Mesozoic Accretion Research Articles

Whitehorse Trough records Late Triassic–Cretaceous accretionary orogenesis in the Northern Canadian Cordillera via detrital mineral thermochronometry

The Whitehorse Trough formed during early Mesozoic accretion of the Intermontane terranes to northwestern North America. Here we investigate its thermal history using detrital mineral thermochronology, including 171 single-crystal (U–Th)/He zircon (ZHe) ages from 35 samples, 158 single-crystal (U–Th)/He apatite (AHe) ages from 33 samples, and apatite fission track (AFT) ages from 12 samples. ZHe single crystal ages range from 222 to 42 Ma and define Triassic–Early Jurassic, Late Jurassic, and Cretaceous–Paleogene age groups. AFT central ages range from 95 to 30 Ma with a dominant age peak at ∼50 Ma, and AHe single crystal ages range from 228 to 13 Ma with a dominant age peak between 50 and 40 Ma. Forward and inverse models of thermochronological data are compatible with two regional burial/heating stages that variably reset He in zircon. Maximum temperatures of the Whitehorse Trough strata locally exceeded 150 °C during Early Jurassic burial and shortening into a fold and thrust belt. Following Middle to Late Jurassic exhumation-related cooling and development of a prominent unconformity, Whitehorse Trough strata were buried again. Temperatures locally exceeded 150 °C during the Cretaceous, suggesting maximum burial of between ∼4 and 7.5 km. Heating and cooling rates during the Early–Middle Jurassic were ∼10 °C/myr, coinciding with deposition, fold and thrust belt development, and regional crustal thickening during the final stages of Intermontane terrane accretion. Maximum heating rates during the Cretaceous were ∼4–7 °C/myr and likely correspond to regional crustal thickening of the northern Cordillera hinterland and establishment of an outboard, Andean-type continental arc system.

Preliminary Study of Accretion Complexes of Mangilu Area, Pangkep Ragency South Sulawesi

The exposure of Mesozoic rocks in the Mangilu area, both as a result of tectonic deformation in the form of mélange, low-high degree metamorphism and deep-sea sediments (flysch, radiolarian cherts, and sediment flow in the form of olistostrome), which formed in the subduction zone indicates complex accretionary rocks. The purpose of this study is to identify and study the accretionary complex trace rocks of the Mesozoic era. The method used in this study was field observation, including collection of geological data, tectonic deformation, rock groups and laboratory analysis (petrography). In general, the accretion complex is divided into 2 categories, namely a). oceanic volcanic groups and sediments of the oceanic crust; b). terigenous sediments from continents with sialic beds. The oceanic volcanic group consists of gabbro, diabase, dolerite, pillow and hyaloclastic lava, and oceanic sediments consist of radiolarian chert and mali limestone. The terigen sedimentary group consists of flysch type sandstones and olistostrome flow sediments with components in the form of granite, granodiorite, dacite and diabase. Other components found in autoclastic breccia rocks in the Mangilu area consist of metaperidotite, serpentinite, quartzite and metachert. Based on the Oceanic Plate Stratigraphy (OPS) arrangement, the autoclastic breccia rocks in the Mangilu area consist of bottom up, namely ultramafic/serpentinite, gabbro, diabase, pillow basaltic lava, hyaloclastic, radiolarian chert, mali limestone and flysch sediments. The mixing of rocks originating from oceanic plates with continental plates in autoclastic breccias indicates that the Mesozoic accretion complex in the Mangilu area has experienced strong deformation in the form of thurst in the Tertiary era.

Correlation of Andesite Complexes in the Southern Framing of the Eastern Part of the Mongol–Okhotsk Orogenic Belt according to Geochronological, Geochemical, and Isotope-Geochemical Data

Abstract —The evolution of the eastern part of the Mongol–Okhotsk Orogenic Belt marks the late Mesozoic accretion–collision processes involving the Siberian and Sino–Korean plates. Tectonic restructuring in the region was accompanied by the formation of igneous complexes, which replaced each other almost without a temporal break. Andesitic magmatism was widely manifested in the southern framing of the eastern part of the belt. Its study is complicated by the isolation of volcanic fields and their confinement to the Amur–Zeya depression. Study of the geochemical and geochronological characteristics of igneous complexes there made it possible to recognize and systematize the temporal stages and substantiate the geodynamic formation of these rocks. However, there are controversial geologic objects, such as the Isikan volcanic field. We present the first data on the isotopic age and chemical and isotope-chemical compositions of the rocks of this field. 40Ar/39Ar dating showed that the integrated age of the dacitic matrix is 113.0 ± 2.6 Ma and the plateau age is 121.0 ± 1.6 Ma. In chemical composition the studied volcanics are similar to rocks of the Poyarkov volcanic complex developed within the Bureya– Jiamusi superterrane, which formed under subduction in the period 120–105 Ma. In geochemical and isotope-geochemical characteristics the volcanics are similar to rocks of igneous belts of Andean-type active continental margins.

Accretion, subduction erosion, and tectonic extrusion during late Paleozoic to Mesozoic orogenesis in NE China

The high-pressure Heilongjiang Complex records a long and complicated evolution from the late Paleozoic to Mesozoic in NE China, which is still subject to controversy. In this paper we present a new detailed study including structural, geochemical and geochronological analyses (Ar–Ar dating on metamorphic minerals, U-Pb SIMS dating on zircons). Two main periods of oceanward accretionary growth are identified between 305 Ma and 195 Ma and around 112–101 Ma. These periods of accretionary growth were associated with the emplacement of voluminous magmatism, slab roll-back with continental back-arc extension, and growth of a forearc accretionary prism. Between these periods of accretionary growth, a major phase of regional subduction erosion took place between 195 Ma and 142 Ma at the eastern edge of the Songliao Block. The main effects of this subduction erosion phase were that regional Jurassic-early Cretaceous extrusion overlapped with Mesozoic accretion and erosion in NE China, which disturbed the initial orogenic architecture. With a review of relevant zircon ages of this accretionary orogen, we unravel the complicated geodynamic history of accretion, subduction erosion and tectonic extrusion that shaped NE China from the late Paleozoic to Mesozoic.

Pamir Plateau formation and crustal thickening before the India-Asia collision inferred from dating and petrology of the 110–92 Ma Southern Pamir volcanic sequence

The formation of the Pamir is a key component of the India-Asia collision with major implications for lithospheric processes, plateau formation, land-sea configurations and associated climate changes. Although the formation of the Pamir is traditionally linked to Cenozoic processes associated with the India-Asia collision, the contribution of the Mesozoic tectonic evolution remains poorly understood. The Pamir was formed by the suturing of Gondwanan terranes to the south margin of Eurasia, however, the timing and tectonic mechanisms associated with this Mesozoic accretion remain poorly constrained. These processes are recorded by several igneous belts within these terranes, which are not well studied. Within the Southern Pamir, the Albian-Turonian volcanic rocks and comagmatic plutons of the Kyzylrabat Igneous Complex (KIC) provide an important and still unconstrained record of the Pamir evolution. Here we provide the age, origin and the geodynamic setting of the KIC volcanics by studying their petrology, zircon U-Pb geochronology, geochemistry and isotope composition. 17 samples from the KIC volcanics yield U-Pb ages spanning from 92 to 110Ma. The volcanics are intermediate to acidic in composition (SiO2=56–69wt%) and exhibit high-K calc-alkaline and shoshonitic affinity (K2O/Na2O=1–2.2wt%). They show enrichment in LILE and LREE and depletion in HFSE and HREE with negative Ta, Ti and Nb anomalies, suggesting an arc-related tectonic setting for their formation. Low εNd(t) values (from −9.1 to −4.7), relatively high 87Sr/86Sr(i) ratios (0.7069–0.7096) and broad range of zircon εHf values (from −22.6 to −1.5) suggest a mixture of different magma sources. These features suggest that volcanics were derived by crustal under- or intraplating of an enriched subduction-related mantle shoshonitic magmas, by heating and partial melting of the lower crust, and by mixing of both magma components. Our results further imply that the KIC volcanics represent a shoshonitic suite typical of an evolution from active continental arc to post-collisional setting with a steepening of the Benioff zone and thickening of the crust toward the back-arc. This setting is best explained by the subduction - collision transition along the Shyok suture due to accretion of the Kohistan island arc to the Karakoram. This suggests that a significant part of the crustal shortening and thickening accommodated in the Pamir occurred in the Mesozoic before the India-Asia collision with implications for regional tectonic models. This further suggests the Pamir was already a major topographic feature with potentially important paleoclimate forcing such as the monsoonal circulation.

Lithospheric Architecture of the Lhasa Terrane and Its Control on Ore Deposits in the Himalayan-Tibetan Orogen

Magmatic-hydrothermal ore deposits in collisional orogens are new targets for modern mineral exploration, yet it is unclear why they preferentially occur in some specific tectonic environments within these orogenic belts. We integrate geologic and geochemical data (especially zircon U-Pb dating and Lu-Hf isotope data) for Mesozoic-Cenozoic magmatic rocks and associated ore deposits in the Lhasa terrane, a highly endowed tectonic unit within the Himalayan-Tibetan orogen, and provide the first example in a continental collision terrane of the application of zircon Hf isotope data to image the lithospheric architecture and its relationship with ore deposits. Three crustal blocks are identified within the Lhasa terrane by the Hf isotope mapping method. They include a central long-lived Precambrian microcontinent with local reworking and two surrounding juvenile Phanerozoic crustal blocks with significant mantle contributions to constituent magmatic rocks. The three crustal blocks are bounded by two E-W–trending terrane-boundary faults, and each block is cut by two N-S–striking concealed faults. Isotopic signatures of zircons from the juvenile crustal blocks indicate that the Phanerozoic continental crust grew from several Mesozoic volcanic-plutonic arcs and by underplating of mantle-derived magmas generated during Mesozoic accretion and Cenozoic collision. Mesozoic subduction-related porphyry Cu-Au deposits and Cenozoic collision-related Cu-Mo deposits are exclusively located in regions with high e Hf(>5) juvenile crust. Cu enrichment during differentiation of high f o2 arc magmas is the key for the formation of Mesozoic subduction-related porphyry Cu-Au. By contrast, remelting of the lower crustal Cu sulfide-rich magmatic cumulates within the juvenile crust is interpreted to have played a key role in the formation of Cenozoic collision-related Cu-Mo deposits. Granite-related Pb-Zn deposits cluster in the oldest crustal regions or developed along the margin of the old crustal block bounded by lithospheric faults. The porphyry Mo deposits are localized along the reworked margins of the old crustal block. It is suggested that crustal reworking released Mo from the old crust to form porphyry Mo deposits, whereas leaching of Pb and Zn from the Paleozoic carbonate cover strata by felsic intrusion-driven fluids is critical to the formation of Pb-Zn ore deposits. Skarn Fe-Cu ore deposits are typically localized along a terrane boundary fault, i.e., lithospheric discontinuity, through which crust-derived felsic melt mixed with Cu-rich mantle-derived mafic magmas ascending upward. Associated granitoid rocks usually bear microgranular mafic enclaves and show a zircon Hf isotope array from negative to positive e Hf values (−7.3 to +6.7), supporting mixing of juvenile mantle and evolved crustal sources. The Hf isotope maps show temporal-spatial relationships between crustal structure and the location of ore deposits, demonstrating that the structure, nature, and composition of the crust controlled the localization of ore deposits and the migration of ore-forming metals in the terrane. This study shows that the lithospheric architecture of an orogenic terrane can be imaged by Hf isotope mapping to provide mappable units which can be used to explore for ore deposits at the terrane scale.

Mesozoic accretion of juvenile sub-continental lithospheric mantle beneath South China and its implications: Geochemical and Re–Os isotopic results from Ningyuan mantle xenoliths

The mechanism for the widespread Mesozoic magmatism in South China has been ascribed to either the paleo-Pacific plate subduction or intra-continental lithospheric extension. Mantle xenoliths entrained in the Jurassic Ningyuan alkaline basalts from southern Hunan Province, including twelve lherzolites and one harzburgite, have been studied to constrain the composition and age of the Mesozoic sub-continental lithospheric mantle. The lherzolites contain 2.06–4.09 wt.% Al 2O 3 and 2.03–3.91 wt.% CaO, which are higher than the abundances of these species in the harbzurgite (1.72 wt.% and 1.17 wt.%, respectively). The Cr# of spinel varies from 0.07 to 0.24. Orthopyroxene contains 3.61–4.96 wt.% Al 2O 3 and 0.51–0.8 wt.% CaO, whereas clinopyroxene contains 4.78–7.32 wt.% Al 2O 3 and 1.04–2.01 wt.% Na 2O. Both whole rock and mineral compositions suggest that the Ningyuan mantle xenoliths have been subjected to low degrees of partial melting. Modeling of Y and Yb contents of clinopyroxene indicates that the lherzolites have been subjected to 3–5% degrees of partial melting, while the harzburgite has experienced about 12% melting. Clinopyroxene in most Ningyuan mantle xenoliths shows variable depletion in incompatible elements, suggesting that they have been weakly enriched after melting. Clinopyroxene in one sample (TYS01-3) displays strong enrichment in incompatible elements, reflecting the local occurrence of melt metasomatism. Equilibrium temperatures estimated by two-pyroxene geothermometry vary from 994 to 1081 °C, indicating that the Mesozoic mantle lithosphere has a hot geotherm. All lherzolites display consistent highly siderophile element (HSE) patterns and have suprachondritic Ru/Ir and Pd/Ir ratios. The harzburgite shows remarkable depletion in Pt, Pd and Re. The lherzolites have 187Os/ 188Os ratios of 0.12116–0.12929, which are higher than that of the harzburgite (0.11681). A correlation between 187Os/ 188Os and Al 2O 3 exists among the Ningyuan mantle xenoliths, which if interpreted as an isochron analog yields a model age of ~ 2.2 Ga. This age is older than the Re depletion age (T RD) of the harzburgite (~ 1.8 Ga), which represents a minimum age of melt depletion. The old ages either support the existence of ancient lithosphere relics beneath the Ningyuan region or reflect the accretion of ancient mantle materials from the asthenosphere during the Mesozoic. Although the tectonic implication of the old ages is ambiguous, compositions of the Ningyuan mantle xenoliths suggest that ancient mantle lithosphere beneath South China has been thinned and replaced by hotter, younger mantle during the Mesozoic, which has led to extensive lithospheric extension and abundant magmatism.

Paleozoic–early Mesozoic gold deposits of the Xinjiang Autonomous Region, northwestern China

The late Paleozoic–early Mesozoic tectonic evolution of Xinjiang Autonomous Region, northwestern China provided a favorable geological setting for the formation of lode gold deposits along the sutures between a number of the major Eastern Asia cratonic blocks. These sutures are now represented by the Altay Shan, Tian Shan, and Kunlun Shan ranges, with the former two separated by the Junggar basin and the latter two by the immense Tarim basin. In northernmost Xinjiang, final growth of the Altaid orogen, southward from the Angara craton, is now recorded in the remote mid- to late Paleozoic Altay Shan. Accreted Early to Middle Devonian oceanic rock sequences contain typically small, precious-metal bearing Fe–Cu–Zn VMS deposits (e.g. Ashele). Orogenic gold deposits are widespread along the major Irtysh (e.g. Duyolanasayi, Saidi, Taerde, Kabenbulake, Akexike, Shaerbulake) and Tuergen–Hongshanzui (e.g. Hongshanzui) fault systems, as well as in structurally displaced terrane slivers of the western Junggar (e.g. Hatu) and eastern Junggar areas. Geological and geochronological constraints indicate a generally Late Carboniferous to Early Permian episode of gold deposition, which was coeval with the final stages of Altaid magmatism and large-scale, right-lateral translation along older terrane-bounding faults. The Tian Shan, an exceptionally gold-rich mountain range to the west in the Central Asian republics, is only beginning to be recognized for its gold potential in Xinjiang. In this easternmost part to the range, northerly- and southerly-directed subduction/accretion of early to mid-Paleozoic and mid- to late Paleozoic oceanic terranes, respectively, to the Precambrian Yili block (central Tian Shan) was associated with 400 to 250 Ma arc magmatism and Carboniferous through Early Permian gold-forming hydrothermal events. The more significant resulting deposits in the terranes of the southern Tian Shan include the Sawayaerdun orogenic deposit along the Kyrgyzstan border and the epithermal and replacement deposits of the Kanggurtag belt to the east in the Chol Tagh range. Gold deposits of approximately the same age in the Yili block include the Axi hot springs/epithermal deposit near the Kazakhstan border and a series of small orogenic gold deposits south of Urumqi (e.g. Wangfeng). Gold-rich porphyry copper deposits (e.g. Tuwu) define important new exploration targets in the northern Tian Shan of Xinjiang. The northern foothills of the Kunlun Shan of southern Xinjiang host scattered, small placer gold deposits. Sources for the gold have not been identified, but are hypothesized to be orogenic gold veins beneath the icefields to the south. They are predicted to have formed in the Tianshuihai terrane during its early Mesozoic accretion to the amalgamated Tarim–Qaidam–Kunlun cratonic block.

Gold deposits in the Xiaoqinling-Xiong'ershan region, Qinling Mountains, central China

The gold-rich Xiaoqinling–Xiong'ershan region in eastern Shaanxi and western Henan provinces, central China, lies about 30–50 km inland of the southern margin of the North China craton. More than 100 gold deposits and occurrences are concentrated in the Xiaoqinling (west), Xiaoshan (middle), and Xiong'ershan (east) areas. Late Archean gneiss of the Taihua Group, and Middle Proterozoic metavolcanic rocks of the Xiong'er Group are the main host rocks for the deposits. Mesozoic granitoids (ca. 178–104 Ma) are present in most gold districts, but deposits are typically hosted in the Precambrian basement rocks hundreds of meters to as far as 10 km from the intrusions and related hornfels zones. Deposits in the Xiaoqinling and Xiaoshan areas are best classified as orogenic gold deposits, with ores occurring in a number of distinct belts both in quartz veins and disseminated in altered metamorphic rocks. Alteration assemblages are dominated by quartz, sericite, pyrite, and carbonate minerals. The ore-forming fluids were low salinity, CO2-rich, and characterized by isotopically heavy δ18O. Four deposits (Dongchuang, Wenyu, Yangzhaiyu, and Dahu) in the Xiaoqinling area each contain resources of about 1 Moz Au. Some of the gold deposits in the Xiong'ershan area represent more shallowly emplaced tellurium-enriched orogenic systems, which include resources of approximately 1–1.5 Moz Au at Shanggong and Beiling (or Tantou). Others are epithermal deposits (e.g., Qiyugou and Dianfang) that are hosted in volcanic breccia pipes. Isotopic dates for all gold deposits, although often contradictory, generally cluster between 172–99 Ma and are coeval with emplacement of the post-kinematic granitoids. The gold deposits formed during a period of relaxation of far-field compressional stresses, clearly subsequent to the extensive Paleozoic–early Mesozoic accretion of arc terranes and the Yangtze craton onto the southern margin of the North China craton. Hydrothermal and magmatic events occurred locally where extension-related Precambrian basement uplifting took place along the craton margin. Fluids for the orogenic gold deposits in the Xiaoqinling, Xiaoshan, and Xiong'ershan areas may have been released from evolving magmas or resulted from prograde metamorphic reactions within the uplift zones. Alternatively, for the epithermal gold deposits at shallower levels in the Xiong'ershan area, gold-transporting fluids were mainly exsolved from coeval magmas, although meteoric water was also involved in these hydrothermal systems.

Understanding Mesozoic accretion in Southeast Asia: Significance of Triassic thermotectonism (Indosinian orogeny) in Vietnam

Results from a zircon U-Pb study of the metamorphic basement of Vietnam reveal that a large part of the continental crust was affected by a short-lived episode of ductile deformation and high-temperature metamorphism between 258 ± 6 Ma and 243 ± 5 Ma. Although coincident with final stages of North-South China collision (Qinling orogenesis), the thermotectonism in Vietnam was caused by accretion of Sibumasu to Indochina–South China. This accretion event (Indosinian orogeny) has regional significance because it contributed to the final stages of North-South China collision, an aspect not explicitly recognized in Qinling orogenic models.

Timing of high-pressure metamorphism in the Yukon – Tanana terrane, Canadian Cordillera: constraints from U – Pb zircon dating of eclogite from the Teslin tectonic zone

Zircon from eclogite near Last Peak in the Teslin tectonic zone yielded a U–Pb isotopic age of 269 + 2 Ma (2σ), the first precise age for such a rock in the Yukon –Tanana terrane of the Canadian Cordillera. Both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin, and the U – Pb age therefore constrains the timing of peak high-pressure metamorphism in this rock. The U – Pb age demonstrates for the first time that an Early Permian high-pressure metamorphic event occurred in rocks now making up the Teslin tectonic zone, and possibly elsewhere in the Yukon – Tanana terrane. This U – Pb age provides a new geochronologic "pin" in the evolution of the Yukon – Tanana terrane prior to its Mesozoic accretion to the North American continental margin and, combined with recent 40Ar/39Ar muscovite data, indicates that high-pressure metamorphism at this time was a relatively short-lived event.

Submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc, Sawyers Bar area, central Klamath Mountains, California, USA

New bulk-rock oxygen isotope data indicate a complicated history of fluid-rock interactions in the upper few kilometers of a basaltic arc, attending its Mesozoic accretion to the western margin of North America. Folded, multiply recrystallized, weakly metasomatized mafic volcanics and interstratified sediments of the Sawyers Bar terrane, an eastern segment of the western Triassic and Paleozoic belt, were investigated. The following scenario can now be reconstructed: (1) island-arc tholeiites (IATs), ocean-island basalts (OIBs), and distal turbidites were deposited in a subsea environment during Permian and Early Mesozoic time (170–245 Ma). Basaltic rocks underwent low-temperature alteration by seawater; recrystallization occurred at 100–200°C and < 1 kbar. Alkali exchange and variable Mg-enrichment were accompanied by increases in bulk-rock δ 18O values of the greenstones from 6 to approximately 10%o, preceeding initial stages of island-arc formation. (2) Middle Jurassic (165–170 Ma) suturing of the seaward oceanic arc structurally beneath a landward, 227 Ma blueschist terrane resulted in regional deformation and greenschist-facies metamorphism. Pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300–425°C, 3 ± 1 kbar. (3) Granitoid plutons, emplaced during late-Middle Jurassic time (160–165 Ma), heated adjacent wallrocks to ∼500–600°C at pressures of approximately 2–3 kbar; thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high δ 18O fluids with intercalated greenstones. δ 18O values in IAT and OIB metavolcanics increased from 9 to 10% along axial portions of NS-trending folds to more than 15%o where metabasalts are intimately interlayered with the metasediments. Construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid-rock isotopic exchanges, reflecting the P- T-time evolution of the complex.

Continental accretion: contrasting Mesozoic and Early Proterozoic tectonic regimes in North America

Juvenile continental crust was accreted to southern and western North America during the Early Proterozoic and Mesozoic, respectively. Graywacke, granite, granodiorite, and basalt comprise most of the accreted Early Proterozoic crust, whereas graywacke, andesite, basalt, and granodiorite comprise most of the Mesozoic crust. In addition, carbonates, ultramafics, pelagic sediments, and tonalite/diorite are minor but important components in the juvenile Mesozoic crust, whereas rhyolites are important in the Early Proterozoic crust. Mesozoic supracrustal rocks vary significantly in chemical composition, while Early Proterozoic supracrustals have a limited compositional range and exhibit a linear relation between many element concentrations suggesting a genetic linkage between accreted terranes. Although SiO 2, Al 2O 3, FeO, and incompatible elements are more enriched in Early Proterozoic than in Mesozoic supracrustal rocks, negative Eu anomalies are typical of rocks of both ages. Early Proterozoic granitoids are enriched in LILE (large ion lithophile elements) compared to Mesozoic granitoids, and granitoids of both ages of are enriched in LILE and have larger Eu anomalies than associated supracrustal rocks. Accreted Mesozoic upper crust is similar to andesite in chemical composition, and the bulk crust is similar to basaltic andesite. In contrast, accreted Early Proterozoic upper crust and bulk crust are similar to granodiorite and andesite, respectively. Incompatible elements are depleted in the Mesozoic compared to the Early Proterozoic crust, but both crustal types have negative NbTa anomalies. Depending on the composition assumed for the lower crust, both ages of crust have either very small or negligible Eu anomalies. Lifespans of the Early Proterozoic terranes (time interval between oldest rocks in a terrane and its collision with North America) are 20–80 My, whereas lifespans of Mesozoic terranes are 50–500 My, with most falling between 50 and 200 My. Within Mesozoic terranes, tectonic setting may differ, whereas in most Early Proterozoic terranes tectonic setting appears to have remained the same. Unlike the Mesozoic terranes, which were fragmented during collision and displaced along transcurrent faults, Early Proterozoic terranes show no evidence of major transcurrent offset. Using accretion age windows of 120 My for the Mesozoic and 115 My for the Early Proterozoic, we obtain total crustal accretion rates of 1.33 km 3/y and 1.73 km 3/y, respectively, for 6000 km of strike length in each case. Early Proterozoic crustal accretion in southwestern North America was strikingly different from that in northwestern North America during the Mesozoic. Mesozoic accretion involves transformation of mafic oceanic terranes into continental crust. In contrast, most of the juvenile Early Proterozoic crust appears to have evolved directly into mature continental crust without passing through an ‘oceanic’ stage. This probably occurred in a continental margin arc system. Our results also indicate that oceanic terranes cannot evolve into continental crust as closed chemical systems. Although some Mesozoic oceanic terranes began to evolve into continental crust before accretion to North America, most of the transition occurred during and shortly after accretion. This may have been accomplished by incompatible element enrichment associated with subduction-related processes beneath collisionally thickened crust. The accreted Mesozoic crust has not yet evolved into mature continental crust and whether it will depends on the duration of subduction processes along the continental margin in the future.

The Border Ranges fault system in Glacier Bay National Park, Alaska: evidence for major early Cenozoic dextral strike-slip motion

The Border Ranges fault system of southern Alaska, the fundamental break between the arc basement and the forearc accretionary complex, is the boundary between the Peninsular–Alexander–Wrangellia terrane and the Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Glacier Bay National Park. Mylonitic rocks in the zone record abundant evidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional correlations of Chugach terrane rocks involved in the deformation constrain this movement between latest Cretaceous and Early Eocene (~50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations between Glacier Bay plutons and rocks of similar ages elsewhere along the Border Ranges fault system suggest that as much as 700 km of dextral motion may have been accommodated by this structure. These observations are consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone following late Mesozoic accretion of the Peninsular–Alexander–Wrangellia terrane to North America.

Late Mesozoic and possible early Tertiary accretion in western Washington State: The Helena-Haystack mélange and the Darrington-Devils Mountain fault zone

The Helena-Haystack melange (HH melange) and coincident Darrington-Devils Mountain fault zone (DDMFZ) in northwestern Washington separate two terranes, the Northwest Cascade System (NWCS) and the western and eastern melange belts (WEMB). The two terranes of Paleozoic and Mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history. The HH melange is a serpentinite-matrix melange containing blocks of adjacent terranes but also exotic blocks of schistose metavolcanic rocks and Jurassic tonalite and associated amphibolite. The HH melange must have formed between early Cretaceous and late middle Eocene time, because it contains tectonic clasts of early Cretaceous Shuksan Greenschist and is overlain by late middle Eocene sedimentary and volcanic rocks. Less certain constraints on its age are a tectonic clast of metarhyolite that yields 90 Ma metamorphic ages and the presumption that the melange was emplaced before the outboard Olympic terrane arrived at about 50 Ma. The apparent continuity of the HH melange and the Decatur terrane of the San Juan Islands suggests that the melange is the strongly tectonized equivalent of the Fidalgo ophiolite. The out-crop pattern suggests that the HH melange overlies rocks of the NWCS and it may have formed when the WEMB terranes were thrust over rocks of the NWCS. Much of the exposed belt of the HH melange is overlain by late middle Eocene feldspathic sandstone and volcanic rocks of the Barlow Pass Volcanics of Vance (1957a), which are cut by numerous faults of the DDMFZ paralleling the melange. The Barlow Pass Volcanics appear to overlie the Straight Creek fault without large offset, but a displaced exotic block of amphibolite with attached early or early middle Eocene(?) sandstone in the melange suggests that strike-slip movement along the DDMFZ was synchronous with movement on the Straight Creek fault, and stretched cobbles in the conglomerates of the Barlow Pass Volcanics suggest post-Straight Creek movement. The possible continuation of the DDMFZ to the northwest as the San Juan and the West Coast faults on Vancouver Island suggests That the structure has had a major role in the emplacement of all the westernmost terranes in the Pacific Northwest. This major suture is strongly bowed to the northeast opposite the great oroclinal bend of the Olympic terrane, suggesting that the emplacement of that terrane may have deformed a once straighter strike-slip zone.

History and modes of Mesozoic accretion in Southeastern Russia

Abstract The history of Mesozoic accretion and growth of the Asia eastern margin, occupied by Southeastern Russia, includes five main events; two main tectonic regimes were responsible for the growth of the continent. In the Triassic‐Jurassic, Early Cretaceous and Late Cretaceous‐Paleogene, the subduction of the oceanic lithosphere resulted in the formation of wide accretionary wedges of the Mongol‐Okhotsk, Khingan‐Okhotsk and Eastern Sikhote‐Alin active continental margins, respectively. These stages of the comparatively slow growth of the continent were broken by stages of rapid growth and drastic changes in the shape of the continent, since at these stages large terranes of various tectonic nature collided with active continental margins. At the end of the Early‐Middle Jurassic, the Bureya terranes collided with the Mongol‐Okhotsk active margin, and at the beginning of the Late Cretaceous there was collision of the Central and Southern Sikhote‐Alin terranes with the Khingan‐Okhotsk active margin.Collision‐related structural styles in all cases are indicative of oblique collision and great strike‐slip motions along the main sutures. The peculiarities of the terrane's geological structure show that prior to collision with the Mongol‐Okhotsk and Khingan‐Okhotsk active margins, they had already accreted to Asia and then migrated along its margins along the strike‐slip faults. The Bureya terranes were squeezed out of the compression zone between Siberia and North China. This compression zone originated after the Paleozoic oceans which divided these cratons had closed. The Khanka terranes and Mesozoic accretionary wedge terranes of the Sikhote‐Alin shifted along the strike‐slip faults subparallel to the Asia Pacific margin. Strike‐slip motions resulted in duplication of the primary tectonic zonation.

Speculations on the Mesozoic accretion of Asia by microblock collisions and wrench movement : Faure, Michel, 1987 C. r. Acad. Sci., Paris, (Sér. II)304(2):93–98. (In French, English abstract.)

Speculations on the Mesozoic accretion of Asia by microblock collisions and wrench movement : Faure, Michel, 1987 C. r. Acad. Sci., Paris, (Sér. II)304(2):93–98. (In French, English abstract.)

Comment and Reply on ‘Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland’

Comment and Reply on ‘Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland’

Comment and Reply on ‘Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland’

Comment and Reply on ‘Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland’

Mesozoic Accretionary Tectonics of Alaska: ABSTRACT

Most of Alaska represents an enormous mosaic of allochthonous tectono-stratigraphic terranes, each characterized by a distinctive pre-Tertiary stratigraphic and tectonic history. More than 40 terranes presently are recognized, ranging in size from thousands of square kilometers to unique tectonic blocks having outcrop areas of only a few square kilometers. With a single exception, all of these pre-Tertiary terranes evidently are allochthonous with respect to North America and to one another. Some are best interpreted as displaced parts of the continental margin, but others--particularly in southern Alaska--may be exotic to North America. Paleomagnetic studies show that some terranes such as Wrangellia, have been transported as much as 3,000 km. Amalgamation of different terranes prior to final emplacement can be documented or inferred in a few places, but most of Alaska was tectonically assembled from individual lithosphere fragments and microplates during late Mesozoic time by complex accretion. Various schemes have been proposed for rotation and/or offset of northern Alaska into its present position. In south-central Alaska, southwest-trending terranes generally parallel the present-day Aleutian trench system. Outcrop patterns of rocks within these terranes point to mainly convergent late Mesozoic accretionary tectonics involving the development of nappes. Subsequent strike-slip faulting of these terranes is relatively minor, but original Mesozoic accretion of these belts may have involved large-scale transform displacemen along the northwest-trending parts of the Tintina trench and ancestral Denali fault systems. Significant tectonic displacement continues only along the active strike-slip faults in southeast Alaska and accretion is limited to the mechanically related Aleutian trench system. End_of_Article - Last_Page 784------------