Combining shock barometry with numerical modeling: Insights into complex crater formation—The example of the Siljan impact structure (Sweden)

Abstract

Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz‐bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best‐fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best‐fit model results in a final crater (rim‐to‐rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak‐ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure.

Estimating average shock pressures recorded by impactite samples based on universal stage investigations of planar deformation features in quartz—Sources of error and recommendations

“…() and Holm‐Alwmark et al. (). In the case of samples with a high number of PDF sets per grain (i.e., ~>3), if only using ANIE and WIP programs, a non‐negligible proportion of PDFs are classified as unindexed, when in fact they would be indexed using the manual graphical method.…”

“…The number following the letters H or V corresponds to depth (in meters) below the surface (see Holm‐Alwmark et al. ). The number following the letter S corresponds to the sample number presented in Holm et al.…”

“…To address this, since measuring features only one time in each quartz grain can be compared to an average method, we have indexed all data presented in Holm‐Alwmark et al. () also with 6°, 7°, and 8° errors (Fig. ).…”

“…() and Holm‐Alwmark et al. (), and are reevaluated in this paper in more detail. The data sets were originally processed only manually, using the traditional graphical method.…”

“…Samples include granites collected in the field as well as granites from the Hättberg and Vålarna drill cores (Holm‐Alwmark et al. ). The PDF data sets were originally published in Holm et al.…”