Tectonic Displacement Research Articles

Differential rotations of north Kyushu Island related to middle Miocene clockwise rotation of SW Japan

Paleomagnetic measurements were made on samples of Cretaceous granites and Tertiary sedimentary rocks from north Kyushu Island in order to clarify the kinematics of clockwise (CW) rotation of the SW Japan tectonic block at 15 Ma. A paleomagnetic direction from Oligocene sedimentary rocks in the northern part of north Kyushu Island indicates a significant CW rotation of the region (R=35.9°±11.0°) relative to eastern Asia, while the direction from Oligocene to early Miocene sedimentary rocks in the western part of north Kyushu Island shows no significant tectonic displacement. In situ directions of secondary magnetization from Eocene sedimentary rocks in the southwestern part of north Kyushu Island, and stable remanences from Cretaceous granites, provide CW deflections of declination. These paleomagnetic data, combined with those previously reported, suggest that the main part of north Kyushu Island (Main Kyushu) was rotated CW (36°) between 30 and 14 Ma relative to eastern Asia, whereas no significant tectonic displacement has taken place since 20 Ma in the western part (Western Kyushu). Considering regional geological structures, the paleomagnetic results indicate that the western extent of the SW Japan tectonic block which experienced middle Miocene CW rotation is truncated west of Main Kyushu. The paleomagnetic data also suggest that Main Kyushu rotated counterclockwise (28° ± 13°) relative to the main part of SW Japan between 30 and 14 Ma. This result implies that the westernmost part of the SW Japan block (Main Kyushu) did not experience as much rotation as its main part during CW rotation of the whole block.

OVERVIEW: Tectonic implications of Cenozoic volcanism in coastal California

Cenozoic volcanic rocks of coastal California were erupted west of the magmatic arc trend related to subduction along the continental margin. Two assemblages, representing discrete pulses of mid-Tertiary (Oligocene–Miocene) and mid-Miocene volcanism, occupied relatively compact tracts, as restored palinspastically prior to disruption and dispersal by tectonic displacements within the Neogene San Andreas transform system. A third assemblage records migratory post–mid-Miocene volcanism at centers located progressively farther north. The distribution and petrologic character of all three assemblages reflect slab-window magmatism triggered by decompression melting of upwelling mantle. Paleogeographic reconstructions, based on analysis of magnetic anomaly patterns offshore and restoration of on-land features prior to San Andreas transform slip and associated transrotational deformation, indicate the paleotectonic positions of the Cenozoic volcanic fields before structural disruption. The pulses of mid-Tertiary and mid-Miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise-trench encounters associated with subduction of the Vancouver-Farallon and Monterey-Arguello plates, respectively, along the continental margin off southern California. Migratory post–mid-Miocene volcanism in central California accompanied the incremental expansion of a growing slab window as the Mendocino triple junction migrated northward along the continental margin. Mid-Miocene Columbia River basalt volcanism in the Pacific Northwest was coeval with the mid-Miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the San Andreas transform as a coherent plate boundary. Columbia River volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the Pacific plate.

Shear zone patterns and strain distribution at the scale of a Penninic nappe: the Suretta nappe (Eastern Swiss Alps)

Shear zone patterns within the Suretta nappe of the Swiss Alps are studied in order to understand finite strain and bulk tectonic displacements. The Suretta nappe is chosen for two main reasons; the availability of late Variscan granites deformed during Alpine tectonics only, and because of continuous outcrop from bottom to top of the nappe. This Alpine nappe recorded two major ductile deformation phases, which developed under low-grade metamorphic conditions. The analysis of shear zone patterns in the Roffna granite reveals that the first deformation is well developed in the entire nappe; the second deformation, however, is more localized in the lower and upper parts. The bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other Penninic nappes in this area: first a stacking of the nappe towards the NW, followed by the top to the E shearing during post-thickening extensional tectonics. The shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale.

SNAKE RIVER, WYOMING, CROSSING WITH THE LEICA NA-3000 DIGITAL LEVEL

A Leica NA-3000 digital level was used to carry a line of precise levelling across the Snake River where it courses a 40 m deep gorge in Grand Teton National Park, Wyoming. Two semi-permanent turning points were established 80 m apart across the river, a shaded instrument was set within 12 cm of the mid-point on a gravel bar in the river, and the height difference between the two turning points was measured 25 times in double-run mode. The standard deviation of a homogeneous set of 25 readings is ±0.10 mm. The standard deviation of all 52 readings is ±0.14 mm. Both results are acceptable in a 30 km-long levelling line otherwise surveyed to First Order, Second Class standards to monitor tectonic displacement across the Teton fault.

SNAKE RIVER, WYOMING, CROSSING WITH THE LEICA NA-3000 DIGITAL LEVEL

AbstractA Leica NA-3000 digital level was used to carry a line of precise levelling across the Snake River where it courses a 40 m deep gorge in Grand Teton National Park, Wyoming. Two semi-permanent turning points were established 80 m apart across the river, a shaded instrument was set within 12 cm of the mid-point on a gravel bar in the river, and the height difference between the two turning points was measured 25 times in double-run mode. The standard deviation of a homogeneous set of 25 readings is ±0.10 mm. The standard deviation of all 52 readings is ±0.14 mm. Both results are acceptable in a 30 km-long levelling line otherwise surveyed to First Order, Second Class standards to monitor tectonic displacement across the Teton fault.

Hillslope development in areas of active tectonics

Tectonic and geomorphic displacements of the Earth's surface control topographic profile development; therefore, their analysis should be combined. In the model presented here, transient finite difference solutions to the continuity equation for material transport determine geomorphic displacements. The material transport rate is a function of distance from the divide to the power m, local slope to the power n, and a rate constant. Values of m and n may be adjusted to simulate processes varying from rainsplash and soil creep (i.e., diffusive; m = 0, n = 1) to slope wash and river flow (m > 0, n > 0). The actual geomorphic displacements may be transport or weathering‐limited, depending on soil profile development. Superimposed edge dislocations in an elastic half‐plane are used to model tectonic displacements. Slip along a normal or reverse fault of any dip, depth and down‐dip length may be incremental (earthquake) or continuous (aseismic creep). Considering climate and material properties constant, the ratio of the transport capacity rate constant to the fault slip rate roughly determines form. This model extends existing morphologic diffusion erosion analyses to include other geomorphic conditions and processes (transport‐ or weathering‐limited conditions, material flux boundary conditions, and the development of gullies and knickpoints) and more heterogeneous spatial and temporal distributions of tectonic displacement (such as those due to slip along buried thrust faults). We advocate calibration of these parameters and processes to provide a quantitative approach to modeling landform development, determining deformation rates, and inferring earthquake hazards.

Improved stability of a deeply anchored geodetic monument for deformation monitoring

A test of two different monument designs used in geodetic networks shows that monuments installed to depths of 5 to 10 meters, and laterally braced, exhibit less environmentally caused displacement than those installed to 2 meters depth. At Parkfield, California, we have been monitoring the lengths of 17 baselines over the past decade with a 2 to 3 day interval between measurements using a two‐color geodimeter with a nominal precision of 0.5 mm over 5 km long baselines. Significant variations are observed on many of these baselines which appear to be related to the seasonally occurring rainfall, with the larger variations approaching 10 mm over the past decade. To test whether we could improve upon the measurements on some of the more susceptible lines, at two sites we installed new monuments within about 30 meters of the original monuments. After 1.5 years of measurements it is evident that the new monuments significantly attenuate the seasonal displacements to less than 1 mm. The use of deeply anchored monuments should improve the ability of fault‐scale geodetic monitoring arrays to detect small tectonic displacements.

Miocene to recent history of the western Altiplano in northern Chile revealed by lacustrine sediments of the Lauca basin (18�15??18�40? S/69�30??69�05?W)

The intramontane Lauca Basin at the western margin of the northern Chilean Altiplano lies to the west of and is topographically isolated from the well-known Plio-Pleistocene lake system of fluvio-lacustrine origin that covers the Bolivian Altiplano from Lake Titicaca to the north for more than 800 km to the Salar de Uyuni in the south. The Lauca Basin is filled by a sequence of some 120 m of mainly upper Miocene to Pliocene elastic and volcaniclastic sediments of lacustrine and alluvial origin. Volcanic rocks, partly pyroelastic, provide useful marker horizons. In the first period (6–4 Ma) of its evolution, the ‘Lago Lauca’ was a shallow ephemeral lake. Evaporites indicate temporarily closed conditions. After 4 Ma the lake changed to a perennial water body surrounded by alluvial plains. In the late Pleistocene and Holocene (2-0 Ma) there was only marginal deposition of alluvial and glacial sediments. The basin formed as a half-graben or by pull-apart between 10 and 15 Ma (tectonic displacement of the basal ignimbrite sequence during the ‘Quechua Phase’) and 6.2 Ma (maximum K/Ar ages of biotites of tuff horizons in the deepest part of the basin). Apart from this early basin formation, there has been surprisingly little displacement during the past 6 Ma close to the Western Cordillera of the Altiplano. Also, climate indicators (pollen, evaporites, sedimentary facies) suggest that an arid climate has existed for the past 6 Ma on the Altiplano. Together, these pieces of evidence indicate the absence of large scale block-faulting, tilt and major uplift during the past 5–6 Ma in this area.

40Ar/39Ar dating of white micas from the epidote to the omphacite zones, northern New Caledonia: tectonic implications

An 40Ar/39Ar study of white micas from high-pressure metamorphic rocks of northern New Caledonia yielded cooling ages of 37 ± 1 Ma for both epidote and omphacite zone samples. Whole-rock samples from the lawsonite zone yielded ages in the range 44–51 Ma with complicated age spectra, probably reflecting both detrital and newly grown micas. The areal extent of the mica samples, over 300 km2, suggests that the epidote and omphacite zone rocks cooled through the muscovite closure temperature, about 350 °C, as a coherent cooling unit. Simple thermal modeling suggests that these rocks could have closed at similar times if the unroofing rate were greater than 2–10 mm∙a−1. Lawsonite zone rocks occur structurally within about 0.5 km of garnet–omphacite rocks, suggesting the possibility of major postmetamorphic tectonic displacement.

Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California

The east Ventura basin originated in the middle Miocene as a rift system bounded on one side by the Oak Ridge-Simi Hills structural shelf and on the other side by a granitic ridge parallel to the San Gabriel fault. This fault began accumulating right slip 10-12 m.y. ago at a rate of 4.5-9 mm/yr (depending on whether total slip is 45 or 60 km), slowing to about 1 mm/yr in the Quaternary. North of the Santa Clara River, rifting ended prior to deposition of the uppermost Miocene-lower Pliocene Towsley Formation. South of the Santa Clara River, the rift axis shifted southwest toward the Oak Ridge-Simi Hills shelf as the Towsley Formation accumulated against a normal-fault ancestor of the Santa Susana fault. A change to contractile tectonics occurred in the Pliocene with depos tion of the Fernando Formation, when the Newhall-Potrero anticline developed as a monocline above a blind reverse fault; the Pico anticline to the southeast and the Temescal and Hopper Ranch-Modelo anticlines to the northwest may have a similar origin. Tectonic inversion and displacement on the southwest-verging Santa Susana fault began about 0.5 Ma based on appearance of locally-derived clasts in the upper Saugus Formation and its equivalents, and continues today, along with the southwest-verging San Cayetano fault farther west. Also active are northeast-verging backthrusts occurring in the east Ventura basin fold belt, and a segment of the San Gabriel fault which now acts as a northeast-dipping oblique-slip reverse fault. Northeast-trending discontinuities and structures divide the present deformation zone into four segments. In the Hopper Canyon segment at the west end of the area in the west Ventura basin, the San Cayetano fault places Miocene Modelo Formation over Pliocene-Pleistocene strata more than 5 km thick, largely at maximum burial. To the southeast, the Newhall-Potrero segment is characterized by north-vergent backthrusts within the east Ventura fold belt and by southward thrusting of the basin sequence (tectonic inversion) over the structural shelf on the Santa Susana fault. Farther southeast, the Placerita segment is marked by reverse faulting on both the Santa Susana fault and the San Gabriel fault. Southeast of the basin in the San Fernando Valley, the Sylmar segment contains a thick Pli cene-Pleistocene sequence overridden by basement rocks of the San Gabriel Mountains as well as the south-verging Mission Hills-Granada Hills and Northridge Hills fault zones.

Thermal history of the Sonnblick Dome, south-east Tauern Window, Austria: Implications for heterogeneous uplift within the Pennine basement

The Sonnblick Dome is one of several domal structures affecting the interface between basement and cover within the Pennine Zone of the Tauern Window in the eastern Alps. Rb-Sr isotopic data, comprising 19 biotite and 22 white mica ages from variably deformed granitic gneisses, provide new evidence of the thermal and tectonic history of the dome and its relationships with other parts of the south-east Tauern Window. White mica ages generally cluster between 26 and 30 Ma although there are values up to 82 Ma, which appear to reflect incomplete equilibration during Tertiary metamorphism under low amphibolite facies conditions; six closely spaced samples from an intensely sheared ‘gneiss lamella’ are more tightly grouped between 26 and 27.6 Ma and provide the best estimate of the age of syntectonic crystallization. Biotite ages are systematically younger, ranging from 19 to 23.5 Ma, reflecting closure during post-metamorphic cooling. Sonnblick Dome and the Hochalm Dome approximately 20 km further east, where closure of Rb-Sr in biotite did not occur until 16.5 Ma; the metamorphic peak here is also probably younger, possibly as late as 22 Ma. The Sonnblick Dome was formed before 27 Ma and the deformation style had changed to extension before biotite closure by 19 Ma. In contrast, rapid updoming in the Hochalm Dome was previously dated at 16.5 Ma and the differences in thermal history can be linked to differences in deformation history. Overall the geochronological data from the south-east Tauern Window demonstrate the heterogeneity of thermal history on a geographical scale of 10 km and emphasize the importance of tectonic displacements in controlling temperature within orogenic belts.

Last interglacial sea levels on the south coast of New South Wales

SUMMARY Remnants of the Last Interglacial shoreline occur at Middle Lagoon on the far south coast of New South Wales. Relict beach sediments can be traced to a height of at least +4.8 m and are indicative of a former mean sea level of about +3 m. Thermoluminescence (TL) ages of 126 ± 13 ka and 114 ± 15 ka were determined for beach and aeolian facies respectively. Sands in the lower part of an exposure on the adjacent Gillards Beach gave TL ages of 108 ± 13 ka, but sands in the upper part of that exposure gave an age of 19.9 ± 3.5 ka. This chronological evidence of a stratigraphic unconformity in what was initially taken as pedogenic differentiation at Gillards Beach is supported by contrasting electron traps and colour centres in crystal lattices of quartz grains in these two samples. No tectonic displacement is apparent. This site provides the first evidence of the Last Interglacial sea level for 1000 km along the coast between Gippsland and Newcastle.

Extensional reactivation of thrust faults accompanied by coseismic surface rupture, southwestern Wyoming and north-central Utah

Extensional reactivation of thrust faults north of the Uinta Mountains in Utah and Wyoming is accompanied by Quaternary surface faulting, tilting, and monoclinal folding. The 40-km-long Bear River fault zone consists of down-to- the-west, right-stepping, en echelon scarps, each about 3.0 to 3.5 km in length, striking N20°W to N20°E. Scarp-derived colluvial deposits record two Holocene surface ruptures with net vertical tectonic displacements ranging from 5 m per event (West, 1989, in press). Radiocarbon ages indicate surface faulting events occurred at about 4.6 ka and 2.4 ka. The length of surface rupture and net slip per event in the Bear River fault zone imply a seismic potential comparable to the Lost River fault, Hebgen Lake fault, and the Wasatch fault zone. A late Quaternary normal scarp is also coincident with the leading edge of the Absaroka thrust fault 7.2 km west of the Bear River fault zone. Scarp-derived colluvial deposits record one surface faulting event over a length of at least 5.0 km. The age of surface rupture is coeval with the Bear River fault zone, ∼2.4 ka. Normal faults displacing Pleistocene surfaces/deposits and tectonic tilt indicate that the Hogsback thrust east of the Bear River fault zone was reactivated in a normal sense but now may be inactive due to subsequent development of the Bear River fault zone. Cumulative normal throw of 200± m on the Hogsback fault may be responsible for postulated tectonic separation of the Bear River and Green River drainage basins Neotectonic deformation results from east-west extension superposed on the Hogsback and Absaroka thrust plates. Extensional reactivation of thrust faults caused propagation of new normal faults over stress points, particularly at the Hogsback ramp to flat transition, and at the thrust leading edges. The ages of surface rupture and cumulative throws indicate normal reactivation of the Hogsback and Absaroka thrusts, and propagation of the intervening Bear River fault zone proceeded from east to west opposite initial development of the thrusts in Late Cretaceous to Paleocene time. Normally reactivated thrust faults and propagation of listric normal faults accompanied by surface rupture have significant implications for the tectonic/seismogenic development of the eastern Basin and Range transition zone.

Paleomagnetism and geochronology of 23 Ma gabbroic intrusions in the Keku Strait, Alaska, and implications for the Alexander Terrane

Samples of Tertiary gabbro from 24 sites in the Keku Strait, Alaska, help constrain the displacement history of the Alexander terrane. Step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant 40Ar/39Ar age of 23.1 ± 1.7 Ma. The characteristic magnetization resides in magnetite, is easily isolated by thermal and alternating field demagnetization, and has both normal and reversed polarities. The mean paleomagnetic pole, with no structural correction, is latitude 87.1°N, longitude 141.6°E, A95 = 10.1°, with N = 20 sites. This pole indicates insignificant tectonic displacement (0.5° ± 8.2° southward) and rotation (0.6° ± 15.2° counterclockwise). We therefore conclude that any northward displacement or vertical axis rotation of the Alexander terrane, and/or tilting in the vicinity of the Keku Strait must have occurred before 23 Ma.

Tectonic style of venus: An analog to polar “icepack tectonics”

Venus and Earth are similar in size, bulk composition, internal heat generation and planetary differentiation. Like Earth, Venusian surface relief is at least partially compensated isostatically at some depth in the mantle and is partially supported by the dynamics of mantle convection. The “continental” crustal rocks of Venus are less dense than its mantle, but are much more globally extensive than their terrestrial counterparts. Based on the observed unimodal elevation distribution, the Venusian “continental” crust would appear to completely cover the planet. This suggests that crustal recycling may consist of simply remelting at its base. Venusian tectonics is fundamentally different from terrestrial tectonics (Solomon et al., 1991) in that there is no observed evidence for large scale subduction of lithosphere into the deep mantle, no large-offset strike-slip faults, and few examples of concave ridges characteristics of cooling and spreading lithosphere. The primary mode of planetary heat loss is not tectonics as on Earth, but instead appears to be hot spot volcanism. The tectonic history of Venus is dominated by a relatively static lithosphere with lateral tectonic displacements of limited scale much like the tectonics once believed to describe the geological development of the Earth. Thus there are fundamental differences between terrestrial and Venusian tectonics in morphology, kinematics and dynamics. Venusian tectonics appears to more closely resemble the ridges and leads in a choked field of Arctic pack ice in that the unimodal crust is buoyant and completely covers the surface, the primary plate driving forces are surface tractions at the base of plates, and there is recycling of crust by basal melting rather than subduction.

Late Gondwanide tectonic rotations within Gondwanaland

Geologic and paleomagnetic evidence from the Ellsworth‐Whitmore mountains crustal block of West Antarctica, and from the Falkland Islands of South America, indicates that tectonic displacement of major portions of the “Samfrau geosyncline” occurred after the early Mesozoic Gondwanide folding but prior to seafloor spreading. Comparison with continental deformation in the collisional Alpine‐Himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the Gondwanian orogeny resulted from collision of a fore‐arc and magmatic arc terrane with the Panthalassic margin of the Gondwana craton during closure of a marginal basin. The displacements had major consequences for the ensuing evolution of the South Atlantic‐Weddell Sea‐Ross Sea region.

A new controlled source tensor AMT system

The Lunigiana basin is a NW trending tectonic depression located in the Northern Apennines of Italy. The basin is bounded by active normal faults, characterized by moderate to strong seismicity, with historical earthquakes reaching up to magnitude 6 (February 14, 1834) and instrumental events with magnitude up to 5.4 (June 21, 2013).Using the classical methodologies of active tectonic studies (i.e. remote sensing and digital topography analysis, field mapping, structural geology, and tectonic geomorphology), the main active faults have been mapped for their geometries, kinematics and evidence of activity. Both fault systems bounding the basin to the northeast and southwest have revealed clear evidence of fault capability, i.e., tectonic displacement of the latest Pleistocene-Holocene sediments and geomorphic markers. Then, the study focused on the southwestern Arzelato-Mulazzo-Tresana fault system (AMT), depicting its Late Quaternary tectono-sedimentary evolution and linkage relationship between important fault segments. The throw measurement of key geomorphic markers on the central fault of the AMT allowed to derive medium- (last 350–250 ka) and short-term (18–13 ka) vertical slip rates: 0.44–0.62 and 0.45–0.68 mm/yr, respectively.Lastly, an exploratory trench for paleoseismological analysis was dug across the fault scarp where geomorphic indicators of active surface faulting were most pronounced, just south of the village of Mulazzo.Its paleoseismological analysis revealed at least two events of surface faulting, with a cumulative throw in excess of 70 cm, occurred in historical times. Radiocarbon dating of the exposed alluvial and slope deposits allowed to bracket the surface rupture events to 14th to 19th century CE and after the 18th century CE. Typically, in the active extensional belt of the Apennines, the observed offsets correspond to earthquakes of magnitude >6.3, also confirmed by the empirical fault-scaling laws. This magnitude is, therefore, higher than that currently inferred for this seismic source (Mw 5.7÷6.0 in the DISS catalog) and is comparable to that experienced in the adjoining Garfagnana basin (Mw 6.5 on September 7, 1920). The findings of this research suggests a need to revise the seismic hazard of the Lunigiana basin, including greater consideration of the surface faulting hazard in the seismic microzonation studies.More generally, our findings provide new insight into the comprehension of a) the still unclear deep geometry (listric or not?) and the hierarchy of bounding faults, and b) provide new elements to decipher the puzzling seismotectonic relationships of Lunigiana with the adjoining basins on both its ends, characterized by quite different structural settings.

Early Proterozoic basement exposures in the southern Canadian Cordillera: core gneiss of Frenchman Cap, Unit I of the Grand Forks Gneiss, and the Vaseaux Formation

The protolith age of high-grade metamorphic rocks exposed in structurally deep parts of the Omineca Crystalline Belt has been the subject of investigation and controversy for decades. We have applied multiple isotopic dating techniques to rocks of three structural culminations: the Monashee complex (which includes the Frenchman Cap and Thor–Odin gneiss domes), the Grand Forks horst, and the Vaseaux Formation, which lies in the footwall of the Okanagan Valley fault.Frenchman Cap core gneisses contain highly radiogenic Sr that scatters about a 2206 ± 117 Ma (1σ) Rb–Sr isochron with 87Sr/86Sr initial ratio of 0.700 ± 0.002. Monazite and zircon dates for the same rocks are 1851 ± 7 to 2103 ± 16 Ma (only U–Pb dates are given with 2σ errors), with lower intercepts from about 100 to 300 Ma. Sm–Nd whole-rock and crustal-residence (TDM) dates are 2.3 ± 0.2 Ga. Mafic–felsic layering in the core gneiss is also of Early Proterozoic age. There is no geochronometric evidence for Late Proterozoic or Mesozoic migmatization.Frenchman Cap mantling gneisses, including samples from above the Monashee décollement, have radiogenic Sr and unradiogenic Nd compositions that are not consistent with current inferences of a Late Proterozoic to Paleozoic depositional age. Two intrusive granitic rocks, which cut mantling gneiss, are either Early Proterozoic or Mesozoic–Cenozoic with a Proterozoic Sr isotopic signature acquired by assimilation of core gneiss. One other intrusive studied is probably Paleocene Ladybird granite. The age of the mantling gneiss is not yet consistently resolved.Grand Forks Gneiss Unit I paragneiss gives radiogenic whole-rock Sr, zircon U–Pb upper intercept, and Sm–Nd whole-rock crustal-residence dates of 1.7 ± 0.4 Ga, 1681 ± 3 Ma (2σ, but the apparent high precision is very dependent on the assumption made about the time of Pb loss), and 1.9 ± 0.3 Ga, respectively. Unit II and younger Grand Forks Gneiss units are Late Proterozoic or Phanerozoic. All isotope systems have been considerably reset on a centimetre to metre scale by Mesozoic–Cenozoic regional metamorphism. Grand Forks Sr, Pb, and Nd isotope data are much like those for Spokane area pre-Purcell basement.Vaseaux Formation micaceous schist and gneiss give radiogenic whole-rock Sr, zircon U–Pb upper intercept, and Sm–Nd crustal-residence dates of 2.1 ± 0.6 Ga, 1899 ± 49 Ma (2σ), and 2.2 ± 0.1 Ga, respectively. Hornblende-bearing schist and gneiss contain much less radiogenic Sr and more radiogenic Nd. The latter are either tectonic intercalations of Late Proterozoic to Paleozoic eugeosynclinal rocks or Mesozoic–Cenozoic mixtures of mantle-derived magma and older crustal rock. The Vaseaux Formation paragneiss is similar isotopically to paragneiss in the Frenchman Cap core gneiss. This may indicate a similar age, or that Vaseaux sedimentary rocks could be much younger and isochemically derived from a basement of Frenchman Cap character. The first alternative is favored because the three isotope systems are usually not preserved in unison through sedimentary processes. Sr isotopes, in particular, do not usually preserve a provenance age.In all three areas, late Mesozoic to early Cenozoic metamorphic monazite, hornblende, muscovite, and biotite dates provide a record of cooling from a Cretaceous to Paleocene culmination of regional metamorphism, with particularly rapid cooling during Paleocene to Eocene crustal extension and tectonic unroofing.The localities studied are tectonic windows on structural culminations that expose basement that we infer to be part of North America. Their ages fit the pattern of basement ages established for the stable craton. Their extent is consistent with the reconstruction of compressed miogeoclinal rocks. The eastern half of the Cordilleran region on both sides of the United States – Canada border is underlain by Early Proterozoic basement that was attenuated in Late Proterozoic time, compressed during Mesozoic–Cenozoic orogeny, and finally extended in early Cenozoic collapse of the thickened crust. During Mesozoic–Cenozoic orogeny the sedimentary cover of that basement was pushed approximately 200 km eastward and replaced by allochthonous terranes. The tectonic displacements documented in the southern Canadian Cordillera are truly exceptional.

Holocene Displacement of the Malibu Coast Fault Zone, Winter Mesa, Malibu, California: Engineering Geologic Implications

The Malibu Coast Fault Zone (MCFZ) is a major tectonic element in southern California that was heretofore considered to be “nonactive” according to State of California criteria. However, our recent investigations indicate that at least some splays are “active,” for they show tectonic displacement in Holocene time. Accordingly, because the MCFZ crosses rapidly-urbanizing portions of the Los Angeles metropolitan area, it is becoming an increasing geotechnical, as well as a geopolitical, concern for developers and for County and City planners.

Late Quaternary faulting and thermoluminescence dating of the East Cache fault zone, north-central Utah

The East Cache fault zone (ECFZ), the southernmost of five en-echelon Quaternary normal faults between the Wasatch and Teton faults, displays evidence of two large paleoearthquakes in the last 15,000 years (15 ka). Stratigraphic data from a trench on the central segment of the ECFZ are augmented with one radiocarbon date, the regional radiocarbon chronology, and 14 thermoluminescence (TL) age estimates. Of the 14 TL analyses performed by Alpha Analytic Inc., only 8 satisfied our criteria for statistical acceptability (< 20 per cent coefficient of variation, < 50 per cent extrapolation to residual TL level). The TL age estimates broadly bracket the earlier faulting event at between 8.7_ 1.0 and 17.4 + 3.0 ka, and the later event at between 2.5 _+ 0.5 and 8.7 _+ 1.0 ka. One radiocarbon age further constrains the earlier event to be younger than 15,540 _ 130 yr BP. Correlation of local Quaternary deposits to the regional radiocarbon chronology provides indirect, but more tightly limiting, age constraints of ca. 4 to ca. 7 ka for the later event, and ca. 13 ka to 15,540 +_ 130 yr BP for the earlier event. Both events had small vertical displacements at the trench site (0.8 to 1.8 m), but the cumulative net vertical tectonic displacement elsewhere is up to 4.2 m, suggesrive of M s 6.9 to 7.1 paleoearthquakes. Single-event displacements and the extent of fault scarps suggest that these two paleoearthquake ruptures were probably limited to the 20-kin-long central segment of the East Cache fault zone near Logan, Utah. The elapsed time since the most recent event on the central segment (ca. 4 to ca. 7 ka) is somewhat less than the estimated recurrence interval between the latest two paleoearthquake (8.6 _ 3.0 ka). However, comparison of only a single poorly constrained recurrence interval with the elapsed time does not result in a confident assessment of future earthquake potential.