Oceanic Spreading Axes Research Articles

Extension across the Laptev Sea continental rifts constrained by gravity modeling

The Laptev Shelf is the area where the Gakkel Ridge, an active oceanic spreading axis, approaches a continental edge, causing a specific structural style dominated by extensive rift structures. From the latest Cretaceous to the Pliocene, extension exerted on the Laptev Shelf created there several deep subsided rifts and high-standing basement blocks. To understand syn-rift basin geometries and sediment supply relationships across the Laptev Shelf, accurate extension estimates are essential. Therefore, we used 2-D gravity modeling and 3-D gravity inversion to constrain the amount of crustal stretching across the North America-Eurasia plate boundary in the Laptev Shelf. The latest Cretaceous-Cenozoic extension in that area is partitioned among two rift zones, the Laptev Rift System and the New Siberian Rift. These rifts were both overprinted on the Eurasian margin that had been stretched by 190-250 km before the Late Cretaceous. While the Laptev Rift System, connected to the Gakkel Ridge, reveals increasing extension toward the shelf edge (190-380 km), the New Siberian Rift is characterized by approximately uniform stretching along strike (110-125 km). The architecture of the Laptev Rift System shows that the finite extension of about 500 km is sufficient to entirely eliminate crystalline continental crust. In the most stretched rift segment, continental mantle is exhumed at the base of the Late Mesozoic basement. The example of the Laptev Rift System shows that extension driven by divergent plate movement is a sufficient cause to produce almost complete continental breakup without an increased heat input from the asthenospheric mantle.

The magmatic evolution of young island arc crust observed in gabbroic to tonalitic xenoliths from Raoul Island, Kermadec Island Arc

We provide new geochemical and O isotope data for minerals and whole rocks of a suite of gabbroic to tonalitic xenoliths from Raoul Island in the Kermadec island arc. The plagioclase, olivine and clinopyroxene compositions are similar to those observed in the Raoul Island lavas supporting a close relationship of the plutonic and volcanic rocks by crystal fractionation. Plagioclase in gabbros is significantly more An-rich than in similar rocks from oceanic spreading axes reflecting higher water contents in the island arc magmas. Incompatible element and O isotope data suggest that the gabbroic rocks formed from accumulation of minerals of the ascending magmas whereas the tonalites represent highly evolved magmas after extreme fractional crystallization. Temperatures of the magmas calculated from O isotope equilibria and pyroxene thermometers range from about 1200 °C in the mafic to 800 °C in felsic rocks. Barometry of the rocks suggests that gabbros formed between 12 and 18 km depth and tonalites shallower which is in agreement with seismic models of island arc crustal layering. The xenolith data from Raoul Island support seismic studies indicating that some portions of the Tonga-Kermadec island arc show similar layering of felsic and mafic rocks to the Izu-Bonin and the fossil Talkeetna island arcs. • Tonalite and gabbro occur in the deeper crust of the Tonga-Kermadec island arc supporting seismic models of a layered crust. • Gabbros represent cumulates and tonalites highly evolved magmas related to the lavas by fractional crystallization processes. • Crystal fractionation in the lower island arc crust leads to formation of andesitic liquids.

Combined use of the GGSFT data base and on board marine collected data to model the Moho beneath the Powell Basin, Antarctica

The Powell Basin is a small oceanic basin located at the NE end of the Antarctic Peninsula developed during the Early Miocene and mostly surrounded by the continental crusts of the South Orkney Microcontinent, South Scotia Ridge and Antarctic Peninsula margins. Gravity data from the SCAN 97 cruise obtained with the R/V Hesperides and data from the Global Gravity Grid and Sea Floor Topography (GGSFT) database (Sandwell and Smith, 1997) are used to determine the 3D geometry of the crustal-mantle interface (CMI) by numerical inversion methods. Water layer contribution and sedimentary effects were eliminated from the Free Air anomaly to obtain the total anomaly. Sedimentary effects were obtained from the analysis of existing and new SCAN 97 multichannel seismic profiles (MCS). The regional anomaly was obtained after spectral and filtering processes. The smooth 3D geometry of the crustal mantle interface obtained after inversion of the regional anomaly shows an increase in the thickness of the crust towards the continental margins and a NW-SE oriented axis of symmetry coinciding with the position of an older oceanic spreading axis. This interface shows a moderate uplift towards the western part and depicts two main uplifts to the northern and eastern sectors.

The Aoyougou Mafic-Ultramafic Complex in the North Qilian Mountains, Northwest China: A Possible Middle Proterozoic Ophiolite along the Southern Margin of the North China Craton

The Aoyougou mafic-ultramafic complex lies in the Paleozoic orogenic belt of the western part of the North Qilian Mountains near Aoyougou valley in Gansu Province, northwestern China. It consists of serpentinite, a cumulate sequence of gabbro and diorite, pillowed and massive lavas, diabase dikes, and chert, an assemblage tentatively interpreted as an ophiolite. SHRIMP dates on zircons from the diabase dikes indicate a crystallization age of 1777 ± 28 Ma. The basalts show light rareearth element enrichment and have relatively high TiO2 and low Al2O3 contents, characteristic of present-day E-MORB or ocean-island lavas. All of the lavas have relatively low MgO contents and Mg numbers [100Mg/(Mg+Fe)], indicating a somewhat evolved character. The diabase dikes have flat chondrite-normalized REE patterns and are significantly more primitive than the lavas. Based on the lava geochemistry, the nature of the serpentinite, and the regional geology, the Aoyougou complex is interpreted as a fragment of oceanic lithosphere that may have formed at an oceanic spreading axis. Later subduction of oceanic lithosphere in the Middle Proterozoic produced a trench-arc-basin system, which is preserved in the North Qilian Mountains. The tectonically dismembered Aoyougou complex is similar in many respects to Phanerozoic ophiolites and suggests that modern-style plate tectonic processes in China may have begun as early as the Middle Proterozoic.

Eurasia spreading basin to Laptev Shelf transition: structural pattern and heat flow

New geophysical data have become available from shipborne and satellite measurements allowing a re-evaluation of the largely unknown junction of the Arctic spreading centre and the northeastern Siberian continental margin where the transpolar mid-ocean Gakkel Ridge abuts against the continental slope of the Laptev Sea. Based on multichannel seismic reflection and gravity data, this sediment-covered spreading axis can be traced to the continental rise where it is cut-off by a transcurrent fault. Further continuation of the extensional axis into the continental slope can be attributed to two asymmetric grabens, which terminate against the prominent Khatanga–Lomonosov Fracture. Remnants of hydrothermal fauna and high heat-flow values of approximately 100 mW m−2 documented around these grabens in the up-slope area are typical for an oceanic spreading axis. Thus we consider these grabens to be morphotectonic termination of the global Atlantic–Arctic spreading system with plate motions shifting to the Khatanga–Lomonosov Fracture. The high heat flow and the distribution of earthquake epicentres allow us to assume that the present-day divergent plate tectonic boundary passes from the Gakkel Ridge to the eastern part of Laptev Sea with an offset of initial rifting along the Bel'kov–Svyatoi Nos Rift to the projected prolongation of the buried spreading axis by 140–150 km.

Deformation processes at fast to ultra-fast oceanic spreading axes: mechanical approach

The morphology of fast to ultra-fast oceanic spreading ridges such as the East Pacific Rise (EPR) is characterized by an axial dome, 5–10 km wide, culminating at 300–500 m above the surrounding seafloor. This dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis. A large summit trough, 200 m to 2 km wide, locally notches the axial high, only where the dome is inflated, indicative of a time-average robust magma supply. This summit trough is thought to represent an elongated axial summit caldera (ASC) created as a result of the subsidence of the top of the axial magma chamber (AMC). Such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension. Analog experiments using small-scale modeling have been performed in order to better constrain the tectonic evolution of the axial region. The experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table. It is capped with a silicone layer representing hot rocks below the brittle–ductile transition and is covered by a sand layer representing the brittle crust. The experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls. Three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution: extension without withdrawal, withdrawal without extension, withdrawal and synchronous extension. We show that the morphology of the EPR axis can be considered as the result of both horizontal and vertical movements. Two symmetrical lateral grabens develop on both sides of a non-deformed axial dome when single extension is applied to a model with a thin silicone layer. Normal faults of the lateral grabens are rooted on two divergent velocity zones (DVZs) located on the edges of the groove. This situation is regarded as an analog of the natural case where the top of the AMC acts as a stress-free boundary that fails to transmit the extensional stresses to the upper brittle layer. An important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults. During synchronous extension and withdrawal, the initiation of the lateral grabens is favored by a balloon deflation, even if such deflation is unable to generate a superficial collapse. This last case is considered as representative of the evolution of EPR segments showing little variations in melt supply into the AMC. Higher deflation rates under continuous extension correspond to EPR segments undergoing strong variations in melt supply. In such experiments, the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation. Finally, we show that DVZs located at the brittle–ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra-fast spreading ridges. The distance from axis and the width of the DVZs directly control the location and the distribution of the lateral grabens.

Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge.

Gold mineralization associated with low-temperature, off-axis, fluid activity in the Troodos ophiolite, Cyprus

Silicification of umbers in the Troodos ophiolite complex in Cyprus provides evidence for low-temperature hydrothermal activity occurring late in the process of ocean-crust formation, after the crust had moved away from the oceanic spreading centre. Umbers, which are Mn–Fe-rich sediments, precipitate some distance away from oceanic spreading axes. The movement of the fluids partially silicifying the umbers must post-date umber formation and therefore also occurs off axis. These fluids may form part of low-temperature hydrothermal circulation similar to those identified by off-axis heat anomalies in the modern oceans. In Cyprus the silicified umbers often are located away from volcanogenic massive sulphide (VMS) deposits and are aligned along sea-floor faults. They are characterized by quartz veins as well as pervasive silicification. Recent analyses have revealed unexpectedly anomalous, 1–5 ppm, concentrations of Au in these silicified umbers. Gold mineralization is known to occur in ophiolite complexes as part of the metal concentrations in VMS deposits. These deposits are thought to be formed by ancient hydrothermal circulation similar to that associated with black smokers observed at modern mid-ocean ridges. The research presented in this paper provides evidence for a second phase of Au mineralization in Cyprus caused by low temperature silicifying fluids resulting from oceanic hydrothermal activity which occurred away from the spreading centre. The mineralizing fluids which silicified the umbers may have been more widespread, impregnating other lithologies, and may have overprinted the first phase of Au mineralization in some VMS deposits after they had become inactive and had been moved away from the spreading centre. The mineralizing fluids passing through these VMS may have used the same structural pathways as the original base-metal-rich hydrothermal solutions which formed the VMS.

Transform tectonics and thermal rejuvenation on the Côte d’Ivoire-Ghana margin, west Africa

Formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break-up, including transpressional crustal thickening, and thermal rejuvenation and igneous underplating during passage of a spreading ridge. ODP drill holes on the Côte d’Ivoire-Ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin. Apatite fission-track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra-continental wrench zone that predated margin formation. Rapid cooling of >120°C at 120–115 Ma corresponds to erosion of 3.5–5 km along the conjugate Brazilian margin, reflecting c. 1 km of tectonically driven uplift, subaerial erosion, and isostatic uplift due to unloading. Following rift initiation at 120 Ma (Aptian), an oceanic spreading axis passed adjacent to this part of the margin at 90 Ma (Cenomanian). Maximum uplift during the ridge-transform intersection was 390 m, considerably less than the 2000+ m predicted by heat conduction models in local isostatic equilibrium. The modern ridge is partially the product of thicker crust (22 km) underlying the ridge than the adjacent Deep Ivorian Basin (19 km), and partially related to flexural unloading of the transform ridge between the end of intra-continental wrenching and ridge-transform intersection. Flexural coupling between the continental and oceanic plates since ridge-transform intersection has caused a progressive depression of the offshore margin, estimated at about 650 m in the study area.

High Molecular Weight Polycyclic Aromatic Hydrocarbons in Hydrothermal Petroleums-Indicators of High Temperatures and Water Washing: ABSTRACT

Hydrothermal petroleums from active sedimented oceanic spreading axes contain polycyclic aromatic hydrocarbons (PAH) as characteristic components of their rapid high temperature genesis. The unsubstituted PAH dominate over their alkyl-substituted homologs and occur at high concentrations relative to the aliphatic components, which distinguishes hydrothermal petroleums from conventional reservoir petroleums. Some of the samples contain PAH with higher molecular weights than coronene (M.W. 300) and here we describe the structures and distributions of these PAH in typical examples of petroleums from Guaymas Basin, Escanaba Trough and Middle Valley. Some of the aromatic fractions (F2) show PAH of up to nine rings [e.g., dibenzo-(e,ghi)perylene, benzo(a)coronene, benzo(pqr)naphtho(8,1,2-bcd)perylene, naphtho(8,1,2-abc)coronene]. Fraction 3 in some of these samples has ovalene, a condensed ten-ring PAH. These PAH are the most thermodynamically stable structures for each ring number, and are most likely indicative of a one-ring build-up mechanism. The samples also contain a few specific PAHs [e.g., the seven-ring PAH dibenzo(cd,lm)perylene] which are not members of this stable class. Its formation mechanism must be very specific through the condensation of a pair of the three-ring phenalene radicals or through the Scholl-condensation of pyrene to a nine-ring PAH with its subsequent hydrogenation and cracking down to dibenzo(cd,lm)-perylene. This is themore » first report of the occurrence of heavy PAH (M.W. >300) in geological samples. They are concentrated in the bitumens by removal of the lower molecular weight PAH due to mixing during hydrothermal fluid/oil transport and selective deposition/solidification of the heavy ends at the seabed with concurrent loss of the more water soluble and lower weight products to the ambient seawater. Additional post-depositional reworking by hot fluids and biodegradation further enhance the enrichment of the heavy PAH.« less

Hydrothermally Derived Petroleum: Examples from Guaymas Basin, Gulf of California, and Escanaba Trough, Northeast Pacific Ocean (1)

Petroleum is associated with sulfide-rich sediment at two active oceanic spreading axes in the northeast Pacific Ocean. In the Guaymas Basin, a spreading axis in the Gulf of California, petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in Quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites. In Escanaba Trough, at the southern end of the Gorda Ridge spreading axis offshore northern California, petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in Quaternary turbiditic river-derived sediment. Comparisons of the distributions of hydrocarbons--n-alkanes, isoprenoids, terpanes, steranes, and aromatics--show that chemical differences among four petroleum samp es are such that two samples from Guaymas Basin can be distinguished from two samples from Escanaba Trough. Distinguishing characteristics resulting from differences in sources include n-alkane distributions and certain sterane ratios; distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons. These oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes, and their generation, expulsion, and migration occurred simultaneously over an instantaneous geological time period. The potential economic significance of hydrothermally derived petroleum is uncertain, but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation, expulsion, and migration.

Ocean ridge origin and tectonic setting of mesozoic sulphide and oxide deposits of the Argolis Peninsula of the Peloponnesus, Greece

Metal-oxide and sulphide deposits, chemically similar to the hydrothermal deposits of modern and ancient oceanic spreading axes, occur in two areas of the Argolis Peninsula of the Peloponnesus, Greece. In the first, in northeastern and central Argolis, manganese ores immediately overlie MORB-type extrusives of mid-Jurassic age. After Triassic rifting this oceanic crust formed during Jurassic time and was then tectonically emplaced over the adjacent continental margin in the Late Jurassic. The manganese ores (Palea Epidavros) were precipitated from low-temperature hydrothermal vent waters on the axis of a relatively unrifted ocean ridge, which was possibly slow-spreading. The deep-sea sediment cover comprises pelagic carbonate accumulated on the ridge flanks, then ribbon radiolarites deposited on the abyssal plain below the CCD, located close enough to a continental margin to allow deposition of shallow water-derived calciturbidites. In the second area, Ermioni, in the southern Argolis Peninsula (Adheres), massive sulphides and metalliferous oxide sediments occur with mafic extrusives and terrigenous turbidites. Although previously regarded as a single stratigraphical succession (“Shale-sandstone Formation”), the mafic lavas and metal deposits are interpreted here as ophiolitic slices that were incorporated into the turbidites by thrusting, possibly as a trench-accretionary complex. The massive sulphides were precipitated from high-temperature vents sited near the axis of a relatively unrifted spreading ridge which was possibly fast-spreading. Ferruginous (ochres) and ferromanganiferous (umbers) oxide sediments were dispersed around these vents, whereas strongly fractionated manganese ores precipitated from separate low-temperature hydrothermal sites. Alternative tectonic scenarios are discussed.

Ocean ridge origin and tectonic setting of mesozoic sulphide and oxide deposits of the Argolis Peninsula of the Peloponnesus, Greece

Metal-oxide and sulphide deposits, chemically similar to the hydrothermal deposits of modern and ancient oceanic spreading axes, occur in two areas of the Argolis Peninsula of the Peloponnesus, Greece. In the first, in northeastern and central Argolis, manganese ores immediately overlie MORB-type extrusives of mid-Jurassic age. After Triassic rifting this oceanic crust formed during Jurassic time and was then tectonically emplaced over the adjacent continental margin in the Late Jurassic. The manganese ores (Palea Epidavros) were precipitated from low-temperature hydrothermal vent waters on the axis of a relatively unrifted ocean ridge, which was possibly slow-spreading. The deep-sea sediment cover comprises pelagic carbonate accumulated on the ridge flanks, then ribbon radiolarites deposited on the abyssal plain below the CCD, located close enough to a continental margin to allow deposition of shallow water-derived calciturbidites. In the second area, Ermioni, in the southern Argolis Peninsula (Adheres), massive sulphides and metalliferous oxide sediments occur with mafic extrusives and terrigenous turbidites. Although previously regarded as a single stratigraphical succession (“Shale-sandstone Formation”), the mafic lavas and metal deposits are interpreted here as ophiolitic slices that were incorporated into the turbidites by thrusting, possibly as a trench-accretionary complex. The massive sulphides were precipitated from high-temperature vents sited near the axis of a relatively unrifted spreading ridge which was possibly fast-spreading. Ferruginous (ochres) and ferromanganiferous (umbers) oxide sediments were dispersed around these vents, whereas strongly fractionated manganese ores precipitated from separate low-temperature hydrothermal sites. Alternative tectonic scenarios are discussed.

Petroleum from Northeast Pacific Ocean Hydrothermal Systems in Escanaba Trough and Guaymas Basin: ABSTRACT

Asphaltic petroleum is associated with polymetallic sulfide in sediment of the northeastern Pacific Ocean at two active oceanic spreading axes - the Escanaba Trough (ET) at southern end of the Gorda Ridge and the Guaymas Basin (GB) in the Gulf of California. In both areas, the petroleum was derived from hydrothermally altered organic matter in the sediment that blankets the ridge axes. A petroleum geochemical comparison between two samples shows differences that can be attributed to the sources and thermal histories of the organic matter. The results, coupled with sedimentological and kinetic considerations, indicate that the organic matter in the sediment at ET is dominantly terrigenous, whereas at GB it is mainly marine. The data are consistent with petroleum formation by intense heating of short duration, but at GB the temperatures may have been lower and the heating times longer than at ET. The future economic implications of these petroleum occurrences are uncertain. A preliminary analysis, however, suggests that hydrothermally derived petroleum at active oceanic spreading axes will not be a particularly promising resource because of the great water depths and distances from shore, and the lack of suitable long-term traps. The geochemistry of hydrothermal petroleum may, however, provide valuablemore » information for understanding petroleum genesis and the origin of organic matter in some sulfide-rich ore deposits.« less

The base of a sheeted dyke complex, Oman ophiolite: implications for magma chambers at oceanic spreading axes

The sheeted dyke complex in the Maydan syncline area of the Oman ophiolite was formed during a single sea-floor spreading episode. In some areas, individual dykes from this sheeted dyke complex have been seen to root in the high-level gabbro, and others are truncated by high-level gabbro, diorite and trondhjemite. However, elsewhere dykes penetrate laterally into the layered part of the plutonic sequence. This situation suggests that active magma chambers and zones of dyke injection were discontinuous along the spreading axis at any one time.

Ocean-ridge metalliferous and pelagic sediments of the Semail Nappe, Oman

The lavas of the Semail ophiolitic nappe of Oman, which formed at and very near a late Cretaceous oceanic spreading axis, are intercalated with, and overlain by a range of metalliferous sediments including cupriferous sulphides, ferruginous ochres and ferro-manganiferous umbers. The lava sequence is divisible into lower and upper informal units. The ‘lower’ lavas contain small massive sulphides and ochres which either formed by sea-floor oxidation of sulphides or were directly precipitated. In one area the basal ‘upper’ lavas contain mounds of ferruginous oxide sediments associated with umbers. The mounds may have resulted from bacterial precipitation of sulphides and/or Fe-oxy-hydroxides. Localized umbers deposited from hydrothermal solutions occur throughout and immediately above the ‘upper’ lavas, including areas where sea-floor faulting produced thick accumulations of volcaniclastic breccias. The uppermost umbers are overlain by radiolarites and foraminiferal chalk. Non-calcareous radiolarites are restricted to successions above umbers in some hollows suggesting that dissolution of calcareous pelagic sediment by acid hydrothermal solutions occurred during their ponded accumulation. Most of the metalliferous deposits and pelagic sediments can be related to a spreading ocean ridge affected by sea-floor fault zones. Others are associated with extrusives which have island-arc affinities and built up as discrete volcanic centres on the sea-floor.

Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting

Research Article| February 01, 1977 Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting JOHN M. PROFFETT, JR. JOHN M. PROFFETT, JR. 1Anaconda Company, 1849 West North Temple, Salt Lake City, Utah 84116 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1977) 88 (2): 247–266. https://doi.org/10.1130/0016-7606(1977)88<247:CGOTYD>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JOHN M. PROFFETT; Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting. GSA Bulletin 1977;; 88 (2): 247–266. doi: https://doi.org/10.1130/0016-7606(1977)88<247:CGOTYD>2.0.CO;2 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 SocietyGSA Bulletin Search Advanced Search Abstract In the Yerington district, western Nevada, pre-Tertiary rocks are overlain by an Oligocene ignimbrite sequence and Miocene andesites. Basin and Range normal faulting began in Miocene time, as andesitic volcanism died out (17 to 18 m.y. ago), and has continued to the present. The faults dip east and are curved, concave upward, with net displacements in a nearly east-west direction. Movement on the curved faults has resulted in steep westward tilting of the Miocene andesites and of all older rocks. Alluvium and 8- to 11-m.y.-old basalt flows deposited during the period of faulting are tilted gently west. The oldest faults, which dipped steeply east when they were active, are now inactive and dip gently eastward as a result of westward tilting on other faults. Younger faults dip more steeply east, and the youngest faults, those responsible for present Basin and Range topography, are the steepest. More than 100 percent of east-west extension has taken place across the district because of normal faulting. The rate of extension was most rapid between 17 and 11 m.y. ago and was slower after 11 m.y. ago. The extension is deep seated rather than thin skinned and apparently involves thinning of the crust. Several theories of origin for Basin and Range structure can be rejected because of the field data at Yerington, and the theory that Basin and Range structure was caused by a continental spreading axis best fits the data. Basin and Range spreading seems to have been most active between the projections of the Mendocino and Murray fractures. It may have first started south of the Great Basin, when these fractures were farther south relative to the continent and when the oceanic spreading axis that had been between these fractures was interacting with the continent. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.