Identification And Tectonic Implications Of The Neoarchean Alpine-Style Nappes In The Central Orogenic Belt,North China Craton

Whether modern-style plate tectonics operated on early Earth is debated due to a paucity of definitive records of large-scale plate convergence,subduction,and collision in the Archean geological record.The Archean Zanhuang Alpine-style sub-horizontal fold/thrust nappes of the Central Orogenic Belt in the North China Craton represents some of the best geological evidence yet described from an Archean orogen that clearly documents large-scale horizontal plate-scale translations in the Archean.The fold/thrust nappe stack is a tectonic analogue to structures formed in the modern plate tectonic regime.Principles of cross-section construction were utilized to construct a detailed structural profile of the Zanhuang fold nappe/thrust belt.Combined with a systematic structural analysis,tectono-stratigraphic restoration,petrologic,geochronological and geochemical studies,this dissertation aims to provide an important constraint on Archean orogenic evolution,and insights into the tectonic style and geodynamics of the lithosphere in the Archean.The Zanhuang fold nappe/thrust belt,or nappe-stack,is located along the tectonic boundary of the middle section of the Central Orogenic Belt and the Eastern Block of the craton.The nappe stack is bounded on the northwest by an Archean tectonic mélange,and on its southeastern margin it is thrust over a passive margin-like sequence that rests unconformably over basement gneisses of the Eastern Block.Based on 1:1000scale mapping,a 10km,northwest-southeast oriented structural and lithological cross section was constructed across the entire Zanhuang fold nappe/thrust belt.Establishing first-order geometric relationships and field-based anatomy enabled us to subdivide the nappe stack into（1）Black Rock Temple nappe;（2）Zhaimen shear zone,containing a giant sheath-folded nappe;（3）Buddha décollement nappe;and（4）passive margin para-autochthon and autochthon,where individual sub-horizontal nappes are stacked upon each other.With a similar scale and first-order geometry,the Zanhuang fold nappe/thrust belt is geometrically similar to the Alpine fold nappe system.Both orogens and their structures have similar scales,durations of development,rock types,and are characterized by the high-degree of internal strain and non-coaxial deformation with strain localization.The uppermost nappe,named the Black Rock Temple nappe,is comprised of metasedimentary rocks,the protolith of which includes greywacke and interbedded pelite,and metabasite rock of picritic-boninitic composition.Detrital zircons from the metagraywacke sample indicate the dominant source area for detritus supplying Black Rock Temple nappe was circa 2692 Ma old,and yielded a maximum depositional age of2502.3±10.18 Ma using the youngest single cluster method overlapping at 2σuncertainty（YC2σ）.The Zhaimen shear zone,structurally underlying the Black Rock Temple nappe,consists of～1-km-long（parallel to the X-axis）sub-horizontal sheath folded nappe,surrounded by other highly-sheared units.Giant sheath folds are a rare and significant geologic phenomenon,typically associated with high shear strains from large displacements encumbered during emplacement of nappe systems in Phanerozoic orogens,such as that in the Monte Rosa nappe in the Alpine orogen.The giant sheath-folded nappe in the Zhaimen shear zone is comprised of metavolcanic rocks（with pillowed structure）,meta-clastic rocks including sandstone and magnetite quartzite,and a schistose mélange unit with tuffaceous clasts and carbonaceous blocks.The detrital zircons demonstrate that the dominant source area for detritus was of2520-2539 Ma in age,and yielded YC2σof 2507±4.67 Ma to 2535.5±5.66 Ma,indicating the maximum depositional age.The basal nappe is the Buddha décollement nappe,which contains metasedimentary rocks,the protolith of which includes greywacke,pelite and chert,and tholeiitic metabasite rocks.The detrital zircons demonstrate the dominant source area for detritus was of 2654-2702 Ma,and yielded YC2σof 2564±5.59 Ma to 2592.4±8.89 Ma,indicating the maximum depositional age.The para-autochthon upon which the nappe stack was thrust contains a metamorphosed coastal clastic silicious stratigraphic sequence of sandstones,siltstones,pelites,conglomerates,and carbonate-shales that were deposited on a shallow marine platform resting unconformably on a crystalline basement,indicative of passive continental margin-like depositional setting.The detrital zircons demonstrate that the dominant source area of the detritus was of 2502-2522 Ma,and yielded YC2σof 2494.5±8.95 Ma to 2516±3.18 Ma,indicating the maximum depositional age.The Zanhuang fold nappe/thrust belt also preserves records of～2.7 Ga mafic magmatism.In the Black Rock Temple nappe,metabasites of the picrite-boninite series are present,the metamorphic assemblage of which contain large grains of hornblende with minor plagioclase and epidote,indicative of amphibolite facies.The generation of picrite-boninite association was constrained to be circa 2692±35 Ma by cross-cuting trondhjemitic dikes.Their geochemistry showed high Mg O（12.90–15.19 wt%）,SiO₂（48.08–54.42 wt%）,and low Ti O₂（0.41–0.48 wt%）.They possess high Mg^#（60.9–75.4）,Cr（963–1450 ppm）that indicate compositional similarities with primary melt;the（Gd/Yb）_cn vs.（Zr/Sm）_pm,and Al₂O₃/Ti O₂ vs.（Zr/Sm）_pmplots corroborated its picrite-boninite composition.In the Buddha décollement nappes,the tholeiites are dark-green,garnet porphyroblast-bearing foliated amphibolites,with a medium-grained prismatic blastic texture.yielding a crystallization age of 2698±30 Ma.The metabasite has a tholeiitic affinity.The tholeiitic metabasites display lower Mg O（6.97–9.66 wt.%）Ni（5.34–90.4 ppm）and Cr（6.71–169 ppm）,contents than the mantle-derived primary melts,suggesting that their precursor magmas are products of crystal differentiation to various degrees.The trace and rare earth elements of the picrite-boninite-tholeiite metabasites from Black Rock Temple nappe and Buddha décollement nappes present various degrees of enriched LILEs and LREEs and show remarkable Nb-Ta negative anomalies,suggesting that their magma sources are lithospheric mantle together with an enriched asthenospheric mantle modified by subduction-related fluids.The picrite-boninite-tholeiitic basalt association is interpreted as a SSZ-affinity subduction-initiation sequence formed in an early Neoarchean forearc setting,based on the study of the mineral petrogenesis and using tectonic discrimination diagrams.Geometric and statistical analysis of structural-fabrics and kinematic indicators illustrate that the fold/thrust nappes of forearc and oceanic floor sequences have been reworked by folding and thrusting with stratigraphic repetition during three deformation phases,resulting in the present geometry of the belt.Composite sheath folds and the giant sheath fold are associated with ductile shear zones between the different stacked nappes and the Buddha décollement,indicative of strain localization.Given that large shear strains（γ≥10）are required by the simple shear mechanism for sheath fold generation,the development of the mega-scale giant sheath fold（≥1 km）corresponds to no less than 10 km of sub-horizontal displacement in this Archean nappe stack.These structures demonstrate that the large-scale displacement of the Zanhuang fold nappe/thrust belt was accomplished by a combination of rigid body translation along shear zones between nappes and the para-authochthon,as well as internal strain by ductile deformation.The rigid body translation is illustrated by the dominance of nappe gliding on the decollement surface for the crustal-scale tectonic transportation.The internal strain related displacement is illustrated by the high-strain recumbent giant sheath-fold.This work explicitly unravels that the forearc-derived nappe system glided on the decollement surface for the crustal-scale tectonic transportation,while subduction continued and the arc accreted eastward to the older eastern continent,finally being emplaced over a passive margin-like shallow marine platform at 2.52-2.50 Ga.The emplacement age is constrained by a combination of crosscutting relationships and the detrital zircon age from the tectonite“schistés lustrés”that marks the shear zone at the base of the Buddha nappe.The circa 2698±30 Ma picrite-boninite and tholeiite units mark the age of subduction initiation,and this age correlates roughly with the age of the oldest known magmatism in the associated Wutai/Fuping arc.Detrital zircons from the major decollement constrains that the nappes were emplaced over the passive margin at2520 Ma,corresponding to the end of subduction and final collision.These data show that the life cycle of the subduction zone and ocean spanned circa 178 million years;conservative plate velocities of 2 cm/year yield a lateral transport distance of subducted oceanic crust of 3560 kilometers.Comparison of the lateral displacements and structural history of the Archean Zanhuang nappes with those of the Swiss Alps reveals remarkable similarities of the orogenic scales,principal tectonic components,the first-order geometry,and the tectono-structural evolution.The similar architecture,including sub-horizontal nappe stacks,and structural history of many orogenic belts from the Archean to Cenozoic implies that Archean orogenic length scales,the duration of orogenic events,orogenic strain rates,crustal rheology,and the fundamental processes of tectonic evolution have remained relatively constant for the past 2.7 Ga.These comparable relationships provide evidences that the modern plate tectonic paradigm is applicable to the Archean,and they support the notion of tectonic uniformitarianism.