Necessary Conditions for Intraplate Seismic Zones in North America

Thomas, William A.; Powell, Christine

doi:10.1002/2017tc004502

Cited by 29 publications

(28 citation statements)

References 63 publications

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“…This distribution of positive

normalΔ σ_{D}^{'}

is not seen in the reference model without the rift faults. These two findings from our models are consistent with the conclusion drawn by Thomas and Powell () that the seismicity in CSZ requires both the damaged impact structure zone and the rift faults.…”

Section: Discussionsupporting

confidence: 93%

Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

2019

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The Charlevoix Seismic Zone (CSZ) is located along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact structure. The impact structure superimposed major, steeply dipping basement faults trending approximately N35 • E. Approximately 250 earthquakes are recorded each year and are concentrated within and beneath the impact structure. Most M4+ earthquakes associated with the rift faults occurred outside the impact structure. Apart from the unique distribution of earthquakes, stress inversion of focal mechanisms shows stress rotations within the CSZ, and in the CSZ relative to the stress orientation determined from borehole breakouts. The primary goal of this research is to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We approach this using PyLith, a finite-element code for simulations of crustal deformation. Adopting the results from recent hypocenter relocation and 3-D tomography studies, we modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. We also discuss the effects of resolved velocity anomalies. We find that the observed stress rotation is due to the combined effect of the rift faults and the impact structure. One-dimensional velocity models of the CSZ with an embedded impact structure and a combination of 65 • -40 • -40 • and constant 70 • fault dip models with a very low friction coefficient of 0.3 and cohesion of 0 MPa can explain the observed seismicity and more than 50% of the stress rotations. Key Points:• The CSZ seismicity and stress orientations are the combined effects of the rift faults and the impact structure • Planar faults we considered can explain the observed seismicity but only 50% of the stress rotation • An impact structure 4 times less elastically stiff than the surrounding crust can explain the seismicity in the CSZ Supporting Information:• Supporting Information S1 Figure 2. (a) Model domain with the impact structure (gray) and crust (red). The three rift faults (black lines), dimensions, boundary conditions and the total amount of displacement are annotated. The outer 10-km-thick layer of crust (green) is added to contain fault edges within the domain, a requirement by PyLith. (b) The orientation of the model domain (red dotted box) and the associated coordinate axes (red arrows) relative to the geographic reference frame. The red dotted box is not the actual model domain but just a box that shows its "orientation." Cyan circles on the geographic map show the inner and outer impact structure boundaries and black solid lines trace rift faults (see Figure 1).for the region. We then try to correlate modeled stress distributions with the recently-relocated hypocenters (Powell & Lamontagne, 2017) and discuss implications for the nonuniform stress orientation around the impact structure that was previously recognized.

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“…This distribution of positive

normalΔ σ_{D}^{'}

Section: Discussionsupporting

confidence: 93%

Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

2019

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“…The pattern of vorticity within the weaker areas matches the different types of crustal flow within the models (Figures and Table ). The orientation of preexisting crustal structures in the SE Arunta Region also plays a role in the strain‐inducing accommodation of compressive stress (Hurd & Zoback, ; Thomas & Powell, ). This can be observed in models where crustal deformation is generally oriented at low angle to the principal stress component (Figures ).…”

Section: Discussionmentioning

confidence: 99%

“…While geophysical and geological data show that many intraplate regions are under compressional stress regimes originating from far-field stresses (Coblentz et al, 1998;Hurd & Zoback, 2012;Zoback et al, 1989), intracontinental orogens are surprisingly rare when compared to their peri-plate located orogen counterparts; hence, the presence of zones of mechanical weakness in plate interiors must be one of the principal requirements to allow intraplate orogenesis. The seismological impact of active intracontinental orogens is closely related to stress transmission and stress accommodation in the lithosphere (Thomas & Powell, 2017); consequently, in-depth studies of this special class of orogen are needed to better understand the relevance of plate boundary stress conditions to intracontinental orogenic activity.…”

Section: Introductionmentioning

confidence: 99%

Intracontinental Orogeny Enhanced by Far‐Field Extension and Local Weak Crust

et al. 2018

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The accommodation of intraplate stresses in preexisting weak regions of plate interiors is here investigated using thin viscous sheet numerical models. The intraplate stresses are governed by multicomponent and multidirectional stresses originating at plate boundaries. The modeled scenarios mimic plate boundary conditions during the intraplate Alice Springs Orogeny (ASO), central Australia, and include (1) a northwest‐southeast zone of weak lithosphere within strong continental blocks to the north and southand (2) a principal south directed stress condition at the northern boundary that causes minor clockwise rotation of the northern block. Alternative tectonic environments are investigated in additional models that include (1) secondary compressional or extensional stresses acting at the eastern boundary, representing the temporally variable stress conditions during the Tasmanides Orogeny, and (2) an eastern wedge‐shaped zone of rheologically weak lithosphere, mirroring rift fill of the Irindina subbasin. Our results highlight that a low angle between major crustal features (e.g., orogenic elongation and preexisting regional structures) and the principal transmitted stresses is highly relevant in the concentration of elevated levels of differential stress and subsequent localization of deformation in plate interiors. Secondary stresses orthogonal to the principal acting stresses may introduce effects that explain the episodic orogenic activity in the case of the ASO. The combination of secondary extensional stresses at the eastern boundary of Australia and weak lithosphere of the preexisting Irindina subbasin strongly influences the observed spatial strain intensity, localization, and kinematics of deformation during the ASO.

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“…100 Ma. If such deep fluids are indeed present, pervasive faulting and fracturing throughout the crust of the Reelfoot paleo‐rift may act as pathways of increased permeability (Thomas & Powell, ) that collect and channel these fluids from the mantle and the lower crust into the brittle crust, where pore fluid overpressure develop, episodically triggering seismic events or swarms.…”

Section: Discussionmentioning

confidence: 99%

A Parametric Analysis of Fault Reactivation in the New Madrid Seismic Zone: The Role of Pore Fluid Overpressure

Leclère

Calais

2019

JGR Solid Earth

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Earthquakes in Stable Continental Regions (SCR), where localized present-day tectonic loadingis negligible, remain difficult to explain. The New Madrid Seismic Zone (NMSZ) is a type-locale for such events, with four M>7 events in 1811-1812 and a regional seismicity that continues to this day. Here, we seek to determine the most favorable conditions for fault reactivation in such a context using 33 earthquakes in the 2.1-4.7 magnitude range with well-determined focal mechanisms while accounting for the vertical gradient of differential stress with depth. To do so, we developed a Mohr-Coulomb-based parametric analysis of fault reactivation that allows us to vary the orientation of the principal stresses, the shape ratio of the stress tensor, the fault friction coefficient, the pore fluid overpressure, and the gradient of differential stress with depth. Doing so, we are able to determine the lowest stress perturbation conditions required for fault reactivation. Our results show that the reactivation of the faults studied here requires pore fluid overpressure, unless their friction coefficient is 0.4 or less. We argue that such weak faults are unlikely and favor a triggering mechanism via deep fluids, possibly upwelling from the upper mantle where a low-velocity seismic anomaly could indicate their presence. This mechanism, documented in other intraplate areas, does not require local tectonic stress or strain accumulation to explain seismicity in active intraplate regions, where elastic strain is drawn from a prestressed crust.

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Necessary Conditions for Intraplate Seismic Zones in North America

Cited by 29 publications

References 63 publications

Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

Intracontinental Orogeny Enhanced by Far‐Field Extension and Local Weak Crust

A Parametric Analysis of Fault Reactivation in the New Madrid Seismic Zone: The Role of Pore Fluid Overpressure

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