From chile to nevada to the athabasca basin: Earthquake-induced geochemical anomalies from near-field to far-field

摘要

In northern Chile, geochemical anomalies are present above porphyry copper deposits buried beneath thick piedmont gravels. Reactivation of basement faults that originally focussed deposits created fracture zones in the indurated gravels, providing a channel along which earthquakes forced ground water to the surface. The composition of the groundwater had been modified by interaction with the deposits, generating anomalies. Northern Chile lies directly above a subduction zone, can similar anomalies be formed in regions of lesser seismicity? Seismologists have described the phenomenon of "triggered earthquakes", where passage of dynamic surface waves from major earthquakes can cause other earthquakes hundreds or thousands of kilometres distant. These waves remove blockages along faults, releasing high pressure water to rise and replace lower pressure water. The resulting increase in pore pressure lowers the effective normal stress, allowing the fault to slip: upsurge of water and fault reactivation are linked. In Nevada, part of the Great Basin, there is inherent seismicity from crustal extension and triggered earthquakes from major earthquakes as far distant as Alaska. Case studies show distinct geochemical anomalies above deeply buried deposits, attributed to groundwater with elements derived from the deposits reaching the surface. The Proterozoic Athabasca Basin contains the largest concentration of high grade uranium deposits in the world; it is within a cratonic setting. Deposits lie near the unconformity between Athabasca sandstones and a metamorphic basement along faults that were repeatedly reactivated by remote earthquakes. Reactivation of faults, coupled with fluid movement, continued long after the formation of the deposits. This allowed the upward movement of material from the deposits during the Proterozoic, including radiogenic lead, which had accumulated in the uranium ore. In places, this material reached several hundred metres to the upper levels of the sandstones, from which elements were transferred to tills, soils and vegetation during Quaternary time.