Tectonic Quiescence Research Articles

Lead Isotope Constraints on the Origin of Nonsulfide Zinc and Sulfide Zinc-Lead Deposits in the Flinders Ranges, South Australia

Hypogene nonsulfide zinc deposits represent a mode of zinc enrichment in the Earth’s crust that until recently has largely been overlooked relative to supergene nonsulfide zinc and sulfide zinc-lead deposits. The Flinders Ranges, a deformed and uplifted portion of the northern Adelaide geosyncline in South Australia, host numerous prominent examples of hypogene nonsulfide zinc mineralization and thus provide an excellent setting in which to investigate the origin of this deposit type. The Flinders Ranges also host numerous syngenetic and Mississippi Valley-type sulfide zinc-lead deposits, allowing a possible genetic relationship between sulfide and nonsulfide mineralization to be examined. In the present study, lead isotope compositions of zinc and lead minerals from nonsulfide zinc and sulfide zinc-lead deposits from throughout the Flinders Ranges were analyzed to investigate these issues. All the lead isotope compositions measured are anomalous with respect to the Holmes-Houtermans and Stacey-Kramers models. Compositions range from 18.332 to 21.125 for 206Pb/204Pb, from 15.729 to 16.022 for 207Pb/204Pb, and from 38.264 to 40.967 for 208Pb/204Pb. The anomalous, highly radiogenic lead isotope compositions probably reflect input from the highly uranium and thorium enriched basement rocks in the region. The data also form a linear trend in both 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb space, indicating that the data either define an isochron from one of multiple possible stages of lead evolution, or that the deposits all contain varying mixtures of the same two end-member leads, in either case implying that the deposits are genetically related. For a mixing scenario, the present data provide little further constraint on the origin of a hypothetical less radiogenic end member. However, further constraints on the origin of the more radiogenic end member can be obtained by considering the slope of the regression line through the data. Based on an instantaneous model of lead growth, the maximum age of formation of the more radiogenic lead component would be between 910 and 1036 Ma. These ages fall within a period of tectonic quiescence in the region but allow for the possibility that the lead may be associated with a younger event, such as the 850 to 600 Ma rifting event that led to the opening of the Adelaide geosyncline. Based on a continuous model of lead growth, the more radiogenic lead component could have evolved in a source with an age between 1313 and 1523 Ma before it was sequestered in the mineral deposits at about 435 Ma. These 1313 to 1523 Ma dates correspond with the age of much of the younger igneous rocks (e.g., the Hiltaba Suite granites) in the Gawler craton that flank the Adelaide geosyncline on the west and may comprise part of the basement of the Adelaide geosyncline. The lead isotope data also show a strong regional pattern consisting of two distinct centers of radiogenic lead, one located in the southeast corner of the study area and a second, broader zone lying near the west-central part of the study area. This pattern may in part be a function of depth to the basement, in which the more radiogenic lead occurs in deposits located near the edges of the basin, where sediment thicknesses are lower, although heterogeneity in basement composition and proximity to sources of less radiogenic lead may also have affected the pattern. Finally, the lead isotope compositions also show some correlation with deposit size, in which the largest deposits in the Flinders Ranges also contain the most radiogenic lead.

Upper Oligocene–Miocene clinoforms of the foreland Austral Basin of Tierra del Fuego, Argentina: Stratigraphy, depositional sequences and architecture of the foredeep deposits

The Upper Oligocene–Miocene deposits of the foreland Austral Basin of Tierra del Fuego represent the youngest foredeep fill, developed in front of the adjacent fold and thrust belt. They consist of superbly exposed, sub-horizontal clastic successions of more than 600 m of sedimentary thickness. The study of 11 sections by means of facies analysis and sequence stratigraphic criteria enabled the identification of five depositional sequences ( SI– SV), bounded by unconformities (dI-dV) involving hiatuses of different magnitudes. The basal sequence ( SI) includes two members: A, mudstone dominated, deposited by cohesive flows; and B, glauconite-rich, sandstone dominated, deposited by episodic turbidity currents. The remaining sequences ( SII– SV) are composed of complex arrangements of fine conglomerates, coarse- to fine-grained sandstones, and mudstones that were deposited mainly by hyperpycnal flows. The basal unconformities of the SI to SIV involve minor hiatuses, while that of the SV is a major order unconformity. Two types of clinofom geometries are recognized in the foredeep sequences. Type a clinoforms present a wedge shaped geometry and characterize the foredeep infill during the compressional tectonic regime. Regarding this clinoform type, SI is situated closer to the orogen and shows variations in the bedding dip with development of internal unconformities. SII to SIV are situated towards the foreland and are characterized by subhorizontal conformable beds of large lateral extension. Type b clinoforms, with sigmoidal geometry, show a clear northeast progradation related to a progressive foredeep fill under tectonic quiescence. This clinoform type characterizes the deposits in SV. The recognition of hyperpycnites and different types of clinoform geometries in these sequences incorporates new concepts in reservoir prospects, which are critical for the evaluation of the petroleum system in the Austral Basin.

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.

Dynamics of cratons in an evolving mantle

The tectonic quiescence of cratons on a tectonically active planet has been attributed to their physical properties such as buoyancy, viscosity, and yield strength. Previous modelling has shown the conditions under which cratons may be stable for the present, but cast doubt on how they survived in a more energetic mantle of the past. Here we incorporate an endothermic phase change at 670 km, and a depth-dependent viscosity structure consistent with post-glacial rebound and geoid modelling, to simulate the dynamics of cratons in an “Earth-like” convecting system. We find that cratons are unconditionally stable in such systems for plausible ranges of viscosity ratios between the root and asthenosphere (50–150) and the root/oceanic lithosphere yield strength ratio (5–30). Realistic mantle viscosity structures have limited effect on the average background cratonic stress state, but do buffer cratons from extreme stress excursions. An endothermic phase change at 670 km introduces an additional time-dependence into the system, with slab breakthrough into the lower mantle associated with 2–3 fold stress increases at the surface. Under Precambrian mantle conditions, however, the dominant effect is not more violent mantle avalanches, or faster mantle/plate velocities, but rather the drastic viscosity drop which results from hotter mantle conditions in the past. This results in a large decrease in the cratonic stress field, and promotes craton survival under the evolving mantle conditions of the early Earth.

Cenozoic thick-skinned deformation and topography evolution of the Spanish Central System

The Spanish Central System is a Cenozoic pop-up with an E–W to NE–SW orientation that affects all the crust (thick-skinned tectonics). It shows antiform geometry in the upper crust with thickening in the lower crust. Together with the Iberian Chain it constitutes the most prominent mountainous structure of the Pyrenean foreland.The evolutionary patterns concerning the paleotopography of the interior of the Peninsula can be established by an analysis of the following data: gravimetric, topographical, macro and micro tectonic, sedimentological (infilling of the sedimentary basins of the relative foreland), P–T–t path from apatite fission tracks, paleoseismic and instrumental seismicity.Deformation is clearly asymmetric in the Central System as evidenced by the existence of an unique, large (crustal-scale) thrust at its southern border, while in the northern one there is a normal sequence of north verging thrusts, towards the Duero Basin, whose activity ended during the Lower Miocene. This deformation was accomplished under triaxial compression, Oligocene–Lower Miocene in age, marked by NW–SE to NNW–SSE shortening. Locally orientations of paleostresses deviate from that of the regional tensor, following a period of relative tectonic quiescence. During the Upper Miocene–Pliocene, a reactivation of constrictive stress occurred and some structures underwent rejuvenation as a consequence of the action of tectonic stresses similar to those of today (uniaxial extension to strike–slip with NW–SE shortening direction). However, the westernmost areas show continuous activity throughout the whole of the Tertiary, with no apparent pulses. At the present time there is a moderate seismic activity in the Central System related to faults that were active during the Cenozoic, with the same kinematic characteristics.

Tectonic Control on the Sequence Stratigraphy of Nonmarine Retroarc Foreland Basin Fills: Insights from the Upper Jurassic of Central Utah, U.S.A.

Abstract Continental successions of the North American Western Interior retroarc foreland basin provide an excellent opportunity to evaluate the tectonic controls on nonmarine sequence stratigraphy. The transition between the Upper Jurassic Brushy Basin Member anastomosed fluvial system of the Morrison Formation and the gravelly braided-river deposits of the Buckhorn Conglomerate has been studied to assess the dispersal of coarse clastics and the development of associated basin-wide unconformities in a sequence stratigraphic framework. The sharp contact between the two members is interpreted to be conformable based on stratigraphic, sedimentologic, and petrologic data collected at and near Cedar Mountain in central Utah, while a regional, mature paleosol at the top of the Buckhorn Conglomerate indicates the presence of a major sequence boundary. These interpretations are combined with paleoflow data and fluvial architectural analysis to reconstruct the evolution of the alluvial equilibrium profiles that controlled deposition of the succession. The Buckhorn Conglomerate is interpreted as an example of a post-orogenic deposit on the basis of (1) its tabular geometry, (2) distance from the contemporaneous thrust belt, (3) shift to transverse paleoflow direction from the preceding Brushy Basin axial drainage, (4) conformable lower contact, and (5) presence of an overlying sequence boundary with a westward-expanding hiatus eroding the coeval foredeep. The onset of a protracted period of low rate of orthogonal convergence between the Farallon and North American Plates resulted in tectonic quiescence and consequent isostatic rebound of the Middle to Late Jurassic thrust belt and the contiguous contemporaneous foredeep, as well as dynamic uplift. The ensuing basin-wide decrease in subsidence rate induced the progradation of the Buckhorn alluvial plain into the Morrison back-bulge depozone, and the subsequent generation of the overlying pedogenic unconformity, which separates the deposits of the Jurassic flexural event from the overlying Sevier tectonostratigraphic unit.

Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa

Research Article| March 01, 2007 Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa Russell Bailie; Russell Bailie Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa, Present address: Paleoproterozoic Mineralisation Research Group, Dept. of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa e-mail: russellb@postgrad.uj.ac.za Search for other works by this author on: GSW Google Scholar Richard Armstrong; Richard Armstrong Research School of Earth Sciences, The Australian National University, Canberra, 0200, A.C.T., Australia e-mail: richard.armstrong@anu.edu.au Search for other works by this author on: GSW Google Scholar David Reid David Reid Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa e-mail: dlr@geology.uct.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information Russell Bailie Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa, Present address: Paleoproterozoic Mineralisation Research Group, Dept. of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa e-mail: russellb@postgrad.uj.ac.za Richard Armstrong Research School of Earth Sciences, The Australian National University, Canberra, 0200, A.C.T., Australia e-mail: richard.armstrong@anu.edu.au David Reid Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa e-mail: dlr@geology.uct.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2006 Geological Society of South Africa South African Journal of Geology (2007) 110 (1): 87–110. https://doi.org/10.2113/gssajg.110.1.87 Article history First Online: 09 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 Russell Bailie, Richard Armstrong, David Reid; Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa. South African Journal of Geology 2007;; 110 (1): 87–110. doi: https://doi.org/10.2113/gssajg.110.1.87 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 SocietySouth African Journal of Geology Search Advanced Search Abstract The central Bushmanland portion of the Namaqua Province of South Africa is underlain by three voluminous suites of granitic gneisses, termed the Achab, Hoogoor and Aroams Gneisses, in the Aggeneys District, the site of major world-class Broken Hill-type Pb-Zn-Cu-Ag deposits. Geochemically the granitic gneisses have similar characteristics with only slight differences in the degree of fractionation present. The gneisses exhibit a predominance of Kibaran magmatic zircon ~1.21 to ~1.17 Ga ages, which correspond closely to a similar tectono-magmatic event in the Okiep District in Namaqualand to the west and to similar aged granitic gneisses in the Natal Province of the Namaqua-Natal Belt. During this period the Aggeneys District was subjected to peak prograde M2 amphibolite facies grade metamorphism associated with peak D2 deformation. This period was characterised by continental collision and accretion throughout the belt. After a 140 m.y. period of tectonic quiescence a discrete tectono-magmatic episode is recorded in U-Pb ages of ~1.04 to ~1.01 Ga Namaquan Orogeny age from low Th/U metamorphic overgrowths also found in the Okiep District in Namaqualand to the west. Unlike Namaqualand and Natal, however, the Aggeneys District was not subjected to extensive magmatism at this time, and metamorphic conditions did not reach granulite facies grade as it did in Namaqualand. All of the granitic gneisses were intruded syn- to post-collision over a short time period. As the granitic gneisses are texturally, petrographically and geochronologically similar it is argued that they were coeval. The generally I- to S-type metaluminous to peraluminous nature of all of the granitic gneisses, along with previously published depleted mantle (TDM) Sm-Nd model ages of between ~2.42 and ~2.05 Ga, suggests derivation by partial melting of pre-existing ~2.0 Ga crust at ~1.2 Ga in a continental collision tectonic setting. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

FAULT CONTROLS ON SEDIMENT DISTRIBUTION PATTERNS, LIMÓN BASIN, COSTA RICA

The Limón back‐arc basin on the Caribbean Seaboard of Costa Rica is an excellent area in which to study the influence of fault activity on patterns of sediment dispersal. The basin can be divided into northern and southern sub‐basins which are characterized by different structural histories. Whereas the North Limón Basin is dominated by extension, the South Limón Basin has been deformed by northeastward directed compression. Data from a grid of seismic lines in the offshore parts of these two sub‐basins allowed us to map the basin‐fill architecture and the key tectonic elements. Sediment thickness maps were used to infer the sediment distribution during the Middle Miocene, Late Miocene, Pliocene and Quaternary.The results of this study show that the extensional North Limón Basin has the characteristics of a passive continental margin. A wide and structurally homogeneous depocentre developed in relative tectonic quiescence. Tectonic activity increased in Plio‐Pleistocene times and pronounced normal faulting led to the formation of structurally‐controlled depocentres. The configuration in the South Limón Basin is different, since compressional tectonics created a heterogeneous pattern of accommodation space and sediment thickness. In this region, a number of small depocentres have been active since the Middle Miocene and their location changed continuously through time. These changes are most obvious in the Plio‐Pleistocene, where a succession of piggy‐back basins evolved in response to offshore activity on the Limón fold‐and‐thrust belt. Today, ongoing thrust tectonics causes topographic breaks at the sea‐floor of the South Limón Basin. These morphological features control the position of recent submarine channels.The data and interpretations presented here document the influence of fault activity on depositional patterns in a sedimentary basin. This study shows that tectonically controlled depocentres can be highly variable in space and time.

Structural evolution of the Penguins Cluster, UK northern North Sea

Abstract The Penguins Cluster is a group of four oil and gas fields in the northern end of the East Shetland Basin. Its structural complexity is caused by the interaction between two or more fault trend populations, fault reactivation and the impact of faulting on the Brent reservoir architecture. This structural picture is further complicated by a NW–SE trending basement lineament interpreted as a Caledonian shear zone. The present day structural configuration is the result of two Mesozoic rifting episodes and their associated thermal subsidence phases. The Permo-Triassic rifting created a number of north–south-trending tilted fault blocks, and was followed by a period of tectonic quiescence until the Middle Jurassic, when a faulting episode coeval with the Brent Group deposition caused footwall rotation, uplift and erosion of the upper Rannoch Formation prior to the deposition of the Etive Formation across the area. The rifting climaxed in the late Jurassic, when the reactivation of pre-existing faults under oblique-slip conditions in the Penguin C Field created small-scale lozenge-shaped transpressional and transtensional fault blocks. The presence of reverse faults in the area is explained with a continuous kinematic model of structural evolution and oblique-slip fault reactivation rather than positive basin inversion.

Planation surfaces in Northern Ethiopia

Planation surfaces are an old-fashioned topic in geomorphology, but they are nevertheless important where they make up much of the landscape. Northern Ethiopia is largely a stepped topography, caused by differential erosion. Exhumation of old planation surfaces that were preserved under sedimentary or volcanic cover is an important process in landscape evolution. The oldest planation surface is of early Palaeozoic age (PS1); the second is Late Triassic (PS2); and the third is of Early Cretaceous age (PS3). The Oligocene Trap Volcanics buried a surface (PS4) of early Tertiary age, which is now widely exposed by erosion as a surface that, where flat enough, is an exhumed planation surface. The surfaces do not relate to the supposed Africa-wide pediplain sequence of King [King, L.C., 1975. Planation surfaces upon highlands. Z. Geomorph. NF 20 (2), 133–148.], either in mode of formation and age. Although the region is tropical, there is scarce evidence of deep weathering and few indications that the surfaces could be regarded as etchplains. These surfaces indicate that eastern Africa underwent long episodes of tectonic quiescence during which erosion processes were able to planate the surface at altitudes not too far from sea level. Only after the onset of rifting processes, uplift became active and transformed a vast lowland plain into the present Ethiopian highlands, largely exceeding 2500 m a.s.l. Some hypotheses and speculations on the genesis of these surfaces are considered here.

Cenozoic exhumation of the northern Sierra Nevada, California, from (U-Th)/He thermochronology

Research Article| November 01, 2006 Cenozoic exhumation of the northern Sierra Nevada, California, from (U-Th)/He thermochronology M. Robinson Cecil; M. Robinson Cecil 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Mihai N. Ducea; Mihai N. Ducea 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Peter W. Reiners; Peter W. Reiners 2Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA Search for other works by this author on: GSW Google Scholar Clement G. Chase Clement G. Chase 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Author and Article Information M. Robinson Cecil 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Mihai N. Ducea 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Peter W. Reiners 2Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA Clement G. Chase 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Publisher: Geological Society of America Received: 22 Jul 2005 Revision Received: 09 Jan 2006 Accepted: 01 Feb 2006 First Online: 03 Oct 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (11-12): 1481–1488. https://doi.org/10.1130/B25876.1 Article history Received: 22 Jul 2005 Revision Received: 09 Jan 2006 Accepted: 01 Feb 2006 First Online: 03 Oct 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. Robinson Cecil, Mihai N. Ducea, Peter W. Reiners, Clement G. Chase; Cenozoic exhumation of the northern Sierra Nevada, California, from (U-Th)/He thermochronology. GSA Bulletin 2006;; 118 (11-12): 1481–1488. doi: https://doi.org/10.1130/B25876.1 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 Apatite and zircon (U-Th)/He ages from a 100-km-long range-perpendicular transect in the northern Sierra Nevada, California, are used to constrain the exhumation history of the range since ca. 90 Ma. (U-Th)/He ages in apatite decrease from 80 Ma along the low western range flanks to 46 Ma in the higher elevations to the east. (U-Th)/He ages in zircon also show a weak inverse correlation with elevation, decreasing from 91 Ma in the west to 66 Ma in the east. Rocks near the range crest, sampled at elevations of 2200–2500 m, yield the youngest apatite helium ages (46–55 Ma), whereas zircon helium ages are more uniform across the divide. These data reveal relatively rapid cooling rates between ca. 90 and 60 Ma, which are consistent with relatively rapid exhumation rates of 0.2–0.8 km/m.y., followed by a long period of slower exhumation (0.02–0.04 km/m.y.) from the early Paleogene to today. This is reflected in the low-relief morphology of the northern Sierra Nevada, where an Eocene erosional surface has long been identified. A long period of slow exhumation is also consistent with well-documented, widespread lateritic paleosols at the base of Eocene depositional units. Laterites preserved in the northern Sierra Nevada are the product of intense weathering in a subtropical environment and suggest an enduring, soil-mantled topography. We interpret this exhumation history as recording a Late Cretaceous to early Cenozoic period of relatively rapid uplift and unroofing followed by tectonic quiescence and erosional smoothing of Sierran topography through the Neogene. Well-documented recent incision appears to have had little effect on (U-Th)/He ages, suggesting that less than ∼3 km has been eroded from the Sierra Nevada since the early Cenozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cenozoic Tectonic Evolution of the Basin and Range Province in Northwestern Nevada

A regional synthesis of new and existing geologic and thermochronologic data document late Cretaceous - early Cenozoic regional erosion, Oligocene - Miocene volcanism, and subsequent late Miocene extension of the Basin and Range Province in northwestern Nevada and northeastern California. Across an ∼220-km-wide region between the Santa Rosa and Warner Ranges, conformable sequences of 35 to 15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ∼23 km of total east-west extension (∼12%). Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11 to 12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. Extension in the Santa Rosa Range to the east took place during the same interval, although the post-6 Ma part of this history is less well constrained. Less complete constraints from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued up to the present. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but may represent westward migration of Basin and Range extension during the Pliocene. Compared to the many parts of the Basin and Range in central and southern Nevada, extension in northwestern Nevada began more recently, is of lesser total magnitude, and was accommodated entirely by high-angle normal faults. Fission-track data document Late Cretaceous unroofing of Cretaceous (115 --100 Ma) granitic basement rocks in northwestern Nevada, followed by a long period of relative tectonic quiescence that persisted through Oligocene and Miocene volcanism until the onset of Basin and Range extension at ∼12 Ma. The low magnitude of extension (12%) and early Tertiary stability suggest that the modern ∼31 km thick crust in northwestern Nevada was only slightly thicker (∼35 km) prior to extension at 12 Ma, and was no thicker than ∼38 km in the Late Cretaceous. This stands in contrast to other parts of the Basin and Range, where the crust was thickened to at least 45 to 50 km by Cretaceous thrusting and subsequently thinned to ∼30 km by large magnitude (\>50%) extension.

A stratigraphical-geochemical study on the Chaco Paraná continental rift basin—An approach study based on regional sedimentology and drill-hole core analyses, South América

A stratigraphical-geochemical study on the Chaco Paraná continental rift basin—An approach study based on regional sedimentology and drill-hole core analyses, South América

Competing processes of clastic deposition and compartmentalized inversion in an actively evolving transpressional basin, western Mongolia

The Dariv Basin is an actively evolving intracontinental transpressional basin located on the eastern flank of the Mongolian Altai. The basin occupies a complex tectonic position between a restraining bend, a thrusted basement block, and two major conjugate strike-slip fault systems. Structures and sedimentary strata exposed within the Dariv Basin suggest a Mesozoic and Cenozoic two-stage evolution. Jurassic–Cretaceous strata fine upward and record alluvial fan, fluvial and lacustrine depositional environments. The distribution of Mesozoic sedimentary rocks and the presence of a suspected Jurassic normal fault array suggest that the Dariv Basin initially formed as an extensional basin. Following Palaeogene tectonic quiescence, Oligocene–Recent basin fill is dominated by alluvial fan sediments derived from basin-flanking ranges. The modern basin is deforming by thrusting, normal fault inversion and folding along discrete belts expressed as intrabasinal ridges and domes. These belts define a rhomboid of active deformation that compartmentalizes the basin. Sediments derived from these discrete deforming belts and from basin flanking ranges continue to accumulate in the basin centre. Thus, modern fans contain reworked older basin fill and competing processes of sedimentation, deformation, erosion and resedimentation can be observed. The Dariv Basin is an excellent example of a transpressional piggyback basin in the early stages of basin inversion and destruction.

Stratigraphic evidence for millennial-scale temporal clustering of earthquakes on a continental-interior fault: Holocene Mississippi River floodplain deposits, New Madrid seismic zone, USA

The earthquake cycles that characterize continental-interior areas that are far from active plate boundaries have proven highly cryptic and difficult to resolve. We used a novel paleoseismic proxy to address this issue. Namely, we reconstructed Holocene Mississippi River channels from maps of floodplain strata in order to identify channel perturbations reflective of major displacement events on the high-hazard and mid-plate Reelfoot thrust fault, New Madrid seismic zone, U.S.A. Only three discrete slip events are currently documented for the Reelfoot fault (∼ AD 900, ∼ AD 1450, and AD 1812). This study extends this record and, thus, illustrates the utility of stratigraphic proxies as paleoseismic tools. We concurrently offer here some of the first quantified response times for tectonically induced channel pattern changes in large alluvial rivers. We identified at least two cycles of pervasive meandering that were interrupted by channel-straightening responses occurring upstream of the Reelfoot fault scarp. These straightening responses initiated at 2244 BC +/− 269 to 1620 BC +/− 220 and ∼ AD 900, respectively, and each records initiation of a period of Reelfoot fault slip after millennia of relative tectonic quiescence. The second (or New Madrid) straightening response was triggered by the previously known ∼ AD 900 fault slip event, and this initial low sinuosity has been protracted until the modern day by the latter ∼ AD 1450 and AD 1812 events. The first (or Bondurant) straightening response began a period of several hundred to ∼ 1400 years of low river sinuosity which evidences a similar period of multiple recurrent displacement events on the Reelfoot fault. These Bondurant events predate the existing paleoseismic record for the Reelfoot fault. These data offer initial evidence that slip events on the Reelfoot fault were temporally clustered on millennial scales and, thus, offers the first direct evidence for millennial-scale clustering of earthquakes on a continental-interior fault. This carries additional ramifications. Namely, faults that have been quiescent and non-hazardous for millennia could re-enter an enduring period of recurrent hazardous earthquakes with little warning. Likewise, the Reelfoot fault also reveals evidence of temporal clustering of earthquakes on short-term cycles (months), as well as evidence for longer-term reactivation cycles (10 4–10 6 years). This introduces the possibility that temporal clustering could be hierarchical on some continental-interior faults.

Seismic Hazards Assessment for Radioactive Waste Disposal Sites in Regions of Low Seismic Activity

A comparative tectonic quiescence and lack of earthquakes make the stable centres of continents attractive for siting long-term radioactive waste storage facilities. The low rates of deformation in such regions, however, make it difficult to characterize their long-term seismotectonic behaviour, leading to uncertain estimates for the very low probability hazard estimates required by society. In an attempt to overcome the deficiency of both contemporary seismicity and paleoseismic data in central Canada, we have used earthquake histories from regions with similar seismotectonic characteristics from around the world. Substituting space for time, we estimate a long-term rate per 106 km2 of 0.004 magnitude ≥6 earthquakes per annum, of which 33–100% might rupture to the surface.

Crustal versus asthenospheric origin of relief of the Atlas Mountains of Morocco

We investigate the respective roles of crustal tectonic shortening and asthenospheric processes on the topography of the High Atlas and surrounding areas (Morocco). The lithospheric structure is modeled with a direct trial‐and‐error algorithm taking into account gravity (Bouguer and free air), geoid, heat flow, and topography. Three parallel cross sections, crossing the High Atlas and Anti‐Atlas ranges, show that the lithosphere is thinned to 60 km below these mountain ranges. An analysis of the effect of the lithospheric thinning allows us to conclude that the whole topography of the Anti‐Atlas, which belongs to the Sahara domain, is due to asthenospheric processes. In the High Atlas the lithospheric thinning explains a third of the relief of the western High Atlas, 500 m for a mean altitude of 1500 m, and half of the relief of the central High Atlas, 1000 m for a mean altitude of 2000 m. At the scale of Morocco the domain affected by lithospheric thinning forms an elongated NE‐SW strip crossing not only the main structural zones but also the Atlantic margin to the south and the Africa‐Eurasia plate boundary to the north. This major lithospheric thinning is associated with Miocene to recent alkaline volcanism and seismicity. We propose that this thermal anomaly is related to a shallow mantle plume, emplaced during middle to late Miocene time, during a period of relative tectonic quiescence.

Berriasian drowning of the Plassen carbonate platform at the type-locality and its bearing on the early Eoalpine orogenic dynamics in the Northern Calcareous Alps (Austria)

The Plassen carbonate platform (Kimmeridgian to Early Berriasian) developed above the Callovian to Tithonian carbonate clastic radiolaritic flysch basins of the Northern Calcareous Alps during a tectonically active period in a convergent regime. Remnants of the drowning sequence of the Plassen Formation have been discovered at Mount Plassen in the Austrian Salzkammergut. It is represented by calpionellid-radiolaria wacke- to packstones that, due to the occurrence of Calpionellopsis oblonga (Cadisch), are of Late Berriasian age (oblonga Subzone). Thus, the Plassen Formation at its type-locality shows the most complete profile presently known, documenting the carbonate platform evolution from the initial shallowing upward evolution in the Kimmeridgian until the final Berriasian drowning. The shift from neritic to pelagic sedimentation took place during Berriasian times. A siliciclastic-influenced drowning sequence sealed the highly differentiated Plassen carbonate platform. The former interpretation of a Late Jurassic carbonate platform formed under conditions of tectonic quiescence cannot be confirmed. The onset, evolution and drowning of the Plassen carbonate platform took place at an active continental margin. The tectonic evolution of the Northern Calcareous Alps during the Kimmeridgian to Berriasian time span and the reasons for the final drowning of the Plassen carbonate platform are to be seen in connection with further tectonic shortening after the closure of the Tethys Ocean.

Transport and concentration of detrital gold in foreland basins

Economic concentrations of detrital gold are rare in young foreland basins due to paucity of significant gold sources, and a paucity of sediment recycling processes during filling of the foreland basin. Gold shed into the foreland basins is typically widely dispersed in an overwhelming volume of immature basin-fill detritus of no economic significance. In the actively forming Canterbury Basin of New Zealand, minor gold concentration occurs at the mountain front in the bed of the Rakaia River, and 60 km downstream on beaches and the crest of foredunes at the river mouth. The Cretaceous–Tertiary Denver and Western Canada Basins in North America also have minor gold concentrations at the mountain front, and minor gold dispersal into the basin. Tectonic quiescence in the middle Tertiary in the Denver Basin kept gold within 20 km of the mountain front, where renewed uplift in late Tertiary caused minor economic concentrations to form in modern streams. Gold has been transported ca. 200 km across the Western Canada basin by progressive recycling of gravel during slow (ca. 10 to 50 m/Ma) middle Tertiary–Recent regional uplift and tilting, but little concentration has occurred. Development of significant placers in a foreland basin, the generally accepted setting for the Witwatersrand Au-U palaeoplacers, appears to require specific tectonic conditions during and/or after basin evolution to drive the sedimentary recycling necessary for significant placer development. Such tectonic conditions have not occurred in an any of the three young foreland basins examined in this study.

Ultrafast oceanic spreading of the Marsili Basin, southern Tyrrhenian Sea: Evidence from magnetic anomaly analysis

Spectral analysis of both shipborne and airborne magnetic maps of the southern Tyrrhenian Sea reveals seven subparallel positive-negative magnetic anomaly stripes over the flat-lying deep floor of the Marsili oceanic basin. This represents the first evidence of oceanic magnetic anomalies in the Tyrrhenian Sea. The central positive stripe is along the Marsili seamount, a superinflated spreading ridge located at the basin axis. The stratigraphy of Ocean Drilling Program Site 650 and K/Ar ages from the Marsili seamount suggest that the Marsili Basin opened at the remarkable full-spreading rate of ∼19 cm/ yr between ca. 1.6 and 2.1 Ma about the Olduvai subchron. This is the highest spreading rate ever documented, including that observed at the Cocos-Pacific plate boundary. Renewed but slow spreading during the Brunhes chron (after 0.78 Ma), coupled with huge magmatic inflation, gave rise to the Marsili volcano. Our new data and interpretation show that backarc spreading of the Tyrrhenian Sea was episodic, with sudden rapid pulses punctuating relatively long periods of tectonic quiescence.