John H. McBride

SEISMIC EXPLORATION OF CONTINENTAL STRIKE-SLIP ZONES

Abstract Seismic exploration of both Palaeozoic and active strike-slip zones shows strike-slip faults that penetrate all or most of the crust. Offsets on the Moho are evident, particularly at young and active zones with a component of compression, such as the Alpine Fault of New Zealand where a change in crustal thickness of about 20 km is observed. Moho offsets for the old Palaeozoic strike-slip zones are usually much less prominent. Careful migration of crustal seismic reflection data from some of these zones shows that instead of sharp offsets, the Moho structure consists of a localized keel-type crustal thickening of a few kilometres in amplitude and occurs over a zone approximately 10 km wide. The large Moho offsets of young strike-slip zones may in some cases partially decay with time. Active strike-slip zones are becoming an important focus of study, but seismic exploration is hampered by complex near-surface geology, 3D structure and the difficulty of imaging steeply dipping structure in the subsurface. In order to meet this challenge, a wide range of seismic techniques is now being deployed. These include wide-angle seismic reflection, refraction, P-wave delays and the study of guided S-waves. Results from California give geophysical images of vertical strike-slip faults that penetrate to the Moho. In contrast, the Alpine Fault of New Zealand appears to be a surface manifestation of an inclined (∼40°) ramp, extending down to the lower crust and along which uplift and exhumation of the continental crust, and possibly strike-slip motion, is taking place.

STYLE AND ORIGIN OF MID-CARBONIFEROUS DEFORMATION IN THE ILLINOIS BASIN, USA : ANCESTRAL ROCKIES DEFORMATION?

Abstract The integration of outcrop, borehole, and seismic reflection data from the Illinois Basin and adjacent eastern Ozark Dome in Illinois and Missouri sheds new light on the style and origin of intra-cratonic deformation. Typical structures of this region are high-angle reverse faults in Precambrian basement that propagated upward to monoclines and asymmetrical anticlines in Paleozoic sedimentary cover. These are compressive-block structures directly analogous to (although smaller than) `Laramide-style structures of the Colorado Plateau and Rocky Mountain foreland. Central Illinois Basin structures were active chiefly during late Chesterian through Atokan (i.e., late Mississippian to middle Pennsylvanian; mid-Carboniferous) time, with continued intermittent movement through the late Pennsylvanian. Both the style and timing of deformation match those of the `Ancestral Rocky Mountains orogeny of the southern Midcontinent and Rocky Mountain region of the USA. Deformation in the central Illinois Basin has generally been attributed to the nearby late Paleozoic Appalachian–Ouachita orogeny, even though the Illinois Basins compressive block structural style is foreign to the Appalachian foreland. We suggest that the Ancestral Rockies event may have played a significant role in the development of Pennsylvanian-age compressive-block structures in Illinois and Missouri.

UPPER CRUST BENEATH THE CENTRAL ILLINOIS BASIN, UNITED STATES

Newly available industry seismic reflection data provide critical information for understanding the structure and origin of the upper crust (0–12 km depth) beneath the central Illinois basin and the seismic-tectonic framework north of the New Madrid seismic zone in the central Mississippi Valley. Mapping of reflector sequences furnishes the first broad three-dimensional perspective of the structure of Precambrian basement beneath the central United States Midcontinent. The highly coherent basement reflectivity is expressed as a synformal wedge of dipping and subhorizontal reflections situated beneath the center of the Illinois basin that thickens and deepens to the northeast (e.g., 0 to ∼3 km thickness along a 123 km south to north line). The thickening trend of the wedge qualitatively mimics the northward thickening of the Late Cambrian Mt. Simon Sandstone; however, other Paleozoic units in the Illinois basin generally thicken southward into the basin center. The seismic data also reveal an anomalous subsequence defined by a spoon-shaped distribution of disrupted reflections located along the southern margin of the wedge. The boundaries of this subsequence are marked by distinct steeply dipping reflections (possible thrust faults?) that continue or project up to antiformal disruptions of lower Paleozoic marker reflectors, suggesting Paleozoic or possibly later tectonic reactivation of Precambrian structure. The areal extent of the subsequence appears to roughly correspond to an anomalous concentration of larger magnitude upper to middle crustal earthquakes. There are multiple hypotheses for the origin of the Precambrian reflectivity, including basaltic flows or sills interlayered with clastic sediments and/or emplaced within felsic igneous rocks. Such explanations are analogous to nearby Keweenawan rift-related volcanism and sedimentation, which initiated during Proterozoic rifting, and were followed eventually by reverse faulting along the rift margins caused by Grenville compression.

Understanding basement tectonics of an interior cratonic basin: southern Illinois Basin, USA

Abstract Although the Illinois Basin is one of the worlds most studied interior cratonic basins, little is known of its deep structure or seismotectonic framework. In this study, seismic reflection profiles over the major structures of the basin reveal reverse faults that penetrate the upper Precambrian crust, disrupt the surface of Precambrian basement, and accommodate folding of Paleozoic sediments. Observations of deformation indicate concordant folding of the basement surface and overlying strata and/or possible `Laramide-style basement-controlled folding. Most profiles show evidence of dual fold vergence and some profiles suggest transpression involving lower Paleozoic strata and the basement surface. The moderate seismicity in the southern Illinois Basin, just north of the New Madrid seismic zone (NMSZ), defines an active deformation regime of interpreted NNE-striking dextral strike-slip and reverse faulting. Upper crustal fault structure, such as shown in this study, could provide a fabric capable of reactivation by stress manifested as contemporary seismicity. A tectonic framework for the possible kinematic linkage of deformation in the NMSZ and the southern Illinois Basin is proposed on the basis of a hypothetical distribution of branched fault (and/or fold) patterns in southern Illinois that broaden northward beyond the NMSZ. Such a dispersive deformation pattern is thought to be associated with the gradual cessation in displacement north of the NMSZ (i.e., displacement is dissipated over many faults). The abrupt change in seismotectonic regime from a relatively discrete, highly seismic NMSZ with infrequent great earthquakes to a multi-stranded fault pattern with more frequent moderate earthquakes in southern Illinois is typical of active strike-slip systems involving a bend or shift in direction.

Seismic Reflection Images of Shallow Faulting, Northernmost Mississippi Embayment, North of the New Madrid Seismic Zone

High-resolution seismic reflection surveys document tectonic faults that displace Pleistocene and older strata just beyond the northeast termination of the New Madrid seismic zone, at the northernmost extent of the Mississippi embayment. These faults, which are part of the Fluorspar Area fault complex in southeastern Illinois, are directly in line with the northeast-trending seismic zone. The reflection data were acquired using an elastic weight-drop source recorded to 500 msec by a 48-geophone array (24-fold) with a 10-ft (∼3.0 m) station interval. Recognizable reflections were recorded to about 200 msec (100–150 m). The effects of multiple reflections, numerous diffractions, low apparent velocity (i.e., steeply dipping) noise, and the relatively low-frequency content of the recorded signal provided challenges for data processing and interpreting subtle fault offsets. Data processing steps that were critical to the detection of faults included residual statics, post-stack migration, deconvolution, and noise-reduction filtering. Seismic migration was crucial for detecting and mitigating complex fault-related diffraction patterns, which produced an apparent `folding of reflectors on unmigrated sections. Detected individual offsets of shallow reflectors range from 5 to 10 m for the top of Paleozoic bedrock and younger strata. The migrated sections generally indicate vertical to steeply dipping normal and reverse faults, which in places outline small horsts and/or grabens. Tilting or folding of stratal reflectors associated with faulting is also locally observed. At one site, the observed faulting is superimposed over a prominent antiformal structure, which may itself be a product of the Quaternary deformation that produced the steep normal and reverse faults. Our results suggest that faulting of the Paleozoic bedrock and younger sediments of the northern Mississippi embayment is more pervasive and less localized than previously thought.

Variable deep structure of a midcontinent fault and fold zone from seismic reflection: La Salle deformation belt, Illinois basin

Deformation within the United States midcontinent is frequently expressed as quasilinear zones of faulting and folding, such as the La Salle deformation belt, a northwest-trending series of folds cutting through the center of the Illinois basin. Seismic reflection profiles over the southern La Salle deformation belt reveal the three-dimensional structural style of deformation in the lower Paleozoic section and uppermost Precambrian(?) basement. Individual profiles and structural contour maps show for the first time that the folds of the La Salle deformation belt are underlain at depth by reverse faults that disrupt and offset intrabasement structure, offset the top of interpreted Precambrian basement, and accommodate folding of overlying Paleozoic strata. The folds do not represent development of initial dips by strata deposited over a preexisting basement high. Rather, the structures resemble subdued “Laramide-style” forced folds, in that Paleozoic stratal reflectors appear to be flexed over a fault-bounded basement uplift with the basement-cover contact folded concordantly with overlying strata. For about 40 km along strike, the dominant faults reverse their dip direction, alternating between east and west. Less well expressed antithetic or back thrusts appear to be associated with the dominant faults and could together describe a positive flower structure. The overall trend of this part of the La Salle deformation belt is disrupted by along-strike discontinuities that separate distinct fold culminations. Observations of dual vergence and along-strike discontinuities suggest an original deformation regime possibly involving limited transpression associated with distant late Paleozoic Appalachian-Ouachita mountain building. Moderate-magnitude earthquakes located west of the western flank of the La Salle deformation belt have reverse and strike-slip mechanisms at upper crustal depths, which might be reactivating deep basement faults such as observed in this study. The La Salle deformation belt is not necessarily typical of other well-known major midcontinent fault and fold zones, such as the Nemaha ridge, over which Paleozoic and younger sediments appear to simply be draped.

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an “acoustic window” into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse.nnThe appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west–oriented profiles, is characteristic of the East Shetland platform. However, a north-south–oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated appearance. Regional seismic velocity models derived from refraction data suggest that this reflectivity correlates with a continuous lower crustal layer that has an intermediate seismic velocity. The lower crustal reflectivity is determined to be older than Mesozoic age by the bending down and truncation of the two reflectivity levels at the western margin of the North Sea Viking graben by a major mantle reflector inferred to be associated with Mesozoic rifting. The results of this study are thus in contrast with orthodox interpretations of the reflective layered lower crust as being caused by mantle-derived igneous intrusion or by deformation fabrics associated with stretching in response to continental rifting.