Investigating the role of transient rapid trench retreat in initiating rifting of a mobile overriding plate during subduction

<p>Trench retreat, or slab roll-back, has been proposed to account for various degrees of extensional deformation within the overriding plate in subduction zones, eg. Izu-Bonin-Mariana, Tonga etc. However, the relationship between trench retreat rate and the degree of extension has not been rigorously tested. Here we obtain a wide range of trench retreat rate by varying the initial age of subducting plate (SP, Age<sup>0</sup><sub>SP</sub>) and overriding plate (OP, Age<sup>0</sup><sub>OP</sub>) met at trench. Then we investigate how much trench retreat rate is needed to initiate rifting in the OP.</p><p>The results show that models would evolve from a non-steady state towards a steady state as the SP sinks to the transition zone at 660 km.</p><p>Before the SP starts to interact with the transition zone, the trench retreat rate accelerates with time reaching a maximum value (v<sub>max</sub>), which can be very high but only lasts a short time (~0.5 Myr). For models with a given OP, v<sub>max </sub>is Age<sup>0</sup><sub>SP</sub>-dependent. The trench retreat rate, on the other hand, determines the extensional extent within the OP. With increasing Age<sup>0</sup><sub>SP</sub>, a minimum trench retreat rate (v<sub>rift</sub>) is needed to initiate rifting within the OP. For models with Age<sup>0</sup><sub>OP </sub>= 20 Myr and Age<sup>0</sup><sub>OP </sub>= 25 Myr, v<sub>rift</sub> is ~19 cm/yr and ~27 cm/yr separately. This implies that an older OP is more resistant to extensional stress field driven by trench retreat. In all, three types of stretching states are observed within the OP in our models: i) minor extension, where v<sub>max</sub><v<sub>rift</sub> and the OP lithosphere has little extension; ii) rift, where v<sub>max</sub>≈v<sub>rift</sub> and the OP would rift but not be torn apart; iii) break-up, where v<sub>max</sub>>v<sub>rift</sub> and the OP would rift when the trench retreat rate reaches v<sub>rift</sub>, then breaks up into two parts after it exceeds v<sub>rift</sub>. We note all three states involve different extents of mantle wedge erosion at ~100 km away from the trench underneath the OP, while rifting and break-up occur >700 km away from the trench. In the break-up cases, the two parts of the OP can be ~250 km apart.</p><p>After the SP reaches the transition zone, the trench retreat rate would drop to a constant magnitude around 2 cm/yr and lose the Age<sup>0</sup><sub>SP</sub>-dependency. This is because the viscosity jump at the transition zone prevents the SP from accelerating into the lower mantle. Meanwhile, the Age<sup>0</sup><sub>SP</sub>-dependent negative buoyancy loses its dominant role in driving the trench retreat.</p><p>We discuss two driving mechanisms to relate the initiation of extension with rapid trench retreat (trench suction): 1) focused upwelling from the transition zone; 2) horizontal basal drag. We conclude that the transient rapid trench retreat can lead to an extensional stress field through basal drag which is strong enough to initiate rifting or even break-up within a mobile overriding plate. A high negative SP buoyancy could play the driving force to generate this transient rapid trench retreat.</p>

The effect of pre-existing faults on the development of extensional basins: insights from wet clay experiments

<p><span>Most of the present-day extensional systems formed in areas that already experienced an older phase of tectonic activity. Therefore, understanding how a pre-existing structural setting may affect the development of an extensional basin is a crucial interplay to decipher. Depending on the kinematics of these phases, the resulting inherited faults can be extensional, contractional, or transcurrent. Consequently, a new extensional basin forms atop or across pre-existing faults that can dip at a low- (e.g., inherited thrust faults) or high-angle (e.g., inherited extensional faults). Furthermore, the inherited structures can have a non-optimal attitude with respect to the new extensional stress field, thereby determining different instances for reactivation. In this study, we analyzed the impact of dip and strike of inherited faults on the development of an extensional basin using wet clay (kaolin) analogue modeling. We reproduced sixteen different setups by varying the dip (30°, 45°, 60°) and the strike (15°, 30°, 45°, 60°, 75°) of the pre-existing faults that we introduced in the experiments before applying extension. The results show that the orientation of pre-existing faults has a direct effect onto the shape of the new extensional basins. When the pre-existing faults are reused to accommodate the new extensional phase, the formed basins are asymmetric and the rate of growth of the new faults is lower.</span></p>

Seismotectonics of the Grand Wash Arizona Area

The Grand Wash basin is located in northwest Arizona adjacent to the physiographic boundary of the Colorado plateau. The area is well mapped geologically and is geophysically similar to the Basin and Range and Transition Zone in structural style and history. The occurrence of a rare swarm of earthquakes in 2016 in the basin area served as an opportunity to perform a seismotectonic analysis of the Grand Wash basin. Results of the analysis indicate the basin is presently undergoing mild east west extension. The east west extension and associated seismicity of the swarm are here suggested to be the result of stress created by negative gravitational potential energy of the Colorado plateau relative to the lower Basin and Range and Transition Zone. This is suggested by the clockwise rotation of the extensional stress at about 36° N on both the plateau and the Basin and Range and Transition zone.