Neotectonic constraint on models of strain localisation within Australian Stable Continental Region (SCR) crust
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<p>The mechanisms that lead to the localisation of stable continental region (SCR) seismicity, and strain more generally, remain poorly understood. Recent work has emphasised correlations between the historical record of earthquake epicentres and lateral changes in the thickness, composition and/or viscosity (thermal state) of the lithospheric mantle, as inferred from seismic velocity/attenuation constraints. Fluid flow and the distribution of heat production within the crust have also been cited as controls on the location of contemporary seismicity. The plate margin-centric hypothesis that the loading rate of crustal faults can been understood in terms of the strain rate of the underlying lithospheric mantle has been challenged in that a space-geodetic strain signal is yet to be measured in many SCRs. Alternatives involving the release of elastic energy from a pre-stressed lithosphere have been proposed.</p><p>The Australian SCR crust preserves a rich but largely unexplored record of seismogenic crustal deformation spanning a time period much greater than that provided by the historical record of seismicity. Variations in the distribution, cumulative displacement, and recurrence characteristics of neotectonic faults provide important constraint for models of strain localisation mechanisms within SCR crust, with global application. This paper presents two endmember case studies that illustrate the variation in deformation characteristics encountered within Australian SCR crust, and which demonstrate the range and nature of the constraint that might be imposed on models describing crustal deformation and seismic hazard.</p><p>The ~0.5 m high 2018 M<sub>W</sub> 5.3 Lake Muir earthquake scarp in southwest Western Australia is representative of a class of ruptures in the Precambrian SCR of Australia where the scarps are isolated from neighbouring scarps and there is little or no landscape evidence for recurrence of morphotectonic earthquakes, or of the construction of regional tectonic relief. In contrast, scarps in the Phanerozoic SCR of eastern Australia typically occur within a scarp-length of neighbouring scarps, and demonstrate extended histories of recurrence of morphotectonic events. For example, the ~75 km-long Lake George fault scarp is associated with a vertical displacement of ~250 m which accrued as the result of many morphotectonic earthquakes over the last ca. 4 Myr. The scarp links into neighbouring scarps, forming a belt-like arrangement that defines the topographic crest of the southeast Australian highlands. The limited data available indicates that recurrence is highly episodic, with periods of fault activity potentially coinciding with changes at the plate boundaries.</p>While relatively rapid and large‐scale extension occurred in the Death Valley region of California during Neogene time, little or no shallow extension occurred in the adjacent Sierra Nevada. This contrast in tectonic history has often been extrapolated to include the entire lithosphere, but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the Sierra than under the Death Valley region. Upper mantle seismic velocities observed beneath the High Sierra are lower than those observed in other regions with comparable surface heat flow. This discrepancy could be resolved if the mantle lithosphere beneath the High Sierra had become warmer, presumably by tectonic thinning, in the last 10 m.y. Upper mantle seismic velocities, averaged topography, and Bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the High Sierra than beneath the California portion of the Basin and Range Province to the east. This suggests that extension of the crust near Death Valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the Sierra Nevada. The extension in the crust of the California Basin and Range Province and the thinning of the mantle lithosphere under the High Sierra appear to share the same bounds in time and space. The uplift of the High Sierra occurred over the past 9 m.y., which coincides with most of the extension that occurred in the California Basin and Range Province. Because the orientation of extension in the California Basin and Range Province is inferred to be approximately N60°W from geologic, geodetic, and in situ stress measurements, the northern and southern edges of the Death Valley extensional subprovince may extend N60°W from the inferred northern and southern limits of west dipping low‐angle normal faults of the Death Valley region. Pronounced changes in the averaged topography and Bouguer gravity anomaly across these two bounds both in the Basin and Range Province and in the Sierra Nevada support a connection between the tectonics of both regions. The geomorphic history of the southern Sierra suggests an up‐to‐the‐north warp of the Sierra across this southern bound during latest Cenozoic time. Hence extension near Death Valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the Sierra Nevada. This geometry requires.
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In this paper, I make the case for widespread lower-crustal detachment and flow in the North American Cordillera. An indicator that geologically recent flow has occurred comes from seismic structure data showing the crust in most of the Cordillera from Mexico to Alaska is uniformly thin, 33 ± 3 km, with a remarkably flat Moho. The flat Moho is in spite of extensive normal faulting and shortening that might be expected to deform the Moho. It has been concluded previously that the high topographic elevations are due to thermal expansion from Cordillera-wide high temperatures compared to stable areas, not due to a crustal root. I argue that the constant crustal thickness and flat Moho also are a consequence of temperatures sufficiently hot for flow in the lower crust. Lower-crust detachment and flow has previously been inferred for Tibet and the high Andes where the crust is thick such that unusually high temperatures are expected. More surprising is the similar conclusion for the Basin and Range of western USA where the crust is thin, but high temperatures have been inferred to result from current extension. There are now adequate data to conclude the Basin and Range is not unique in crustal thickness or in temperature. The crust in most of the Cordillera is similarly hot in common with many other backarcs. Five thermal constraints are discussed that indicate that for most of the Cordillera, the temperature at the Moho is 800–850 °C compared to 400–450 °C in stable areas. At these temperatures, the effective viscosity is low enough for flow near the base of the crust. The backarc Moho may be viewed as a boundary between almost 'liquid' lower crust over a higher viscosity, but still weak upper mantle. The temperatures are sufficiently high for the Moho to relax to a nearly horizontal gravitational equipotential over a few tens of millions of years. The inference of a weak lower crust also suggests that topography over horizontal scales of over 100 km must be short lived over a similar timescale, after the generating forces relax. A weak lower crust in the Cordillera is also shown by the effective elastic thickness, Te, which indicates significant strength only in the upper crust. Other indicators of lower-crust flow or detachment are seismic reflectors in the lower crust that are interpreted to result from horizontal shearing, and outcrop sections exhumed from the deep crust that exhibit horizontally sheared fabric.
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It has long been recognized that regional topographic gradients may give rise to tectonic (non-lithostatic) stresses in the lithosphere (Artyushkov, 1973). The elevation of a buoyantly-uplifted region represents a balance between these stresses and the strength of the lithosphere. This study uses existing data on crustal and lithospheric structure in the western United States to test the hypothesis that the topographically high (1.5--2.2 km) northern Basin and Range is spreading under it own weight. Following England and Jackson (1989), the total deviatoric tensile force (Fl) in the northern Basin and Range (NBR) due to the regional high topography is the difference between the vertically-integrated lithostatic stress in the NBR and in western California. Using available velocity models for the crust and upper mantle, and empirically-derived velocity-density relationships, calculated values of Fl range between 1--3 [times] 10[sup 12] N/m. Assuming a visco-elastic rheology for the lithosphere, an average heat flow of 90 m W/m[sup 2], and a crustal thickness of 35 km, values of Fl ranging from 1--3 [times] 10[sup 12] N/m may result in horizontal extension rates of approximately 10[sup [minus]15]/s to 10[sup [minus]15]/s to 10[sup [minus]16]/s. This is comparable to the rate of seismically-released strain in the NBR, andmore » to extension rates of 8--9 mm/yr across the region determined from geologic and geodetic data. These results imply that shear tractions on the base of the lithosphere from mantle convection are not necessary to explain NBR extension. In addition to driving active extension, the weight of the topographically high NBR may exert a compressive force on surrounding lowlands. If so, this may account for some of the active shortening in western California, and the state of horizontal compressive stress in the western Great Plains.« less
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Abstract We investigate probabilistic seismic hazard analysis (PSHA) in low-seismicity regions in which epistemic uncertainties are largely due to the sparsity of data, with a focus on Finland, northern Europe. We investigate the sensitivity of site-specific PSHA outcomes to different choices of basic input parameters, starting from preexisting PSHA models of the nuclear licensees in the country, without producing a final hazard curve. The outcome shows that the parameters and models needed to estimate future seismicity rates from actual observations, in particular the b value, seismicity rates, and the largest possible magnitude, M max , as well as the median ground-motion prediction equation, play significant roles. The sensitivity also depends on the spectral frequency; for example, the effect of M max is significant especially for a low-frequency hazard at annual frequency of exceedance 10 −5 but more moderate for peak ground acceleration. The delineation of seismic source zones (SSZs) remains ambiguous in regions of low seismicity. This, combined with the dominance of the host SSZ and its seismicity parameters, may have a substantial impact on the outcome. Our results are quantitatively applicable to Finland, but may also be of relevance to other low-seismicity regions in Europe and elsewhere. For future work we recommend the exploration of PSHA sensitivity with focus on the host SSZ with its immediate vicinity and the b value around the site of interest.
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New geological and geophysical information from the eastern Great Basin sheds light on how Cenozoic extensional strain is partitioned throughout the crustal column and provides broad constraints on how much new material was added to the crust as a consequence of synextensional magmatism. The total amount of extension across the eastern half of the northern Basin and Range province is estimated to be 141 km or 77%, but varies dramatically on more local scales. Individual domains, several tens of kilometers across, have experienced supracrustal extensional strains that vary from 15% to 300%. Volcanism in this region initiated in the early Oligocene and has continued sporadically to the present. The major eruptions appear to be both temporally and spatially associated with large‐magnitude extension. A COCORP deep seismic reflection profile across the eastern Great Basin indicates a uniform present crustal thickness of 30 to 35 km. The preextensional crustal thickness is inferred to have tapered westward from the 40‐ to 45‐km thickness of the Colorado Plateau to a maximum of 50 km in the thickened root zone of the Sevier fold and thrust belt. Simple mass‐balance calculations employing the present and inferred preextensional crustal thicknesses together with the amount of extension suggest that approximately 5 km of the present crustal thickness was added to the crust during Cenozoic extension and magmatism. The lack of relief on the reflection Moho suggests that the crust as a whole may have stretched quite uniformly. A model for crustal stretching that incorporates a heterogeneously deforming (brittle) upper crust decoupled from a more uniformly deforming (ductile) middle and lower crust, together with a significant flux of mantle‐derived magmas into the crust, best explains the geological and geophysical observations from the eastern Great Basin. This open‐system, two layer stretching model has important thermal and mechanical implications and helps account for many of the enigmatic aspects of Cenozoic extension and magmatism in the western United States.
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Problems about the nature of extended crust in the Basin and Range include the role of plastic flow during extension, the possibility of fluids in the crust, and the amount of mantle‐derived material added to the crust. To address these problems, the University of Wyoming conducted a multicomponent wide‐angle seismic experiment in the Ruby Mountains of the Basin and Range. The northern Ruby Mountains expose upper and middle crustal rocks of a metamorphic core complex, whereas the southern Ruby Mountains consist of low‐grade miogeoclinal rocks. The wide‐angle experiment consisted of a 95 km long N‐S profile that extended from the southern to the northern Ruby Mountains. Travel time inversion of the wide‐angle reflection data reveals relatively high seismic velocities in the midcrust to lower crust which suggest that fluid‐filled pores, if they exist, do not reduce the seismic velocities significantly. Therefore the midcrustal to lower crustal porosity is probably much smaller than 1–2 vol %. The average P wave velocity for the crust is 6.4–6.5 km/s in the north and 6.1–6.2 km/s in the south. This difference suggests that larger amounts of mantle‐derived material were added during the Tertiary to the core complex crust of the northern Ruby Mountains than to the “normal” Basin and Range crust of the southern Ruby Mountains. The velocity profile is consistent with a maximum of 6–7 km of mantle‐derived material in the south and a maximum of 12–15 km of mantle‐derived material in the north. Thus volumetric mafic intrusions probably accommodated a maximum of 7 km of core complex exhumation. Shear wave splitting indicates that the upper to middle crust is seismically anisotropic probably due to superposed Tertiary plastic flow patterns. Doming and inflation of a midcrustal layer in the seismic model are compatible with Tertiary flow of material from the south to the north that compensated about 2–5 km of core complex exhumation. Precritical Moho reflections (0–70 km offset, 10–11 s normal two‐way travel time) are weaker and less continuous than midcrustal reflections (7–9 s normal two‐way travel time). Strong and continuous Moho reflections occur at postcritical (>80 km) offsets only. Modeling shows that the Moho depth increases from about 32.5 km in the south to about 34.5 km in the north.
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Deviation from isostasy is commonly believed to be caused by the strength of the Earth's lithosphere. An analysis of crustal compensation dynamics suggests that the deviation may have a dynamic origin. The analysis is based on analytic models that assume that (1) the medium is incompressible and has a layered and linear viscoelastic rheology and (2) the amplitude of topography is small compared with its wavelength. The models can describe topographic relaxation of different density interfaces at both small (e.g., postglacial rebound) and large time‐scales. The models show that for a simple crust‐mantle system with topography at the Earth's surface and Moho representing the only mass anomalies, while the crust always approaches the isostatic state at long wavelengths (>800 km), crustal isostasy may not be an asymptotic limit at short wavelengths, depending on crustal and lithospheric rheology. For a crust with viscosity smaller than lithospheric viscosity, at wavelengths comparable with widths of orogenic belts (i.e., <300 km), the crust tends to approach a state with significant overcompensation (i.e., excess topography at the Moho) within a timescale of about 10 7 years, and this characteristic time depends on wavelengths and crustal viscosity. This overcompensation is greater for weaker crust and stronger lithosphere. A thicker crust or lithosphere also enhances this overcompensation. If crustal and lithospheric viscosities are both large and comparable, the asymptotic state for the crust displays a slight undercompensation. For an elastic and rigid upper crust, the crust eventually becomes undercompensated after a characteristic decay time of topography at the Moho. The characteristic time is dependent on viscosity and thickness of the lower crust. The deviation from isostasy arises because these viscosity structures result in a ratio of vertical velocity at the surface to vertical velocity at the Moho which in the asymptotic state for short wavelengths differs from the ratio of density contrast at the Moho to that at the surface.
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We develop a map of crustal thickness variations across the Great Basin, Colorado Plateau, Rocky Mountain, and Great Plains Provinces of the western United States using common conversion point stacking of teleseismic receiver functions. Below the Rocky Mountains and High Plains in Colorado we find the thickest crust in the region at 45–50 km thick. Beneath the Basin and Range, thinner, between 30 and 40 km, crust is found. Thin, 30 km thick, crust is present in the northern portion of Nevada and Utah despite elevations similar to those farther south. Crustal thickness across the Colorado Plateau can be characterized as a broad transitional region between the thin crust of Basin and Range to the thicker crust of the Rocky Mountains. The impedance contrast across the Mohorovicic discontinuity decreases below the Colorado Plateau, as converted arrivals recorded in this region appear weak compared to surrounding areas. Variations in V P / V S across the region indicate higher values along the western boundary of the Basin and Range, in the Rocky Mountains, and in the western Great Plains. We are not able to characterize V P / V S in the Colorado Plateau. We find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole. Differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western United States.
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