Master Decollement Research Articles

Interseismic Strain Accumulation on Faults Beneath Los Angeles, California

AbstractGeodetic data show that the Los Angeles metropolitan area is undergoing 8–9 mm/year of north‐south tectonic shortening associated with the Big Bend of the San Andreas Fault. This shortening has been linked to multiple damaging twentieth century thrust earthquakes as well as possible Mw ≥ 7.0 Holocene thrust events beneath central Los Angeles. To better characterize this seismic hazard, we assess how this shortening is being accommodated by interseismic strain accumulation on subsurface faults, incorporating detailed seismology‐ and geology‐based models of fault geometry and the low‐stiffness Los Angeles sedimentary basin. We find that strain accumulation on local strike‐slip faults likely contributes no more than 1–2 mm/year of the shortening. We formally invert the geodetic data for the pattern of interseismic strain accumulation on the north dipping Sierra Madre, Puente Hills, and Compton thrust faults and a master decollement. We explore the impact of the assumed material model, strain accumulation on faults to the west and east, and other model assumptions. We infer that the three faults slip at 3–4 mm/year over the long term and are currently partially or fully locked and accruing interseismic strain on their upper sections. This locking implies an annual deficit of seismic moment, 1.6 + 1.3/−0.5 × 1017 Nm/year in total, which is presumably balanced over the long‐term average by the moment released in earthquakes. The depth distribution of moment deficit accumulation rate matches that of seismicity rates in Los Angeles to first order, in part, because the models incorporate the blind nature of the Puente Hills and Compton Faults.

An upper-crustal fold province in the hinterland of the Sevier orogenic belt, eastern Nevada, U.S.A.: A Cordilleran Valley and Ridge in the Basin and Range

Overprinting Cenozoic extension hinders analysis of Cordilleran contractional deformation in the hinterland of the Sevier thrust belt in Nevada. In this study, a 1:250,000 scale paleogeologic map of eastern Nevada, showing spatial distributions of Paleozoic–Mesozoic rocks beneath a Paleogene unconformity, is combined with dip magnitude maps for Paleozoic–Mesozoic and Tertiary rocks, published sedimentary thickness records, and a published reconstruction of extension, in order to define and regionally correlate thrust faults and folds, and estimate the pre-extensional amplitude, wavelength, and limb dips of folds. A new structural province, the Eastern Nevada fold belt, is defined, consisting of a 100-km-wide region containing five first-order folds that can be traced for map distances between 100 and 250 km and have amplitudes of 2–4 km, wavelengths of 20–40 km, pre-extensional limb dips typically between 10° and 30°, and deform rocks as young as Jurassic and Early Cretaceous. No regional-scale thrust faults or decollement horizons breach modern exposure levels in the Eastern Nevada fold belt. First-order folds of the Eastern Nevada fold belt are interpreted to have formed above a deep (≥10 km below the Paleogene unconformity), blind decollement or shear zone, perhaps the westward projection of the master decollement of the Sevier thrust belt. Three hinterland structural provinces, the Central Nevada thrust belt, Western Utah thrust belt, and the intervening Eastern Nevada fold belt, collectively record low-magnitude (a few tens of kilometers), upper-crustal shortening that accompanied Cretaceous translation of the Cordilleran passive-margin basin ∼220 km eastward during the Sevier orogeny. Low deformation magnitudes in the hinterland are attributed to the rheological competence of this thick basin.

Late to post-Appalachian strain partitioning and extension in the Blue Ridge of Alabama and Georgia

Structural observations and U-Pb and 40 Ar/ 39 Ar isotopic age dates are reported for shear zones and metamorphic rocks in the southernmost Appalachian Blue Ridge. Two major mylonite zones, the Goodwater-Enitachopco and Alexander City fault zones, have retrograded peak amphibolite facies fabrics and assemblages in rocks of the ancient Laurentian margin. Both faults are within a zone of transition between Carboniferous (Alleghanian) west-directed thrusts in the foreland and synchronous strike-parallel dextral shear zones in the hinterland. The 40 Ar/ 39 Ar hornblende and muscovite dates record late Mississippian cooling and exhumation from the Late Devonian (Neoacadian orogeny, 380–340 Ma) peak. Retrograde mylonites of the Goodwater-Enitachopco fault are of two types. Earlier formed, type 1, upper greenschist to lower amphibolite facies shears are roughly coplanar with the dominant schistosity of the country rock, and show northwest-southeast stretching. Later formed, type 2 shears are discrete, steeply dipping, middle to upper greenschist facies shear zones that cut across the type 1 shears, displacing them in an oblique dextral and normal slip sense. A 366.5 ± 3.5 Ma U-Pb SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) date on zircon from a prekinematic trondhjemite dike that is cut by a type 2 shear zone places a maximum age on the time of movement along the Goodwater-Enitachopco fault. The 40 Ar/ 39 Ar cooling dates place a minimum on the timing of extensional movement along the type 1 shears of between ca. 334 and 327 Ma. The Goodwater-Enitachopco fault coincides at depth with a basement step up that has been interpreted as a Cambrian rift fault formed along the ancient Laurentian margin, possibly reflecting its reactivation during Mesozoic rifting of Pangea. The Alexander City fault zone is a middle greenschist facies, dextral strike-slip fault rather than a west-vergent thrust fault, as was previously thought. This fault zone is obliquely cut and extended by more east-trending, subvertical, cataclastic faults (Mesozoic?) characterized by intense quartz veining. These brittle faults resemble those in other parts of the Blue Ridge, Inner Piedmont, and Pine Mountain terrane and, together with the Goodwater-Enitachopco and Towaliga faults, they appear to form a broad graben-like structure across the entire piedmont. Shoulder rocks flanking the Alexander City fault zone contain earlier formed (peak to late peak metamorphic) dextral shear zones. A 369.4 ± 4.8 Ma U-Pb thermal ionization mass spectrometry date on zircon records the time of crystallization of a dike that cuts one of the shears bracketing the peak metamorphic fabric to between ca. 388 and 369 Ma and places a minimum age for right-slip shearing. Similar kinematics, geometries, tectonostratigraphic positions, and timing indicate that these Devonian shears are more southern counterparts to the system of Neoacadian dextral faults exposed in the North Carolina Blue Ridge. Kinematic analysis of the Goodwater-Enitachopco and Alexander City faults document that dextral strains in the Alabama and western Georgia Blue Ridge are partitioned much farther toward the foreland than is reported to the northeast, likely as a consequence of the southern Appalachian master decollement having passed obliquely across a several kilometer step up along the Cartersville transform. The top-to-the-south-southeast normal-slip component of movement along the Goodwater-Enitachopco fault is unusual, considering its position far toward the foreland. Loose timing constraints for this extensional event (late Carboniferous to Early Jurassic) leave room for several tectonic explanations, but we favor the following. (1) Late Pennsylvanian to Early Permian crustal thickening created a wedge of Blue Ridge rocks bound above by the Goodwater-Enitachopco, below by the decollement, and to the northwest (present-day direction) by a topographically steep mountain front. (2) Further convergence and crustal thickening caused this wedge to gravitationally collapse with southward-driven motion. (3) Mesozoic rifting reactivated some of the faults as the Gulf of Mexico began to open.

Wide-angle seismic imaging of the Elberton granite, Georgia: A pilot study using instantaneous blasts at dimension-stone quarries

Wide-angle recordings of small, multiple-hole blasts at dimension-stone quarries in the Elberton granite show coherent reflections at offsets between 7 and 15 km. In contrast with much larger blasts at crushed-stone quarries, this type of blast is fired without delays between holes. The data were recorded with an array of twenty portable seismographs using three-component, 4.5-Hz geophones. Migration of source gathers for two of the blasts shows a number of subhorizontal interfaces at depths between 2 and 4 km that may represent a layered complex near the base of the granite. Other reflectors include southeast-dipping interfaces at depths between 6 and 9.3 km and a more gently southeast-dipping complex at 9.5–11 km that may represent the master decollement. The results of the pilot study show that this type of quarry blast can be useful for imaging structure within the upper crust. Although preliminary, the wide-angle results support the interpretation that the Elberton granite is a tabular body less than 4 km thick.

Contrasts in Tectonic style of the Central and Southern Appalachians, United States: Insights from seismic reflection data

Two reflection seismic transects, one across the central Appalachians in Virginia and the other across the southern Appalachians in Georgia, reveal a significant contrast in mid- and lower crustal reflectivity from east to west. Data from east of the Blue Ridge geologic province in Virginia and to the east of the Inner Piedmont in Georgia show a highly reflective crust extending from the near-surface to the Moho, including zones of east-dipping reflections, a sub-horizontal reflection signature at ∼7 seconds, and a west-dipping Moho. Reflection seismic data from west of the Blue Ridge in Virginia and Inner Piedmont farther south are characterized by reflector geometries related to deformation above a master decollement, leading to classic ‘thin-skinned’ tectonic structures in the overlying allochthon, and few if any apparent structures in the underlying basement. The location of the Iapetan rifted margin, the preexistence of favorably oriented structures to the east of this point, and sub-horizontal weak zones within the lower Paleozoic shelf strata have played critical roles in the distribution of seismic reflector geometry. Seismic reflection signatures seen in the southeastern United States are a result of multiple episodes of deformation from the early Paleozoic through the middle Mesozoic. Oblique stresses during late Paleozoic time produced transpression that manifested itself as predominantly strike-slip faulting to the east of the Blue Ridge/Inner Piedmont. Onlapping lower Paleozoic shelf strata responded to tectonic stresses through thin-skinned deformation above a master decollement during the late Paleozoic Alleghanian orogeny, aided in part by sub-horizontal zones of weakness in the strata. This partitioning of strain was supported via tectonic buttressing provided by Precambrian continental crust that was little deformed in the Taconic orogeny. During the Alleghanian orogeny, the variations in Valley and Ridge deformational style between the central and southern Appalachians were controlled by the original shape of the continental edge. Further deformation during Mesozoic extension occurred to the east of the Precambrian rift margin in the region where favorably oriented faults were reactivated, leading to the rotation of the fault zones from more steeply dipping initial orientations, the merging of the mid-crustal reflection zone with the Moho, and the formation of Mesozoic basins and antiformal reflections in the seismic sections.

Pseudotachylyte from an ancient accretionary complex: Evidence for melt generation during seismic slip along a master décollement?

Pseudotachylyte is present in the Shimanto accretionary complex in Shikoku, Japan. The pseudotachylyte occurs as a vein in a fault zone and displays flow structure, TiO 2 microlites, and other melting-related textures. The mineral assemblage of the pseudotachylyte suggests that the temperature was raised at least 450 °C above the background levels in the host rocks. This is the first direct evidence of seismogenic fault rock from an accretionary complex. The pseudotachylyte-bearing fault is a roof thrust above underthrusted melange, which is overlain by an offscraped package of coherent units. The geologic setting of the pseudotachylyte is consistent with melt generation during seismic slip along the master decollement in the subduction zone.

Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California

A seismic refraction and low-fold reflection survey, known as the Los Angeles Region Seismic Experiment (LARSE), was conducted along a transect (line 1) extending from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. The chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern California that have been debated for decades and have important implications for earthquake hazard assessment. The results (or constraints) are as follows. (1) The maximum depth of the Los Angeles basin along line 1 is 8–9 km. (2) The deep structure of the Sierra Madre fault zone in the northern San Gabriel Valley is as follows. The Duarte branch of the Sierra Madre fault zone forms a buried, 2.5-km-high, moderately north dipping buttress between the sedimentary and volcanic rocks of the San Gabriel Valley and the igneous and metamorphic rocks of the San Gabriel Mountains. (For deeper structure, see following.) (3) There are active crustal decollements in southern California. At middle-crustal depths, the Sierra Madre fault zone appears to sole into a master decollement that terminates northward at the San Andreas fault and projects southward beneath the San Gabriel Valley to the Puente Hills blind thrust fault. (4) The dip and depth extent of the San Andreas fault along line 1 dips steeply (∼83°) northward and extends to at least the Moho. (5) The subsurface lateral extent of the Pelona Schist in southern California is as follows. Along line 1, the Pelona Schist underlies much, if not all of the San Gabriel Mountains south of the San Andreas fault to middle-crustal depths. North of the San Andreas fault, it is apparently not present along the transect.

Foreland crustal structure of the New York recess, northeastern United States

A new structural model for the northeast part of the Central Appalachian foreland and fold-and-thrust belt is based on detailed field mapping, geophysical data, and balanced cross-section analysis. The model demonstrates that the region contains a multiply deformed, parautochthonous fold-and-thrust system of Paleozoic age. Our interpretations differ from previous ones in which the entire region north of the Newark basin was considered to be allochthonous. The new interpretation requires a substantial decrease in Paleozoic tectonic shortening northeastward from adjacent parts of the Central Appalachian foreland and illustrates the common occurrence of backthrusting within the region. During early Paleozoic time northern New Jersey consisted of a Taconic orogenic foreland in which cover folds (F1) involved lower Paleozoic carbonate and flysch overlying Middle Proterozoic basement. F1 folds are open and upright in the foreland and more gently inclined to recumbent southeastward toward the trace of the Taconic allochthons. F1 structures were cut and transported by a fold-and-thrust system of the Alleghany orogeny. This thrust system mostly involves synthetic faults originating from a master decollement rooted in Proterozoic basement. Antithetic faults locally modify early synthetic overthrusts and S1 cleavage in lower Paleozoic cover and show out-of-sequence structural development. The synthetic parts of the regional thrust system are bounded in the northwestern foreland by blind antithetic faults interpreted from seismic-reflection data. This antithetic faulting probably represents Paleozoic reactivation of Late Proterozoic basement faults. Tectonic contraction in overlying cover occurred by wedge faulting where synthetic and antithetic components of the foreland fault system overlap. S2 cleavage in the Paleozoic cover stems from Alleghanian shortening and flattening and commonly occurs in the footwall of large overthrust sheets. Paleozoic structures in Proterozoic basement include fault blocks bounded by high-angle faults and low- to moderate-angle shear zones that locally produce overlying cover folds. Broad and open folds in basement probably reflect shear-zone displacement of subhorizontal foliation. Our cross-section interpretations require limited involvement of lower Paleozoic cover folds in the footwalls of major overthrust faults. Palinspastic restoration of F1 folds produces an arched passive-margin sequence. The tectonic contraction for the Valley and Ridge province and southeastern Pocono Plateau is about 25 km, and tectonic wedge angles are 8°–11°.

Alleghanian development of the Goat Rock fault zone, southernmost Appalachians: Temporal compatibility with the master decollement

The Goat Rock and associated Bartletts Ferry fault zones, which mark the eastern margin of the Pine Mountain Grenville basement massif, are controversial due to the suggestion that they are rare exposed segments of the late Paleozoic southern Appalachian master decollement. The controversy in part stems from reported middle Paleozoic (Acadian) radiometric dates postulated as the time of movement along these fault zones. Ultramylonite samples from the type area at Goat Rock Dam yield a 287 [plus minus] 15 Ma Rb-Sr isochron interpreted as the time of Sr isotopic rehomgenization during mylonitization. This date is corroborated by Late Pennsylvanian-Early Permian [sup 40]Ar/[sup 39]Ar mineral ages on hornblende (297-288 Ma) and muscovite (285-278 Ma) from neomineralized and dynamically recrystallized rocks within and straddling the fault zone. These Late Pennsylvanian-Early Permian dates indicate the time of right-slip movement (Alleghenian) along the Goat Rock fault zone, which is compatible with the timing suggested by COCORP for thrusting along the southern Appalachian master decollement.

Extension in the Cretaceous Sevier orogen, North American Cordillera

Recent geologic studies in the western United States Cordillera provide evidence of extensional deformation in Cretaceous time, during development of the Sevier foreland fold and thrust belt. This evidence comes from the region roughly bound on the east by the fold and thrust belt and on the west by the Mesozoic continental arc. We postulate that extension in this Internal Zone accommodated gravitational collapse of the. evolving Sevier orogen. Major surface-breaking normal faults and extensional basins that characterize other regions of syncompressional gravitational collapse (such as the Himalayan orogen) have not been documented in the Cretaceous record of the Internal Zone. Consequently, if extension was widespread in the Internal Zone, then it had little surface expression. We propose a conceptual model of the Sevier orogen, consistent with geologic constraints and supported by simplistic rheological arguments, in which westward movement of a mid-crustal extensional allochthon was driven by buoyancy stresses in the over-thickened Internal Zone. Bound at the top by the master decollement of the Sevier fold and thrust belt and at the bottom by a west-dipping, normal-sense shear zone, this extensional allochthon would have been effectively decoupled from the upper and lower crust and free to accommodate gravitational collapse during continued convergence across the orogen.

Crustal evolutionary model for the Variscides of Ireland and Wales from SWAT seismic data

The SWAT seismic reflection profiles recently acquired across the Celtic Sea demonstrate that the crust, which underlies the Munster and Pembroke Variscan foldbelts, contains extensive south-dipping reflections interpreted as Hercynian foreland-directed ramp thrusts largely involving the pre-Devonian basement. These large-scale crustal ramps developed without any intervening flat-lying decollement levels, and are thought to have originated in a deep infra-crustal decoupling zone not imaged on the SWAT profiles because it lies within the present-day layered lower crust at approximately 25 km depth. The pre-existing extensional basinal pattern is assumed to have highly influenced the geometry of these deeply rooted thrusts, and in particular that of the frontal ramp which emerges along a discontinuous N100°-trending thrust zone located above the faulted extensional margin of the Munster, Pembroke and Mendips Devonian basins. Shortening of the Irish and Welsh external zones is hence assigned to dominantly crustal stacking achieved along steep ramps dying out upwards, and giving way to mainly undisrupted folds in the thick overlying Devonian infill. In addition, a shallow thin-skinned fold-thrust system developed above an intra-Namurian master decollement still preserved in the footwall of the frontal ramp where it either stays blind (Ireland) or emerges (Pembroke) above a dominantly wrench-faulted Caledonian basement. Compiling SWAT seismic data and structural geology leads to a crustal evolutionary model for the Irish and South Welsh external Variscides from a Devonian basin template to a final deformation pattern.

Paleozoic and Grenvillian Structures in the southern Appalachians: Extended interpretation of seismic reflection data

Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500‐km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge‐Inner Piedmont allochthon, including the locations of the Blue Ridge master décollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master décollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge‐Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper‐to‐Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic‐Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post‐Grenvillian, pre‐Early Cambrian low‐density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time‐structure contour maps with available gravity data and two‐dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low‐density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.

Regional Factors Affecting Petroleum Potential of the Aleutian Accretionary Prism: ABSTRACT

Two first-order factors influencing the petroleum potential of large accretionary prisms are (1) their structural contact (backstop) with the margin's bedrock framework, and (2) that beneath a master decollement, the volume of the sediment that has underthrust the prism and passed through the petroleum-generation window (PGW). In the Aleutian prism, the backstop contact slopes landward because the accretionary body has underrun and elevated the seaward edge of the ridge's arc massif. Within the thickened landward part of the prism, the overlying lid of the underrun bedrock slab and its mantle of thick (2-4 km) forearc deposits has served to constrain vertical fluid movements and enhance the thermal environment and circumstances that favor the generation of hydrocarbons and their channelized migration toward higher structural levels. The thickness of the underthrusting or subdecollement section commonly is greater than 1.0 km. Most of this sequence of fine-grained sediment is transported below the landward-dipping slab or forearc basement to depths in excess of 8 km. A substantial mass is transported farther landward and deeper, and is subducted beneath the base of the arc massif. Deeply underthrust and subducted sedimentary deposits thus are transported through the PGW - a circumstance that should sustain the continuousmore » generation of petroleum from deep within and below the accreted pile. During the past 5-6 m.y., more than 50% of the 1 to 3-m-thick sedimentary section entering the subduction zone (at roughly 80 km/m.y.) has been subducted and, therefore, passed through the PGW. Evidently, since the late Miocene, larger volumes of hydrocarbons have been produced and possibly entrapped in high-level structures within either the accretionary complex or the forearc section mantling the underthrust basement slab.« less

A geophysical study of the Earth's crust in central Virginia: Implications for Appalachian crustal structure

A regional seismic reflection line (I‐64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I‐64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.

Results from regional vibroseis profiling: Appalachian ultra-deep core hole site study

In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians. The major points in the interpretation are: 1) the Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) the Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures (‘duplex tuning wedges’) connected by other thrust faults that duplicate shelf strata to a thickness of 4-5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged. The ADCOH data were originally recorded with 14-56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments.

Heat flow at the proposed Appalachian Ultradeep Core Hole (ADCOH) Site: Tectonic implications

The heat flow in northwestern South Carolina at the Appalachian Ultradeep Core Hole (ADCOH) site area is approximately 55 mW/m². This data supplements other data to the east in the Piedmont and Atlantic Coastal Plain provinces where heat flows > 55 mW/m² are characteristic of post‐ and late‐synmetamorphic granitoids. Piedmont heat flow and heat generation data for granites, metagranites, and one Slate Belt site, in a zone approximately parallel to major structural Appalachian trends, define a linear relation. Tectonic truncation of heat‐producing crust at a depth of about 8 km (a depth equal to the slope of the heat flow‐heat production line) is proposed to explain the linear relation. Using the value of reduced heat flow estimated from this empirical relation, and assuming thicknesses of heat‐producing crust defined by new ADCOH seismic data, the heat flow and heat production at the ADCOH site are consistent with a depth to the base of the Inner Piedmont crystalline allochthon of about 5.5 km. Seismic data at the ADCOH site confirm that the Inner Piedmont is tectonically truncated at about 5.5 km by the Blue Ridge master decollement. Temperatures at 10 km at the ADCOH site are predicted to be less than 200 °C.

Inferences on sediment strength and fault friction from structures at the Aleutian Trench

Research Article| June 01, 1987 Inferences on sediment strength and fault friction from structures at the Aleutian Trench Dan M. Davis; Dan M. Davis 1Department of Earth and Space Sciences, State University of New York at Stony Brook Stony Brook, New York 10964 Search for other works by this author on: GSW Google Scholar Roland von Huene Roland von Huene 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Geology (1987) 15 (6): 517–522. https://doi.org/10.1130/0091-7613(1987)15<517:IOSSAF>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Dan M. Davis, Roland von Huene; Inferences on sediment strength and fault friction from structures at the Aleutian Trench. Geology 1987;; 15 (6): 517–522. doi: https://doi.org/10.1130/0091-7613(1987)15<517:IOSSAF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The structural relations of conjugate thrust faults and the master decollement imaged in a research-level, processed, multichannel seismic reflection record across the eastern Aleutian Trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone. These estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms. They suggest the presence of high pore-fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment. Therefore, coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic Signatures of Tectonic Lithofacies from Regional Lines, Appalachian Ultradeep Core-Hole Site Area: ABSTRACT

A highly reflective upper crust is imaged on regional seismic data between Hayesville, North Carolina, and Fair Play, South Carolina. Above Eocambrian(.) rift basins, reflections originate from probable lower Paleozoic shelf strata at 8-10 km (3 sec). Reflections at 1.2-2.6 sec east of Hayesville also might be from lower Paleozoic rocks, implying a (repeated) thickness of about 5 km of shelf strata. Among the most prominent seismic signatures are wedge-shaped packages of reflections opening to the northwest from depths of 6 km (2 sec) and 9 km (3.2 sec) between Hayesville, North Carolina, and Westminster, South Carolina. These reflections may indicate locations where decollements ramp up to shallow crustal levels, and where local stacking of thin beds has resulted in tuning of reflections with a two to three fold increase in reflection amplitudes. The horizontal bases of such packages may indicate the crustal levels of decollements that are usually acoustically transparent. By one interpretation, the Blue Ridge master (largest horizontal displacement) decollement lies above these signatures at a depth of 3-5 km. Just northwest of Westminster, reflections from the Brevard zone may be truncated near or at this master decollement at 5.5 km.

The cheyenne belt: analysis of a proterozoic suture in Southern Wyoming

The Cheyenne belt of southeastern Wyoming is a major shear zone which separates Archean rocks of the Wyoming province to the north from 1800-1600 Ma old eugeoclinal gneisses to the south. Miogeoclinal rocks (2500-2000 Ma old) unconformably overlie Archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift-formed continental margin. Intrusive tholeiitic sills and dikes are interpreted as rift-related intrusions and a date of 2000 Ma on a felsic differentiate of these intrusions gives the approximate age of rifting. There are no known post-2000 Ma felsic intrusions north of the Cheyenne belt. Volcanogenic gneisses and abundant syntectonic calc-alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks. The volcanics are a bimodal basalt-rhyolite assemblage. Plutons include large gabbroic complexes and quartz diorite (1780 Ma), syntectonic granitoids (1730-1630 Ma) and post-tectonic anorthosite and granite (1400 Ma). There is no evidence for Archean crust south of the Cheyenne belt. Structural data (thrusts in the miogeoclinal rocks, vertical stretching lineations, and the same fold geometries north and south of the shear zone) suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane (island arc) over the northern terrane (craton and miogeocline), probably as a continuation of the south-dipping subduction which generated calc-alkaline plutons of the southern terrane. A metamorphic discontinuity across the shear zone, with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south, also suggests thrusting of the southern terrane (deeper crustal levels) over the northern terrane (shallower levels). The Cheyenne belt may be a deeply-eroded master decollement, perhaps analogous to a ramp in the master decollement in the southern Appalachians. This interpretation of the Cheyenne belt as a Proterozoic suture zone provides an explanation for the geologic, geochronologic, geophysical, metallogenic, and metamorphic discontinuities across the shear zone.

Termination of Southern Appalachian Overthrust: ABSTRACT

Autochthonous sediments equivalent to Valley and Ridge formations are interpreted to exist beneath crystalline rocks of the entire Blue Ridge and western portion of the Piedmont. Although hydrocarbon potential has not been determined, we have defined the southeastern extent of sediments beneath the allochthonous southern Appalachians. Truncation of underlying sediments by a subsurface fault occurs at 10 km (6 mi) depth approximately 55 km (34 mi) southeast of the Brevard zone. Farther southeast, a basal detachment might exist, but it would be within mylonitized Grenville basement. Blue Ridge basement rocks probably originated from this area. The interpretation is primarily based on detailed analysis of reprocessed COCORP seismic data, modeling potential fields, and the un isputable fact that Grenville rocks must be cut by a deep fault someplace beneath the southern Appalachians. All previous theories which attempted to correlate with the original interpretation of a regional detachment underlying the entire southern Appalachians should be reexamined. Although detachments and major thrust faults undoubtedly exist throughout the southern Appalachians, they do not form one continuous overthrust sheet and are not underlain by sediments deposited on the ancient continental margin of North America. Geophysical studies of the southern Appalachians have determined them to be largely allochthonous. Seismic data show that a continuous master decollement underlies the Valley and Ridge, Blue Ridge, and Piedmont provinces. Other forms of geophysical data are consistent with the hypothesis of an extensive overthrust system. The question remains, however, how far east can we define a continuous master decollement? This question has been debated in many conferences in recent years where two extreme positions have developed. One side believes that the Brevard zone represents a cryptic suture continuing to depth beneath the inner Piedmont and terminating the master decollement at depth. The alternative position draws a detachment continuous from the western Valley and Ridge, beneath the ent re Appalachians to the southeast, deep beneath the coastal plain, and possibly out to the edge of the continent in the Atlantic Ocean. In this paper, a moderate position is presented which places a sloping master decollement root zone beneath the eastern Piedmont. The adjoining province of island arc assemblages is thus regarded as an accreted terrain which is generally regarded as autochthonous. Evidence for such a model is primarily from reflection profiles across Georgia. Additional support comes from gravity, aeromagnetics, magnetotellurics, and surface geology. Figure End_of_Article - Last_Page 487------------