Constraints on the size of the Vredefort impact crater from numerical modeling
Constraints on the size of the Vredefort impact crater from numerical modeling
复制标题
数值模拟对 Vredefort 撞击坑大小的限制
DOI：
10.1111/j.1945-5100.1998.tb01652.x
复制
发表时间：
1997
影响因子：
2.2
通讯作者：
E. Pierazzo
中科院分区：
文献类型：
--
作者：
E. Turtle;E. Pierazzo
摘要：南非的弗里德堡构造是由大约20亿年前的一次陨石撞击形成的。从那时起，陨石坑已经被严重侵蚀；因此，为了估计它的原始大小，研究人员不得不严重依赖与其他陆地撞击结构的比较。最近对原始陨石坑直径的估计从160公里到400公里不等。在这项研究中，我们结合了氢代码和有限元建模的能力，使用前者来预测撞击产生的冲击波的压力在哪里会高到足以形成平面变形特征（pdf）和粉碎锥体，后者来跟踪这些冲击等压线在火山口坍塌期间的后续位移。我们通过将弗里德堡周围pdf文件的观测位置与我们对不同尺寸的弹丸撞击的模拟结果进行比较，建立了对弹丸和瞬态弹坑大小（以及最终弹坑大小）的限制。这些模拟表明，直径约10公里的岩石弹丸，以20公里/秒的速度垂直撞击，产生的冲击压力与弗里德堡周围pdf分布一致。这些弹丸参数对应于直径约80公里的瞬态弹坑或直径约120-160公里的最终弹坑。考虑到建模过程中的不确定性，我们认为最终的陨石坑直径为120至200公里，与在弗里德堡观测到的pdf位置一致。与破碎锥形成相对应的冲击压力轮廓在接近表面时几乎是水平的，这使得这些特征的位置对陨石坑大小的限制作用减弱。然而，它们可能会对撞击后发生的侵蚀量提供限制。
Abstract— The Vredefort structure in South Africa was created by a meteorite impact about two billion years ago. Since that time, the crater has been deeply eroded; so to estimate its original size, researchers have had to rely heavily upon comparison to other terrestrial impact structures. Recent estimates of the original crater diameter range from 160 km to as much as 400 km. In this study, we combined the capabilities of both hydrocode and finite‐element modeling, using the former to predict where the pressure of an impact‐generated shock wave would have been high enough to form planar deformation features (PDFs) and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater. We established constraints on the sizes of the projectile and the transient crater (and, thus, on the size of the final crater) by comparing the observed locations of PDFs around Vredefort to the results of our simulations of impacts by projectiles of various sizes. These simulations indicate that a rocky projectile with a diameter of ∼10 km, impacting vertically at a velocity of 20 km/s generates shock pressures that are consistent with the distribution of PDFs around Vredefort. These projectile parameters correspond to a transient crater ∼80 km in diameter or a final crater ∼120–160 km in diameter. Allowing for uncertainties in our modeling procedures, we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of PDFs at Vredefort. The shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface, making the locations of these features less useful constraints on the crater size. However, they may provide a constraint on the amount of erosion that has occurred since the impact.