuration in profile and fault arrays in plane are analyzed, furthermore, the formation mechanism of the faults is discussed. It is concluded that late Cenozoic faults were formed by extension other than strike-slip movement, which was controlled by the mechanism of pure shear in the lower crust during the period. In addition, the relationship between Late Cenozoic faults and shallow hydrocarbon accumulation is discussed, it indicates that the faults controlled the formation of shallow closures and constituted the effective hydrocarbon pathways connecting deep source rock and shallow traps.

Late Cenozoic tectonic stress fields of the Mongolian microplate

The article deals with the Late Cenozoic fault tectonics and crustal stress state in the Mongolian block (Central and West Mongolia). An analysis was made of the stress field reconstructions, tectonic deformations in active fault zones of the central part of the block (Khangai dome), adjacent uplifts, and intermountain basins, i.e., Mongolian Altai, Gobi Altai, Valley of Lakes, and Khan-Taishir-Nuruu, Khan-Khukhei and Bolnai uplifts. Deformations associated with a general northeast-trending collision compression are concentrated along the periphery of the Mongolian block, the maximum compression occurring on the western and southern block boundaries and giving rise to the right- and left-lateral transpressive structures of Mongolian and Gobi Altai. The deformations associated with the contraction of the Earth's crust affect not only bordering ranges but also the intermountain depressions separating the Gobi and Mongolian Altai from the Khangai and the southern Khangai domes. A variety of deformations in the central Khangai dome is associated with the interaction between extension occurring under dynamic influence of the mantle anomaly and regional north-east compression. Tectonic stiffness of the Khangai block appears to be somewhat favorable to the deformation transfer to its northern termination, i.e., the North Khangai strike-slip fault. This latitudinal zone shows that the compression increases toward the west where it merges with the transpressive structures of the Mongolian Altai, whereas the extension increases toward the east. The formation of releasing bend strike-slip features may be due to the proximity of the Baikal rift zone that resulted from north-west extension associated to a general southwestward movement of the Amur plate.

Circular geomorphologic and geological features in the Japanese islands

The available geomorphologic, geological, geophysical, and paleogeographic data were used to identify and reconstruct, to varying degrees of accuracy, dome-block and tectono-magmatic circular uplifts within the Japanese island arc, which contain the main mountain massifs and the centers of orogenic granitoid magmatism on Kyushu, Honshu, Shikoku, and Hokkaido islands. By analogy with the magmatic domes of mountain features in the adjacent continental land (Sikhote-Alin’), they are identified with Cretaceous/Paleogene granitoid focus systems of regional rank. It was found that these elements of the structural setting have preserved their significance up to the present, in spite of the fact that destructive tectonogenetic tendencies have been dominant since the Late Oligocene/Miocene. We also hypothesize that relicts of a major dome-block feature (Nampo) exist within the northern Philippine Sea. The local circular features on these islands reflect the entire diversity of focus formations at volcanic arcs and in the volcano-plutonic belts at active continental margins. Our study showed that the morphotectonic base of the Japanese Islands is not a chaotic collage of terranes, but rather a consolidated system of Cretaceous/Paleogene central-type orogenic uplifts that are evolving in an inherited manner with superimposed Late Cenozoic magmatic, block, and fault features. The Japanese Islands contain an abundance of circular features of varying ranks and ages, indicating the essential control of deep injected dislocations and of the magmatic factor in the structure and evolution of the region.

Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

The Blue Lake Fault Zone occurs at a major crustal boundary between Otago Schist and Torlesse greywacke. This zone has been active since the middle Cretaceous (c. 112 Ma), when extensional exhumation of the Otago Schist belt was initiated. The Blue Lake Fault Zone is dominated by a set of normal faults that have caused juxtaposition of rocks of different metamorphic grade, from prehnite-pumpellyite facies turbidites to pervasively recrystallised greenschist facies schists. This metamorphic transition has been thinned from c. 15 km to c. 2 km by normal fault motion on the scale of kilometres. This condensed section is the narrowest such section on the Otago Schist margin. The faults in this condensed section are defined by gouge zones that are hundreds of metres wide and constitute about 30% of the section. Gouge zones were locally reactivated as thin Late Cenozoic reverse faults on the centimetre to metre scale.

Late Cenozoic fields of the tectonic stresses in Western and Central Mongolia

The paper addresses the Late Cenozoic fault tectonics and the stress state of the Earth’s crust within the Mongolian microplate, embracing Central and Western Mongolia. We analyze the results of reconstructing the stress fields and the tectonic deformations in the zones of active faulting, located at the uplands and in the intermountain trenches, which bound the microplate (Mongolian Altai; Gobi Altai; Dolinoozersk trough; Khan-Taishir-Nuruu, Khan-Houkhei, and Bolnai uplands) and the Khangai dome. Deformations related with the northeastern general-scale collisional compression are concentrated along the periphery of the Mongolian microplate, and the maximum compression is focused on its western and southern boundaries, thus forming the right- and the left-lateral transpressive structures of the Mongolian and Gobi Altai. The deformations associated with the shortening of the Earth’s crust involve not only the mountain ridges framing the block, but also the intermountain troughs that separate the Gobi and Mongolian Altai from the Khangai dome, and the southern portion of the Khangai Uplift. The diversity in the deformations within the central Khengai region ensues from the coupling of tension caused by the dynamical impact of the mantle anomaly, which is located east of 100°E, with a regional NE compression. Owing to the relatively rigid Khangai block, the deformations are transferred to the northern bound of this structure, namely the seismically active North Khangai fault. The role of compression increases to the west of the zone, where it conjugates with the transpressive structures of the Mongolian Altai. The tension becomes more important in the western part of this zone where the releasing bends are formed. A region characterized by extra tension is localized also to the east of 100° E. In terms of the gradient in the lithosphere thickness and the structure types of the upper crust, the submeridional line running along 100°E is interpreted as the key interblock boundary.

Tectonic control on southern Sierra Nevada topography, California

In this study we integrate the apatite (U‐Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low‐relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by age‐elevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N–S and approximately NW. Quaternary west‐side‐up normal faulting along the N–S Breckenridge–Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning Neogene‐Quaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region.

Is the track of the Yellowstone hotspot driven by a deep mantle plume? — Review of volcanism, faulting, and uplift in light of new data

Geophysical imaging of a tilted mantle plume extending at least 500 km beneath the Yellowstone caldera provides compelling support for a plume origin of the entire Yellowstone hotspot track back to its inception at 17 Ma with eruptions of flood basalts and rhyolite. The widespread volcanism, combined with a large volume of buoyant asthenosphere, supports a plume head as an initial phase. Estimates of the diameter of the plume head suggest it completely spanned the upper mantle and was fed from sources beneath the transition zone, We consider a mantle–plume depth to at least 1,000 km to best explain the large scale of features associated with the hotspot track. The Columbia River–Steens flood basalts form a northward-migrating succession consistent with the outward spreading of a plume head beneath the lithosphere. The northern part of the inferred plume head spread (pancaked) upward beneath Mesozoic oceanic crust to produce flood basalts, whereas basalt melt from the southern part intercepted and melted Paleozoic and older crust to produce rhyolite from 17 to 14 Ma. The plume head overlapped the craton margin as defined by strontium isotopes; westward motion of the North American plate has likely “scraped off” the head from the plume tail. Flood basalt chemistries are explained by delamination of the lithosphere where the plume head intersected this cratonic margin. Before reaching the lithosphere, the rising plume head apparently intercepted the east-dipping Juan de Fuca slab and was deflected ~ 250 km to the west; the plume head eventually broke through the slab, leaving an abruptly truncated slab. Westward deflection of the plume head can explain the anomalously rapid hotspot movement of 62 km/m.y. from 17 to 10 Ma, compared to the rate of ~ 25 km/m.y. from 10 to 2 Ma. A plume head-to-tail transition occurred in the 14-to-10-Ma interval in the central Snake River Plain and was characterized by frequent (every 200–300 ka for about 2 m.y. from 12.7 to 10.5 Ma) “large volume (> 7000 km 3)”, and high temperature rhyolitic eruptions (> 1000 °C) along a ~ 200–km-wide east–west band. The broad transition area required a heat source of comparable area. Differing characteristics of the volcanic fields here may in part be due to variations in crustal composition but also may reflect development in differing parts of an evolving plume where the older fields may reflect the eruption from several volcanic centers located above very large and extensive rhyolitic magma chamber(s) over the detached plume head while the younger fields may signal the arrival of the plume tail intercepting and melting the lithosphere and generating a more focused rhyolitic magma chamber. The three youngest volcanic fields of the hotspot track started with large ignimbrite eruptions at 10.21, 6.62, and 2.05 Ma. They indicate hotspot migration N55° E at ~ 25 km/m.y. compatible in direction and velocity with the North American Plate motion. The Yellowstone Crescent of High Terrain (YCHT) flares outward ahead of the volcanic progression in a pattern similar to a bow-wave, and thus favors a sub-lithospheric driver. Estimates of YCHT-uplift rates are between 0.1 and 0.4 mm/yr. Drainage divides have migrated northeastward with the hotspot. The Continental Divide and a radial drainage pattern now centers on the hotspot. The largest geoid anomaly in the conterminous U.S. is also centered on Yellowstone and, consistent with uplift above a mantle plume. Bands of late Cenozoic faulting extend south and west from Yellowstone. These bands are subdivided into belts based both on recency of offset and range-front height. Fault history within these belts suggests the following pattern: Belt I — starting activity but little accumulated offset; Belt II — peak activity with high total offset and activity younger than 14 ka; Belt III — waning activity with large offset and activity younger than 140 ka; and Belt IV — apparently dead on substantial range fronts (south side of the eastern Snake River Plain only). These belts of fault activity have migrated northeast in tandem with the adjacent hotspot volcanism. On the southern arm of the YCHT, fault activity occurs on the inner, western slope consistent with driving by gravitational potential energy, whereas faulting has not started on the eastern, outer, more compressional slope. Range fronts increase in height and steepness northeastward along the southern-fault band. Both the belts of faulting and the YCHT are asymmetrical across the volcanic hotspot track, flaring out 1.6 times more on the south than the north side. This and the southeast tilt of the Yellowstone plume may reflect southeast flow of the upper mantle.

Coupling of late cenozoic faulting of the Siberian Platform margin and Baikal Rifting

The relationship between the Late Cenozoic tectonic deformations at the Siberian Platform (SP) margin and the adjacent Baikal Rift System is considered. This study was aimed at estimation of the stress state and conditions of neotectonic reactivation of faults in the sedimentary cover of the eastern Irkutsk Amphitheater (the Angara‐Lena Uplift and Fore-Baikal Trough coupling zone). On the basis of geological, structural, and geomorphic methods, the Late Cenozoic fault assemblages were investigated and the related stress field was reconstructed to provide evidence for interrelated deformation of the Baikal Rift and the SP margin. The dynamic interaction of the SP block with the surrounding fold systems at the neotectonic stage and the evolution of the Late Cenozoic faulting remain poorly studied. A.G. Zolotarev developed the concept of fore-rift troughs [5 among others]. The brittle fracturing of the upper Pleistocene sediments was documented in the Fore-Sayan Foredeep [1, 8]. The existence of recent seismic activity of the platform has been proven in [2]. Seminsky et al. [8 and others] attempted to demonstrate the interrelation between the seismic activity and deformations in the Baikal Rift. However, these issues remain insufficiently studied because of short-term instrumental measurements and the absence of seismic station network on the platform. The northeastern part of the Irkutsk Amphitheater of the SP is characterized by medium-intensity neotectonic uplifting with an amplitude of 800 m relative to the initial surface [4]. Such values have been estimated for the Angara‐Lena Uplift. The Fore-Baikal Trough, which extends on the eastern side along the SP margin, ascended less intensely (amplitude of vertical movements was not greater than 200 m). The difference in the present-day hypsometric position of these structures suggests a rather high velocity gradient of neotectonic vertical movements in their coupling zone, providing prerequisites for differential motions along fault zones. The schematic map of faults in the Fore-Baikal Trough and Angara‐Lena Uplift coupling zone (Fig. 1) is based on the interpretation of satellite images and aerial photographs, analysis of a digital 3D model of topography, results of field geostructural and geomorphic observations, and compilation of the State Geological Mapping data. The faults in the study territory make up a rather dense network in comparison with inner sectors of the SP, probably owing to the closeness with the Sayan‐ Baikal mobile region. Paleozoic folds and faults significantly controlled the development of local neotectonic structures. The northern part of the territory is dominated by submeridional faults that reflect the general strike of older linear structures in the study region (Kirenga fold zone [3]). The NE-trending faults inherit structures of the Zhigalovo‐Tulukmur fold zone. The neotectonic reactivation stage is characterized by nonuniform manifestation over the study area (Fig. 1). Judging from the density of neotectonic faults, the northern Fore-Baikal Trough and the zone of its coupling with the Angara‐Lena Uplift were most active. The maximums of fault density are confined to the Khanda (southern end), Baldakhin’ya, and Novoselovo basins. The platformal sedimentary cover in the northern Angara‐Lena Uplift is marked by minimum faulting.

Active faults and crustal stress in the northeastern flank of the Baikal rift system

We discuss the patterns of Late Cenozoic faulting and crustal stress in the northeastern flank of the Baikal rift system. The Late Cenozoic faults are mainly of NE and ENE strikes. Faults of these trends, along with W–E faults, have been principal seismogenic structures. They have normal or left-lateral oblique geometry with different amounts of horizontal motion. The N–S, NW, and NNW faults bear signature of mostly right-lateral strike slip, and those of the WNW direction are left-lateral strike-slip faults, often with a normal component. The rift basins in this part of the rift system have had different evolution dynamics, with the most rapid faulting and sedimentation in the Muya basin. According to structural and seismological data, regional stress in the area has been stable and dominated by NW extension through the Pleistocene-Holocene and the Present. The directions and obliquity angles of principal normal stresses and percentages of stress types show local lateral variations. The evolution of the northeastern flank of the Baikal rift system can be explained by a model of oblique rifting which accounts for the architecture of rift basins, the pattern of Late Cenozoic active faults, and the stress pattern derived from structural and seismicity data. The model is consistent with centrifuge models of magma emplacement during continental oblique rifting. Oblique extension associated with underplated magma can be maintained in the area by mantle plumes beneath the Kichera, Upper Angara, and Chara rift basins. The presence of subcrustal magma chambers may explain why the three basins formed before other basins in the NE flank of the rift system.

Late Cenozoic fault pattern and stress fields in the Barguzin rift (Baikal region)

New structural and tectonophysical data, combined with the published geophysical and seismological evidence, were used to map the Late Cenozoic fault pattern and crustal stress in the Barguzin rift. Faults striking in the NE direction are the most abundant elements of the rift structure. A special part in the Late Cenozoic patterns of faults and stresses belongs to an over 400 km long N-S lineament which shows up as a system of separate fault segments between 110° and 110°30′ E. The Late Cenozoic evolution of the rift has been controlled mainly by extension punctuated with local shear stresses derived from the regional extension stress and accommodated by strike slip, combined with the dominant normal motion, along NE or N-NE faults and/or along their cross faults. Extension was of a relatively stable NW-SE direction, almost rift-orthogonal. The obtained fault pattern and stress maps can be used for reference in mapping seismic hazard associated with ongoing faulting in an active and changeable stress field.

PO19-593 SALIVARY IMMUNOGLOBULINS AND RISK OF CORONARY ARTERY DISEASE

The Maule, Chile, (Mw 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the Mw 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), Mw 5.8 Catamarca (1400 km; nine days), Mw 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.

“Sarsen Stones” at German Hill, Central Otago, New Zealand, and Their Potential for In Situ Cosmogenic Isotope Dating of Landscape Evolution

Abstract 10Be concentrations measured in silica‐cemented Tertiary sandstones in Central Otago have yielded minimum exposure ages of up to 1400 ka, some of the oldest ever recorded outside the dry valleys of Antarctica and some arid deserts. The silica‐cemented sandstones outcrop as boulders in a region where their exposure is caused by the growth of anticlinal ridges above blind reverse faults. Initial studies using a combination of in situ cosmogenic isotope measurements and geomorphological observations on one range have demonstrated the potential of this technique to investigate Late Cenozoic fault growth and erosion rates. The best‐exposed and preserved occurrence of the quartz‐rich boulders is at German Hill on North Rough Ridge, where their stratigraphic context and their sedimentary and diagenetic origin, together with their method of emplacement and preservation on the modern land surface, can be studied in detail. These are all features that affect their suitability for cosmogenic isotope studies...

Colorado Late Cenozoic Fault and Fold Database and Internet Map Server: User-Friendly Technology for Complex Information

Research Article| February 01, 2005 Colorado Late Cenozoic Fault and Fold Database and Internet Map Server: User-Friendly Technology for Complex Information KAREN S. MORGAN; KAREN S. MORGAN 1Colorado Geological Survey, 1313 Sherman Street, Room 715, Denver, CO 80203 Search for other works by this author on: GSW Google Scholar GERALD J. PATTYN; GERALD J. PATTYN 2Farragut Systems, Inc., 1455 Dixon Avenue, Suite 310, Lafayette, CO 80026 Search for other works by this author on: GSW Google Scholar MATTHEW L. MORGAN MATTHEW L. MORGAN 3Colorado Geological Survey, 1313 Sherman Street, Room 715, Denver, CO 80203 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2005) 11 (2): 155–162. https://doi.org/10.2113/11.2.155 Article history first online: 02 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 KAREN S. MORGAN, GERALD J. PATTYN, MATTHEW L. MORGAN; Colorado Late Cenozoic Fault and Fold Database and Internet Map Server: User-Friendly Technology for Complex Information. Environmental & Engineering Geoscience 2005;; 11 (2): 155–162. doi: https://doi.org/10.2113/11.2.155 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 SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Internet mapping applications for geologic data allow simultaneous data delivery and collection, enabling quick data modification while efficiently supplying the end user with information. Utilizing Web-based technologies, the Colorado Geological Survey's Colorado Late Cenozoic Fault and Fold Database was transformed from a monothematic, nonspatial Microsoft Access database into a complex information set incorporating multiple data sources. The resulting user-friendly format supports easy analysis and browsing. The core of the application is the Microsoft Access database, which contains information compiled from available literature about faults and folds that are known or suspected to have moved during the late Cenozoic. The database contains nonspatial fields such as structure type, age, and rate of movement. Geographic locations of the fault and fold traces were compiled from previous studies at 1:250,000 scale to form a spatial database containing information such as length and strike. Integration of the two databases allowed both spatial and nonspatial information to be presented on the Internet as a single dataset (http://geosurvey.state.co.us/pubs/ceno/). The user-friendly interface enables users to view and query the data in an integrated manner, thus providing multiple ways to locate desired information. Retaining the digital data format also allows continuous data updating and quick delivery of newly acquired information. This dataset is a valuable resource to anyone interested in earthquake hazards and the activity of faults and folds in Colorado. Additional geologic hazard layers and imagery may aid in decision support and hazard evaluation. The up-to-date and customizable maps are invaluable tools for researchers or the public. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structure and topographic evolution of the Main Divide in the Landsborough‐Hopkins area of the Southern Alps, New Zealand

The Landsborough‐Hopkins area of the Southern Alps lies between the tectonically active central Southern Alps and the south Southern Alps which were most active in the Miocene. The Landsborough‐Hopkins area is underlain by interleaved tectonic slices (kilometre scale) of textural zones 2A, 2B, and 3 schist surrounding a thin sliver of unfoliated greywacke. Foliation and bedding are steeply dipping and northeast striking. This structural pattern is a northern extension of Miocene deformation style which dominates to the south. Late Cenozoic faults, particularly the Main Divide Fault Zone of the Mt Cook area to the north, are not well developed in the Landsborough‐Hopkins area. The northeast structural grain controlled pre‐Quaternary drainage orientation of the Landsborough and Clarke Rivers, and these rivers flowed into major river systems to the east of the topographic divide at that time. These rivers have since been captured by the west‐flowing Haast River, and the topographic divide has stepped progressively to the east in this area. This eastward stepping of the topographic divide was driven by tectonic shortening in the northern part of the area. The Landsborough River capture was facilitated by erosion at a sinistral cross‐fault, the Creswicke Fault. At the capture zone, both Landsborough River and Main Divide cut sharply across the regional structural grain imposed by Miocene deformation. Uplift at the capture zone has raised a c. 2 km high divide since capture. In contrast, after the Clarke River was captured by the Haast River, the Wills River (a former Clarke tributary) maintained the Haast Pass as a topographically low capture point. Hydrothermal alteration and vein formation occurred in the immediate area of the Landsborough River capture zone. Fluid flow was structurally controlled, and deposited ankerite, quartz, and/or sulfide minerals in faults and associated fractures.

Biostratigraphy and Structure of Paleozoic Host Rocks and Their Relationshipto Carlin-Type Gold Deposits in the Jerritt Canyon Mining District, Nevada

The Jerritt Canyon mining district in the northern Independence Range, northern Nevada, contains multiple, nearly horizontal, thrust masses of platform carbonate rocks that are exposed in a series of north- to northeast-elongated, tectonic windows through rocks of the Roberts Mountains allochthon. The Roberts Mountains allochthon was emplaced during the Late Devonian to Early Mississippian Antler orogeny. These thrust masses contain structurally and stratigraphically controlled Carlin-type gold deposits. The gold deposits are hosted in tectonically truncated units of the Silurian to Devonian Hanson Creek and Roberts Mountains Formations that lie within structural slices of an Eastern assemblage of Cambrian to Devonian carbonate rocks. In addition, these multiply thrust-faulted and folded host rocks are structurally interleaved with Mississippian siliciclastic rocks and are overlain structurally by Cambrian to Devonian siliciclastic units of the Roberts Mountains allochthon. All sedimentary rocks were involved in thrusting, high-angle faulting, and folding, and some of these events indicate substantial late Paleozoic and/or Mesozoic regional shortening. Early Pennsylvanian and late Eocene dikes also intrude the sedimentary rocks. These rocks all were uplifted into a northeast-trending range by subsequent late Cenozoic Basin and Range faulting. Eocene sedimentary and volcanic rocks flank part of the range. Pathways of hydrothermal fluid flow and locations of Carlin-type gold orebodies in the Jerritt Canyon mining district were controlled by structural and host-rock geometries within specific lithologies of the stacked thrust masses of Eastern assemblage rocks. The gold deposits are most common proximal to intersections of northeast-striking faults, northwest-striking dikes, and thrust planes that lie adjacent to permeable stratigraphic horizons. The host stratigraphic units include carbonate sequences that contained primary intercrystalline permeability, which provided initial pathways for fluid flow and later served as precipitation sites for ore minerals. Alteration, during, and perhaps prior to mineralization, enhanced primary permeability by dissolution, by removal of calcite, and by formation of dolomite. Ore-stage sulfide minerals and alteration minerals commonly precipitated in pore spaces among dolomite grains. Microveinlets and microbrecciation in zones of intense alteration also provided networks of secondary permeability that further enhanced fluid flux and produced additional sites for ore deposition.

Late Cenozoic tectonics of the northern Los Angeles fault system, California

The northern Los Angeles fault system along the southern range front of the Santa Monica Mountains includes potentially seismogenic faults directly beneath the Los Angeles metropolitan area. For a better assessment of seismic hazards, we mapped late Cenozoic faults and folds in the northern Los Angeles basin using an extensive set of oil-well and surface geologic data. The northern Los Angeles fault system developed through early to late Miocene transrotational and transtensional regimes and a Pliocene and Quaternary transpressional regime. The Santa Monica, San Vicente, and Las Cienegas faults are early to late Miocene normal faults that were later reactivated as reverse faults, suggesting that the orientation of reverse faults is largely controlled by Miocene extensional tectonics rather than by the post-Miocene stress field. Tectonic inversion occurred at the beginning of Pliocene time with the reactivation of Miocene normal faults and initiation of reverse faults. Many Pliocene contractile structures became inactive by the middle Pleistocene, and younger deformation is taken up by new active structures, including the West Beverly Hills lineament and an active strand of the Santa Monica fault. The West Beverly Hills lineament is the northernmost segment of the Newport- Inglewood fault zone, which may have propagated northward to the Santa Monica Mountains in Quaternary time. The lineament acts as a segment boundary for the active left-lateral Santa Monica−Hollywood fault system and bounds the Hollywood basin to the west. Uplift of an oxygen- isotope substage 5e marine terrace north of the city of Santa Monica and an assumed dip of >45° for the Santa Monica Mountains thrust fault underlying and uplifting the Santa Monica Mountains suggest that an average dip-slip rate for the fault is <1.3 mm/yr. Crustal shortening across the northern Los Angeles fault system accounts for less than a third of the current rate of shortening between the San Gabriel Mountains and Palos Verdes Hills based on global positioning system observations.

Determination of escarpment age using morphologic analysis: An example from the Galilee, northern Israel

We used topographic and structural data and very limited age control to perform quantitative morphometric analyses and to determine relative ages of escarpments bounded by late Cenozoic normal faults in the Galilee, Israel. The Galilee is an extensional zone composed of a series of uplifted and tilted blocks forming large escarpments built mainly of carbonate rocks. Two parameters used to discriminate tectonic stages are the ratio between the height of the escarpment and the total stratigraphic displacement ( L ) and the degree of concavity of escarpment slopes relative to a reference slope. The only dated reference slope is Mount Tur9an, ∼300 m high and formed by the Tur9an fault system, which has a total stratigraphic displacement of 625 m. A basalt flow that delimits the age of the Tur9an escarpment is dated to 4.23 ± 0.23 Ma and displaced 300 m, which is identical to the present-day topographic expression of this escarpment. The L value for this escarpment is ∼0.5. The Tur9an fault system was active prior to 4.23 Ma at slow uplift rates that enabled erosion to maintain the gentle slope over which the basalt flowed. Increased offset rates following the basalt extrusion led to the formation of the escarpment. The preservation of the basalt at the top of the escarpment indicates that erosional lowering of the upper surface of the Tur9an block has been minor since its formation. The L values indicate two stages of uplift; an early stage during which offset rates were probably low enough that they did not form topography, and a later stage that formed topography, which is preserved. The timing of the change in displacement rates from a slow continuous stage to a fast, topography-forming stage was determined by comparing the shape of the dated slope of Tur9an to that of other slopes. We conclude the following: (1) generally, the topographic profiles of different parts of each individual escarpment have similar shapes indicating similar ages; (2) escarpments having slopes that are more concave or convex than the reference Tur9an escarpment are older or younger than 4 Ma, respectively; and (3) the Galilee escarpments did not form simultaneously. A few escarpments were already major morphologic features by the early to middle Pliocene, whereas the rest formed during the late Pliocene. Morphometric analysis is a useful method for studying the geologic history of a landscape controlled by normal fault uplift and characterized by the absence of sediment deposition and where carbonate dissolution is the main erosional process. This and similar approaches can be used to discriminate tectonic stages and understand the relationship between tectonic activity and surface processes in other extensional regions.

Late Cenozoic Reverse Faulting in the Fall Zone, Southeastern Virginia.

A set of en-echelon reverse faults cut Paleozoic metamorphosed igneous rocks of the Piedmont and overlying late Cenozoic sediments at the Old Hickory Heavy Mineral Deposit in the Fall Zone of southeastern Virginia. Diorite of the eastern Slate Belt was faulted over nearshore to shore-face deposits of the Pliocene Yorktown Formation. These NW-SE-striking faults experienced oblique dip-slip movement with a maximum displacement of up to 6 m on individual faults. Faults tip out along strike and are overlain by distinct cobble beds, suggesting that sediment deposition and faulting were contemporaneous. Deformation at Old Hickory may have been formed by reactivation of existing Paleozoic structures under a regionally extensive compressional stress field parallel to the modern one.

Morphologic dating of scarps formed by repeated slip events along the San Andreas Fault, Carrizo Plain, California

Morphologic dating of fault scarps determines late Cenozoic fault activity by comparing observed topographic profiles with those determined using a calibrated hillslope development model. We postulate that the material transport rate along the profile is a function only of local slope and is transport‐limited. Morphologic dating of hillslopes bounded by continuously dropping boundaries or cut by continuously slipping faults is used to determine the material transport rate constant, the time since the downdrop was initiated, or the fault began to slip. We calibrated the hillslope development model on the southwest facing scarp southeast of Wallace Creek along the San Andreas Fault (SAF) in the Carrizo Plain, California. The scarp has been exposed by right‐lateral offset of a southeast sloping shutter ridge located on the southwest side of the SAF and by vertical offset related to secondary deformation. We assume that all the observed offset occurs after initial exposure of the scarp by passage of the shutter ridge. Forward modeling of profile development yielded a κ (mass diffusivity) of 8.6±0.8 m2 kyr−1. Normal fault slip rates were determined for two graben‐bounding faults in the Northern Elkhorn Hills in the southeastern Carrizo Plain by applying the calibrated κ to the degradation of the scarps to determine the scarp age. One fault scarp began to form about 12 kyr ago and the other about 63 kyr ago. Those ages and estimates of the dip slip along the faults result in slip rates of 1–2 mm yr−1.

Geometry and style of partitioned deformation within a late Cenozoic transpressional zone in the eastern Gobi Altai Mountains, Mongolia

The Gobi Altai is the easternmost extension of the Mongolian Altai and consists of topographically discontinuous E-W-trending ranges with peaks averaging 2000–3000 m in elevation. The region is seismically active and characterized by prominent E-W left-lateral strike-slip faults that localize transpressional deformation and uplift along their lengths and at stepover zones. This report summarizes structural field investigations made in the easternmost Gobi Altai to document the structural geometry and style of late Cenozoic transpressional deformation in the region in order to better understand processes of intracontinental mountain building and the distant intracontinental strain response to the Indo-Eurasian collision. The Artsa Bogd range marks the northeastern terminus of the Gobi Altai and is topographically asymmetric with a high northern margin marked by N-vergent thrust faults and left-lateral oblique-slip faults. The northern side of the range is also bounded by a foreland basin that contains N-vergent thrust faults and folds that deform Quaternary sediments. The southern margin of Artsa Bogd appears tectonically inactive but contains S-vergent thrust faults and left-lateral wrench zones. The range appears to have a flower structure cross-sectional geometry that may reflect transpressional inversion of a Mesozoic basin. The isolated, high and narrow Tsost Uul range south of Artsa Bogd occupies a restraining bend position along the left-lateral Tsost Uul strike-slip fault system. Major faults within the range define a half-flower structure cross-sectional geometry. To the south of the Tsost Uul range, the Gobi Bulag left-lateral strike-slip fault system is marked by small push-up ridges and one major restraining bend mountain where the fault steps to the right near its western end. Throughout the region, Late Cretaceous-Tertiary basalts and Tertiary and Quaternary sediments are deformed by the major fault systems indicating late Cenozoic fault activity. These fault systems and the ranges formed along them occur at fairly regular intervals (approximately 20 km) between the North Gobi Altai fault system and the Gobi Tien Shan fault system, two major left-lateral strike-slip faults that cut across southern Mongolia. Together the faults define a parallel array of discrete linear belts of Cenozoic E-W left-lateral transpressional deformation south of the Hangay Dome. The regular spacing of the fault systems may suggest more uniform distributed left-lateral flow at depth. Eastward-directed lower crustal and lithospheric mantle flow is suggested by existing seismic anisotropy data for the eastern Gobi Altai and is believed to be the driving force for the upper crustal deformation.