Anatomy of a subduction complex: architecture of the Franciscan Complex,California, at multiple length and time scales

The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.     

新疆伊吾阿勒吞昆多蛇绿混杂岩特征

前人将唐巴勒-卡拉麦里缝合线置于伊吾县以南哈尔里克山塔水河一线。1993年巴瑞士等在伊吾西北大黑山发现超镁铁岩-镁铁岩洋壳残片,认为卡拉麦里蛇绿岩从伊吾西北大黑山一带向东延伸。2009年笔者参与伊吾县阿尔通盖一带1∶5万区域地质矿产调查工作,于伊吾县以北阿勒吞昆多一带发现阿勒吞昆多蛇绿混杂岩,该蛇绿岩中斜长花岗岩形成于早石炭世((351±6)Ma),大地构造为哈萨克斯坦-准噶尔板块及准噶尔微板块、唐巴勒-卡拉麦里古生代沟弧体系。在研究前人资料基础上,提出阿勒吞昆多蛇绿混杂岩为卡拉麦里蛇绿岩(属唐巴勒-卡拉麦里缝合线)向东延伸部分,唐巴勒-卡拉麦里缝合线(沟弧体系)西起唐巴勒,向东经达拉布特-卡拉麦里-塔克扎勒-大黑山至伊吾县阿勒吞昆多一带,可能延伸至蒙古境内。     

内蒙古乌斯尼黑蛇绿混杂岩带形成时代的地质新证据

贺根山-乌斯尼黑蛇绿混杂岩带的属性和形成时代存在明显的认识上的分歧.早期认为是二叠纪侵入的超镁铁质岩体.该超镁铁质岩体侵入了下二叠统格根敖包组,并被哲斯组地层不整合覆盖,形成时代为二叠纪.20世纪80年代以来,通过中国北方板块构造研究,在硅质岩中采集到的一系列化石资料表明,贺根山地区蛇绿混杂岩的形成时代应为中-晚泥盆世...     

内蒙古北山地区百合山蛇绿混杂岩带的厘定及其洋盆俯冲极性——基于1∶5万清河沟幅地质图的新认识

造山带内蛇绿混杂岩带结构与组成的精细研究可为古板块构造格局重建和古洋盆演化提供最直接证据。北山造山带内存在多条蛇绿混杂岩带,记录了古亚洲洋古生代以来的俯冲和闭合过程,然而其大地构造演化长期存在争议。红石山—百合山蛇绿混杂岩带位于北山造山带北部,主要由蛇绿(混杂)岩和增生杂岩组成,具典型的"块体裹夹于基质"的混杂岩结构特征,发育紧闭褶皱、无根褶皱、透入性面理和双重逆冲构造。蛇绿混杂岩带中岩块主要由超镁铁质-镁铁质岩(变质橄榄岩、辉石橄榄岩、异剥辉石岩、蛇纹岩)、辉长岩、玄武岩、斜长花岗岩、硅质岩等洋壳残块以及奥陶纪火山岩、灰岩等外来岩块组成,基质则主要为蛇纹岩、砂板岩及少量的绿帘绿泥片岩;在蛇绿混杂岩带北侧发育有台地相灰岩与深水浊积岩组成的沉积混杂块体,具滑塌堆积特征。蛇绿混杂岩带内发育三期构造变形,前两期为中深构造层次下形成的透入性变形,第三期为浅表层次的脆性变形,未形成区域性面理。空间上,由增生杂岩和蛇绿(混杂)岩组成的百合山蛇绿混杂岩带共同仰冲于绿条山组浊积岩之上,具有与红石山地区蛇绿混杂岩带相似的岩石组成、构造变形和时空结构特征。百合山蛇绿混杂岩带南侧发育同期的明水岩浆弧,由晚石炭世石英闪长岩-花岗闪长岩-二长花岗岩以及白山组岛弧火山岩组成,其与百合山蛇绿混杂岩带共同构成了北山造山带北部石炭—二叠纪的沟-弧体系,指示了红石山—百合山洋盆向南俯冲的极性。     

Sedimentary serpentinite and chaotic units of the lower Great Valley Group forearc basin deposits,California: updates on distribution and characteristics

ABSTRACT

Sedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.     

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen,northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.     

北山造山带白云山地区蛇绿混杂岩结构、组成特征与形成时代

红柳河-洗肠井蛇绿混杂岩带位于内蒙古北山造山带中部,呈北西西向展布,向东延伸至白云山-洗肠井蛇绿混杂岩带,向西延伸至牛圈子-红柳园蛇绿混杂岩带。在白云山地区,蛇绿混杂岩由不同类型的岩块与基质组成,基质具有分带性,南侧以砂板岩基质为主,北侧以蛇纹岩基质及绿泥钠长片岩基质为主,反映了俯冲增生杂岩的特征。其中基性熔岩大面积发育,岩性以绿泥钠长片岩及变质玄武岩为主,TAS及AFM图解显示基性熔岩具有拉斑玄武岩的特征,微量元素蛛网图中具有弱Nb-Ta负异常及弱Sr正异常,稀土元素配分曲线显示了轻重稀土基本未分馏的曲线型式,微量元素系列判别图解显示了类似于MORB-like玄武岩的地球化学特征。由此可见,白云山地区基性熔岩为MORB-like玄武岩,反映了蛇绿混杂岩形成于弧前的构造背景。此外,本次工作获得侵入基性熔岩中的斜长花岗岩锆石²⁰⁶Pb/²³⁸U加权平均年龄为519.8±2.1Ma,ε_Hf（t）-t图解显示了斜长花岗岩为地幔分异的产物。微量元素蛛网图中Nb-Ta亏损、右倾的稀土配分曲线及Th/Yb-Nb/Yb图解反映了斜长花岗岩具有岛弧岩浆岩的特征。对比研究区晚寒武世岛弧钙碱性辉长岩,我们认为白云山地区早寒武世发育MORB-like玄武岩,中-晚寒武世为岛弧钙碱性辉长岩及斜长花岗岩,反映了初始俯冲-正常俯冲的岩浆作用。     

Serpentinites and serpentinites: Variety of origins and emplacement mechanisms of serpentinite bodies in the California Cordillera

Most serpentinitized peridotite in orogenic belts is derived from oceanic lithosphere, but the emplacement mechanisms of these rocks vary greatly, as illustrated by the nature of these rock bodies and their contacts. The diverse emplacement mechanisms have important implications for connecting ophiolitic rock occurrences to large‐scale orogenic processes. In the California Cordillera, the largest bodies of ultramafic rocks are parts of ophiolite sheets, such as the Coast Range ophiolite (CRO), that were part of the upper plate of an oceanic subduction system. Such units differ from smaller bodies within subduction complexes such as the Franciscan Complex that were transferred from the subducting plate to the subduction complex during accretion. Some intra‐subduction complex ultramafic rocks occur as nearly block‐free sheets within the Franciscan Complex, and as a part of mafic–ultramafic imbricates or broken formations within the Shoo Fly Complex of the northern Sierra Nevada. Franciscan Complex serpentinite also occurs as sedimentary serpentinite mélange that was partly subducted after deposition in the trench via submarine sliding. Such mélanges include blocks that record older and higher grade metamorphism than the matrix. Sedimentary serpentinite mélange that includes high‐pressure metamorphic blocks is also found in the basal Great Valley Group forearc basin deposits depositionally overlie the CRO. Distinguishing the different serpentinite origins is difficult in the California Cordillera even though a terminal continental collision did not affect this orogenic belt. In more typical orogenic belts with greater post‐subduction disruption, distinction between the types of serpentinite occurrences presents a greater challenge.     

The metamorphic history of rocks buried,accreted and exhumed in an accretionary prism: an example from the Otago Schist,New Zealand

Although subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low‐grade metamorphic assemblages. In this study a multi‐technique thermobarometric study of the Chrystalls Beach Complex mélange, which is located within the Otago Schist accretion‐collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench‐fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo‐Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions in the range 400–550 MPa and 250–300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b₀ spacing analyses of white mica. Rare lawsonite occurs in a post‐foliation vein, and illite ‘crystallinity’ measurements indicate a thermal overprint during exhumation. These P–T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P–T path. Based on variability in P–T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher‐T than experienced along the prograde path. Observed subduction‐related subgreenschist assemblages are therefore likely to indicate lower‐P than experienced during subduction, as higher‐P mineral compositions re‐equilibrate during exhumation. The P–T path inferred in this study is similar in shape to P–T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.     

Trace-element geochemistry of transform-fault serpentinite in high-pressure subduction mélanges (eastern Cuba): implications for subduction initiation

The Sierra del Convento and La Corea mélanges (eastern Cuba) are vestiges of a Cretaceous subduction channel in the Caribbean realm. Both mélanges contain blocks of oceanic crust and serpentinite subducted to high pressure within a serpentinite matrix. The bulk composition of serpentinite indicates spinel-harzburgite and -herzolite protoliths. The samples preserve fertile protolith signatures that suggest low melting degrees. High concentration of immobile elements Zr, Th, Nb, and REE contents (from ~0.1 to ~2 CI-chondrite) point to early melt–rock interaction processes before serpentinization took place. Major- and trace-element compositions suggest an oceanic fracture-zone–transform-fault setting. A mild negative Eu anomaly in most samples indicates low-temperature fluid–rock interaction as a likely consequence of seawater infiltration during oceanic serpentinization. A second, more important, serpentinization stage is related to enrichment in U, Pb, Cs, Ba, and Sr due to the infiltration of slab-derived fluids. The mineral assemblages are mainly formed by antigorite, lizardite, and chlorite, with local minor talc, tremolite, anthophyllite, dolomite, brucite, and relict orthopyroxene. The local presence of anthophyllite and the replacements of lizardite by antigorite indicate a metamorphic evolution from the cooling of peridotite/serpentinite at the oceanic context to mild heating and compression in a subduction setting. We propose that serpentinites formed at an oceanic transform-fault setting that was the locus of subduction initiation of the Proto-Caribbean basin below the Caribbean plate during early Cretaceous times. Onset of subduction at the fracture zone allowed the preservation of abyssal transform-fault serpentinites at the upper plate, whereas limited downward drag during mature subduction placed the rocks in the subduction channel where they tectonically mixed with the upward-migrating accreted block of the subducted Proto-Caribbean oceanic crust. Hence, we suggest that relatively fertile serpentinites of high-pressure mélanges were witness to the onset of subduction at an oceanic transform-fault setting.