1. SPATIAL VARIATION IN FOLD-AND-THRUST BELT STRUCTURAL GEOMETRY AND RESTORED SYN-TECTONIC LOADS: PENNSYLVANIA VALLEY AND RIDGE
EVANS, Mark A., Department of Physics and Earth Science, Central Connecticut State University, New Britain, CT 06050
ABSTRACT:
In fold-and thrust belts, syn-tectonic loads from overthrusting and/or sedimentation may drive forward propagation of thrust sheets. In order to evaluate the contribution of syn-tectonic loads in the development of the Pennsylvania Valley & Ridge province, vein samples were collected along four transects from the southern Valley and Ridge province to the anthracite belt. Fluid inclusion microthermometry and barometry of the vein minerals provides estimates of syntectonic overburdens.
Four line-balanced structural cross sections constructed through the region show that the structural geometry of the fold-and-thrust belt varies markedly from the ~030°-striking southern segment to ~060°-striking Juniata culmination. In the south, the eastern part of the belt is defined by a series of imbricated Cambro-Ordovician carbonate horses with leading-edge fault-propagation style folds. These give way to the Broadtop synclinorium in the central part of the belt, and then two additional carbonate horses with similar leading-edge folds toward the Appalachian Structural Front (ASF). The deformed length of the duplex is 80 km, with a retrodeformed length of the carbonate strut of 130 km. In the central and eastern parts of the salient, the structural geometry is defined by a duplex with imbricate horses of Cambro-Ordovician carbonates, that transition to an antiformal stack of carbonate thrust sheets near the ASF. The deformed length of the duplex is these areas is ~98 km, with a retrodeformed length of 205 km.
Based on retrodeformed sections and fluid inclusion microthermometry data of CH4±CO2 and aqueous fluid inclusions, syn-Alleghenian sediment and/or thrust loads also vary across the region. In the southern segment, estimated post-Carboniferous loads range from 4 to 5 km in the east and central parts of the belt to less than 1.5 km at the ASF. Similarly, in the central and easternmost sections, maximum loads are 4 to 5 km toward the hinterland. However, sample sites at and near the ASF in the central part of the salient indicate little post-Carboniferous load. This area corresponds to a low maturity region in the Middle Devonian section in the Plateau province. It is possible that the leading duplex structure in the salient was a persistent topographic high and Permian sediments were deflected to the northeast and west.
DETERMINING OVERBURDEN:
Microthermometric analysis of aqueous and methane fluid inclusions (Figure 2) in pre- to syn-folding quartz and calcite veins were used to determine the trapping pressure and trapping temperature of the inclusions. These values were then used to estimate the paleo-overburden during vein formation. Only primary inclusions were used for this analysis. The estimated overburdens were then plotted on restored structural cross sections of the Valley & Ridge province based on the stratigraphic position of the sample. Published stratigraphic thicknesses for the Paleozoic cover rock sequence were used where available. Restored overburdens that exceeded the known thickness to the top of the Mississippian were assumed to be syn-Alleghenian tectonic load. A A’ C B A
Figure 2. Examples of fluid inclusions.
A) Coeval primary aqueous and methane
Inclusions, B) primary aqueous inclusions,
and C) primary methane inclusions. B B’
Figure 3. Examples of methods used to determine trapping pressure and overburden.
Figure 5. Burial history diagrams for the Devonian Shale interval for seven subareas shown on Figure 1. Dark gray shading is oil generation window. Wide gray bar is timing of the Valley & Ridge folding based on Stamatakos et al. (1996) and Evans (2010). Narrow vertical tan bar is timing of the Appalachian Wide Stress Field from Engelder and Whitaker (2006). Geothermal gradients were varied from 25° km-1 during stable deposition to 20° km-1 during rapid sedimentation. C C’ A GL
Figure 6. Map of the Valley & Ridge province showing the estimate of average maximum Alleghenian syn-tectonic load (in kilometers). Shaded area is where estimated load is greater than 4km. Map will become better defined as more data is added.
AB&BE B EB RA C
CONCLUSIONS:
The structural geometry of the Pennsylvania Valley & Ridge Province changes markedly from the southern part of the salient to the northeastern part.
In the south, the eastern part of the belt is defined by a series of imbricated Cambro-Ordovician carbonate horses with leading-edge fault-propagation style folds. These give way to the Broadtop synclinorium in the central part of the belt, and then two additional carbonate horses with similar leading-edge folds toward the Appalachian Structural Front (ASF). In this area, the duplex exhibits ~40% shortening.
In the central and eastern parts of the salient, the structural geometry is defined by a duplex with imbricate horses of Cambro-Ordovician carbonates, that transition to an antiformal stack of carbonate thrust sheets near the ASF. In these areas, the duplex exhibits ~48 to 53% shortening.
Restored overburdens based on fluid inclusion microthermometry are quite variable when plotted in deformed sections, but on restored sections reflect a general pattern of high syn-Alleghenian tectonic load toward the southeast of up to 5+ km. This load may be due to structural overthrusting and/or to sediment shedding off of uplifted mountains.
The syn-tectonic load on the restored sections decreases rapidly toward the hinterland, with ~2 km in the center of the fold-and-thrust belts and <1 km at the ASF. However, this is based on limited data, and with the acquisition of addition data, this pattern may change.
The <1 km loads at the ASF are reflected in the presence of liquid hydrocarbon inclusions in veins along the central ASF. These low maturity inclusions are not found anywhere else in the Valley & Ridge. D’ D A’
H&BR B’ C’
BED D B’
Figure 4. Line-balanced structural cross sections across the Pennsylvania Valley & Ridge province. Also, at half-scale, restored sections. Red filled circles indicate locations of fluid inclusion microthermometry data used in determining paleo-overburden values. Estimated paleo-overburden given in restored sections as ranges. Blue lines are data from calcite, black are data from quartz.
Figure 1. Geologic map of the study area. Shown are field stations, most of which have vein mineral samples for fluid inclusion microthermometry. Also shown are the locations of the four cross sections shown in Figure 3 and he burial history curves in Figure 4. Inset map shows location of the study area. To the left is a stratigraphic column of the study area, color-coded to the geologic map. Major detachment horizons are shown with an arrow.