Brian J. Darby

Geometric and temporal evolution of an extensional detachment fault, Hohhot metamorphic core complex, Inner Mongolia, China

The Early Cretaceous Hohhot metamorphic core complex and its master Hohhot detachment fault are ;400 km west of Beijing in the Daqing Shan (Mountains) of Inner Mongolia. The complex developed across the east-trending Yinshan fold-and-thrust belt within ,4 m.y. following cessation of thrusting ca. 125 Ma (see note added in proof in main text). Postcontractional extension was initiated within a mid-crustal zone of mylonitic and ductile shear that was in part controlled by Carboniferous(?) strata sandwiched between its Proterozoic and Archean crystalline basement and an overlying thrust sheet of similar crystalline rocks. The Hohhot detachment fault appears to have rooted into deep, kinematically active levels of the mid-crustal shear zone. Higher, inactive levels of the mylonitic section were transected by the fault and carried upward in its footwall. Geometries of the footwall mylonitic rocks indicate localized ramp-flat geometries of the fault within and across them. The crosscut top of the mylonitic sequence defines a mylonitic front that departs from the gently south dipping detachment fault and dips northward into its footwall. Early Cretaceous extension was widespread elsewhere in northern China, and was particularly pronounced in the Yunmeng Shan core complex north of Beijing. The gravitational collapse of orogenically thickened crust acting in concert with localized centers of deep-seated plutonism appear to have led to the development of isolated metamorphic core complexes within a broad region of more distributed extensional deformation.

Mesozoic contractional deformation in the middle of the Asian tectonic collage: the intraplate Western Ordos fold–thrust belt, China

Abstract Intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of strain concentrations into weak zones. These weak zones may be pre-existing crustal heterogeneities such as faults, thermally weakened zones due to magmatism and/or the effects of a thick sedimentary cover, or mechanical contrasts between adjacent crustal blocks. Intraplate deformation is not exclusively related to continent–continent collision; it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries. Asia has many examples of active intraplate deformation that are thought to be related to the Cenozoic Indo-Asian collision. Although much attention is placed on active examples, discerning controls on pre-Cenozoic Asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of Asia. Situated on either side of the Yellow River along the western margin of the Ordos Plateau, the intraplate Western Ordos fold–thrust belt involves units as old as Archean basement and as young as Late Jurassic(?). The fold–thrust belt is spectacularly exposed in two ranges, the Helan Shan and the Zhuozi Shan. Cross-sections drawn through the north-trending fold–thrust belt suggest 30% minimum shortening. A paleocurrent reversal within the Lower–Middle Jurassic section is interpreted to mark the onset of contraction and mountain building. A Late Jurassic(?) synorogenic boulder conglomerate records the final deformation in the fold–thrust belt. Synchronous with contraction, and cutting across the fold–thrust belt, is an ∼E–W-striking strike-slip fault that displays right-lateral drag and sub-horizontal striae. We interpret this fault as an accommodation structure in the thrust belt. Following ∼E–W contraction, the thrust belt was dismembered by an ∼N15°W-trending left-lateral strike-slip fault. The left-lateral fault displaces the frontal portion of the thrust belt (the Zhuozi Shan) 62 km to the north, relative to a more internal portion (the Helan Shan). We propose that the anomalous orientation and intraplate location of the Western Ordos fold–thrust belt is a function of mechanical contrasts between a previously deformed area and a stable crustal block (Ordos) to the east. Pre-existing crustal weakness or anisotropy along the western margin of the Ordos block is most likely related to either a Late Proterozoic–Early Paleozoic aulacogen, a Triassic fault system, or to both. Although its intraplate location and the complexities of Mesozoic Asian plate interactions make it difficult to relate deformation in the Western Ordos fold–thrust belt to a specific contemporaneous plate boundary, its N–S trend may suggest a link with paleo-Pacific subduction along the eastern margin of Asia.

Late Paleozoic Sedimentation on the Northern Margin of the North China Block: Implications for Regional Tectonics and Climate Change

The Late Paleozoic collision between the North China continental block and the Altaid arc terranes of Mongolia represents one of the earliest and most fundamental tectonic events in the ongoing construction of Asia. New detrital zircon provenance data from Carboniferous-Permian nonmarine strata on the northern margin of North China imply that the northern margin of the North China block constituted a continental margin arc prior to this collision (~400-275 Ma) and that collision took place via south-directed subduction beneath North China. A significant and widespread climate change took place in North China in mid-Permian time, and is recorded by a change from Carboniferous and Lower Permian humid-climate, coal-bearing sedimentary facies to Upper Permian and Lower Triassic arid-climate redbeds. In northern North China, this climate change is accompanied by a paleocurrent reversal, which indicates the onset of uplift on the northern margin of the North China block. The temporal association of climate change and uplift suggests that aridification of North China may have been caused by a rainshadow effect from topography related to the convergence and ultimate collision between the North China block and the Altaid arc terranes of Mongolia. Alternatively, climate change may have occurred as a result of northward drift of the North China block through arid subtropical latitudes.

EVIDENCE OF MIOCENE CRUSTAL SHORTENING IN THE NORTH QILIAN SHAN FROM CENOZOIC STRATIGRAPHY OF THE WESTERN HEXI CORRIDOR, GANSU PROVINCE, CHINA

New sedimentologic, stratigraphic, and compositional data from the Paleogene-Neogene stratigraphic succession exposed in the northwest Hexi Corridor and within the North Qilian Shan, provide evidence to suggest that crustal shortening in the North Qilian Shan fold-thrust belt initiated during the Miocene. The section is composed of four lithostratigraphic units: Oligocene-Miocene fine- to coarse-grained Unit 1, Miocene conglomeratic Unit 2, and Pliocene-Pleistocene conglomeratic Units 3 and 4. Unit 3 lies in angular unconformity over both Units 1 and 2, and Unit 4 contains a progressive unconformity. The onset of conglomerate deposition at the base of Unit 2 suggests an increase in depositional energy, which we interpret as the result of proximal orogenesis in the North Qilian Shan fold and thrust belt. Supporting evidence includes the appearance of strongly northeast-trending paleocurrents, indicating paleoflow away from the Qilian Shan, clast lithologies that match sources in the North Qilian Shan, and sandstone with detrital framework modes that indicate a recycled orogen source. In contrast, Unit 1 contains paleocurrent indicators that are variable but generally trend northward and sandstone and clast compositions which are more diagnostic of a continental block source. Detrital zircon age determinations from Unit 1 are also not consistent with a source in the North Qilian Shan; rather, they suggest a provenance in hinterland regions within the South Qilian Shan and North Qaidam terranes. In sum, these results are all consistent with initiation of proximal uplift of the North Qilian Shan during deposition of the gradational transition from Unit 1 to Unit 2, demonstrating shortening in the Qilian Shan before the late Miocene. This comprehensive study tightens our understanding of when far-field stress related to the India-Eurasia continent-continent collision reached the northeastern edge of the Tibetan Plateau.

Tectonic implications of detrital zircon data from Paleozoic and Triassic strata in western Nevada and Northern California

U-Pb analyses of detrital zircons from various allochthonous assemblages of Paleozoic and early Mesozoic age in western Nevada and northern California yield new constraints on the sediment dispersal patterns and tectonic evolution of western North America. During early Paleozoic time, a large submarine fan system formed in slope, rise, basinal, and perhaps trench settings near the continental margin, west of continental shelf deposits of the Cordilleran miogeocline. Our detrital zircon data suggest that most of the detritus in this fan system along the western U.S. segment of the margin was derived from the Peace River Arch region of northwestern Canada, and some detritus was shed from basement rocks of the southwestern United States or western Mexico. In most cases, the detritus in the allochthonous assemblages was recycled through platformal and/or miogeoclinal sedimentary units prior to accumulating in offshelf environments. Lower Paleozoic rocks of the Roberts Mountains allochthon, Shoo Fly Complex, and Yreka terrane are interpreted to have been parts of this fan complex that accumulated along the central U.S. segment of the continental margin, probably within 1000 km of the miogeocline. During the mid-Paleozoic Antler orogeny, parts of the lower Paleozoic fan complex were deformed and uplifted, and strata of the Roberts Mountains allochthon were tectonically emplaced onto the continental margin. This orogeny was apparently driven at least in part by convergence of the Sierra-Klamath arc with the continental margin, as has been proposed by many previous workers, because these arc terranes are overlain by Mississippian clastic strata derived from the Roberts Mountains allochthon. Our data are not sufficient, however, to determine the polarity of the arc, or whether the arc formed along the continental margin or was exotic to western North America. Detrital zircon data indicate that following the Antler orogeny, clastic sediments derived from the Roberts Mountains allochthon were deposited both on the continental margin to the east and within intra-arc and backarc basins to the west. The occurrence of this detritus in terranes of western Nevada and northern California indicates that they were proximal to each other and to the continental margin during late Paleozoic time. The presence of upper Paleozoic volcanic and plutonic rocks and arc-derived detrital zircons in strata of the northern Sierra, eastern Klamath, and Black Rock terranes records the existence of a west-facing magmatic arc near the continental margin during late Paleozoic time. Our data are not supportive of scenarios in which these arc terranes were located farther north or thousands of kilometers offshore of the Nevada continental margin during late Paleozoic time. Following a second phase of uplift, erosion, and allochthon emplacement during the Permian-Early Triassic Sonoma orogeny, Middle and Upper Triassic strata now preserved in west-central Nevada accumulated in a backarc basin. Our data indicate that the basinal assemblages contain detritus from arc terranes to the west as well as the craton to the east.

Mesozoic Tectonics and Sedimentation of the Giant Polyphase Nonmarine Intraplate Ordos Basin, Western North China Block

The Mesozoic Ordos Basin is a large intracontinental basin that is characterized by a thick, undeformed stratigraphic succession over a wide region that is bounded by deformed polyphase orogenic belts on all margins. Structural and stratigraphic studies document a diverse history of Late Palaeozoic through Cretaceous shortening, extension and strike‐slip on the northern and western basin margins, and a longer‐lived contractile setting in the southern part of the basin. In response to these structural episodes, associated basins formed within, or adjacent to, the marginal mountain belts and were filled by a variety of nonmarine depositional processes. Synchronously, subsidence across the whole of the Ordos block resulted in a large, integrated basin, also filled by nonmarine systems, that spanned across distinct structural domains. These stratigraphic and structural characteristics are interpreted to suggest that mechanical contrasts between the basin interior and the marginal mountain belts fundamentally control the timing, distribution and styles of deformation and basin formation on the margins of the Ordos block. This mechanical contrast between the Ordos Basin and its margins encouraged the repeated, polyphase deformation on its margins, whereas the vast area of the Ordos block continued to subside uniformly. These characteristics are typical of many large basin systems found on relatively recently assembled continents, like southern Eurasia. Examples that may be similar in many respects to the Ordos Basin include other collisional successor basins of China, such as the Tarim, Junggar and Sichuan basins.

Provenance and paleogeography of the Black Rock Terrane, northwestern Nevada: Implications of U-Pb detrital zircon geochronology

U-Pb ages have been determined for 50 detrital zircon grains from Mississippian and Triassic strata of the Black Rock terrane, northwestern Nevada. The Devonian(?) to Mississippian Pass Creek unit has three broad age groups: 976-1132 Ma (n = 8), 1595-1927 Ma (n = 10), and 2504-2660 Ma (n = 3). The Triassic Bishop Canyon formation contains a dominant group of grains between 268 and 441 Ma (n = 11), a cluster of ages between 1868 and 1925 Ma (n = 5), and scattered ages between 1184 and 1813 Ma (n = 10) and between 2183 and 3183 Ma (n = 3). Most of the ages in these samples match well with the ages of grains present in basement provinces and off-shelf assemblages in the western United States. Grains in the Pass Creek unit were most likely recycled from lower Paleozoic strata of the Roberts Mountains allochthon and from strata exposed in the Salmon River arch region of Idaho, western Montana, and eastern Washington. These provenance links suggest that the Black Rock terrane was located along the northern Nevada-Idaho segment of the Cordilleran margin, near its current location, during late Paleozoic time. Because the upper Paleozoic stratigraphy of the Black Rock terrane is similar to that found in more outboard arc assemblages, including those of the Klamath Mountains and Sierra Nevada, this relation provides an indirect but important link between the U.S. continental margin and the more outboard arc assemblages. Zircon grains in the Upper Triassic Bishop Canyon formation were derived from a source containing both upper Paleozoic igneous rocks and clastic strata bearing 1.1-3.2 Ga detrital zircons. The most likely source for this combination of grains is Paleozoic basement rocks of Mesozoic arc assemblages in the eastern Klamath Mountains and Black Rock terrane. These provenance links provide evidence of uplift and erosion of arc basement in the western Cordillera during early Mesozoic time, and support interpretations that lower Mesozoic arc assemblages in this region of the Cordillera were isolated from the continental margin by a Triassic backarc basin.

Ordos Basin Gas Reservoir Outcrop Analogs: Permian Braided Fluvial Sandstone of the Zhuozi Shan and Helan Shan, China

Permian sandstone outcrops along the eastern margin of the Zhuozi Shan, Inner Mongolia China, represent the most geographically proximal analogs for subsurface natural gas reservoirs of the north-central Ordos basin in the emerging Permian sandstone play; they lie along depositional strike, within the same structural province 90-125 km WNW of the Sulige gas field. These units were deposited by south-flowing braided fluvial depositional systems, and are dominated by channel and macroform deposits consisting of fine to very coarse sandstone. Beds are typically amalgamated both vertically and laterally to form laterally extensive sandstone bodies that are between 8 and 60 m thick; the sandstone bodies are thinner, and less continuous laterally near the top of the Permian. Amalgamated sandstone bodies are separated by mudstones that reach 10 m in thickness, and the Permian sandstone interval overall is bracketed between Permo-Triassic mudstone above and Carboniferous mudstone below. These sedimentologic characteristics control the heterogeneity and geometry of reservoir facies at the interwell scale, and will exert influences on the productivity of reservoirs in the subsurface of the Ordos basin.