Duplex style and triangle zone formation: insights from physical modeling

Late Paleozoic deformation within interior North America has produced a series of north-northwest- to northwest-trending elongate basins that cover much of Oklahoma, Texas, New Mexico, Colorado, and Utah. Each basin thickens asymmetrically toward an adjacent region of coeval basement uplift from which it is separated by synsedimentary faults with great vertical relief. The remarkable coincidence in timing, geometry, and apparent structural style throughout the region of late Paleozoic deformation strongly suggests that these paired regions of basin subsidence and basement uplift form a unified system of regional deformation, the greater Ancestral Rocky Mountains. Over this region, basin subsidence and basement uplift were approximately synchronous, beginning in the Chesterian-Morrowan, continuing through the Pennsylvanian, and ending in the Wolfcampian (although minor post-Wolfcampian deformation occurs locally). The basement uplifts show evidence for folding and faulting in the Pennsylvanian and Early Permian. Reverse faults and thrust faults have been drilled over many of the uplifts, but only in the Anadarko region has thrusting of the basement uplifts over the adjacent basin been clearly documented. Extensive basement-involved thrusting also occurs along the margins of the Delaware and Midland basins, and suggests that the entire greater Ancestral Rocky Mountains region probably formed as the result of northeast-southwest-directed-intraplate shortening. Deformation within the greater Ancestral Rocky Mountains was coeval with late Paleozoic subduction along much of the North American plate margin, and has traditionally been related to emplacement of thrust sheets within the Ouachita-Marathon orogenic belt. The nature, timing, and orientation of events along the Ouachita-Marathon belt make it difficult to drive the deformation of the greater Ancestral Rocky Mountains by emplacement of the Ouachita-Marathon belt along the southern margin of North America. We speculate the deformation was driven instead by events within a late Paleozoic Andean margin along the southwestern margin of North America. Evidence for the existence of this previously unrecognized Andean margin comes from east-central Mexico, where a Pennsylvanian and Permian volcanic arc indicates that a northeast-dipping subduction boundary lay to the south and west. The interpretation that deformation throughout the greater Ancestral Rocky Mountains occurs by basement-involved overthrusting on gently to moderately dipping thrust faults suggests that potential hydrocarbon reserves beneath crystalline thrust sheets may be much greater than is generally supposed.

Late Paleozoic Deformation of Interior North America: The Greater Ancestral Rocky Mountains

[1] We have obtained shear velocity structure beneath the northeastern United States and southeastern Canada using Rayleigh wave phases and amplitudes. Thin crust (36–42 km) is observed along the Atlantic coast and in the eastern Appalachian orogen, and thick crust (42–46 km) is imaged in the western Appalachians and in the western New York portion of the Grenville Province. The variation of crustal thickness correlates well with the observed Bouguer gravity anomalies. In the upper mantle, the high-velocity continental keel of cratonic North America is present in the western part of the study area, while a broad low-velocity region is imaged in New England from the Hudson River valley to the White Mountains. This low-velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the Monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel. In addition to lateral variations in velocity, we estimate the azimuthal dependence of phase velocity. Strong and relatively uniform shear wave splitting is observed in the study region, but at periods of 100 s or less, the average azimuthal anisotropy of Rayleigh waves is less than 1% and is not significantly different from zero at any individual period. This small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere. Much of the source of the shear wave splitting must lie deeper than 200 km.

/pdf/shear-velocity-structure-and-azimuthal-anisotropy-beneath-1ck7pj62d7.pdf

Shear velocity structure and azimuthal anisotropy beneath eastern North America from Rayleigh wave inversion

Basement-involved structures commonly occur as long, irregular chains of uplifts in foreland basins. These structures commonly contain significant hydrocarbon accumulations, with most major fields located on the broad crests of these structures. The search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures. Characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip, and gently dipping frontlimbs and backlimbs. Fault slip in the basement is usually accommodated in the cover by a triangular, widening-upward deformation zone on the forelimb, with the nature of deformation controlled primarily by the mechanical stratigraphy. If the cover contains interlayered competent and incompetent units, the incompetent units are characterized by significant penetrative deformation, whereas the competent units are faulted after a relatively small amount of penetrative deformation. Depending on the competency contrast between the basement and cover, the nature of basement, and the physical conditions of deformation, the deformation zone may also propagate downward into the basement. Gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault, respectively. Many basement faults are characterized by a number of synclinal fault bends within the basement, which result in long and gently dipping backlimbs. Forelimb panels are related to anticlinal bends that typically occur at the basement-cover interface, as well as at one or more locations in the sedimentary cover. Case studies of well-constrained examples of structures from the Bighorn and Uinta basins and the Central Basin platform, demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy. These models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data.

Foreland Basement-Involved Structures

Except in regions of recent crustal extension, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter or mineral reactions. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO2/3He ratios and delta13C(CO2) values observed in CO2-rich natural gases in the west Texas Val Verde basin and also the mantle 3He/22Ne ratios observed in other basin systems. Regional changes in CO2/3He and CO2/CH4 ratios can be explained if the CO2 input pre-dates methane generation in the basin, which occurred about 280 Myr ago. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago appears to be the only tectonic event with appropriate timing and location to be the source of the magmatic CO2. Our identification of magmatic CO2 in a foreland basin indicates that the origin of CO2 in other mid-continent basin systems should be re-evaluated. Also, the inferred closed-system preservation of natural gas in a trapping structure for approximately 300 Myr is far longer than the residence time predicted by diffusion models.

300-Myr-old magmatic CO2 in natural gas reservoirs of the west Texas Permian basin.

The Rome trough, a northeast-trending graben, is that part of the Cambrian interior rift system that extends into the central Appalachian foreland basin in eastern North America. On the basis of changes in graben polarity and rock thickness shown from exploration and production wells, seismic lines, and gravity and magnetic intensity maps, we divide the trough into the eastern Kentucky, southern West Virginia, and northern West Virginia segments. In eastern Kentucky, the master synthetic fault zone consists of several major faults on the northwestern side of the trough where the most significant thickness and facies changes occur. In southern West Virginia, however, a single master synthetic fault, called the East-Margin fault, is located on the southeastern side of the trough. Syndepositional motion along that fault controlled the concentrated deposition of both the rift and postrift sequences. The East-Margin fault continues northward into the northern West Virginia segment, apparently with less stratigraphic effect on postrift sequences, and a second major normal fault, the Interior fault, developed in the northern West Virginia segment. These three rift segments are separated by two basement structures interpreted as two accommodation zones extending approximately along the 38th parallel and Burning-Mann lineaments. Computer-aided interpretation of seismic data and subsurface geologic mapping indicate that the Rome trough experienced several major phases of deformation throughout the Paleozoic. From the Early(?)-Middle Cambrian (pre-Copper Ridge deposition), rapid extension and rifting occurred in association with the opening of the Iapetus-Theic Ocean at the continental margin. The Late Cambrian-Middle Ordovician phase (Copper Ridge to Black River deposition) was dominated by slow differential subsidence, forming a successor sag basin that may have been caused by postrift thermal contraction on the passive continental margin. Faults of the Rome trough were less active from the Late Ordovician-Pennsylvanian (post-Trenton deposition), but low-relief inversion structures began to form as the Appalachian foreland started to develop. These three major phases of deformation are speculated to be responsible for the vertical stacking of different structural styles and depositional sequences that may have affected potential reservoir facies, trapping geometry, and hydrocarbon accumulation.

/pdf/along-axis-segmentation-and-growth-history-of-the-rome-3olihfb7cu.pdf

Along-Axis Segmentation and Growth History of the Rome Trough in the Central Appalachian Basin

Structural cross sections and a regional tectonic map show that the Central Basin uplift consists of upthrust blocks of alternating vergence. The uplift is separated from the adjacent Delaware basin by a major fault that changes throw and structural style along strike, accompanying changes in block vergence found along the uplift. The west-facing basement blocks were thrust westward over the Delaware basin, whereas the adjacent blocks face eastward. The distribution and style of structures suggest that small amounts of left-lateral movement occurred along west-trending and northwest-trending faults that separated rising blocks of the uplift. Transfer of lateral movement from vertical, west-trending faults on the Central Basin uplift and the Ozona arch to the west-trending Mid-basin fault in the Delaware basin rotated blocks of the uplift in a clockwise sense. Faults and folds within the study area have trends and slip directions that are generally similar to those within the Reagan and Washita Valley fault zones in the Arbuckle uplift adjacent to the Anadarko basin. The difference in the style of deformation between the two uplifts relates to varied angular relationships between preexisting structure and the direction of late Paleozoic stress. The identity of structural trends within these two uplifts suggests that other late Paleozoic basins and uplifts of the Ancestral Rocky Mountains may have formed in a similar stress field and that crustal rotation was pervasive along the southwestern margin of the continent during the late Paleozoic Ouachita-Maratho deformation.

Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas (1)

Basement structure of the Appalachian foreland in West Virginia: Its style and effect on sedimentation

Interpretation of seismic data from the central Appalachians Valley and Ridge province reveals that a large blind thrust, the Broad Top thrust, is much more extensive than previously realized. This thrust sheet extends across the 15 mi (24 km) width of the Nittany anticlinorium, a dominate structural element that lies along the western margin of the Valley and Ridge province adjacent to the Plateau province in the Appalachian foreland. It extends for a distance along strike of at least 100 mi (160 km), and duplicates a 7800 ft (2380 m) thick sequence of Cambrian and Ordovician limestone and dolomite referred to as the Cambrian-Ordovician lithotectonic unit. Thinning of the duplicated Cambrian-Ordovician sequence along the length of the Nittany anticlinorium by as much as 2500 ft (760 m) observed on seismic lines is attributed to removal of less brittle intervals from the base of the upper sheet and top of the lower sheet. The missing section from this inter-sheet detachment zone may have been transported toward the foreland and incorporated into the leading parts of the thrust sheet. The tectonic thinning solves space problems associated with duplication of the lithotectonic unit beneath what have previously been considered abnormally deep synclines of the anticlinorium. The abnormal depth of these synclines occurs where tectonic thinning is located on the seismic lines. Structural traps in a triangle zone along the Allegheny structural front have been attributed to the intense shortening associated with emplacement of the Broad Top thrust sheet. The presence of the thrust sheet along the length of the structural front in West Virginia and Virginia enhances the probability of successful exploration for triangle zone structures along the entire length of the structural front. Structures produced by variable tectonic thinning in the inter-sheet detachment zone may also produce additional traps within the anticlinorium in the form of closure or associated fracture systems.

Broad Top Thrust Sheet: An Extensive Blind Thrust in the Central Appalachians (1)

A block diagram of the Cambrian-Ordovician lithotectonic unit illustrates three-dimensional structural relationships within that sequence along the length of the central Appalachian Valley and Ridge and High Plateau provinces. The diagram shows that the Valley and Ridge province is divisible into areas within which shortening is relatively constant in the Cambrian-Ordovician lithotectonic unit. These areas are bounded by zones across which significant differences in shortening occur. These transition zones contain major cross-strike structural discontinuities in surface structure; in some instances, these discontinuities extend across the Valley and Ridge province and into the High Plateau province. Increases in fold amplitude and number occur in the cover of the Plateau, across strike from the more intensely deformed areas of the Valley and Ridge, where shortening within the Cambrian-Ordovician unit is significantly greater than elsewhere within that province. Structurally controlled gas accumulations are more prevalent in these areas of the Plateau.

Three-Dimensional Structural Interrelationships Within Cambrian-Ordovician Lithotectonic Unit of Central Appalachians

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