Early Paleozoic North Research Articles

A Late Cambrian Continental Convergent Margin in the North Qilian Orogenic Belt, Northwestern China: Geochemical and Geochronological Evidence from Hongtugou Mafic Rocks

The tectonic setting and subduction polarity of the early Paleozoic North Qilian Orogenic Belt (NQOB) in northwestern China is poorly constrained due to complex tectonic deformation. Mafic and ultramafic rocks in the South Ophiolite Belt of the NQOB are interpreted to be middle ocean ridge ophiolite or suprasubdcution zone ophiolite. To address this, we have conducted geochemical and geochronological investigations of the mafic rock sequence (cumulate gabbros, diabases, isotropic gabbros, and basalts) in Hongtugou in the South Ophiolite Belt. Trace element characteristics of the pillow basalts and the isotropic gabbros with enrichment of Th and La relative to Nb on the N-MORB normalized multi-element diagram are consistent with a suprasubduction setting, where similarities with the Panamanian proto-arc rocks suggest they formed shortly after subduction initiation. Major element modelling for cumulate gabbros and basalts indicates the hydrous condition of crystallization which further supports a suprasubduction setting. The Proterozoic zircon crystals captured in a cumulate gabbro and a diabase suggest this suprasubduction zone is a continental convergent margin. A weighted mean zircon SHRIMP age of 507 ± 6 Ma from an isotropic gabbro is consistent with crystallization ages of other mafic rocks in this belt. This suggests the North Qilian oceanic lithosphere subducted beneath the continent in the late Cambrian. Mafic rocks in this study along with the serpentinized peridotite do not fall into the category of ophiolite, despite displaying an ophiolite sequence.

Hydrothermal systems with radiogenic Sr in the North Qaidam ultrahigh-pressure metamorphic belt, NE Tibetan Plateau and implications for regional dissolved Sr budget

Hydrothermal systems, including hot springs and hydrothermal calcite, are common features of orogenic belts. The hot springs and hydrothermal calcite in the Himalayas are characterized by radiogenic Sr related to metamorphic processes. Such metamorphism-driven processes have contributed significant amounts of radiogenic Sr to rivers and groundwater, which has affected the evolution of global seawater 87Sr/86Sr during the Cenozoic. However, whether such hydrothermal systems with highly radiogenic Sr occur in other continental collisional belts with intense metamorphism remains unknown, which hinders a better understanding of regional and global Sr cycles. Compiled 87Sr/86Sr ratios in surface water around the Tibetan Plateau show that the Qilian-North Qaidam region displays a higher 87Sr/86Sr ratio than other regions, except for the Himalayas. Here, we report 87Sr/86Sr ratios of a hydrothermal system on the northern margin of the early Paleozoic North Qaidam ultrahigh-pressure metamorphic belt, NE Tibetan Plateau. The high 87Sr/86Sr ratios (0.757–0.734) of the hot spring water and the acetic acid leachates of the surrounding rocks (gneiss and schist) with other isotopic tracers (e.g., oxygen, hydrogen, lithium, and boron isotopes) indicate that 1) Sr was exchanged during metamorphism between carbonate and silicate phases in crustal rocks and the high 87Sr/86Sr ratio of the hot spring water was acquired from a deep water-rock reaction process; and 2) hydrothermal systems around the Qilian-North Qaidam region operate like those in the Himalayas. Mass balance calculations indicate that the metamorphism-forced hydrothermal system plays a key role in delivering radiogenic Sr to rivers and lakes and yielding higher 87Sr/86Sr ratios in surface water around the Qilian Shan region than in the surrounding areas. The reactivated early Paleozoic Qilian orogenic belts with ultrahigh-pressure metamorphism caused by the India-Asia collision have significantly impacted the regional water 87Sr/86Sr ratio, suggesting that the reactivated old orogenic belts with intense metamorphic processes formed by continental subduction/collision can play a pivotal role in regulating past seawater 87Sr/86Sr evolution.

Melt‐Fluxed Melting of the Heterogeneously Mixed Lower Arc Crust: A Case Study from the Qinling Orogenic Belt, Central China

AbstractAdakitic rocks with high MgO content and lower crust affinity at convergent margins provide important constraints on arc crust evolution and regional tectonics. An integrated study on the only adakitic pluton with high MgO content and its mafic microgranular enclaves (MMEs) in the Early Paleozoic North Qinling arc terrane was conducted in this paper. Zircons from 2 host rock samples yielded identical weighted mean ages of 428 Ma. The MMEs contain two types of zircons: type 1 zircons share similar morphology, chemical composition, and indistinguishable age with those from the host rock; type 2 zircons yielded an older age of 450 Ma. The similarity in zircon Hf isotopic composition for these zircons indicates a cognate origin of the MMEs. However, the lower Ce4+/Ce3+, higher crystallization temperature and the LREE overabundance of type 2 zircons indicate that mafic intrusions in the lower arc crust were subjected to a post‐magmatic event at ∼450 Ma. Samples from the Tangzang pluton are characterized by typical adakitic geochemical features, and high MgO (Mg#: 50–52), Cr, Ni, and Ba contents. Considering their similar isotopic compositions with the Neoproterozoic metabasites in the North Qinling terrane, we argue that the Tangzang pluton was generated by crustal anatexis of two sources with contributions from the metasomatized mantle wedge. The mantle connection was achieved by aqueous melt‐fluxed melting of a “hot zone” in the lower arc crust. This implies that andesitic continental crust with Mg# of ∼0.5 may also be associated with fluxed‐melting of the lower arc crust.

Timing of amalgamation of the Alxa Block and the North China Block: Constraints based on detrital zircon U–Pb ages and sedimentologic and structural evidence

Detrital zircon LA-ICP-MS U–Pb ages of the Paleozoic strata in the southeastern Alxa Block indicate that the primary provenance did not change between the Devonian and the Early Carboniferous. Except for a sample from the Cambrian strata, the Early Paleozoic North Qilian Orogenic Belt may have been a major source of Paleozoic sediments in the study area. The main source of Lower–Middle Devonian sediments was the Early Paleozoic North Qilian Orogenic Belt. The source of the Upper Devonian sediments was the Alxa Block, while the North China Block was a minor contributor. The deformation of the Cambrian and Devonian Systems in the southeastern Alxa Block indicates that a strong east–west compression event occurred in the study area before the Early Carboniferous. The paleocurrents of the Upper Devonian in the southeastern Alxa Block indicate that the source was located to the north and was not the North Qilian Orogenic Belt to the south. Moreover, the deposition of the Upper Devonian in the southeastern Alxa Block was a response to a strong deformation event that occurred along the eastern boundary of the Alxa Block during the Late Devonian and Early Carboniferous. The North China Block became the primary source during the Early Carboniferous, and the Alxa Block was a minor source. A regional stratigraphic comparison also indicates that similar depositional environments were present until the Carboniferous and Permian on the Alxa Block and North China Block. All of these data indicate that amalgamation of the Alxa Block and the North China Block occurred between the Late Devonian and the Early Carboniferous.

Late Paleozoic subduction–accretion along the southern margin of the North Qinling terrane, central China: Evidence from zircon U-Pb dating and geochemistry of the Wuguan Complex

The Qinling Orogen separating the North China plate from the Yangtze plate is a key area for understanding the timing and process of aggregation between the two plates. Two competing and highly contrasting tectonic models currently exist to explain the timing and nature of collision; one advocates a Devonian continental collision while the other favors a Triassic collision. The Wuguan Complex, between the early Paleozoic North Qinling and the Mesozoic South Qinling terranes, can provide important constraints on the late Paleozoic evolutionary processes of the Qinling Orogen. Metamorphosed sedimentary rock of the Wuguan Complex have a detrital zircon age spectrum with two major peaks at 453Ma and 800Ma, several minor age populations of 350–430Ma and 1000–2868Ma, and a youngest weighted mean age of 358±3Ma, indicating a mixed source from the North Qinling terrane. The recrystallized zircons yield a weighted mean age of 333±2Ma, representing the metamorphic age. Geochemical analyses imply that the sedimentary rocks were originally deposited in an active continental margin dominated by an acidic-arc source with a subordinate mafic-ultramafic source. The youngest population of detrital zircons (358Ma) suggests that the Wuguan Complex developed as forearc basin along the southern accreted margin of the North Qinling terrane during the early Carboniferous, whereas the ca. 520–460Ma mafic rocks with E-MORB, N-MORB, OIB or island arc basalt signatures probably derived from the Danfeng Group. In combination with regional data, we suggest that the depositional age of the Wuguan Complex is ca. 389–330Ma, but it was subsequently incorporated into tectonic mélange by the northward subduction of the Paleo-Qinling Ocean. A long-lived southward-facing subduction-accretionary system in front of the North Qinling terrane probably lasted until at least the early Carboniferous.

U–Pb zircon ages and geochemistry of the Wuguan complex in the Qinling orogen, central China: Implications for the late Paleozoic tectonic evolution between the Sino-Korean and Yangtze cratons

The tectonic evolution of the Qinling orogen, central China, is the key to understanding the assembly of the Sino-Korean and Yangtze cratons. The Wuguan complex, between the early Paleozoic North Qinling and Mesozoic South Qinling tectonic belts, can provide important constraints on the late Paleozoic evolutionary processes in the Qinling orogen. U–Pb zircon analyses, using laser ablation-multicollector-inductively coupled plasma-mass spectrometry, reveal protolith ages of 446±2Ma for a garnet amphibolite, 368±3Ma for a meta-andesite, and 351±2Ma for a mylonitized granitic dike from the Wuguan complex. Elemental geochemistry indicates typical island arc affinities for all the above rocks, but some amphibolites of unknown age have E-MORB signatures. Detrital zircons from a metaquartzite have an age spectrum with a major peak at 462Ma, two subordinate peaks at 828 and 446Ma, and a youngest weighted mean age of 423±5Ma. This suggests that at least some of metasedimentary rocks from the Wuguan complex belong to the part of the Devonian turbidite sequence of the Liuling Group, which was deposited in a fore-arc basin along the southern accreted margin of the Sino-Korean craton, whereas the late Ordovician precursors of the amphibolite might be derived from the Danfeng Group. The occurrence of late Devonian–early Carboniferous arc-related rocks in the Wuguan complex implies penecontemporaneous oceanic subduction, and therefore the Paleo-Qinling Ocean was not finally closed until the early Carboniferous. On the other hand, metamorphic zircon grains from two amphibolites yielded ages of 321±2 and 318±3Ma. Hence, the Wuguan complex in the Qinling orogen and the Guishan complex in the Tongbai orogen constitute a medium-pressure Carboniferous metamorphic belt that is more than 500km long, and which was formed in the hanging wall of a subduction zone.

Adakitic (tonalitic-trondhjemitic) magmas resulting from eclogite decompression and dehydration melting during exhumation in response to continental collision

Modern adakite or adakitic rocks are thought to result from partial melting of younger and thus warmer subducting ocean crust in subduction zones, with the melt interacting with or without mantle wedge peridotite during ascent, or from melting of thickened mafic lower crust. Here we show that adakitic (tonalitic-trondhjemitic) melts can also be produced by eclogite decompression during exhumation of subducted and metamorphosed oceanic/continental crust in response to continental collision, as exemplified by the adakitic rocks genetically associated with the early Paleozoic North Qaidam ultra-high pressure metamorphic (UHPM) belt on the northern margin of the Greater Tibetan Plateau. We present field evidence for partial melting of eclogite and its products, including adakitic melt, volumetrically significant plutons evolved from the melt, cumulate rocks precipitated from the melt, and associated granulitic residues. This “adakitic assemblage” records a clear progression from eclogite decompression and heating to partial melting, to melt fractionation and ascent/percolation in response to exhumation of the UHPM package. The garnetite and garnet-rich layers in the adakitic assemblage are of cumulate origin from the adakitic melt at high pressure, and accommodate much of the Nb–Ta–Ti. Zircon SHRIMP U–Pb dating shows that partial melting of the eclogite took place at ∼435–410Ma, which postdates the seafloor subduction (>440Ma) and temporally overlaps the UHPM (∼440–425Ma). While the geological context and the timing of adakite melt formation we observe differ from the prevailing models, our observations and documentations demonstrate that eclogite melting during UHPM exhumation may be important in contributing to crustal growth.

Detrital zircon record of continental collision: Assembly of the Qilian Orogen, China

The Qilian Orogen, Northwest China, preserves a record of Early to Middle Paleozoic subduction and collision between the Central Qilian Block and North China Craton through closure of the intervening Late Neoproterozoic to Early Paleozoic North Qilian Ocean. Ordovician strata in the Hexi-Corridor Basin document the initial collisional history with a change in tectonic setting from retro-arc to foreland basin. Ages of zircons from the Upper Ordovician strata range from Archean to early Paleozoic with prominent peaks at around 2.5 Ga, 1.6 Ga, 1.0 Ga, and 0.47 Ga. The early Paleozoic ages are consistent with the time of magmatic arc activity related to northward subduction of the Qilian Ocean. Precambrian grains are restricted to the upper part of the sampled sections, with the proportion of Paleozoic grains descending up the section. This trend is interpreted to reflect the gradual termination of subduction zone magmatism and the swamping of the basin by material from the colliding Central Qilian Block and North China Craton. Archean and the early Proterozoic detritus were likely derived from the North China Craton to the north, whereas the Central Qilian Block to the south provided the late Proterozoic grains. Silurian and Devonian strata within the Hexi-Corridor Basin continue to show this mixed source provenance with depositional environment changing from a flysch to molasse setting.

Granitic Magmatism in the North Qaidam Early Paleozoic Ultrahigh-Pressure Metamorphic Belt, Northwest China

Three suites of granitic rocks are present in the early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt, which has eclogite ages of 466-495 Ma. The first suite consists of quartz monzodiorite + granodiorite + monzogranite, the second of monzogranite + two-mica granite + muscovite granite + syenogranite, and the third of granodiorite + monzogranite + biotite granite. The country rocks into which these bodies were intruded consist chiefly of medium- and high-grade Proterozoic gneisses and schists and Paleozoic sedimentary and volcanic rocks. Monzogranite of the first suite has a zircon SHRIMP age of 473 Ma, whereas syenogranite in the second suite has an age of 446 Ma, and granodiorite in the third suite has an age of 397 Ma. Rocks of the first and third suites have similar mineralogies consisting of plagioclase, hornblende, quartz, alkali feldspar, and biotite, whereas those of the second suite consist of potassium feldspar, quartz, muscovite, biotite, and plagioclase. Rocks of the first and third suites have I-type compositions with SiO2 = 61-69 wt%, Na2O/K2O >1, ANK <1, and Eu/Eu* = 0.7-1.0, whereas the second suite consists of S-type granites, with SiO2 = 70-76 wt%, Na2O/K2O <1, ANK >1, and Eu/Eu* = 0.1-0.3. The first suite is interpreted as island arc or active continental margin magmatism, the second as igneous activity related to collision between the Qaidam and Qilian blocks, and the third as post-collisional plutonic activity.

Comparison of some sediment-hosted, stratiform barite deposits in China, the United States, and India

Shifts in world barite production since the 1980s have resulted in China becoming the world's largest barite-producing country followed by the US and India. Most barite produced for use in drilling fluids is derived from black shale- and chert-hosted, stratiform marine deposits. In China, Late Proterozoic to Early Cambrian marine barite deposits occur on the oceanic margins of the Yangtze platform, in the Qinling region in the north and the Jiangnan region in the south. Most US ore-grade deposits are in the Nevada barite belt; most commercial deposits occur in Ordovician and Devonian marine rocks along the western margin of the early Paleozoic North American continent. Production in India is predominantly from a single Middle Proterozoic deposit in a sedimentary basin located on Archean basement in Andrah Pradesh. The geologic and geochemical characteristics of the deposits are consistent with origins from a variety of sedimentary-exhalative processes, with biogenic processes contributing to the concentration of some seafloor barite. Linear distributions of clusters of lenticular deposits suggest a geographic relationship to syndepositional seafloor fault zones. Sulfur isotope data of the barite deposits range from values that are similar to coeval seawater sulfate to significantly higher δ 34S values. Strontium isotope values of continental-margin-type deposits in Nevada and China are less radiogenic than those of cratonic-rift deposits (e.g. Meggen and Rammelsberg). Comparison of La n /Ce n ratios of barite in the Qinling region of China with marine chert ratios suggests a relationship to hydrothermal fluids, whereas ratios from the Jiangnan region and Nevada can be interpreted as reflecting a biogenic influence. The California Borderland provides a potential modern analog where hydrothermal barium is being deposited on the seafloor in fault-block-bounded basins. Anoxic to dysaerobic conditions on some marine basin floors result from upwelling, nutrient-rich currents and high productivity in surface waters. In this setting, biogenic processes could contribute to the concentration of barium from hydrothermal sources.

Provenance of Paleozoic mudstones in a contact metamorphic aureole determined by rare earth element, Th, and Sc analyses, Sierra Nevada, California

Research Article| April 01, 1993 Provenance of Paleozoic mudstones in a contact metamorphic aureole determined by rare earth element, Th, and Sc analyses, Sierra Nevada, California Gary H. Girty; Gary H. Girty 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar Andrew D. Hanson; Andrew D. Hanson 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar Aaron S. Yoshinobu; Aaron S. Yoshinobu 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar Charles Knaack; Charles Knaack 2Department of Geology, Washington State University, Pullman, Washington 99164 Search for other works by this author on: GSW Google Scholar Diane Johnson Diane Johnson 2Department of Geology, Washington State University, Pullman, Washington 99164 Search for other works by this author on: GSW Google Scholar Author and Article Information Gary H. Girty 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Andrew D. Hanson 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Aaron S. Yoshinobu 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Charles Knaack 2Department of Geology, Washington State University, Pullman, Washington 99164 Diane Johnson 2Department of Geology, Washington State University, Pullman, Washington 99164 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (4): 363–366. https://doi.org/10.1130/0091-7613(1993)021<0363:POPMIA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gary H. Girty, Andrew D. Hanson, Aaron S. Yoshinobu, Charles Knaack, Diane Johnson; Provenance of Paleozoic mudstones in a contact metamorphic aureole determined by rare earth element, Th, and Sc analyses, Sierra Nevada, California. Geology 1993;; 21 (4): 363–366. doi: https://doi.org/10.1130/0091-7613(1993)021<0363:POPMIA>2.3.CO;2 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 SocietyGeology Search Advanced Search Abstract Rare earth element (REE), Th, and Sc analyses of mudstones from the contact metamorphic aureole of the Middle Jurassic Emigrant Gap composite pluton support the idea that the lower Paleozoic Lang, Black Oak Spring, and Zion Hill sequences are composed of detritus derived from a continental landmass. On REE distribution diagrams, our samples exhibited (1) no systematic change as a result of varying metamorphic grade, (2) light-REE enrichment trends, and (3) Eu anomalies that vary from 0.46 to 0.93 and average 0.66. On a La-Th-Sc ternary diagram, our data cluster within the fields of passive-margin sediment and post-Archean Australian shale (PAAS). These new data—in conjunction with the quartz-rich character of sandstones, the absence of volcanic material, and Precambrian detrital zircon ages—suggest that clastic material in the Lang, Black Oak spring, and Zion Hill sequences was derived from a continental landmass that may have been North America or, alternatively, some continental fragment oceanward of early Paleozoic North America. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ordovician bimodal magmatism in the Ogcheon belt (South Korea): intracontinental rift-related volcanic activity

The Ogcheon belt of South Korea has been interpreted to be an oceanic suture zone; nevertheless, all lines of evidence are consistent with an intraplate evolution since early Paleozoic times. The early Paleozoic Ogcheon Supergroup is composed of a Cambrian to early Ordovician quartzite and carbonate platform sequence overlain by a thick volcano-sedimentary sequence; the volcanic rocks have been emplaced in close association with thick debris flow or flysch-like deposits. The metavolcanic complex displays a bimodal association of metabasalts or metadolerites associated with metatrachytes and rare metarhyolites; most of the basic volcanic rocks were emplaced as hypabyssal complexes (sills, dykes and laccoliths) while the acidic volcanic sequences are mainly pyroclastic flow or fall deposits and volcano-derived sedimentary rocks. The scarcity of rocks of intermediate composition, the occurrence of Fe- and Ti-rich basic facies and the high level of incompatible elements are all features of tholeiitic transitional suites. ϵNd values close to zero for basic rocks and trachyte are diagnostic of intracontinental lavas. The negative ϵNd values of the rhyolite suggest that crustal contamination occurred during magma genesis or ascent. Taking into account both the overall characteristics of the sedimentary environment and the petrological and geochemical features of the volcanic component, the sequences appear to have evolved in an intracontinental rift and do not represent oceanic remnants as has been previously interpreted. Thus, the interpretation of the Ogcheon belt as a suture zone is definitely ruled out. Consequently, the limit between early Paleozoic North and South China continents might be located elsewhere, perhaps farther north in the Imjingang belt of central Korea.

Biofacies Evidence for Late Cambrian Low-Paleolatitude Oceans, Western United States and Central Asia

Biofacies that formed on carbonate platform-margin slopes adjacent to an early Paleozoic, low-paleolatitude paleoocean are contained in the Upper Cambrian Swarbrick Formation, Tyby Shale, and Upper Cambrian-lowest Ordovician Hales Limestone of the Hot Creek Range, Nevada, and the Upper Cambrian-lowest Ordovician part of the Shabakty Suite of the Malyi Karatau, southern Kazakhstan. These in-situ limestones formed in platform-margin slope and basin-plain environments. Shoal-water faunal assemblages occur in carbonate-turbidite and debris-flow deposits interbedded with in-situ deeper water assemblages of the submarine-fan facies. Abundant sponge spicules, geographically widespread benthic trilobites, and rare ostracodes occur in some of the in-situ beds. In contrast, the shoal-water platform environments were well oxygenated and contain mainly endemic trilobite assemblages. These biofacies characteristics support an interpretation that Late Cambrian oceans were poorly oxygenated, but not anoxic, below the surface mixing layer and that benthic trilobite faunas were widely distributed in response to the more-or-less continuous deep water, low-oxygen habitats. Elements of the Late Cambrian low-oxygen biofacies are widespread in the Tien Shan structural belt of China and the Soviet Union, in central and eastern China, and along the western margin of early Paleozoic North America. This facies distribution pattern defines the transition from low-paleolatitude, shoal-water carbonate platformsmore » to open oceans which have since been destroyed by pre-Late Ordovician and pre-middle Paleozoic Paleotectonic activity.« less

Geometry and Style of Post-Obduction Thrusting in a Paleozoic Orogen: The Taconic Frontal Thrust System

The Taconic Frontal Thrust System, associated with the later stages of the medial Ordovician Taconic orogeny, a continental margin-volcanic arc collision, defines the present western boundary of the Taconic Allochthon in eastern New York and west-central Vermont. The Frontal Thrust emplaces regionally folded and foliated Taconic rocks over deformed, but essentially unmetamorphosed, flysch. The Frontal Thrust is temporally and kinematically related to the Champlain and Orwell faults to the north, that emplace variably deformed sections of Early Paleozoic shelf rocks over partially coeval sections of the Early Paleozoic platform. Major movement on these faults occurred after initial obduction of the Allochthon onto the Early Paleozoic North American continental shelf and prior to being unconformably overlapped by the Early Devonian. This fault system is considered to be part of the final phase of a continuum of thrusts that evolved during the Taconic Orogeny, and therefore records only the final emplacement geometry of the Allochthon. Detailed mapping has revealed a complex, fine-scale imbricate structure at and near the front of the Allochthon. The imbricate structure in front of the Allochthon is defined by slivers and small thrust sheets of Lower and Middle Ordovician shelf carbonate and overlying Middle Ordovician shales, flysch and wildflysch (debris flow deposits), and melange. These imbricates have dimensions of hundreds of meters to kilometers parallel with strike and a few to several hundred meters structural thickness across strike. Ramps and tear faults within the imbricate stack control the final geometry of the otherwise gently east-dipping Frontal Thrust surface. Complex superposed fold patterns occur locally within the overlying regionally folded Taconic sequence at these frontal structures. At Bald Mtn. and other localities to the north, the carbonate slivers have an internal duplex structure, with the Frontal Thrust forming the roof fault to the duplex. In some areas the Frontal Thrust appears to follow the earlier, pre-cleavage thrusts (e.g., Basal Taconic Thrust) along which initial obduction occurred. Microscopic and mesoscopic fault-related deformation associated with the Frontal Thrust appear to have developed in fully lithified sedimentary sequences. Deformation involving partially lithified rock is limited to the local incorporation of olistostromes into tectonic melange at the leading edge of the Allochthon. The pattern of essentially coeval thrusting, lithification, cleavage development, low-grade metamorphism deduced here may be generally representative of the final stages of continental margin-island arc collision.