[SCG59-P07] Subsidence in an inverted failed rift basin, Niigata Basin, northern Honshu, Japan
Keywords:local isostasy, inversion tectonics, crustal properties, Niigata Basin, backstripping, subsidence analysis
In this study, we aim to examine the reason for this subsidence by calculating the total subsidence of the basin over time, and the subsidence of the Niigata basin due to sediment and water loading alone using 1D Airy backstripping. The difference between these calculations represents the contribution of thermo-tectonic effects on the vertical movements of the basin. Wireline logs, rocks samples, and biostratigraphic data from oil corporation records for eleven wells drilled in the Niigata Basin between 1960 and 2000 are used to derive the parameters needed for the model: paleobathymetry, age-depth relations, and exponential porosity-depth relations for different lithologies.
The results show rapid tectonic subsidence of 1200 – 2000 m between approximately 17 – 15 Ma that is interpreted as large scale syn-rift crustal thinning. Between 15 – 3.5 Ma, the tectonic subsidence exponentially decreases totaling at approximately 1000 m of subsidence. We attribute this stage to the post-rift thermal relaxation of the crust. At 3.5 Ma in several wells, accelerating subsidence of up to 3500 m is observed, due to both tectonic and sediment loading processes. At approximately 1 Ma, differential behavior between the northern and southern clusters of wells is observed. The southern wells show gradual uplift in both the tectonic and total subsidence curves, in accordance with the prevailing compressional stress regime [Sato, 1994]. In the northern wells, gradual tectonic uplift is observed, whereas we observed a net downwards vertical motion due to sediment loading alone.
The results indicate that at the time of inversion, a densification of the crustal column in the Niigata Basin took place. Possibly the mafic block as described by [Sato et al., 2012] in the basin’s lower crust experienced high pressure phase transitions, dynamic recrystallization or internal faulting. When the subsidence curves stabilize around 1 Ma the crustal column of the Niigata Basin becomes less dense, possibly due to two granitic crustal thrust wedges as described by [Sato et al., 2012] that close in on the basin from the eastern and western sides along outwards dipping reverse faults. We used a back-of-the-envelope analytical model, to calculate the isostatic response of the crust to the onset of such a thrust wedge. We found that this process contributes to the isostatic uplift that is observed in most wells at 1 Ma. We attribute the ongoing total subsidence that still occurs in the northern Niigata Basin to crustal loading due to rapid sedimentary infill of the area’s depressions.
This study demonstrates the importance of prevalent crustal properties in determining the subsidence behavior of an inverted failed rift basin. More research needs be done on the on the flexural rigidity of the crust beneath the Niigata Basin in order to further constrain the isostatic response of the basin to loading. This information can then be used to validate the assumptions of local isostasy that were made in this study.