Search

The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone--the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift--yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons are dominated by 1550-1300 Ma (50%) and 1300-1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300-1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800-1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment. Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850-600 Ma) and Cambrian (541-487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (>60%) by 1550-1300 Ma grains with up to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of >1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup. [ABSTRACT FROM AUTHOR]

Detrital zircon (DZ) geochronology has become a popular tool in provenance studies during the past two decades. However, similar zircon crystallization ages from different source regions greatly hamper the interpretation of sediment dispersal and recycling processes. The Alleghenian–Ouachita–Marathon (AOM) foreland and vicinity in North America is a region where some dominant DZ age groups could come from both the southern Appalachians in the eastern United States and the Gondwanan terranes in Mexico. In this study, we present 1045 new DZ U–Pb ages and 81 DZ core–rim age pairs in lower Permian sandstone in the Permian Basin and Miocene sandstone in the eastern Gulf of Mexico (GOM). These new data were integrated with published DZ single U–Pb age and core–rim ages from syn‐ to post‐orogenic strata in the Permian Basin, Marathon foldbelt, southern Appalachian foreland basin and eastern GOM to interpret the sediment‐dispersal models in the AOM foreland and eastern GOM. Our models show that during the Leonardian Stage, sediments derived from the Appalachians were first delivered to the US midcontinent and then recycled to the Permian Basin; during the Miocene, sediment from the Appalachians fluxed to the eastern GOM, with no longshore mixing from the western GOM. These models based on the integration of single U–Pb and core–rim ages are consistent with published results that used other methods, including zircon single U–Pb age, zircon Hf isotopic data, zircon (U–Th)/He age, sedimentology and stratigraphy. Our results demonstrate that although some limitations exist, zircon core–rim age is a powerful tool, adding an extra constraint on the interpretation of sediment‐dispersal systems. This tool is particularly applicable to the post‐orogenic stage, during which the sediment pathways are more complicated because of the dominant input from distal sources. Insights gained in this study imply that this novel strategy of using core and rim ages could be integrated with other methods to better understand sediment dispersal. [ABSTRACT FROM AUTHOR]

Miocene extension in the back arc of the retreating Hellenic subduction zone resulted in metamorphic core complex formation and exhumation of the Cycladic HP‐LT rocks. The extension was accommodated by bivergent detachment systems, generally occupying the interface between the Cycladic Blueschist Unit (CBU) and overlying Pelagonian units. However, the nature and kinematic history of the contact between the CBU and underlying Cycladic Basement (CB) in the southern Cyclades remains debated due to the presence of both top‐to‐the‐N and top‐to‐the‐S kinematics and ambiguities dating the final exhumation stages. Zircon and apatite (U‐Th)/He data and thermal modelling for the CB and CBU on Ios and Sikinos show that both units were likely exhumed as a coherent footwall block in response to rapid slip along the bivergent Miocene Santorini and Naxos‐Paros detachment systems. [ABSTRACT FROM AUTHOR]

Middle to Upper Devonian strata of the Franklinian Basin in the Canadian Arctic contain a rich record of Phanerozoic tectonic events along the northern margin of Laurentia. We report detrital thermochronometric zircon (U-Th)/He ages (ZHe; number of aliquots, n = 72) and apatite (U-Th)/He ages (AHe; n = 38) from these Paleozoic strata in an effort to better understand the sediment source regions and the exhumation history of the basin. Detrital ZHe ages are older than corresponding stratigraphic ages and were not reset during subsequent burial, thus constraining both maximum burial depths ( Les strates du Devonien moyen a superieur dans le bassin franklinien de l'Arctique canadien contiennent un enregistrement important d'evenements tectoniques (Phanerozoique) le long de la bordure nord de la Laurentie. Nous rapportons des ages thermochronometriques (U-Th) sur des zircons detritiques (ZHe; n = 72) et a partir d'apatites (U-Th)/He (Ahe; n = 38) de ces strates paleozoiques ain de mieux comprendre les regions sources des sediments et l'historique de l'exhumation du bassin. Les ages ZHe detritiques sont plus anciens que les ages stratigraphiques correspondants et ils n'ont pas ete rajeunis durant l'enfouissement subsequent, limitant ainsi les profondeurs maximales d'enfouissement (, Introduction The geologic record of the Canadian Arctic Islands provides important insights into the tectonic evolution of the northern Laurentian margin as well as the development of the adjacent Amerasian [...]

Laser ablation--inductively coupled plasma--mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic--Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520-420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200-1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syndepositional magmatic sources (420-380 Ma), and distal sources of chiefly Mesoproterozoic basement (1650-900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic--Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous--Permian (310-250 Ma) and Late Triassic--Early Jurassic (220-180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased influence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050-950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850-1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous--Paleocene (90-55 Ma), Jurassic (190-150 Ma), and limited Permian--Triassic (280-220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene--Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180-40 Ma) and shield-derived sediments (mostly 1850-950 Ma). Late Miocene--Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic--Cenozoic zircons and increased proportions of 1650-900 Ma zircons emblematic of Cretaceous strata. doi: 10.1130/B30118.1

[1] Apatite and zircon (U-Th)/He cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the High Zagros fold-thrust belt. Single-grain cooling age data were collected from (1) Cambrian sandstone in various thrust sheets, (2) sandstone and basement clasts derived from structurally controlled salt plugs or fault-bounded slices, and (3) syntectonic Neogene siliciclastics strata. In the northwestern (Kuhrang) and central (Kuh-e Lajin) High Zagros, apatite (U-Th)/He (AHe) ages range from ~26.7 to ~0.38 Ma. Most cooling and exhumation occurred in the early to middle Miocene, constrained by AHe ages ~19-15 Ma from the High Zagros thrust sheet, localized faults, and reset cooling ages from Bakhtiyari deposits. In the southeastern High Zagros (Kuh-e Dinar), cooling occurred later with AHe ages ranging from ~16.5 to ~0.79 Ma. Here most cooling and exhumation occurred in the late Miocene, constrained by AHe ages ~12-8 Ma from the High Zagros fault, and exhumed Paleozoic clasts in synorogenic strata. Zircon (U-Th)/He (ZHe) ages from bedrock samples across the High Zagros are reflective of the precollisional thermal history. The preservation of precollisional ZHe ages limits the pre-Miocene maximum burial temperature of the exhumed strata to < 180 [degrees]C, and indicate < 7-9 km of maximum exhumation in the central Zagros. This study shows that thrust activity in the High Zagros and continental suturing along the Zagros suture was underway by at least 19 Ma, and initiated no later than latest Oligocene to early Miocene time (~23 Ma). Citation: Gavillot, Y., G. J. Axen, D. F. Stockli, B. K. Horton, and M. D. Fakhari (2010), Timing of thrust activity in the High Zagros fold-thrust belt, Iran, from (U-Th)/He thermochronometry, Tectonics, 29, TC4025, doi: 10.1029/2009TC002484.

The hinterland of the Sevier orogenic belt of western North America is widely interpreted as a Cretaceous to Paleogene orogenic plateau. Although evidence for mid-crustal extension of Late Cretaceous age within the Sevier hinterland is widespread, coeval surface-breaking normal fault systems have not been documented. New 1:12,000-scale mapping within the type section of the latest Cretaceous to Eocene Sheep Pass Formation of east-central Nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of Late Cretaceous and Paleogene stratigraphic throw. Intrabasinal normal faulting caused lateral thickness variations within the basal Sheep Pass Formation, although upper members are largely unaffected. An extensional basin setting best explains the fanning of bedding dips, the deposition of megabreccia, and the presence of syndepositional normal faults within the Sheep Pass Formation. Deposition of the basal member of the Sheep Pass Formation is bracketed between ca. 81.3 [+ or -] 3.7 Ma and 66.1 [+ or -] 5.4 Ma, based on the (U-Th)/ He cooling ages of detrital zircons, and on a U-Pb carbonate age derived from the overlying lacustrine limestone member. These new data provide the strongest evidence to date for the existence of Late Cretaceous, surface-breaking normal faults in the Sevier hinterland. Normal faulting was coeval with mid-crustal hinterland extension and with continued contraction within the Sevier foreland to the east.

[1] The Salton Trough is the northernmost segment of the active Gulf of California oblique rift system. The main rift-related structure in the western Salton Trough is the low-angle west Salton detachment fault (WSDF). Footwall and hanging wall apatite and zircon (U-Th)/He ages record a significant contrast in thermal history across the WSDF, confirming Pliocene normal slip along the fault. Apatite (U-Th)/He ages record rapid exhumational cooling from ~5 to ~2 Ma, consistent with the timing of accelerated tectonic subsidence recorded in WSDF hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern San Andreas fault in the Salton Trough. Our results indicate that the WSDF caused at least 2.3-4 km of exhumation and >8-10 km of approximately E directed horizontal extension since ~5 Ma. Slip rate of the WSDF at Yaqui Ridge was likely in the range of ~2.3-5 km/Ma from ~7 to ~2 Ma. The WSDF may have been active well before ~5 Ma. Middle Miocene apatite (U-Th)/He ages from samples at high elevations have steep age-elevation gradients and suggest that exhumation may have initiated by ~12 Ma, about when subduction ceased along a large segment of the Baja California margin and consistent with the age of onset of extension in northeastern Baja California. doi: 10.1029/2007TC002172.

New geologic map, tectonic, geomorphologic, and terrestrial cosmogenic nuclide (TCN) geochronologic data document the geometry, style, kinematics, and slip rates on late Quaternary faults within the Queen Valley, California-Nevada area. These data provide important insight into the kinematics of fault-slip transfer from the dextral White Mountains fault zone northward into the Mina deflection. Queen Valley is an ~16-km-long, NE-trending basin bounded to the south by the White Mountains and underlain by four major Pleistocene to Holocene alluvial-fan surfaces. Four different fault types and orientations cut and offset all but the youngest surfaces: (1) The normal-slip Queen Valley fault, which consists of a set of NE-striking, NW- and SE-dipping normal fault scarps that cut across the SE side of the valley and offset all but the youngest surfaces; (2) discontinuons NE-striking, sinistral faults exposed on the north side of the valley; (3) the NW-striking dextral Coyote Springs fault, which merges into (4) a set of E-W--striking thrust faults. Measured offsets across normal fault scarps developed within [sup.10]Be TCN-dated surfaces yield minimum late Pleistocene horizontal extension rates of 0.1-0.3 mm/yr. Documented fault geometries and slip orientations across Queen Valley suggest that fault-slip transfer models, such as the extensional displacement transfer, block rotation, and simple shear models, within the dextral fault system proposed for the eastern California shear zone--Walker Lane belt are not applicable to this part of the Mina deflection. Rather, dextral fault slip is transferred by both a restraining westward step and a releasing eastward step. Restraining and releasing bends have been extensively documented at a range of scales in strike-slip fault tectonic settings globally, and they have been simulated in analog models; thus, it is not surprising to document both within the ~630-km-long dextral shear zone that makes up the northern eastern California shear zone--Walker Lane belt. Our results, combined with published slip rates for the dextral White Mountain fault zone and the eastern sinistral Coaldale fault, suggest that transfer of dextral slip into the Mina deflection is partitioned into three different components: horizontal extension along the Queen Valley fault, thrust faulting that merges into the dominantly dextral slip along the Coyote Springs fault, and dominantly sinistral slip along the Coaldale fault. A velocity vector diagram of fault-slip partitioning across Queen Valley predicts a small component of contraction across the Coyote Springs and western Coaldale faults. Contraction across the Mina deflection is consistent with global positioning system data. An observed reduction in late Pleistocene fault-slip rates at the northern end of the eastern California shear zone and across the southwestern part of the Mina deflection may be explained by distribution of slip across a much broader zone than generally thought. Keywords: normal faults, strike-slip faults, thrust faults, terrestrial cosmogenic nuclide geochronology, fault-slip transfer, Walker Lane belt, Eastern California shear zone.

New geologic mapping, tectonic geomorphologic, [sup.10]Be terrestrial cosmogenic nuclide, and (U-Th)/He zircon and apatite thermochronometric data provide the first numerical constraints on late Cretaceous to late Quaternary exhumation of the Inyo Mountains and vertical slip and horizontal extension rates across the eastern Inyo fault zone, California. The east-dipping eastern Inyo fault zone bounds the eastern flank of the Inyo Mountains, a prominent geomorphic feature within the western Basin and Range Province and eastern California shear zone. (U-Th)/He zircon and apatite thermochronometry yield age patterns across the range that are interpreted as indicating: (1) two episodes of moderate to rapid exhumation associated with Laramide deformation during the late Cretaceous/ early Tertiary; (2) development of a slowly eroding surface during a prolonged period from early Eocene to middle Miocene; (3) rapid cooling, exhumation, and initiation of normal slip along the eastern Inyo fault zone, accommodated by westward tilting of the Inyo Mountains block, at 15.6 Ma; and (4) rapid cooling, exhumation, and renewed normal slip along the eastern Inyo fault zone at 2.8 Ma. Fault slip continues today as indicated by fault scarps that cut late Pleistocene alluvial fan surfaces. The second episode of normal slip at 2.8 Ma also signals onset of dextral slip along the Hunter Mountain fault, yielding a Pliocene dextral slip rate of 3.3 [+ or -] 1.0 mm/a, where a is years. Summing this dextral slip rate with estimated dextral slip rates along the Owens Valley, Death Valley, and Stateline faults yields a net geologic dextral slip rate across the eastern California shear zone of 9.3 + 2.2/-1.4 to 9.8 + 1.4/-1.0 mm/a.

Grenville-age (1.3--0.9 Ga) zircons represent one of the most ubiquitous detrital zircon (DZ) age modes on Earth. In North America, given the widespread occurrence of Grenville basement, Grenville DZs are commonly viewed as nondiagnostic with regard to source region in provenance studies. Systematic recovery of DZ core-rim U-Pb ages makes it possible to identify and differentiate previously indistinguishable basement source terranes by leveraging their multistage tectono-magmatic evolution. Our analysis demonstrates that Grenville DZs exhibit distinct rim ages in different parts of the North American Paleozoic Appalachian-Ouachita-Marathon foreland. Whereas Grenville DZ grains in the eastern foreland, sourced from the southern Appalachian orogen in the eastern United States, exhibit Taconian and Acadian (490--350 Ma) rims, grains in the western foreland, derived from Mexico, mainly show Neoproterozoic (750--500 Ma) rim ages. This difference permits differentiation of non-diagnostic core ages by their distinctive rim ages. Furthermore, core-rim paired ages can illuminate potential genetic relationships among coexisting age components in DZ spectra, thereby indicating whether the DZs are derived from separate sources or from a single source with multistage tectono-magmatic histories. Thus, DZ rim-core ages can provide critical insights into reconstructing global source-to-sink systems and elucidating genetic linkages within multistage orogenic systems. [ABSTRACT FROM AUTHOR]