Structural style of the Marathon thrust belt, West Texas

The Marathon portion of the Ouachita thrust belt consists of a highly deformed allochthonous wedge of Cambrian-Pennsylvanian slope strata (Marathon facies) that was transported to the northwest and emplaced over Pennsylvanian foredeep sediments. The foredeep strata in turn overlie early-middle Paleozoic shelfal sediments which are deformed by late Paleozoic basement-involved reverse faults. The Dugout Creek thrust is the basal thrust of the allochthon. Shortening in this sheet and overlying sheets is 80%. Steep imbricate faults link the Dugout Creek thrust to upper level detachments forming complex duplex zones. Progressive thrusting and shortening within the allochthon folded the upper level detachments and associated thrust sheets. The Caballos Novaculite is the most competent unit within the Marathon facies and controlled development of prominent detachment folds.Deeper imbricate sheets composed of the Late Pennsylvanian foredeep strata, and possibly early-middle Paleozoic shelfal sediments developed concurrently with emplacement of the Marathon allochthon and folded the overlying allochthon. Following termination of thrusting in the earliest Permian, subsidence and deposition shifted northward to the Delaware, Midland and Val Verde foreland basins.     

Basement-cover relations in the Appalachian fold and thrust belt

The external massifs along the Appalachian orogen include Precambrian basement rocks with attached cover. To the northwest (cratonward), in the Appalachian foreland fold and thrust belt, Palaeozoic sedimentary rocks, but no basement rocks, are exposed; that belt was the subject of the classic debate about thin-skinned (deformed cover rocks detached from undeformed basement) and thick-skinned (basement deformed with attached cover) structural styles. Presently available data indicate detached cover rocks and thin-skinned style in the fold and thrust belt: large-scale thrusting occurred late in the orogenic history. In the external basement massifs, late Precambrian graben-fill sedimentary and volcanic rocks indicate early basement faults; and within the craton, steep basement faults bound graben blocks of Cambrian age. Distribution of known basement faults suggests that basement rocks beneath the fold and thrust belt may also be faulted. Local episodic synsedimentary structural movement through much of the Palaeozoic is documented by stratigraphy in the fold and thrust belt. Axes of early synsedimentary structures are approximately coincident with axes of late folds and thrust fault ramps, but stratigraphic data show that magnitude of the early structures was much less than that of the late structures. These relations suggest the interpretation that early low-magnitude structures formed in cover rocks over basement faults and that the early structures, or the basement faults, significantly influenced the geometry of later detachment structures during large-scale horizontal translation.     

扎格罗斯褶皱冲断带构造变形特征

扎格罗斯褶皱冲断带是扎格罗斯碰撞造山带的前陆褶皱冲断带, 也是波斯湾周缘前陆盆地的楔顶带, 自北东到南西垂直于构造线方向可分为高扎格罗斯冲断带和扎格罗斯简单褶皱带, 自北西到南东沿构造线方向可分为洛雷斯坦区(Lorestan)、迪兹富勒湾区(Dezful Embayment)和法尔斯区(Fars)。扎格罗斯褶皱冲断带的形成始于晚白垩世阿拉伯板块的洋壳向北俯冲到欧亚板块之下, 褶皱冲断构造从北东部缝合带向南西方向伸展, 并在上新世基本定型。本文选取了横切扎格罗斯褶皱冲断带的3条地质剖面和两条局部地震剖面进行构造变形分析。剖面分析显示研究区垂向上由一条大滑脱面将扎格罗斯褶皱冲断带剖面分为上、下两个构造层, 褶皱冲断变形从北东到南西向由强变弱。研究区发育走滑、挤压和拉张3种构造变形, 挤压构造变形占主导地位。挤压构造变形又包括滑脱褶皱、断展褶皱、断弯褶皱和双重构造等。     

西昆仑山前东段新生代褶皱冲断带及其锥形楔机制

西昆仑山前东段的和田-柯克亚构造带与塔里木盆地内的麻扎塔格构造带相距为200 km,两者近似平行,遥相对应,麻扎塔格构造带是和田-柯克亚构造带的前沿,山前挤压变形量沿古近系阿尔塔什组膏泥岩向北传递约200 km到麻扎塔格,形成往北东逆冲的断层传播褶皱,在盆地腹部表现为一条显著的北西向地形隆起。因此,整个西昆仑山前东段新生代褶皱冲断带宽约为270 km,长约为220 km,由和田-柯克亚构造带、麦盖提斜坡和麻扎塔格构造带组成。中部近200 km的麦盖提斜坡内没有明显构造变形,且整个褶皱冲断带的锥形楔角度远小于活动造山带的临界锥形角,说明古近系底部阿尔塔什组膏泥岩以及新生代沉积体的形态是这个褶皱冲断带锥形楔形成的主控原因,膏泥岩控制了锥形楔的顶面坡度α基底构造形态及上覆沉积楔形体则控制了锥形楔的底部滑脱层倾角β。     

准南逆冲褶皱带超压与逆冲断层持续活动

天山北缘准南地区的褶皱带为自新生代以来一直持续活动的逆冲构造带,由于逆冲断层的持续活动,形成了现今断层和相关褶皱。钻井资料显示,准南逆冲褶皱带内的超压层主要发育在古近纪安集海河组泥岩和紫泥泉子组泥岩之中,而该泥岩同时又成为逆冲断层发育的主滑脱面。通过多年来对准南地区地面地质调查、二维地震和三维地震资料的解释以及钻井证实,我们统计出准南逆冲褶皱带现存的逆冲断层倾角分别集中在两个区间: 30±5°和50±5°区间。应力分析表明,在持续挤压应力作用下,超压层(泥岩、页岩和煤系地层)中和超压层之下地层中发育的早期逆冲断层与晚期最大主压应力之间的夹角处在30±5°之间时,作用在断层面上的最大主应力与最小主应力比达到最小值,因此该断层最容易再次活动,形成最大的流体压力,因而断层周围的流体就会沿着最大主应力方向发生流动,断层本身就会成为流体运移的主要通道; 而早期逆冲断层与晚期最大主压应力之间的夹角处在50±5°之间时,作用在断层面上的最大主应力与最小主应力比较大,断层重新活动所需要的流体压力较高,导致断层作为流体运移的通道因被挤压而闭合。应力分析和钻井实测应力均指出,准南逆冲褶皱带发育的超压为挤压构造应力形成的超压。这些研究表明,准南逆冲褶皱带的逆冲断层持续活动,导致早期发育的断层在晚期应力作用下,断层倾角聚集在两个优势区间,油气沿最大主压应力方向运移,聚集油气则沿断层滑动面发育形成构造超压,导致该区域油气长期处于运移与聚集的动平衡状态。     

Antithetic fault linkages in a deep water fold and thrust belt

Deep water fold and thrust belts consist of both forethrusts and backthrusts that can link along strike to form continuous folds in the overburden. The interaction of faults of opposing dip are termed ‘antithetic thrust fault linkages’ and share the common feature of a switch in vergence of overlying hangingwall anticlines. Using three-dimensional seismic data, on the toe-of-slope of the Niger Delta, linkages are classified into three distinct structural styles. This preliminary classification is based on the vertical extent of faulting within a transfer zones relative to the branch line of the antithetic faults. The stratigraphic level of the lateral tip of the fault, the shape of lateral tip region of a fault plane and the stratal deformation within the transfer zones is also distinctive in each type of fault linkage. A Type 1 linkage comprises faults that overlap exclusively above the level of the branch line. A ‘pop-up’ structure forms within the transfer zone with sediments below remaining planar. The lower tip lines of faults climb stratigraphically towards the linkage zone creating asymmetric, upward-tapering lateral tip regions. In Type 2 linkages fault overlap occurs lower than the level of the branch line such that lateral fault tips are located within the footwall of the counterpart fault. Faulting is thus limited to the deeper section within the transfer zone and creates unfaulted, symmetric, bell-shaped folds in the overburden. Upper tip lines of faults lose elevation within the transfer zone creating asymmetric, downwards-tapering lateral tip regions. In Type 3 linkages both faults continue above and below the branch line within the transfer zone resulting in cross-cutting fault relationships. Horizon continuity across the folds, through the transfer zones, varies significantly with depth and with the type of fault intersection.     

A tertiary fold and thrust belt in the Valle del Cauca Basin, Colombian Andes

Surface geology and heophysical data, supplemented by regional structural interpretations, indicate that the Valle del Cauca basin and adjacent areas in west-central Colombia form a west-vergent, basement-involved fold and thrust belt. This belt is part of a Cenozoic orogen developed along the west side of the Romeral fault system. Structural analysis and geometrical constraints show that the Mesozoic ophiolitic basement and its Cenozoic sedimentary cover are involved in a “thick-skinned” west-vergent foreland style deformation. The rocks are transported and shortened by deeply rooted thrust faults and stacked in imbricate fashion. The faults have a NE---SW regional trend, are listric in shape, developed as splay faults which are interpreted as joining a common detachment at over 10 km depth. The faults carry Paleogene sedimentary strata and Cretaceous basement rocks westward over Miocene strata of the Valle del Cauca Basin. Fold axes trend parallel or sub parallel to the thrust faults. The folds are westwardly asymmetrical with parallel to kink geometry, and are interpreted to be fault-propagation folds stacked in an imbricate thrust system. Stratigraphic evidence suggests that the Valle del Cauca basin was deformed between Oligocene and upper Miocene time. The kinematic history outlined above is consistent with an oblique convergence between the Panama and South American plates during the Cenozoic.A negative residual Bouguer anomaly of 20–70 mgls in the central part of the Valle del Cauca basin indicates that a substantial volume of low density sedimentary rocks is concealed beneath the thrust sheets exposed at the land surface. The hydrocarbon potential of the Valle del Cauca should be reevaluated in light of the structural interpretations presented in this paper.     

Structure of an active foreland fold and thrust belt,Papua New Guinea

Geological structure of the active foreland fold and thrust belt of Papua New Guinea has been interpreted using high-quality seismic-reflection data. Three en échelon anticlines, the Strickland, Cecilia and Wai Asi, are located along the frontal margin of the Papuan Fold Belt. All three are foreland-vergent and cut by hinterland-dipping thrust faults that sole into a common detachment beneath the Oligocene to Miocene Darai Limestone. Two of the anticlines are linked by a right-lateral transfer zone. Folding occurs primarily in the upper 2000 m of strata, which consist of Darai Limestone overlain by Miocene to Quaternary siliciclastic sedimentary rocks. Beneath the Darai Limestone lies the less-competent shaly Ieru Formation, which exhibits disharmonic folding and variable bed thickness. Seismic-reflection data clearly show that the Plio-Pleistocene upper Era Beds are deformed to the same extent as the underlying Darai Limestone, demonstrating that most of the observed deformation has occurred during the Late Pliocene and Pleistocene.     

库车冲断带秋里塔格构造带东段构造样式与构造演化

秋里塔格构造带位于库车褶皱冲断前缘,其东段包括东秋里塔格背斜和库车塔吾背斜。野外调查和地震剖面解释表明:秋里塔格构造带东段盐下发育断层转折褶皱; 盐上东秋里塔格背斜为滑脱箱状背斜,库车塔吾背斜核部为南倾逆冲断层所破坏。演化剖面显示秋里塔格构造带东段在侏罗纪断陷期发育了正断裂,其后为平静期,直到库车晚期后逆冲断层和褶皱快速发育,背斜最终形成。膏盐岩及古构造对构造变形具有重要影响,一方面作为滑脱层,分割了盐下层与盐上层,导致二者形成不同的构造样式; 另一方面塑性流动充填于背斜核部。由于膏盐岩的厚度差异,东秋里塔格背斜盐上发育褶皱,而库车塔吾背斜核部被逆冲断层破坏,膏盐层厚度还影响了膏盐层上下构造高点的相对位置。盐下构造的发育受侏罗纪古构造控制,进而影响了盐上构造的发育。     

Deepwater fold and thrust belt classification,tectonics, structure and hydrocarbon prospectivity: A review

Deepwater fold and thrust belts (DWFTBs) are classified into near-field stress-driven Type 1 systems confined to the sedimentary section, and Type 2 systems deformed by either far-field stresses alone, or mixed near- and far-field stresses. DWFTBs can occur at all stages of the Wilson cycle up to early stage continent continent collision. Type 1 systems have either weak shale or salt detachments, they occur predominantly on passive margins but can also be found in convergent-related areas such as the Mediterranean and N. Borneo. Examples include the Niger and Nile deltas, the west coast of Africa, and the Gulf of Mexico. Type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin DWFTBs. Continent convergence zones cover DWFTBs developed during continent–arc or continent–continent collision, and those in a deepwater intracontinental setting (e.g. W. Sulawesi, Makassar Straits). Active margins include accretionary prisms and transform margins. The greatest variability in DWFTB structural style occurs between salt and shale detachments, and not between tectonic settings. Changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to DWFTB type. In comparison with shale, salt detachment DWFTBS display a lower critical wedge taper, more detachment folds, long and episodic duration of deformation and more variation in vergence. Structures unique to salt include canopies and nappes. Accretionary prisms also standout from other DWFTBs due to their relatively long, continuous duration, rapid offshore propagation of the thrust front, and large amount of shortening. In terms of petroleum systems, many similar issues affect all DWFTBs, these include: the oceanward decrease in heat flow, offshore increase in age of mature source rock, and causes of trap failure (e.g. leaky oblique and frontal thrust faults, breach of top seal by fluid pipes). One major difference between Type 1 and Type 2 systems is reservoir rock. High quality, continent-derived, quartz-rich sandstones are generally prevalent in Type 1 systems. More diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in Type 2 systems, or many are simply poor in well-developed turbidite sandstone units. However, some Type 2 systems, particularly those adjacent to active orogenic belts are partially sourced by high quality continent-derived sandstones (e.g. NW Borneo, S. Caspian Sea, Columbus Basin). In some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts, coupled with uplift due to collision, results in accretionary prism related fold belts that pass laterally from sub-aerial to deepwater conditions (e.g. S. Caspian Sea, Indo-Burma Ranges). The six major hydrocarbon producing regions of DWFTBs worldwide (Gulf of Mexico, Niger Delta, NW Borneo, Brazil, West Africa, S. Caspian Sea) stand out as differing from most other DWFTBs in certain fundamental ways, particularly the very large volume of sediment deposited in the basins, and/or the great thickness and extent of salt or overpressured shale sdetachments.     

Tectonothermal evolution of a foreland fold and thrust belt: the Cantabrian Zone (Iberian Variscan belt,NW Spain)

Analysis of the conodont colour alteration index and the Kübler index of illite allowed us the characterization of four types of very low‐ or low‐grade metamorphism in the Cantabrian Zone (CZ) and determination of their regional and temporal distribution. These types are: (1) an orogenic Variscan metamorphism present only in restricted areas of the western and north‐western parts of the CZ where epizonal conditions are reached; (2) a burial metamorphism that appears in the basal part of some nappes, where anchizonal conditions are sometimes achieved; the thermal peak preceded emplacement of the nappes; (3) a late‐Variscan metamorphism in the southern and south‐eastern parts of the CZ; a cleavage, cutting most of the Variscan folds, is associated with this metamorphism, which has been related to an extensional episode; (4) a contact metamorphism and hydrothermal activity associated with minor intrusive bodies. The extension continued after the Variscan deformation giving rise to hydrothermal activity during Permian times.     

Influence of structural position on fracture networks in the Torridon Group,Achnashellach fold and thrust belt,NW Scotland

In fold-and-thrust belts rocks undergo deformation as fold geometries evolve. Deformation may be accommodated by brittle fracturing, which can vary depending on structural position. We use 2D forward modelling and 3D restorations to determine strain distributions throughout folds of the Achnashellach Culmination, Moine Thrust Belt, NW Scotland. Fracture data is taken from the Torridon Group; a thick, coarse grained fluviatile sandstone deposited during the Proterozoic. Modelling infers a correlation between strain and simple curvature; we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt.In high curvature regions, such as forelimbs, fracture intensities are high and fractures are short and oriented parallel to fold hinges. In low curvature regions fractures have variable intensities and are longer. Fracture orientations in these regions are scattered and vary over short distances. These variations do not relate to strain; data suggests lithology may influence fracturing. The strain history of fold structures also influences fracturing; structures with longer deformation histories exhibit consistent fracture attributes due to moderate-high strain during folding, despite present day low curvature. This is in contrast to younger folds with similar curvatures but shorter deformation histories. We suggest in high strain regions fracturing is influenced by structural controls, whereas in low strain regions lithology becomes more important in influencing fracturing.     

川东褶皱带构造发育深度层次与变形样式

地壳上层的结构特征和变形样式在垂直方向上的变化远远大于其在水平维度上的分异。川东褶皱带自晚古生代以来地史演化统一、地层展布稳定,之后新生代盆、山演化分异幅度较大,使得不同深度的地层和基底得以出露,不同地壳深度层次的构造样式得以展示,这为研究应力的垂向分异,提供了很好的条件。本文基于地壳垂直方向上变形几何的不守恒、构造脱耦以及构造层次的概念,通过野外构造现象的详细解析、野外脆性破裂产状统计、断裂之间交切关系以及活动性质观察等综合分析,结合遥感图像研究,对川东褶皱区隔挡式褶皱和隔槽式褶皱形成提出了新的解释模型。以华蓥山与齐岳山为界,川东褶皱带被分为3个呈叠置关系的区域。研究表明华蓥山以西(Ⅰ区)没有发生强烈的构造变形,变形深度最小(<2 km); 华蓥山与齐岳山之间(Ⅱ区)构造样式为在北北东向剪切作用下形成的陡立构造面理,变形深度为2～5 km; 而齐岳山以东(Ⅲ区)的构造样式是发育轴向北东的宽缓褶曲,变形深度为4～6.6 km。研究分析后得出,川东褶皱带在晚古生代以来,没有经历过大幅度的地壳垂向运动和明显的旋转运动,而白垩纪以后,发育了早期北北东向和晚期北东向的两期构造变形。Ⅱ、Ⅲ两区的构造样式发育于同一应力场(北西-南东主应力场),而晚期北东向断裂活动是形成上述3个区域呈现出断块并置的原因。另外,由于后期不同断块抬升和剥露的差异,使3个区域迥异的构造样式呈现在地表。这一认识对研究油气相关的构造圈闭、固体多金属矿产相关的矿床深度问题以及大地构造学等问题都有创新意义。     

Kinematic analyses of orogen-parallel L-tectonites from Pelling-Munsiari thrust of Sikkim Himalayan fold thrust belt: Insights from multiple,incremental strain markers

Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (W_k) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (∼56–96%) that progressively became simple-shear dominated (∼29–54%), as recorded by the oblique fabric and subgrains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen.The L-tectonites record ∼30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.     

Superposed lateral ramps in the Pell City thrust sheet, Appalachian thrust belt, Alabama

In the Appalachian thrust belt in Alabama, thrust sheets of Paleozoic strata generally strike northeastward and are imbricated northwestward; four transverse zones cross the regional strike of the thrust belt. The large-scale Pell City thrust sheet ends southwestward at an oblique lateral ramp within the Harpersville transverse zone, where the leading edge of the thrust sheet (the Pell City fault) curves abruptly 55° counterclockwise. The northwest-striking segment of the Pell City fault conforms to the geometry of an oblique lateral ramp in the footwall. Furthermore, the Pell City fault cuts up section in the hanging wall southwestward toward the transverse zone, indicating a hanging-wall lateral ramp emplaced over the footwall oblique lateral ramp.In the hanging wall adjacent to the northwest-trending segment of the Pell City fault, a pervasive train of upright, isoclinal folds (with 50% apparent shortening) trends N15°W, oblique to the regional translation direction. The fold train is limited to the southwestern part of the Pell City thrust sheet; farther northeast, the regional northeasterly strike prevails. The isoclinal folds in the hanging wall indicate contractional crowding perpendicular to the footwall oblique lateral ramp.     

A Late Archean foreland fold and thrust belt in the North China Craton: Implications for early collisional tectonics

The Archean North China craton is divided into the Western and Eastern blocks along the Central Orogenic belt. A 1600 km long Archean foreland basin and thrust belt fringes the eastern side of the Central Orogenic belt. Rocks in the orogen form tectonically-stacked east-vergent fold and thrust sheets including foreland basin sediments, 2.50 Ga ophiolitic mélange, and an island arc complex. Foreland basin sediments overlie a passive margin sequence, and include a 2.50 Ga deep-water turbidite sequence that grades upward and westward into shallow-water molasse, now disposed in structurally imbricated east-verging thrusts and asymmetric folds that gradually migrated craton-ward with deformation, uplift, and erosion of the orogen. There is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the North China craton along the Central Orogenic belt at 2.50 Ga. The Qinglong foreland basin and Central Orogenic belt of the North China craton represents one of the best-preserved Archean orogen-to-craton transitions in the world. Its classic internal to external zonation, and flexural response to loading, demonstrate that convergent tectonics in the Archean were broadly similar to Phanerozoic convergent margin processes.     

Tectonic burial and exhumation in a foreland fold and thrust belt: the Monte Alpi case history (Southern Apennines,Italy)

The Monte Alpi area of the Southern Apennines represents the only sector of the thrust belt where the reservoir rocks (i.e. Apulian Platform carbonates) for major hydrocarbon accumulations in southern Italy are interpreted to crop out. Tectonic evolution and exhumation of this area were analysed by integrating stratigraphic and structural data with different organic and inorganic parameters which record the burial and thermal evolution of the sediments (vitrinite reflectance, fluid inclusions, and I/S mixed layers in clayey sediments). Our analyses suggest that the presently exposed Monte Alpi structure suffered a loading of ca. 4000 m, owing to the emplacement of allochthonous units in Early Pliocene times. Available geological data indicate that erosion of the tectonic load occurred since the Late Pliocene, when the area first emerged. This implies an average exhumation rate in excess of 1 mm/year. A model can be constructed which matches the maturity indices and also takes into account intermediate stages of the evolution, resulting from combined structural and fluid inclusion data. By this model, a first stage of exhumation would have taken place at an average rate of about 0.36 mm/year. This was controlled by uplift and erosion associated with both: (i) thrusting at depth within the Apulian carbonates (Late Pliocene), and (ii) strike-slip faulting (Early Pleistocene). A second exhumation stage would have occurred in the last 700 ky at a much faster rate (ca. 4 mm/year) as a result of extensional tectonics.     

Ductile-to-brittle transition in shear during thrust sheet emplacement,Southern Appalachian thrust belt

Mesoscopic structures in anchimetamorphic (T = 200–300°C) strata of the Pulaski thrust sheet, Southern Appalachian thrust belt, developed in progressive, heterogeneous simple shear near the ductile-to-brittle transition. Shear (γ≤3) was localized in weak, anisotropic pelitic rocks (Rome Formation) along the base of this 5–11 km thick thrust sheet. Folds, which vary from upright and open to isoclinal and NW-facing, developed during ductile shearing and display a correlation between tightness and axial-surface dip. Movement along brecciated thrust zones, which evolved progressively from zones of greatest ductile strain, resulted in low-angle truncation of fold axis trends, coaxial refolding of earlier structures, and imbrication of the thrust sheet.Transient variations in fluid pressure (P_f) controlled the mechanical behavior of the thrust sheet. Systematic veins imply P_f >σ₃ + T (T = tensile strength) during ductile deformation, whereas later non-systematic vein arrays in high strain zones record periods of nearly hydrostatic stress. Elevated P_f, which led to fracturing, dilation, and fault initiation, appears confined to pelitic zones within the Rome Formation. This, coupled with decreasing temperature, resulted in the transition from ductile folding to brittle thrusting. Changing physical conditions probably reflect erosional unroofing during uplift and late Paleozoic thrust sheet emplacement.