Northwest-trending Structures Research Articles

Transverse faulting in the Paradox Basin, Colorado Plateau, USA: Active, intermittent faulting over a 300 million year history of strain accommodation and fluid flow

The Paradox Basin (Colorado Plateau, USA) is dominated by major, first-order northwest-trending structures commonly 40 km or more in trace length, including (1) regional salt wall corridors related to passive diapirism during Pennsylvanian to Jurassic time, (2) gentle upright folds produced by Laramide shortening during the Late Cretaceous through early Cenozoic, (3) late Laramide normal faults, and (4) normal faults representing Neogene salt dissolution collapse. Less obvious at a regional scale are the fault zones aligned perpendicular (northeast-trending) to the dominant structural grain. There are 16 such faults zones, marked by short trace lengths (3−12 km), small offsets (10−100 m), and predominantly extensional kinematics. Based on published geological maps, field observations of fault zone properties (including fluid flow indicators), and kinematic analysis, we interpret these structures as transverse accommodation faults. Co-spatial structural associations reveal the transverse fault zones were active intermittently, likely as a means of minimizing along-strike strain incompatibility that accrued along the first-order structures as they evolved during late Paleozoic to Late Jurassic halokinesis, (mild) Late Cretaceous to Eocene folding, late Laramide normal faulting, and Neogene to recent collapse faulting. Locally, transverse faulting was influenced by reactivation of northeast-striking faults that offset sub-salt formations, including basement. Active, intermittent transverse faulting over the past ∼300 m.y. is consistent with the synthesis of published interpretations and age determinations focusing on the timing of diverse fluids that exploited the permeability of the transverse fault zones. The Paradox Basin, with its enormous subsurface salt volume and enduring fluid flow, has been an ideal dynamic environment for producing intermittently active transverse accommodation faulting.

The structurally‐controlled polyphase Au mineralization in the southern Youjiang Basin: Insights from magnetotelluric imaging

The Youjiang Basin, located on the southwestern margin of the Yangtze Craton, hosts significant Upper Triassic and Lower Cretaceous gold (Au) deposits. Isotope studies suggest that the polyphase Au mineralization in the Youjiang Basin is derived from Indosinian and Yanshanian magmatic events, but the structural plumbing system beneath the basin remains poorly understood. We collected 32 magnetotelluric (MT) stations extending from the southern margin of the Youjiang Basin which hosts the Zhesang, Badu, and Jinya Au deposits at the western margin of the Jiangnan Orogenic (JNO) belt. The full impedance tensor of MT data is utilized to generate a high-precision lithospheric resistivity model by three-dimensional (3-D) inversion. Importantly, our MT data reveals several crustal conductors (<30 Ω·m) underlying major structures in the northwestern part of the study area. These crustal conductive anomalies spatially overlap with high-magnetic anomalies and polymetallic Au-Sn deposits. We interpret these anomalies as solid conductive materials such as graphite and/or sulfides related to the structurally controlled Mesozoic Au mineralization. Additionally, we interpret these conductive anomalies beneath the known Au deposits as the conduits of magmas and solidified paleo-magma chambers that were emplaced along deep-seated northwestern faults and likely documenting the genetic relationship between Au mineralization and Mesozoic magmatism. The synthesis of the electrical resistivity structure and metallogenic considerations is presented in a schematic sketch model illustrating that the deep-seated northwest-trending structures likely acted as a plumbing system controlling both the migration and emplacement of magmas and related oxidizing fluids that subsequently precipitated the polyphase Au deposits.

Matangkaka manganese deposit, Ambitle Island, Feni Island Group, Papua New Guinea: a Quaternary epithermal stratabound manganese oxide deposit

The Matangkaka manganese deposit is an unusual Quaternary epithermal stratabound manganese oxide deposit from Ambitle Island in the Feni Island Group, New Ireland Province, Papua New Guinea. Ambitle is a small volcanic island that has undergone Quaternary explosive and effusive volcanic activity. Oldest volcanic rocks on the island consist of mafic–intermediate lavas and epiclastic rocks of the Pleistocene Ambitle Volcano (Waramung Volcanics). These rocks rest unconformably on Oligocene calcilutites of the New Ireland Basin. The summit of Ambitle Volcano is now modified as a semi-circular topographic rim around Nanum Valley, known as the Nanum Valley Crater. This crater formed during gravity induced failure (or sector collapse) of the southwest flanks of the summit of Ambitle Volcano. Ambitle Crater is a resurgent Pleistocene–Holocene crater located within the summit crater of Ambitle Volcano. Deposits of resurgent volcanism include block and ash deposits, mafic–intermediate lavas, trachyte lavas, tephra and lacustrine sediments (Babamis Volcanics). Two new members are described from the Babamis Volcanics (Nanmikit Trachyte Member and Danmagal Tephra Member). The tephra member is the youngest deposit in the Plio-Pleistocene Tabar–Lihir–Tanga–Feni alkalic volcanic arc. Three new intrusive units are described (Kabang Syenite, Matangkaka Monzonite and Saddle Monzonite). The Saddle hydrothermal system (new name) is an active geothermal system, elongated along the arc-parallel northwest-trending Niffin structural zone. The Matangkaka occurrence is classified as a Mn–Fe–Cu–Mo deposit. Active northwest-trending structures and northeast-trending faults allowed sea water to circulate to depth, leach metals from basement and overlying volcaniclastic rocks and ascend to shallow levels. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units and forming 3–4 m-thick bedded deposits. Circulating neutral chloride waters of the Saddle hydrothermal system played a key role in the deposition of manganese in porous and permeable volcaniclastic units. Mineralising fluids were probably 120 °C. KEY POINTS The Matangkaka manganese deposit is an unusual Quaternary epithermal stratabound manganese oxide deposit on Ambitle Island in the Feni Island Group, Tabar–Lihir–Tanga–Feni alkalic volcanic arc, New Ireland Province, Papua New Guinea. Ambitle Island is a small volcanic island that has undergone Pleistocene–Holocene explosive and effusive volcanic activity. Active fractures have allowed sea water to circulate to depth, leach metals from basement and overlying volcaniclastic rocks and convect to shallow levels. Manganese mineralisation was deposited by neutral chloride waters, probably 120°C. Y/Ho ratios indicate there was a sea water component to mineralising fluids. The manganese occurrence is classified as a Mn–Fe–Cu–Mo deposit. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units, and forming 3–4 m-thick bedded deposits.

An Analysis of Distributed Strike‐Slip Shear Deformation of the Qaidam Basin, Northern Tibetan Plateau

AbstractThe interior structure of the Qaidam Basin records kinematic information related to the regional deformation of the northern Tibetan Plateau. Based on balanced cross sections and depth structure maps covering two individual anticlines in the western Qaidam Basin, we calculate shortening and strike‐slip amounts on the structures. Strain analyses reveal a complex deformation mechanism for them, comprising ~7°–10° clockwise rotation and oblique convergence, with block rotation having contributed more to the sinistral slip than has oblique convergence. Considering the similarities in geometry, kinematics, and onset time of the pervasive northwest trending structures within the basin, we propose that they were developed during distributed dextral shear that has affected the Qaidam Basin since the early Miocene. The bulk shear strikes at ~N55°E, which is consistent with the northeastward decrease in gravitational potential energy away from the central Tibetan Plateau, and is facilitated by the movement on the Altyn Tagh and eastern Kunlun faults.

Structure and Stratigraphy of Rumbli and Panjar Areas of Kashmir and Pakistan with the Aid of GIS

ABSTRACT The project area lies in the southern part of the Hazara Kashmir syntaxis. The Hazara Kashmir syntaxis is an antiformal structure. The project area includes Rumbli, Namb, Chatrora, Chachan, Panjar, Barathian and Utrinna areas of Rawalpindi and Sudhnoti districts. The southeastern limb of the Hazara Kashmir syntaxis is imbricated along Punjal thrust, Main Boundary thrust and Riasi fault. The Jhelum fault truncates the western limb of Hazara Kashmir syntaxis. The core of syntaxis comprises of Himalayan molasse deposits. These molasse deposits represent the part of cover sequence of Indian plate. These Himalayan molasse deposits include the Early to Middle Miocene Kamlial Formation, Middle to Late Miocene Chinji Formation, Late Miocene Nagri Formation and Late Miocene Dhok Pathan Formation. The area is highly deformed resulting folds and faults. The major folds in the project area are the Panjar anticline, Barathian syncline, Barathian anticline, Rumbli anticline, Chatrora antiformal syncline and Namb syncline. The folds are either northwest-southeast trending or southwest-northeast trending. The folds are asymmetric, open, and gentle and close in nature. The folds are southwest, northeast or southeast vergent. The Jhelum fault truncates the northeast and northwest trending structures. The folds and faults are the result of northeast-southwest or northwest-southeast Himalayan compression.

New insights from geophysical data on the regional structure and geometry of the southwest Thelon Basin and its basement, Northwest Territories, Canada

Detailed analysis of gravity and aeromagnetic data covering the southwest Thelon Basin, Northwest Territories, Canada, has provided insight into basement geology that has significance to exploration for uranium and possibly other economic metals in a remote frontier region. Interpretation of basement geology has been constrained by the calibration of gravity and magnetic signatures with Precambrian geology adjacent to the basin and sparse seismic data within the basin, creating the first basement geologic map of the southwest Thelon Basin. The basement to the overlying sedimentary units is dominated by magnetic felsic and mafic bodies variably overlying and intruding the gneissic crystalline basement. Supracrustal belts located outside the basin margins are interpreted to continue below the basin fill. Major structures have been delineated geophysically including the Howard Lake Shear Zone and the Bathurst and McDonald fault systems. Northwest-trending structures forming part of the Bathurst fault system appear to control the unconformity surface morphology and the location of basin depocenters. The geologic interpretations are corroborated by joint gravity and magnetic modeling of profiles that reveal the deepest part of the Thelon Basin reaches depths of [Formula: see text] in an area of subdued magnetic and gravimetric response to the north. The basin is a focus of active exploration for uranium, and we have found that areas along the south and eastern margins underlain by U-rich granitoid rocks may have significant potential where intersected by reactivated faults.

Proterozoic accretionary tectonics in the east Kimberley region, Australia

The east Kimberley region contains well-preserved tectonic structure dating back to the Earth’s most significant stage of continental growth: the assembly of the Nuna supercontinent. An integrated geological–geophysical investigation of this region has been conducted and reveals insight into its tectonic evolution, including potential influence of significant crustal-scale structures in the development of regional architecture, the emplacement of magma, and the relationship of these structures to large-scale deformation. Some newly interpreted features include a north-trending structure, and three north-west trending structures that segment the north-east trending orogen. The central segment of the orogen is a zone of higher metamorphic grade, and is host to a distinct gravity high. This gravity high can be explained by excess mass in the mid-crust. This anomaly is consistent with either a large mafic–ultramafic intrusion or a high-density crustal fragment. Possible tectonic models to explain the geophysical and metamorphic anomalies involve, in the latter case, the accretion of a crustal fragment to the Kimberley Craton prior to the 1865–1850Ma Hooper Orogeny or, in the former case, intrusion of voluminous mafic magmas into the middle crust. Whether by igneous or structural means, we consider the development of this anomalous region to be a result of along-strike variations in subduction dynamics. These were perhaps driven by variations in slab-geometry accommodated by the orogen-normal structures we identify. The orogen-normal structures are interpreted to be crustal-scale faults, along which significant vertical displacement occurred when a crustal fragment collided with Kimberley Craton and exhumed high-grade metamorphic rocks to the surface.

High‐resolution mapping of two large‐scale transpressional fault zones in the California Continental Borderland: Santa Cruz‐Catalina Ridge and Ferrelo faults

AbstractNew mapping of two active transpressional fault zones in the California Continental Borderland, the Santa Cruz‐Catalina Ridge fault and the Ferrelo fault, was carried out to characterize their geometries, using over 4500 line‐km of new multibeam bathymetry data collected in 2010 combined with existing data. Faults identified from seafloor morphology were verified in the subsurface using existing seismic reflection data including single‐channel and multichannel seismic profiles compiled over the past three decades. The two fault systems are parallel and are capable of large lateral offsets and reverse slip during earthquakes. The geometry of the fault systems shows evidence of multiple segments that could experience throughgoing rupture over distances exceeding 100 km. Published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures. Historical and recent focal mechanisms obtained from first‐motion and moment tensor studies confirm regional strain partitioning dominated by right slip on major throughgoing faults with reverse‐oblique mechanisms on adjacent structures. Transpression on west and northwest trending structures persists as far as 270 km south of the Transverse Ranges; extension persists in the southern Borderland. A logjam model describes the tectonic evolution of crustal blocks bounded by strike‐slip and reverse faults which are restrained from northwest displacement by the Transverse Ranges and the southern San Andreas fault big bend. Because of their potential for dip‐slip rupture, the faults may also be capable of generating local tsunamis that would impact Southern California coastlines, including populated regions in the Channel Islands.

Structural overprinting of Mesozoic thrust systems in eastern California and its importance to reconstruction of Neogene extension in the southern Basin and Range

Geologic observations from the Resting Spring and Nopah Ranges (California, USA) together with a synthesis of regional data indicate that previous reconstructions of the Death Valley extensional terrane need to be revised because they do not account for three-dimensional, preextensional structures that complicate structural markers used in these reconstructions. This conclusion arises from detailed mapping that indicates structural overprinting of early northeast-trending fold-thrust systems of the Sevier orogenic belt by younger northwest-trending structures. The northwest-trending fold-thrust system includes a series of folds and a major thrust that includes the Nopah Peak and the Gerstley faults. This thrust fully involved crystalline basement, and there are extensive basement exposures in the hanging wall of the thrust in the southern Nopah Range. Basement involvement presumably produced a large ramp anticline to the southwest of the Nopah Range in what is now the Death Valley extensional terrane. The existence of this ramp anticline is supported by occurrences of successively older rocks to the southwest, beneath the Tertiary unconformity. The northwest-trending fold-thrust system is also recognizable to the north and east of the Gerstley–Nopah Peak fault as a series of west-northwest– to northwest-trending folds that have been known for more than 30 years, but were overlooked in earlier reconstructions. We use our observations together with recently published data on the State Line fault and extensional structure in Pahrump Valley to consider two preliminary map-view restorations of the Resting Spring and Nopah Ranges relative to the Spring Mountains, a relatively unextended block to the east. These restorations place the northwest-trending thrust system along strike from the southern Spring Mountains, where a similar overprinting is observed, supporting the basic restoration. One restoration that strictly adheres to published estimates for motion across the Pahrump Valley suggests, however, that the widely cited correlation of the Wheeler Pass and Chicago Pass thrust system is highly suspect. Our preferred interpretation concludes that the Chicago Pass–Shaw thrust system of the Resting Spring∼Nopah Ranges is an along-strike equivalent of the Lee Canyon thrust; the Wheeler Pass thrust correlates to the Montgomery Mountains thrust; and both thrust systems have an uncertain continuation into the Death Valley extensional terrane. The Resting Spring–Nopah–Spring Mountains restoration provides a template for future restorations. We emphasize that the three-dimensional preextensional geometry, together with other markers like Mesozoic magmatic belts and older sedimentary facies trends, provides an opportunity for using modern visualization and database systems to develop high-precision reconstructions using the abundance of crosscutting markers. Thus, although this study, along with other recent studies, indicates that previous reconstructions are not workable, future studies that include the full three-dimensional data could lead to a nearly unique solution to the preextensional paleogeography.

Petroleum implications of the REI09 2D Reconnaissance Survey, Reinga/Northland Basin, northwest New Zealand

The Reinga/Northland Basin is located offshore northwest of New Zealand, adjacent to the petroleum producing Taranaki Basin. Water depths range from shelfal to more than 2,000 m. Analysis of a large multichannel 2D seismic-reflection dataset, tied to Taranaki Basin, constrains seismic stratigraphic units and unconformities formed during successive deformational events in the region. Five phases are identified. Phase one: extension that created major northwest-trending structures. The age of the graben-filling sediments is inferred to be Jurassic and/or Cretaceous. Source rocks are interpreted to have been deposited widely during this phase. Phase two: Late Cretaceous to Late Eocene regional subsidence with increasing marine influence in the Reinga region. Source and reservoir successions are inferred to have been deposited during this interval. Phase three: Late Eocene compression resulted in folding of the northwest Reinga region. Phase four: Oligocene to Early Miocene regional subsidence and emplacement of the Northern Allochthon and eruption of the Northland Volcanic Arc in earliest Miocene time. The Early Miocene deformation and volcanism relate to development of the subduction zone to the northeast of the basin. Reservoir facies were deposited in regions affected by deformation and seal facies were deposited in distal regions. Phase five: From the mid-Miocene to the present day subsidence continued in the Reinga Basin with deposition of pelagic sediments. Uplift of the Wanganella Ridge, in the northwest part of the Reinga region in the Middle Miocene and intermittent deformation in the northeast through to Pliocene time resulted in localised deposition of channel and fan reservoir facies.

Modelling spatial anisotropy of gold concentration data using GIS-based interpolated maps and variogram analysis: Implications for structural control of mineralization

Linear trends of anomalously high gold values in the Florida Canyon gold deposit, Nevada have been identified using a combination of contour maps of gold (Au) concentration developed with a geographic information system (GIS) and variogram maps created using a geostatistical analysis package. These linear trends are interpreted to represent major fault zones that exerted a prinicipal control on gold mineralization and therefore imparted a spatial anisotropy to gold concentrations. Oxidation state information such as oxide, sulfide or mixed was used initially to map and contour the lower limit of the oxidation zone. Linear trends on this surface suggest the location and trend of major structural elements in the deposit that guided late oxidizing fluids. Subsequently, four contour maps of gold concentrations in oxidized rocks were produced, each map representing 500 ft vertical intervals starting at 3500 ft above mean sea level (msl). Relatively high concentrations of Au that form linear trends on these maps suggest the presence of structural features, such as shear zones that controlled mineralization. Finally, to validate the observed trends, variogram maps of gold concentrations were derived through geostatistical analysis and the major axes of anisotropy were determined for each map. The results that emerge suggest linear trends of northeast, northwest and, less prominently, north–south orientations. The north–south and northeast trends match those of known and mapped major structures associated with the Florida Canyon deposit. However, the results imply a stronger control on mineralization by northwest-trending structures than previously recognized and the location of possible structures of all trends not previously mapped. They also serve to identify faults that controlled both early hydrothermal fluids and late oxidizing fluids since the gold distribution represents the time integrated effects of both fluid events. The linear trends derived by spatial analysis (contour maps, variogram maps) of geochemical data (i.e., gold concentration), combined with the results of the field observations prove to be advantageous in understanding the structural control of gold mineralization. Such spatial analyses of geochemical concentration data are particularly useful in the field of mineral exploration.

Structural Evolution of the Rock Creek Gold Deposit, Seward Peninsula, Alaska

Sheeted quartz veins at Rock Creek contain an indicated gold resource of 404,000 ounces (oz) at a grade of 1.31 grams per ton (g/t). The deposit, located on the Seward Peninsula, 12 km north of Nome, Alaska, occurs in variably graphitic greenschist-facies schist, fine-grained quartzite, and dirty marble of the lower Paleozoic to upper Proterozoic Nome Group. The veins at Rock Creek, as at many prospects in the southwestern Seward Peninsula, are hosted by the most carbonaceous part of the Nome Group. These strata underwent complex polyphase deformations that were progressively less penetrative through time. They include four distinct styles of folding, followed by an episode of thrust faulting. The Mid-Cretaceous dilational vein system formed during the onset of D4 deformation along the northeast-trending Albion fault. The overall shape of the vein system in relationship to controlling faults shows that a preexisting structural fabric may have influenced its formation; tension veins lie along but slightly oblique to the northeast-striking Albion fault in a left-stepping array. Younger “Albion” veins parallel and lie within the Albion fault zone. The highest ore tenor and greatest concentration of veins occur near the intersections of northeast and northwest structures. The veins diminish in frequency along strike across the northwest-trending structures. The vein system occurs primarily in the hanging wall of a curviplanar dextral thrust fault that splays from the high-angle Albion fault and becomes gently dipping in its upper reaches. Vein concentration diminishes abruptly below this curviplanar transpressional structure. Small stratigraphic offsets show that the vein system was not displaced by the structure but rather formed preferentially above it. The orientations and sequencing of structural elements show that the tension vein system formed during a period of highly focused dilation. The Albion fault zone subsequently propagated through the maximum zone of dilation as it evolved to a throughgoing dextral strike-slip fault system. Vein chemistry changed from primarily Au-As (tension veins) to Sb-Au-Ag-As-Pb (Albion-style veins) contemporaneously with a 10° counterclockwise migration of vein orientation. These changes mark a period of highly focused extension immediately prior to the onset of throughgoing transpression along the Albion fault, which resulted in a collapse of the hydrothermal system. The left-stepping geometry of tension veins and later evolution of the overall structural system to compression suggest that it formed within a restraining bend along the dextral strike-slip fault system. The general shape of the vein system, the positions and orientations of controlling structures, and their vectors and styles of displacement define a sidewall ripout that flanks the Albion fault zone.

Tectonic Framework and Evolution of the Gawler Craton, Southern Australia

The Gawler craton is the major crustal province in the southern Australian Proterozoic and is pivotal in models seeking to describe the evolution of Proterozoic Australia. The craton is host to the Olympic Dam iron oxide Cu-Au-U-REE deposit, as well as a number of other iron oxide copper-gold (IOCG), Au, and iron ore deposits. The evolution of the Gawler craton is dominated by two major phases of tectonic activity, Late Archean and late Paleoproterozoic-early Mesoproterozoic, in total spanning ca. 1 billion years. The Late Archean (2560–2500 Ma) basin development was coeval with arclike felsic magmatism and mafic-ultramafic magmas, including komatiites. Collisional deformation between 2480 and 2420 Ma led to ca. 400 m.y. of tectonic quiescence, conceivably within an early Paleoproterozoic continental interior. The second major phase of tectonic activity was in the middle to late Paleoproterozoic and early Mesoproterozoic (2000–1450 Ma). Paleoproterozoic tectonism was initially dominated by rift-related events, which produced a series of basins over the interval ca. 1900 to 1730 Ma, some of which contain significant iron ore reserves. Transient contractional deformation at ca. 1850 Ma led to batholith-scale granitic magmatism, in part derived from melting of the late Archean basement. Major basin development was finally terminated by the 1730 to 1690 Ma Kimban orogeny, the effects of which are widespread and include the formation of crustal-scale shear systems, granitic magmatism, and low- to high-grade metamorphism. The Kimban orogeny was followed by a renewed period of extension between ca. 1680 and 1640 Ma, leading to local magmatism and sedimentation which appears in part to have been coeval with the high-grade ca. 1650 Ma Ooldean event in the western Gawler craton. At ca. 1620 to 1615 Ma, the generation of the arc-related St. Peter Suite in the southern Gawler craton implies that craton components were located at an active plate margin. The arc environment was superseded by a transition to a continental interior regime, which coincided with the voluminous Gawler Range Volcanics (1595–1590 Ma) and Hiltaba Suite granitoids (1595–1575 Ma). The Hiltaba Suite is dominated by high T fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement. Emplacement of the Hiltaba Suite coincided with the major mineralizing interval in the Gawler craton. Regionally, two distinct mineral systems have been recognized: the Olympic IOCG province in the eastern part of the craton, and the gold-dominated systems within the central Gawler gold province. The spatial distribution of IOCG versus Au-dominated mineral systems appears to reflect regional variations in crustal composition and Hiltaba Suite petrogeneses. Hiltaba-aged granites in the Olympic IOCG province are isotopically more evolved, richer in U and Th, and oxidized compared to similar aged granites in the Au-dominated province. Modern-day heat flow in the IOCG province is significantly higher (90 ± 10 mWm−2) compared to the Au-dominated province (54 ± 5 mWm−2), suggesting there are important lithospheric compositional differences between the two metallogenic provinces, which may reflect an older phase of craton assembly. Widespread northwest-southeast contractional deformation coeval with the emplacement of the Hiltaba Suite is expressed by the formation and/or reactivation of shear zones ranging up to crustal scale. Within this regime, northwest-trending structures such as those in the vicinity of the Olympic Dam deposit are likely to have accommodated dilation associated with strike-slip movements. Intersection between these structures and northeast-trending contractional faults may have formed suitable structural traps for 1590 to 1580 Ma mineralization. The timing of syn-Hiltaba deformation overlaps with transpression and medium to high-grade metamorphism associated with the ca. 1570 to 1540 Ma Kararan orogeny which dominates the geophysical architecture of the central northern part of the craton. The youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca. 1470 and 1450 Ma and regional cooling.

Nature, origin and tectonic significance of anomalous transverse structures, southeastern Skeena Fold Belt, British Columbia

Abstract An anomalous domain of complex structure occurs in the eastern Skeena Fold Belt at Mosque Mountain, at the northwestern end of a regional belt of Cretaceous Sustut Group exposure. Folds and contractional faults with northwest- and northeast- to east-trends, that are parallel and at a high angle to the regional structural trend respectively, deform Jurassic Hazelton and Bowser Lake groups and unconformably overlying Sustut Group strata in this area. Anomalous structural trends at Mosque Mountain appear to reflect a history of superposed deformation. The older deformation occurred prior to and during deposition of the Middle Albian or younger basal Sustut Group. It caused the formation of a northwest-trending basement uplift of Hazelton and Bowser rocks whose abrupt northwest termination directly underlies the Mosque Mountain area. The main post late Campanian to early Maastrichtian contractional deformation formed northwest- and northeast- to east-trending structures in the Mosque Mountain area. Structural geometries and cross-cutting relationships suggest these nearly orthogonal contractional structures formed alternately. These relationships are most reasonably attributed to deformation on alternately active detachments or (blind) thrusts. Northwest-trending structures conform to regional strain patterns. Northeast- to east-trending structures were most likely caused by detachment deflection at the transverse, north end of the basement uplift; by movement up an oblique ramp formed by the geometric inversion of the north end of the basement uplift along a blind thrust, and/or reactivation of the north margin of the basement uplift. Geometric inversion of the early formed basement uplift on a footwall flat probably produced the south-facing monocline that localized the northwestern limit of preserved Sustut strata to the Mosque Mountain area. Late dextral oblique-slip and extensional faults cross-cut Skeena Fold Belt structures and indicate that transtensional deformation, common in the Central Interior of British Columbia, extended northward into the Mosque Mountain area.

Bald Mountain Gold Mining District, Nevada: A Jurassic Reduced Intrusion-Related Gold System

The Bald Mountain mining district has produced about 2 million ounces (Moz) of Au. Geologic mapping, field relationships, geochemical data, petrographic observations, fluid inclusion characteristics, and Pb, S, O, and H isotope data indicate that Au mineralization was associated with a reduced Jurassic intrusion. Gold deposits are localized within and surrounding a Jurassic (159 Ma) quartz monzonite porphyry pluton and dike complex that intrudes Cambrian to Mississippian carbonate and clastic rocks. The pluton, associated dikes, and Au mineralization were controlled by a crustal-scale northwest-trending structure named the Bida trend. Gold deposits are localized by fracture networks in the pluton and the contact metamorphic aureole, dike margins, high-angle faults, and certain strata or shale-limestone contacts in sedimentary rocks. Gold mineralization was accompanied by silicification and phyllic alteration, ±argillic alteration at shallow levels. Although Au is typically present throughout, the system exhibits a classic concentric geochemical zonation pattern with Mo, W, Bi, and Cu near the center, Ag, Pb, and Zn at intermediate distances, and As and Sb peripheral to the intrusion. Near the center of the system, micron-sized native Au occurs with base metal sulfides and sulfosalts. In peripheral deposits and in later stages of mineralization, Au is typically submicron in size and resides in pyrite or arsenopyrite. Electron microprobe and laser ablation ICP-MS analyses show that arsenopyrite, pyrite, and Bi sulfide minerals contain 10s to 1,000s of ppm Au. Ore-forming fluids were aqueous and carbonic at deep levels and episodically hypersaline at shallow levels due to boiling. The isotopic compositions of H and O in quartz and sericite and S and Pb in sulfides are indicative of magmatic ore fluids with sedimentary sulfur. Together, the evidence suggests that Au was introduced by reduced S-bearing magmatic fluids derived from a reduced intrusion. The reduced character of the intrusion was caused by assimilation of carbonaceous sedimentary rocks. Tertiary faults dismember the area and drop down the upper part of the mineralizing system to the west. The abundant and widespread kaolinite in oxide ores is relatively disordered (1A polytype) and has δD and δ18O values suggestive of a supergene origin. The deep weathering and oxidation of the ores associated with exhumation made them amenable to open-pit mining and processing using cyanide heap leach methods.

2001-2002 Upper Three Runs Sequence of Earthquakes at the Savannah River Site, South Carolina

On 8 October 2001 a small felt earthquake ( mbLg 2.60) occurred near Upper Three Runs Creek in the north central area of the Savannah River Site, South Carolina. It was located at 33.32°N, 81.67°W with a focal depth of 3.9 ± 0.8 km and an origin time of 00:23:01.12 UTC. Seven very small aftershocks ( MD ≤ 1.4) followed the main event with the last one occurring 6 March 2002. All activity occurred within a small area of 6.0-6.5 km2. Further analysis of collected data indicates a correlation of this low-level seismic activity with a small northwest-trending structure observed in detailed gravity and magnetic data. Both single-event and composite focal mechanisms were derived using local and regional stations. Results indicated predominantly dip-slip motion along a fault striking north-northwest at 335° and dipping 41° to the southwest. A 3D plot of the eight hypocenters clearly defines a fault plane nearly analogous to that obtained from the focal solutions. The Upper Three Runs series of events is another example of a separate class of earthquakes that occur within the central Piedmont and upper Coastal Plain of South Carolina. The Upper Three Runs sequence of events demonstrates that shallow intersections of structures interpreted from potential field data can be the foci for localized stress concentrations where microearthquake activity (M ≤ 3.0) can occur. These earthquakes are attributable to small-scale faults associated with pockets of relatively high stress concentrations and are generally accompanied by loud noises. Their shallow depths and small epicentral areas suggest that these earthquakes are extremely localized and are not attributable to any large-scale regional features.

The Padaeng Supergene Nonsulfide Zinc Deposit, Mae Sod, Thailand

The Padaeng deposit near Mae Sod in western Thailand was the first supergene nonsulfide zinc deposit in the world to be developed as a large modern mining operation. The mine and associated zinc smelter, operated by Padaeng Industry Public Company Ltd. since 1984, went into production with reserves of 4.59 Mt at a grade of 28.9 percent zinc with a 10 percent zinc cutoff. Current resources are 5.14 Mt at a grade of 12.0 percent zinc with a 3 percent zinc cutoff. The Padaeng deposit is hosted by a mixed carbonate-clastic sequence of Middle Jurassic age. The deposit occurs in the hanging wall of the Padaeng fault, a major northwest-trending structure that was active through Cretaceous and Tertiary tectonism and uplift. Nonsulfide zinc ore comprises dominant hemimorphite with minor smithsonite and hydrozincite. Strata-bound ore zones occur within a northwest-dipping, deeply weathered, dolomitic sandstone; steeply dipping and irregular karstic zones in underlying massive, silty dolomite are controlled by north-trending fracture zones. Sulfide zinc-lead mineralization of Mississippi Valley type occured extensively in the vicinity of the Padaeng mine, most notably the small resources at Pha De and Hua Lon. Mineral deposits are typically sphalerite rich with minor galena and pyrite, forming small-scale open-space fillings, veins, and replacements within hydrothermal dolomite. Mineralization is dominantly strata bound within a horizon of intense hydrothermal dolomitization that forms the stratigraphic hanging wall to the nonsulfide ore zones at Padaeng. The only significant sulfide at the Padaeng mine is within this unit. Only trace sulfide occurs peripheral to, or down dip of, strata-bound or steeply dipping, nonsulfide orebodies. Sulfide mineralization is believed to have accompanied Cretaceous uplift and deformation, related to the onset of oblique subduction beneath the western margin of the Shan-Thai terrane. The nonsulfide deposit is believed to have formed when a substantial body of sulfide ore was uplifted on the margin of the Mae Sod Tertiary intermontane basin, commencing in the middle to late Miocene. Zinc-bearing acidic supergene fluids, generated by oxidation of the precursor sulfide body, reacted with carbonate in the underlying stratigraphic section to precipitate hemimorphite and smithsonite. Fluids were channeled by permeable dolomitic sandstones and by steep fracture and fault zones. Acidic fluids promoted deep weathering and karst formation, allowing mineralization to extend down dip in sandstone units for at least 150 m and vertically for a similar distance in steep structural zones. Transport of zinc out of the precursor sulfide body was facilitated by a falling water table, owing to uplift of the Padaeng fault block and a change from wet tropical to monsoonal or semiarid climatic conditions. There is no evidence for significant in situ replacement of sulfide deposits, and the leached remnants of the precursor sulfide body have been removed by erosion. The supergene process of dissolution and reprecipitation of zinc in the host rocks increased zinc grades and separation of zinc from lead, producing an economically attractive deposit. Successful exploration for this type of deposit requires a good understanding of the controls on primary sulfide mineralization and a good knowledge of local neotectonism, uplift history, hydrogeology, climatic evolution, and weathering history.

Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and establishment of northwest- and north-trending tectonic grains in the southwestern United States

The Grand Canyon Supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late Mesoproterozoic and Neoproterozoic. New 40 Ar/ 39 Ar age determinations indicate that the Mesoproterozoic Unkar Group was deposited between ca. 1.2 and 1.1 Ga. Basins in which the Unkar Group was deposited and the related northwest-striking faults were created by northeast-southwest extension, which was contemporaneous with regional northwest-southeast “Grenville” contraction. New U-Pb data indicate that the Neoproterozoic Chuar Group was deposited between 800 and 742 Ma. Sedimentary and tectonic studies show that Chuar deposition took place during east-west extension and resulting normal slip across the Butte fault. This event is interpreted to be an intracratonic response to the breakup of Rodinia and initiation of the Cordilleran rift margin. Laramide monoclines of the Grand Canyon region have north and northwest trends, reactivate faults that originated at the time of Unkar and Chuar deposition, and can be traced for great distances (hundreds of kilometers) from the Grand Canyon. We use the distribution of monoclines in the Southwest to infer the extent of Proterozoic extensional fault systems. The 1.1 Ga northwest-trending structures and ca. 800–700 Ma north-trending extensional structures created regional fault networks that were tectonically inverted during formation of the Ancestral Rocky Mountains and Laramide contraction and reactivated during Tertiary extension.

Sediment distribution and transport along a rocky, embayed coast: Monterey Peninsula and Carmel Bay, California

Abstract Field measurements of beach morphology and sedimentology were made along the Monterey Peninsula and Carmel Bay, California, in the spring and summer of 1997. These data were combined with low-altitude aerial imagery, high-resolution bathymetry, and local geology to understand how coastal geomorphology, lithology, and tectonics influence the distribution and transport of littoral sediment in the nearshore and inner shelf along a rocky shoreline over the course of decades. Three primary modes of sediment distribution in the nearshore and on the inner shelf off the Monterey Peninsula and in Carmel Bay were observed. Along stretches of the study area that were exposed to the dominant wave direction, sediment has accumulated in shore-normal bathymetric lows interpreted to be paleo-stream channels. Where the coastline is oriented parallel to the dominant wave direction and streams channels trend perpendicular to the coast, sediment-filled paleo-stream channels occur in the nearshore as well, but here they are connected to one another by shore-parallel ribbons of sediment at depths between 2 and 6 m. Where the coastline is oriented parallel to the dominant wave direction and onshore stream channels are not present, only shore-parallel patches of sediment at depths greater than 15 m are present. We interpret the distribution and interaction or transport of littoral sediment between pocket beaches along this coastline to be primarily controlled by the northwest-trending structure of the region and the dominant oceanographic regime. Because of the structural barriers to littoral transport, peaks in wave energy appear to be the dominant factor controlling the timing and magnitude of sediment transport between pocket beaches, more so than along long linear coasts. Accordingly, the magnitude and timing of sediment transport is dictated by the episodic nature of storm activity.

Clay alteration and gold deposition in the Genesis and Blue Star deposits, Eureka County, Nevada

The Genesis and Blue Star sedimentary rock-hosted gold deposits occur within the 40-mile-long Carlin trend and are located in Eureka County, Nevada. The deposits are hosted within the Devonian calcareous Popovieh Formation, the siliciclastic Rodeo Creek unit and the siliciclastic Vinini Formation. The host rocks have undergone contact metamorphism, decalcification, silicification, argillization, and supergene oxidation.Detailed characterization of the alteration patterns, mineralogy, modes of occurrence, and associated geochemistry of clay minerals resulted in the following classifications: least altered rocks, found distal to the orebody, consisting of both metamorphosed and unmetamorphosed host rock that has not been completely decalcified; and altered rocks, found proximal to the orebody that have been decalcified. Altered rocks are classified further into the following groups based on clay mineral content: silicic, 1 to 10 percent clay; silicic-argillic, 10 to 35 percent clay; and argillic, 35 to 80 percent clay. Clay species identified are 1M illite, 2M 1 illite, kaolinite, halloysite, and dioctahedral smectite.An early hydrothermal event resulted in the precipitation of euhedral kaolinite and at least one generation of silica. This event occurred contemporaneously with decalcification which increased rock permeability and porosity. A second clay alteration event resulted in the precipitation of hydrothermal 1M illite which replaced hydrothermal kaolinite and is associated with gold deposition. Silver and silica deposition is also associated with this phase of hydrothermal alteration.Hydrothermal alteration was followed by supergene alteration which resulted in the formation of supergene kaolinite, halloysite, and smectite as well as the oxidation of iron-bearing minerals. Supergene clays are concentrated along faults, dike margins, and within rocks containing carbonate. Gold mineralization is not associated with supergene clay minerals within the Genesis and Blue Star deposits.Rocks classified as silicic-argillic in the Popovich Formation represent the most significant gold host. Silicic-argillic rocks commonly exhibit bedding-parallel alteration zones. This pattern of alteration indicates that stratigraphy as well as northwest-trending structures played a significant role in the migration of gold-bearing fluids. Based on K-Ar age determinations of hydrothermal 1M illite associated with gold, the main event of mineralization in the Genesis and Blue Star deposits occurred between 93 and 100 Ma, during mid-Cretaceous time.