Previously Interpreted Research Articles

Timing of Postglacial Cirque Reoccupation in the Northern Uinta Mountains, Northeastern Utah, U.S.A.

Radiocarbon-dated organics from lacustrine cores and colluvial basins, combined with measurements of lichens growing on moraines previously interpreted as Neoglacial in age, argue that the extent of post-Pleistocene ice in the northern Uinta Mountains was limited. An AMS date of 12,190 ± 120 14C yr BP on basal organic sediment from a tarn indicates that deglaciation was locally complete by ~14 ka BP. Additional radiocarbon ages of ~10 ka BP from two colluvial basins behind end moraines suggest that these moraines date to the latest Pleistocene. Cross-cutting moraines from a fourth site indicate that an active cirque-glacier system was present at some point after the terminal Pleistocene deglaciation. Maximum diameters of Rhizocarpon geographicum lichens on these moraines, however, suggest they may predate Little Ice Age deposits in the Wind River Range and Sierra Nevada. The convergence of evidence suggests that glaciers in the northern Uintas during the Holocene were extremely limited in extent, and restricted to the middle Neoglacial period. This preliminary conclusion contradicts some previous interpretations of the upper subalpine geomorphology of the range, which postulated widespread post-Pinedale glaciation, and multiple stades of Holocene Neoglaciation.

Timing of Postglacial Cirque Reoccupation in the Northern Uinta Mountains, Northeastern Utah, U.S.A.

Radiocarbon-dated organics from lacustrine cores and colluvial basins, combined with measurements of lichens growing on moraines previously interpreted as Neoglacial in age, argue that the extent of post-Pleistocene ice in the northern Uinta Mountains was limited. An AMS date of 12,190 ± 120 14C yr BP on basal organic sediment from a tarn indicates that deglaciation was locally complete by ~14 ka BP. Additional radiocarbon ages of ~10 ka BP from two colluvial basins behind end moraines suggest that these moraines date to the latest Pleistocene. Cross-cutting moraines from a fourth site indicate that an active cirque-glacier system was present at some point after the terminal Pleistocene deglaciation. Maximum diameters of Rhizocarpon geographicum lichens on these moraines, however, suggest they may predate Little Ice Age deposits in the Wind River Range and Sierra Nevada. The convergence of evidence suggests that glaciers in the northern Uintas during the Holocene were extremely limited in extent, and restricted to the middle Neoglacial period. This preliminary conclusion contradicts some previous interpretations of the upper subalpine geomorphology of the range, which postulated widespread post-Pinedale glaciation, and multiple stades of Holocene Neoglaciation.

Seismic Interpretation of the Deep Structure of the Wabash Valley Fault System

Interpretations of newly available seismic reflection profiles near the center of the Illinois Basin indicate that the Wabash Valley Fault System is rooted in a series of basement-penetrating faults. The fault system is composed predominantly of north-northeast-trending high-angle normal faults. The largest faults in the system bound the 22-km wide 40-km long Grayville Graben. Structure contour maps drawn on the base of the Mount Simon Sandstone (Cambrian System) and a deeper pre-Mount Simon horizon show dip-slip displacements totaling at least 600 meters across the New Harmony fault. In contrast to previous interpretations, the N-S extent of significant fault offsets is restricted to a region north of 38° latitude and south of 38.35° latitude. This suggests that the graben is not a NE extension of the structural complex composed of the Rough Creek Fault System and the Reelfoot Rift as previously interpreted. Structural complexity on the graben floor also decreases to the south. Structural trends north of 38° latitude are offset laterally across several large faults, indicating strike-slip motions of 2 to 4 km. Some of the major faults are interpreted to penetrate to depths of 7 km or more. Correlation of these faults with steep potential field gradients suggests that the fault positions are controlled by major lithologic contacts within the basement and that the faults may extend into the depth range where earthquakes are generated, revealing a potential link between specific faults and recently observed low-level seismicity in the area.

Does the Great Glen fault really disrupt Moho and upper mantle structure?

Many previous interpretations of deep seismic reflection profiles across the Great Glen strike‐slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the Moho discontinuity. Such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the British Isles. Reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the “Moho step” interpretation. Diffraction analysis and seismic migration applied to reflection data north of Shetland across the northward continuation of the Great Glen fault (Walls Boundary fault) indicate a narrow synform or “keel” developed on the Moho directly beneath the fault. This keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault. North and west of Ireland, analyses of amplitudes, frequencies, and the geometrical behavior of reflections upon migration show that structures previously interpreted as Moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin. In one location north of Ireland where some three‐dimensional control exists, the vertically downward projection of the Great Glen fault intersects, without disrupting, dipping structure in the upper mantle. This observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp (or blind thrust) in the uppermost mantle, somewhat analogous to models developed for the San Andreas fault that indicate displacement along the fault to be laterally offset within the middle crust. One of the principal conclusions of this study, that major vertical steps on the Moho beneath the Great Glen fault are difficult to support from the available seismic data, is consistent with Theologically based studies which predict that Moho “topography” such as vertical steps is unlikely to be preserved over long periods of geologic time.

Crustal structure of central Saudi Arabia determined from seismic refraction profiling

Based on a two-dimensional ray path interpretation of travel time and wave amplitude ratios, a detailed crustal velocity structure of central Saudi Arabia has been derived using the deep-seismic refraction profile of Saudi Arabia. The crustal model derived significantly differs from previous interpretations carried out on the same data set since more energetic crustal phases were considered in this analysis. The two-dimensional crustal model derived shows that the crust consists of four distinct layers. It is approximately 42 km thick under the Arabian platform and thins gradually in a southwest direction to about 38 km under the shield. The upper crust consists of two layers. The first one has a P-wave velocity of about 6.1 km/s, and is about 3 km thick thinning to about 1 km under the platform. The second layer has a P-wave velocity of about 6.2 km/s and is about 14 km thick but thins to about 7 km under the platform. The intermediate crustal layer has a P-wave velocity of about 6.4 km/s, is about 7.5 km thick under the shield; under the platform it is about 16 km thick with a velocity of about 6.3 km/s. The lower crust has a P-wave velocity of about 6.9 km/s and is about 15 km thick. A decrease in the upper mantle Pn velocity from 8.2 to 8.1 km/s seems to accompany the crustal thinning. The Moho discontinuity is sharply defined beneath the shield and gently dips towards the northeast. A significant crustal inhomogeneity in the region separating the Afif terrane from the Ar Rayn terrane is interpreted as a thrusting system, rather than a distinct vertical discontinuity in the crustal structure, as previously interpreted. This region between the two terranes, which is of different crustal composition, is probably the result of thrusting and emplacement of nappes of upper crustal materials of the Afif terrane onto the Ar Rayn foreland. The nature and location of this zone were constrained by travel time and wave amplitude ratio measurements. The detailed velocity crustal model derived in this study is of significant importance in earthquake studies, tectonics, and determining the evolution of the Arabian Plate.

Tectonic implications of subthrust structures revealed by seismic profiling of Appalachian‐Ouachita Orogenic Belt

A growing body of deep seismic reflection profiles across the Appalachian‐Ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting, of later thrusting, or of some combination of rifting and thrusting. The subthrust features can be placed into three general categories: (1) small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading, perhaps reactivating earlier rifting faults, (2) moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland, interpreted in part as strata deposited during the earlier rifting, and (3) hinterland‐dipping reflection sequences beneath more interior portions of the mountain belt, previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin. These interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings. The comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults, the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process. These dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the Appalachian foreland. The comparisons also suggest an alternative to interpretations of the hinterland‐dipping sequences as thrust‐related, in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins. The implication of this new interpretation is that the actual continent/ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the Appalachians and Ouachitas.

Regional structure and syntectonic granite intrusion in the Dalradian of the Gweebarra Bay area, Donegal

6 phases of deformation are recognized in the Appin Group (Lower Dalradian) metasediments around Gweebarra Bay. The major early folds are F 2 in age (including the Crohy and Maghery Anticlines) facing up to the N or NE. S 2 transposes an earlier fabric (S 1 ) which is only rarely preserved. No major or minor F 1 folds are recognized and the facing of S 1 is unknown. D 3 produces structures verging S on S 2 but does not produce major folds comparable with D 2 . The Maghery Anticline, previously interpreted as changing from an open upright fold in the SE to overturned in the NW, is shown to be overturned throughout. Previous interpretation involving the re-folding of the Maas Syncline by the upright Mulnamin Anticline is not supported and both major folds are now interpreted as D 4 , in age, with the axial plane of the Mulnamin Anticline dipping moderately or gently S. D 4 , is a regional deformation phase synchronous with the intrusion of the Ardara Granite. D 5 , structures are upright or face to the N. The development of a major NE–SW trending sinistral shear zone adjacent to the SW end of the Main Donegal Granite is synchronous with D 5 , deformation, F 5 , folds rotating into sub-parallelism with the shear planes.