| Title | Lithospheric strength and stress revisited: pruning the Christmas tree |
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| Author | Ellis, S; Wang, K |
| Source | Earth and Planetary Science Letters vol. 595, 117771, 2022 p. 1-13, https://doi.org/10.1016/j.epsl.2022.117771 |
| Image |  |
| Year | 2022 |
| Alt Series | Natural Resources Canada, Contribution Series 20220167 |
| Publisher | Elsevier |
| Document | serial |
| Lang. | English |
| Media | paper; digital; on-line |
| File format | pdf |
| Subjects | tectonics; Science and Technology; lithosphere; stress analyses; craton; plate boundaries; tectonic environments |
| Illustrations | models; graphs; lithologic sections; tables |
| Program | Public Safety Geoscience Assessing Earthquake Geohazards |
| Released | 2022 08 07 |
| Abstract | Whether lithospheric stress can reach the maximum level predicted by the Christmas-tree strength envelope is a fundamental question but with controversial answers. There is little controversy that a
deforming lithosphere in high heat flow regions is likely critically stressed, i.e., at full yield at all depths, as described by the envelope. But different conceptual frameworks offer opposite views for very cold lithosphere, either at full yield
or far below yield. Here, we use simple numerical models to investigate stresses in end-member cold cratonic lithosphere (e.g., Canadian Craton) in comparison with end-member warm plate-boundary lithosphere (e.g., Canadian Cordillera). The two key
elements of our modelling are (1) that lithospheric stress builds up elastically with horizontal tectonic loading not only in the elastic-frictional brittle regime but also in the viscoelastic ductile regime, and (2) that the stress level is limited
by the available tectonic force. In a cratonic lithosphere, the limiting tectonic force is sustained by competent rock material over a large depth range, represented by the competent thickness Tcthat exceeds 90 km. The lithosphere undergoes mostly
elastic deformation at a stress level of a few tens of MPa. While weakly stressed strong lithosphere can still produce limited earthquakes at shallow depths due to structural and stress heterogeneity, the lithospheric stress under horizontal tectonic
loading is theoretically predicted to be orders of magnitude lower than predicted by the Christmas-tree envelope. Stresses in a real lithosphere may substantially deviate from this theoretical level because of spatiotemporal variations in rheology
and structure. For example, the stress memory of past loading history in cold lithosphere may or may not be erased by more recent tectonic stresses. Because much of previous scientific debates on lithospheric stress levels and comparison with
seismicity were focused on topographically induced flexural stress, we also investigate the effect of vertical loading. We show that the effective elastic thicknessTederived from the flexural response is a reasonable proxy for Tcderived from
horizontal tectonic loading; a very large Tesuch as >80 km is generally associated with very low tectonic stress far below yield. However, the flexure-induced bending stress is not directly comparable with seismicity because it may either enhance or
suppress seismogenic stress in the crust. |
| Summary | (Plain Language Summary, not published)
A fundamental but extremely controversial issue in understanding intraplate earthquake processes and hazard is the level of tectonic stress that drives
earthquakes. According to the most widely held view, the stress is sufficiently high everywhere such that the crust is always on the verge of seismic failure, i.e., ready to produce earthquakes upon small perturbations. In this theoretical study, we
demonstrate that in old cratonic regions such as much of eastern Canada the stress is very low, and the crust generally is far from failure. Earthquake occurrence under very low background tectonic stress reflects structural heterogeneity with either
locally high stress or locally weak faults. The findings provide a conceptual framework for studying natural and induced earthquakes within tectonic plates. |
| GEOSCAN ID | 330336 |
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