Minimum Throw Research Articles

Mechanical Stratigraphy Controls Normal Fault Growth and Dimensions, Outer Kwanza Basin, Offshore Angola

AbstractMechanical stratigraphy controls the growth patterns and dimensions of relatively small normal faults, yet how it influences the development of much larger structures remains unclear. Here, we use 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to constrain the geometry and kinematics of several normal faults formed in a deep‐water clastic succession. The faults are up to 6.3‐km long and 1.9‐km tall and have up to 44 m of throw. Aspect ratios and lower‐tip throw gradients are greater for faults that terminate downward at a c. 100 m thick, mass‐transport complex (MTC; up to 5.2 and 0.12) than for those that offset it (up to 2.7 and 0.01). Faults that offset the MTC invariably have >30 m of throw. Based on their geometric properties and throw patterns, we interpret that the faults nucleated above the MTC and propagated down toward it. Upon encountering this unit, which we infer behaved in a more ductile manner than encasing strata, tip propagation was halted until tip stresses were sufficiently high (corresponding to minimum throw of c. 30 m) to breach it. Faults with smaller throw were unable to breach the MTC. We argue that using only geometric criteria to determine fault growth patterns can mask the significant control mechanical stratigraphy has on fault kinematics. Mechanical stratigraphy is therefore a key control on the growth of large, seismic‐scale normal faults, in a similar way to that observed for far smaller structures.

The Río Grío–Pancrudo Fault Zone (central Iberian Chain, Spain): recent extensional activity revealed by drainage reversal

AbstractThe NNW–SSE-trending extensional Río Grío–Pancrudo Fault Zone is a large-scale structure that obliquely cuts the Neogene NW–SE Calatayud Basin. Its negative inversion during the Neogene–Quaternary extension gave rise to structural and geomorphological rearrangement of the basin margin. Geological mapping has allowed two right-relayed fault segments to be distinguished, whose recent extensional activity has been mainly characterized using a deformed planation surface (Fundamental Erosion Surface (FES) 3; 3.5 Ma) as a geomorphic marker. Normal slip along the Río Grío–Lanzuela Fault Segment has induced hanging-wall tilting, subsequent drainage reversal at the Güeimil valley after the Pliocene–Pleistocene transition, as well as morphological scarps and surficial ruptures in Pleistocene materials. In this sector, an offset of FES3 indicates a total throw ofc. 240 m, resulting in a slip rate of 0.07 mm a–1, while retrodeformation of hanging-wall tilting affecting a younger piedmont surface allows the calculation of a minimum throw in the range of 140–220 m after the Pliocene–Pleistocene transition, with a minimum slip rate of 0.07–0.11 mm a–1. For the late Pleistocene period, vertical displacement ofc. 20 m of a sedimentary level dated to 66.6 ± 6.5 ka yields a slip rate approaching 0.30–0.36 mm a–1. At the Cucalón–Pancrudo Fault Segment, the offset of FES3 allows the calculation of a maximum vertical slip of 300 m for the last 3.5 Ma, and hence a net slip rate close to 0.09 mm a–1. Totallingc. 88 km in length, the Río Grío–Pancrudo Fault Zone could be the largest recent macrostructure in the Iberian Chain, probably active, with the corresponding undeniable seismogenic potential.

Active near-surface growth faulting and late Holocene history of motion: Matagorda peninsula, Texas

The structural framework of the northern Gulf of Mexico coastal zone includes numerous growth fault systems. Neotectonic processes in coastal environments here have been shown to be important contributors to subsidence and relative sea-level rise, as well as having significant influences on sedimentary accretion processes. One active growth fault at East Matagorda Peninsula, Texas was subjected to detailed study to address the hypotheses that (1) this fault was present and active before the peak of regional hydrocarbon recovery in the mid-20th century, and that (2) slip along this fault has been episodic over the resolved period of record. To characterize the late Holocene behavior of this fault, three core transects were established normal to the surface fault trace, and stratigraphic analysis was conducted with the use of radiocarbon (14C) for chronological control. Correlation of time-equivalent stratigraphic boundaries reveals a maximum total late Holocene offset of 0.7 m. The Matagorda fault was in place long before the peak of regional oil and gas extraction, but experienced a late Holocene rejuvenation, initiated between 2500 and 1500 years BP. The kinematic behavior of the fault expressed as temporally and spatially variable measures of expanded stratigraphic thicknesses on the downthrown extent suggest that sediment loading from the transgressive Matagorda barrier complex may have contributed to this rejuvenation, and that slip along the fault persists to the present. Radiocarbon chronology also revealed clear differences in the magnitude of fault throw during the late Holocene, with maximum displacement (0.7 m) occurring at the southwestern-most transect and minimum throw at the northeastern-most transect (0.2 m).

Investigating fault propagation and segment linkage using throw distribution analysis within the Agbada formation of Ewan and Oloye fields, northwestern Niger delta

Throw distribution analysis of the key stratigraphic surfaces (sequence boundaries and maximum flooding surfaces) across faults has allowed detailed investigation of the tectonic history within the Ewan and Oloye fields, northwestern Niger delta. The structure in the studied area is dominated by growth fault systems which are listric in cross section and concave to the basin in plan-view. Generally, the faults are active down to 2000 m depth before they die out or sole into the underlying shale. The hanging-wall blocks of growth faults are deformed into broad rollover anticlines, with some synthetic and antithetic faults initiated from the anticline crests, and fault splays off major faults, further complicating these structures. Stratigraphic key surfaces within the syn-faulting succession range in age from 16.7 to 10.35 Ma.Periods of maximum and minimum throw are established from 2-Dimensional throw distribution on the growth fault plane. Throw distribution allows analysis of growth fault nucleation, propagation and linkage. Each fault nucleated at different and a distinct interval within the stratigraphic section, as a result of the paleo-stress distribution between the interacting faults. Nucleation and linkage positions can be identified at points of maximum and minimum throw respectively. Following nucleation, faults propagated radially and linked to form the present geometry. Within the study area, fault propagation and segment linkage (lateral and vertical) are important features of the fault system. Understanding of growth fault evolution and linkage has greatly improved prediction of seal potential, trap geometry and migration. The accurate timing of the segment linkage has helped to evaluate the seal risk.

Rapid extensional unroofing of a granite–migmatite dome with relics of high-pressure rocks, the Podolsko complex, Bohemian Massif

AbstractThe Podolsko complex, Bohemian Massif, is a high-grade dome that is exposed along the suprastructure–infrastructure boundary of the Variscan orogen and records snapshots of its protracted evolution. The dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high- to ultrahigh-pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards. Our new U–Pb zircon ages indicate that the leucocratic migmatites were derived from Early Ordovician (c. 480 Ma) felsic igneous crust; the same age is inferred for melting the proto-source of the metapelitic migmatites. The relics of high- to ultrahigh-pressure rocks suggest that at least some portions of the complex witnessed an early Variscan subduction to mantle depths, followed by high-temperature anatexis and syntectonic growth of the Podolsko dome in the middle crust at c. 340–339 Ma. Subsequently, the dome exhumation was accommodated by crustal-scale extensional detachments. Similar c. 340 Ma ages have also been reported from other segments of the Variscan belt, yet the significance of this tectonothermal event remains uncertain. Here we conclude that the 340 Ma age post-dates the high-pressure metamorphism; the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and, finally, the extensional unroofing of the complex requires a minimum throw of about 8–10 km. We use this as an argument for significant early Carboniferous palaeotopography in the interior of the Variscan orogen.

Elevator tectonics and orogenic collapse of a Tibetan-style plateau in the European Variscides: the role of the Bohemian shear zone

Variscan collision of peri-Gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the Bohemian Massif. This crustal thickness resulted in a highly elevated Bohemian plateau with a topographic height >3–4 km. The Bohemian plateau was covered with unmetamorphic Paleozoic strata, all of which are today well preserved in the Tepla–Barrandian unit because of crustal-scale vertical slip along the Bohemian shear zone (BSZ). The BSZ forms a subvertical, ca. 500-km long and up to 2-km wide belt of dip–slip mylonites which show several 90° deflections in map view. Tepla-Barrandian-down movements were active under retrograde metamorphic conditions, starting with granulite and ceasing with greenschist facies conditions. As slip along the BSZ was largely vertical and led to a minimum throw of 10 km, this type of crustal-scale deformation is referred to as elevator tectonics. The elevator-style movements caused the juxtaposition of the supracrustal Tepla–Barrandian lid (the “elevator”) against high-grade rocks of the extruding orogenic root. The BSZ has further governed the foci of mantle-derived plutonism. New U–Pb zircon and monazite TIMS dating of six plutons suggest that emplacement of mantle-derived melts along the BSZ lasted for at least 20 m.y., starting with the emplacement of the Klatovy granodiorite at 347 +4/−3 Ma and ceasing with the emplacement of the Drahotin pluton at 328 ± 1 Ma. When taking into account the new ages of synkinematic plutons, the simultaneous vertical slip along the individual segments of the BSZ (North, West, and Central Bohemian shear zone) is bracketed to the period 343–337 Ma. Elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating-up of the orogenic root. The overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow, vertical extrusion, fast uplift, and exhumation of the orogenic root.

Seismic hazard scenarios from the longest geologically constrained active fault of the Aegean

Several seismic hazard scenarios are examined concerning the seismic potential of the North Aegean Basin (NAB), based for the first time purely on geological data. Detailed analysis of the bathymetry incorporated by seismic reflection profiling has recently revealed the neotectonic structure of the NAB and demonstrates that the dominant structure is a 160 km long NE–SW trending fault, comprising of four major segments. This is the longest geologically constrained active fault in the Aegean Sea. Following deterministic fault specific analyses, six major seismic sources are identified. Firstly, a ∼55 km oblique normal fault segment towards the southwestern end of the basin, that can accommodate a M=7.1 event. Such an event could produce a maximum vertical offset up to 3–4 m posing also a tsunami threat. Based on geological data a minimum throw rate of 1.2±0.2 mm/yr is estimated for this segment, implying a recurrence interval of 1100±350 yrs. Secondly, a sub-vertical ∼70 km long, E–W dextral trending shear zone northwards the southwestern corner of the N. Aegean basin that can generate a M=7.2 strike–slip event. Sources 3 and 4 towards the central and eastern segment of the fault are expected to produce seismic strike slip events of M=7.1, whereas the fifth source suggests the simultaneous activation of sources 3 and 4, producing a 105 km long rupture and a M=7.4 event. Finally, the sixth source implies a multi-segment rupture involving the entire 160 km long south marginal fault, producing a M max=7.6, which represents the worst-case scenario. This value limits all uncertainties posed on the maximum expected magnitude by other published estimates (ranging from M=7.2 up to 8.5) that are based solely on seismological and/or geodetic approaches.

3D stratigraphic and structural synthesis of the Dannemarie basin (Upper Rhine Graben)

Abstract The Dannemarie basin is the southwesternmost depocentre of the Upper Rhine Graben, which belongs to the West-European Tertiary rift system. It is bounded to the west by the Vosges mountains, to the south by the folded Jura belt and to the east by the Mulhouse block. The rifting reached its maximum activity during the Priabonian and early Rupelian (35-31 Ma). In the framework of the GeoFrance 3D project “Fossé rhénan”, a 3D geometrical model of the Dannemarie basin was built in the gOcad 3D modeler. It incorporates the BRGM well database and geological maps, and 40 seismic cross-sections. These data are used to study the structure and geological history of the area. Seismic data have been converted from time to depth using a 1D time-to-depth polynomial law deduced from the analysis of the Bellemagny borehole. The Dannemarie basin is bounded to the west by the Vosges fault zone and to the east by the Illfurth fault zone. On both borders, basin subsidence was controlled by normal faults and associated syn-rift flexures. The minimum throw on the Vosges fault zone is about 1400 m to the north of the model, decreasing to the south, where it is replaced by a syn-rift flexure. On the Illfurth fault zone, subsidence is accommodated by faults (with about 1000 m throw) and by a flexure (about 300 m). Stratigraphic data indicate that these flexures were active during Priabonian and early Rupelian extension. These monoclinal flexures are interpreted as fault-propagation folds that developed above upward propagating normal faults in the basement. As displacement accumulates, the fault propagates upwards and cuts the overlying fold. Similar fault-fold geometries have been described on the western border of the Rhine graben, close to Colmar and in other extensional tectonic contexts. In the Colmar area, the Vosges fault zone cuts through the basin margin fold, while further south along the western border of the Dannemarie basin, displacement on the fault decreases and subsidence is accommodated on a major flexure. Flexure locations correspond to gravimetric discontinuities attributed to Variscan structures, suggesting reactivation of deep structures during rifting. The Illfurth fault zone displays upwardly divergent fault geometries that resemble “flower structures”. The data can be interpreted as follows, either that (a) the Illfurth fault zone accommodated a minor sinistral strike-slip component due to a post-Miocene NW-SE compressive regional stress field or (b) these faults developed in association with the fault propagation folds.

10 km Minimum throw along the West Bohemian shear zone: Evidence for dramatic crustal thickening and high topography in the Bohemian Massif (European Variscides)

The West Bohemian shear zone (WBSZ) forms a steep collapse structure along which east-side- down normal movements led to the juxtaposition of the relatively cold Cadomian basement of the Tepla-Bar- randian unit against high grade Moldanubian rocks. Syn- kinematic plutons straddle the WBSZ. The Mutenin plu- ton intruded into Moldanubian crust at a depth of 23±4 km as derived by using Al-in-hornblende barome- try. The Tepla-Barrandian Babylon pluton intruded at <12 km depth as indicated by phengite barometry and petrogenetic considerations. Both emplacement depths, together with mineral cooling ages, result in a minimum vertical displacement of 10 km between 340 and 320 Ma. This large throw could be explained by over- thickened crust that was weakened from below. The al- kaline signature of the Mutenin diorite indicates that mantle melting was important to thermally weaken the crust at 340 Ma. The cold Tepla-Barrandian upper crust sank into its weak, partly molten Moldanubian substra- tum, resulting in elevator-style movements, not only along the WBSZ, but also along the Hoher Bogen and Central Bohemian shear zone. All these ductile normal shear zones were active simultaneously during the Low- er Carboniferous and dip steeply towards the Tepla-Bar- randian unit that probably formed a highly elevated pla- teau at this time.

Seismic Stratigraphic Constraints on Neoarchean - Paleoproterozoic Evolution of the Western Margin of the Kaapvaal Craton, South Africa

The Kaapvaal Craton is partially covered by the supracrustal sequences of remnants of the Witwatersrand, Ventersdorp and Transvaal-correlated Griqualand West Basins that span about 800 million years (~3.0 to 2.2 Ga). This study describes and interprets eight newly available seismic reflection profiles of the Ventersdorp and younger cover sequences, acquired by the vibroseis method to 6 seconds TWT (~ 17km), and totalling ~720km in length. New stratigraphic and structural features are identified across three main regions: the Kaapvaal Craton’s western margin, the craton’s northern margin, (southwards from the Thabazimbi-Murchison Lineament across the western extremity of the Bushveld Igneous Complex) and the craton’s interior. Across the craton’s western margin, the volcano-sedimentary succession is shown to thicken towards its edge from 0 km to >17km over 150km. The Ventersdorp Supergroup underlies the Transvaal Sequence right to the western extremity of the craton. Close to the margin, we recognise a normal fault, here named the Moshaweng fault, with >6km normal displacement and an unknown distance of strike-slip displacement. Three prominent unconformities are identified, as well as ~300% tectonic thickening of the Hartley Formation in response to east-verging, Paleoproterozoic (~1.8 Ga) thrusting across the craton. Folding and thrusting of Griqualand West and Olifantshoek Supergroups across the craton are also apparent. A tectonic model for the evolution of the craton’s western margin involves the formation of an Atlantic-type margin by extension, volcanism and rifting over ~70 Ma, followed by 160 Myrs of thermal subsidence. Subsequently, Paleoproterozoic (1.7 to 1.9 Ga) compression involved folding and thrusting eastward across the margin. Along the northern margin of the craton, the seismic profiles traverse a syncline which trends east to west over several hundred kilometers and involves mainly Transvaal-aged rocks and sporadic Bushveld outcrop. Evident from these profiles are discrete Ventersdorp half grabens, up to 3km in depth, with bounding listric normal faults that trend east to west, subparallel to the Thabazimbi-Murchison Lineament (TML), and downthrow predominantly to the north. These faults are truncated by the well-imaged Black Reef unconformity. Across the craton interior, horst and graben structures, involving the Witwatersrand and lower Ventersdorp Formations, dominate the seismic profiles. Bounding listric normal faults (one example with minimum throw of 9.5km), trend east to west and northeast to southwest and downthrow to the north, south, northwest and southeast. The Black Reef unconformity is again distinctive, as is a Paleozoic unconformity (the Beaufort unconformity). The orientation of boundary faults of Ventersdorp-aged half-grabens along the craton’s margins and interior, south of the TML, suggests a complex process of extension tectonism for ~70 Myrs that overlaps in time with northward thrusting of volcano-sedimentary sequences to the north of the TML. On a regional scale, the data presented are consistent with a model for the Kaapvaal Craton that resembles, both in space and timing, the evolution of the Argentine Atlantic margin. This margin developed during an initial 40 to 70 Ma period of regional extension across a ~600 km wide volcano-tectonic rift zone behind an active (Andean) subduction zone, followed by regional lithospheric cooling and subsidence. Our results also confirm that the Kaapvaal Craton was part of a larger Meso-Archean landmass and thus supports growing realisation that models of Archean continental growth rates based on the present day aerial extent of Archean cratons, represent only minimum values.

Nature of the southwestern boundary of the central Mojave Tertiary province, Rodman Mountains, California

Tertiary volcanic and sedimentary rocks in the central Mojave Desert are largely confined to the Barstow-Bristol trough, an elongate tectonic depression filled with lower Miocene strata. The southwestern boundary of the trough is sharply defined and separates thick (∼2 km) Tertiary sequences from an area to the southwest that is virtually devoid of Tertiary rocks. Segments of this boundary have been interpreted as a buttress unconformity, as a right-slip sidewall fault that formed the lateral boundary of an early Miocene extensional terrane, and as a dip-slip fault caused by flexure of the North American plate above the subducted Mendocino fracture zone. The Silver Bell fault in the Rodman Mountains, central Mojave Desert, is the best exposed segment of the boundary, and relations exposed along it can be used to test these hypotheses. The Tertiary sequence north of the Silver Bell fault is at least 2.3 km thick and comprises at least 1.9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault. Stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed Tertiary section. Tuff layers intercalated in the lower part of the clastic deposits are 23–24 m.y. old, indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central Mojave metamorphic core complex. Structural and stratigraphic data indicate that the Silver Bell fault is primarily a dip-slip fault. The Tertiary section is folded into a west-trending, upright to overturned syncline located near the southern edge of exposed Tertiary rocks. The syncline records late Cenozoic north-south shortening along the northwest-striking Calico fault, which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip. Southwestward-dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension. However, bedding dip does not decrease upward in the Tertiary section, paleocurrent directions trend obliquely updip, and northeast-dipping normal faults are rare. Stratal tilting postdated the accumulation of the exposed section, which therefore was not deposited during rotational normal faulting. The new data contradict interpretation of the Silver Bell fault as a major right-slip fault bounding the central Mojave extensional terrane, although they permit small-magnitude extension across the fault. The amount of early Miocene crustal extension in this area therefore is apparently small in spite of stratal tilting, and the lateral limit of large-magnitude crustal extension in the central Mojave block is farther northwest. The data are consistent with the fault having formed by failure during plate flexure above the subducted Mendocino fracture zone.

Structural and metamorphic evidence of local extension along the Vivero fault coeval with bulk crustal shortening in the Variscan chain (NW Spain)

The Vivero fault is a W-dipping, N-S-striking ductile shear zone separating the Ollo de Sapo antiform in its western hangingwall and the Lugo dome in its eastern footwall. Two stages of deformation ( F 1 and F 2) produced nearly coaxial folds with sub-horizontal axes. A crenulation cleavage S 2 transposes an older S 1. Three sets of shear bands in the hangingwall define a pervasive fabric consistent with an E-W bulk shortening perpendicular to a composite S 1–2 foliation and NNE-stretching parallel to L 2. The Vivero fault zone is marked by a mylonitic foliation with a steeply NW-plunging stretching lineation and extensional crenulation cleavage (ECC) indicating normal slip. In the vicinity of the fault, sub-horizontal NNE-trending F 3 folds, with a crenulation cleavage S 3, deform earlier-formed fabrics, including a mylonitic foliation. Pressure-temperature conditions obtained from mineral assemblages on both sides of the Vivero fault yield a minimum throw of 5.5 km. Andalusite-bearing pelite in the hangingwall was infolded by an F 2 synform into the kyanite field at 450–500°C. The eastern edge of these rocks was later accreted to the footwall and heated to andalusite-staurolite conditions at ∼600°C. Slip on the Vivero and Valdoviño faults is kinematically related. East-west shortening during F 2 involved folding and sinistral strike-slip on the Valdoviño fault which induced local extension along the newly generated Vivero fault. Synkinematic emplacement of granitoids along the Vivero fault is favoured by extension. Coeval slip on both faults took place during the later stages of F 2 folding. Geometrical constraints caused northwards escape of the crustal block bounded by the Valdoviño and Vivero faults, recorded by NNE-stretching defined by L 2.

Evidence of Northwest-Southeast Directed Compression of Late Cenozoic Age in the Summit Diggings, San Bernardino County, California

Field examination of a low angle thrust fault which strikes essentially east-west at the principal outcrop reveals overthrusting of diotire onto late Pliocene-early Pleistocene fanglomerates. As is typical with the orientation of the trace trend of the fault tracer is east-northeast-west-southwest with a south-southeastward dip. This implies general northwest-southeast compression in the late Cenozoic which is contrary to the generally accepted tectonic regime for this area. Four distinct periods of movement on the fault can be distinguished from the outcrop with a minimum throw of 56.4 m. The broad structural fabric in this area has a dramatically different trend than either that found in the northeast portion of the Mojave block or in the Basin and Range block immediately to the north. It is possible that the compressional features in this area and the different orientation of mesoscale structures are reflections of the fact that compensation is being made in the crust for differential motions between Mojave and Basin and Range microplates.

Refraction seismic investigations of the northern midcontinent gravity high

Eighty-seven seismic refraction profiles have been obtained to define the geologic structure in the upper crust associated with the midcontinent gravity high in Minnesota and Wisconsin. The seismic measurements were taken across a fixed spread of seven geophones from distances up to 13 km. A structural section was prepared for each profile by interpretation of the travel-time graph, and the individual sections were compiled onto regional cross sections. Measured seismic velocities in bedrock fall in the 2.5–7.1 km/sec range. Observed velocities can be assigned to seven categories corresponding to Paleozoic, upper, middle, and lower Upper Keweenawan strata, Middle Keweenawan volcanics, pre-Keweenawan felsic-to-intermediate intrusive rocks, and pre-Keweenawan mafic intrusive rocks. These groups display good continuity through the area and suggest the lithologic equivalence of rock bodies between geologic provinces. The St. Croix horst and its flanking basins underlie the midcontinent gravity high and its parallel gravity lows north of Minneapolis. Minimum throw along the western and eastern boundary fault zones reaches about 3.0 and 2.0 km. Sedimentary rocks in the eastern basin reach a thickness of at least 2.6 km. A complex horst-like structure also underlies the midcontinent gravity high in southern Minnesota; an uplifted basaltic body is bordered by sedimentary basins having strata about 3.0 km thick. Middle Keweenawan basalts are present locally in the eastern and western basins underlying the Upper Keweenawan strata. Rocks probably lithologically equivalent to the Oronto group are rare in the western basin, common in small basins on the St. Croix horst, and abundant in the eastern basin. Rocks probably lithologically equivalent to the Bayfield group are extensive in the western and eastern basins, but they have not been found on the St. Croix horst. The Bayfield group seems to be several kilometers thick across Douglas County north of the Douglas fault, and it does not appear to increase in thickness under the Bayfield peninsula.

Seismic-Refraction Measurements in Jackson Hole, Wyoming

Three reversed seismic-refraction profiles were recorded in the Jackson Hole, Wyo-ming, area during July 1964. The seismic model which was developed consists of three layers with velocities of 2.4 km/sec for Tertiary and Cretaceous rocks above the Cleverly Formation (Lower Cretaceous), 3.8 km/sec for rocks from Lower Cretaceous down to lower Paleozoic, and 6.1 km/sec for lower Paleozoic (limestones and dolomites) and Precambrian rocks. The maximum thickness of sediments in Jackson Hole is 5 km, and the minimum throw of the Teton fault in the area covered by this survey is about 7 km.

The Ceja Del Rio Puerco: A Border Feature of the Basin and Range Province in New Mexico: I. Stratigraphy and Structure

The triangular highland known as the Llano de Albuquerque lies in Bernalillo and Valencia counties, New Mexico, between the Rio Grande and the Rio Puerco. Westward it breaks off in a low escarpment called the Ceja del Rio Puerco, which in part forms the western boundary of the Basin and Range Province. The northern 30 miles of the Ceja is a fault-line scarp which marks a series of nearly north-south en echelon faults between the late Tertiary Santa Fe formation and the older rocks of the Plateau Province. Part I.-The Santa Fe consists of fine-grained, alluvial fan deposits. Part of the material was derived from areas of volcanic rocks, but the major portion of it came from the sedimentary rocks of the Plateau Province to the northwest. The boundary of the basin of deposition lay some 10 miles or more northwestward. Ten miles east of this area the Santa Fe consists of river gravel, indicating that the fan deposits were laid down in the western portion of a basin containing a through-flowing river comparable to the present Rio Grande. Four main faults or fault zones, successively offset to the west, separate the Santa Fe from the older rocks. In addition, there are three fault zones within the Santa Fe. Because the Santa Fe is here divisible into three lithologic units, Lower Gray, Middle Red, and Upper Buff members, it has been possible to estimate the minimum throw of these faults and to speculate on the amount of depression of the basin as a result of this faulting.