A uniquely shaped impact structure, the Hailin impact crater, has been discovered in northeast China. The crater was formed on a granodiorite hillside and is an oval depression with asymmetric rim height and a maximum diameter of 1360 m. The bottom of the crater is filled by Quaternary sediments with large amounts of rock fragments underneath. The discovery of quartz planar deformation features in rock clasts on the crater floor provides diagnostic evidence for the impact origin of the structure. The shape of the crater is largely due to the impact having occurred on a ridge terrain. The impact event probably occurred in the late Cenozoic Era. The Hailin impact crater is the fourth confirmed Chinese impact crater.
China’s land area accounts for 6.4% of the world’s land area and the fourth largest in the world. Previously, only three impact craters have been discovered in China1–3 [Fig. 1(A)]. Impact craters4 are natural sites to look for material that has experienced extreme pressures and explosive energy release. When a large extraterrestrial object traveling at hypervelocity impacts the Earth’s surface, it excavates material at the target region, and ejects the fragments to pile up around the rim, thereby forming an impact crater.5 Unless the impact angle to the horizontal is very low (<10°), the resulting crater has an approximately circular shape.6 There are currently a few exceptions, such as the Río Cuarto impact structures, which are the only examples of elliptical impact craters, resulting from low-angle impact and ricochet of a meteorite.7 Here, we report the new discovery of a uniquely shaped impact crater in northeastern China.
(A) Location map of Hailin impact crater. The crater is situated in the southeast of Heilongjiang province. The red dots on the map represent the locations of four confirmed meteorite craters in northeast China: 1, Xiuyan impact crater; 2, Yilan impact crater; 3, Baijifeng impact crater; 4. Hailin impact crater. (B) Panoramic view of Hailin impact crater. The oval crater is located on the hillside and has a maximum rim-to-rim diameter of 1360 m. The insert in the upper right corner is a bird’s-eye view of the crater. A state-run forest farm is stationed in the crater, and many buildings can be seen in the picture. Image: Google CNES/Airbus Maxar technologies Landsat/Copernicus data SIO, NOAA, U.S. Navy, NGA, GEBCO. (C) Geological map of Hailin impact crater and its surrounding area, modified from the literature:9 1, Neoproterozoic strata (Pt1); 2, Upper Jurassic strata (J3); 3, Quaternary strata (Q); 4, Neoproterozoic granodiorite (γδPt3); 5, Neoproterozoic diorite (δPt3); 6, Neoproterozoic gabbro (νPt3); 7, Late Permian monzonitic granite (ηγP3); 8, Late Jurassic orthoclase granite (ξγT3); 9, fault; 10, impact crater. (D) Granodiorite fragments exposed on the crater floor.
(A) Location map of Hailin impact crater. The crater is situated in the southeast of Heilongjiang province. The red dots on the map represent the locations of four confirmed meteorite craters in northeast China: 1, Xiuyan impact crater; 2, Yilan impact crater; 3, Baijifeng impact crater; 4. Hailin impact crater. (B) Panoramic view of Hailin impact crater. The oval crater is located on the hillside and has a maximum rim-to-rim diameter of 1360 m. The insert in the upper right corner is a bird’s-eye view of the crater. A state-run forest farm is stationed in the crater, and many buildings can be seen in the picture. Image: Google CNES/Airbus Maxar technologies Landsat/Copernicus data SIO, NOAA, U.S. Navy, NGA, GEBCO. (C) Geological map of Hailin impact crater and its surrounding area, modified from the literature:9 1, Neoproterozoic strata (Pt1); 2, Upper Jurassic strata (J3); 3, Quaternary strata (Q); 4, Neoproterozoic granodiorite (γδPt3); 5, Neoproterozoic diorite (δPt3); 6, Neoproterozoic gabbro (νPt3); 7, Late Permian monzonitic granite (ηγP3); 8, Late Jurassic orthoclase granite (ξγT3); 9, fault; 10, impact crater. (D) Granodiorite fragments exposed on the crater floor.
The Hailin impact crater is an oval depression structure with asymmetric rim height, located on the hillside of the Weihushan National Forest Park, Chaihe Town, Hailin City, Heilongjiang Province, China [Fig. 1(B)]. The latitude and longitude of the crater are 45°18′00″N and 129°25′22″E, respectively. It is located in a middle–low mountainous area at the east foot of Zhangguangcailing in the Changbai Mountain Range. Most of this region is covered with dense forests. In geotectonic terms, the Zhangguangcailing area is located in the Xingmeng Orogenic Belt in the eastern part of the Central Asia Orogenic Belt.8 Since the Proterozoic Era, this area has been experiencing active tectonic movement and magmatism. The main strata in this area include Proterozoic, Paleozoic, Mesozoic, and Cenozoic, and large amounts of Proterozoic, Paleozoic, and Mesozoic intrusive and extrusive rocks are exposed.9 The basement of the Hailin impact crater is a Neoproterozoic granodiorite batholith [Fig. 1(C)]. This shows that the impact took place at a target of granodiorite bedrock.
The crater is located on a hillside at altitude of 900 m [Fig. 1(B)]. The diameter of the rim is 1360 m in the north–south direction and 1150 m in the east–west direction. The height of the crater rim varies greatly from north to south, depending on the topography. The highest point on the northern rim is 560 m above sea level, and that on the southern rim is 450 m. The center of the crater is located on the ridge line at an altitude of 377 m. The apparent depth of the crater, i.e., the height difference from the center of the crater to the highest point at the rim, is 183 m. Most of the eastern rim is missing, but the rest of the rim is well preserved. The lowest point (365 m) on the eroded eastern rim is 12 m lower than the center of the crater.
The crater rim is mostly covered by soil and vegetation. Significant amounts of angular granodiorite rock fragments (up to 2 m in size) are exposed on the western and southern edges. The bottom of the crater is mostly covered by Quaternary sediments, mainly composed of soil, rock clasts, and silt. The sediments in the central area are characterized by lacustrine deposits, composed of peat-rich black silt and fine-grained rock clasts with high water content. The sediments in the marginal area of the bottom are relatively thin, while the thickness of those in the central area is not revealed. A large number of angular granodiorite fragments up to several meters in size are locally exposed in the marginal zone of the crater floor [Fig. 1(D)]. This shows that the bottom of the crater contains large amounts of rock fragments underlying Quaternary sediments.
To reveal the origin of the depression structure, we investigated the possible shock metamorphic phenomena in target rocks. The rock clasts were collected from the sediments at the crater floor. A total of 100 thin sections were prepared from the clasts. All these thin sections were examined with an optical petrographic microscope to investigate planar deformation features (PDFs) in quartz. More than 30 PDF-bearing quartz grains have been identified in detail. Our observations show that most quartz grains contain one set of PDFs [Figs. 2A(a) and 2A(b)]. Some grains contain two or three sets of PDFs [Figs. 2A(c) and 2A(d)]. PDFs in quartz occur as thin lamellar layers with straight planes. The thickness of a single layer of PDFs is less than 2 μm. The spacing between adjacent thin layers in the same set of PDFs is less than 10 μm, mostly 2–5 μm. The PDF layer extends throughout the grain thickness, or occurs locally within the grains. Some PDFs in quartz are decorated with tiny fluid inclusions [Figs. 2A(a) and 2A(d)]. Crystallographic orientations of the PDF planes in quartz were investigated using a four-axis universal stage and a stereographic projection method with the aid of a Wulff net.10–12 The most common forms of PDFs with Miller indices are ω{103} [Figs. 2(B) and 2(C)]. Other forms of PDFs include c(0001), r, z{101}, π{102}, and ξ{112} [Figs. 2(B) and 2(C)]. The formation of PDFs in quartz corresponds to shock pressures ranging from 5 to 30 GPa.10,13 The PDF forms of ω{103} in quartz correspond to a shock pressure of more than 10 GPa, whereas the PDF forms of π{102} correspond to a shock pressure of more than 20 GPa.13 The impact effect in natural rocks usually exhibits heterogeneity. The predominant occurrence of ω{103} in quartz in the investigated samples demonstrates a shock pressure between 10 and 20 GPa. The discovery of PDFs in quartz provides the key evidence for shock metamorphism of the granodiorite target rocks, thus supporting an impact origin of the crater.
Planar deformation features (PDFs) in quartz. (A) Optical microscope images of PDFs in quartz (Qz) under cross-polarized light. (a) One set of (103) PDFs in a quartz single crystal. The quartz grain contains one set of decorated PDFs. (b) Two adjacent quartz crystals, each with (103) PDFs, showing an angular unconformity contact relationship between the PDFs. (c) Two sets of (103) and (013) PDFs in a quartz single crystal. (d) Two sets of dense (103) and (013) PDFs in a quartz single crystal. The quartz grain contains two prominent decorated PDF sets. (B) Frequency percent and angles between c axis and PDF planes, binned at 5°, showing crystallographic orientations of PDFs in 30 quartz grains. (C) PDFs and crystallographic indices indexed in 30 quartz grains.
Planar deformation features (PDFs) in quartz. (A) Optical microscope images of PDFs in quartz (Qz) under cross-polarized light. (a) One set of (103) PDFs in a quartz single crystal. The quartz grain contains one set of decorated PDFs. (b) Two adjacent quartz crystals, each with (103) PDFs, showing an angular unconformity contact relationship between the PDFs. (c) Two sets of (103) and (013) PDFs in a quartz single crystal. (d) Two sets of dense (103) and (013) PDFs in a quartz single crystal. The quartz grain contains two prominent decorated PDF sets. (B) Frequency percent and angles between c axis and PDF planes, binned at 5°, showing crystallographic orientations of PDFs in 30 quartz grains. (C) PDFs and crystallographic indices indexed in 30 quartz grains.
According to the classification of terrestrial impact craters,14 the Hailin impact crater can be classified as a simple crater. A simple crater usually appears as a bowl-shaped depression with a diameter less than 2–4 km, depending on lithology. Because of the relatively small scale of the simple crater, its shape would be greatly influenced by target topography. For the Hailin impact crater, the impact point was on the slope of the ridge. The height of the crater rim in the uphill direction should be greater than that in the downhill direction, and the measured height difference between the northern and southern rims reaches 110 m. In addition, the oval shape of the crater is also closely related to the ridge topography. The mountain ridge is a strip-like ridge landform formed by two opposite slopes. The terrain on both sides of the ridge gradually declines. Therefore, the impact at the ridge resulted in a relatively small rim-to-rim distance across the ridge, and a large rim-to-rim distance along the ridge line. A simple crater bottom usually contains a thick unit of impact breccia. During the impact cratering, the bottom would be partially filled with rock fragments that slipped from the transient crater rim wall and fell back after being ejected. The perfect oval shape and the most well-preserved rim of the Hailin impact crater demonstrate a weakly eroded impact structure. The present rock fragments at the bottom indicate the existence of a complete impact breccia unit in the crater.
Since the Late Cretaceous, the Zhangguangcailing area has experienced two significant surface uplifts from 90 to 70 Ma and from 20 Ma to the present day. The current mountain terrains are considered to be the result of uplifts during the past 20 Ma.15 The Hailin impact crater should have been created after the current mountain was formed. Therefore, the impact event took place during the Miocene epoch or later. The exact age of the crater requires further research. The lacustrine deposits at its bottom have recorded the history of the formation of an impact crater lake, which was subsequently drained after the breakdown of the eastern rim due to external geological forces during its later evolution.
In summary, the oval shape of the Hailin impact crater is related to an asteroid impact on the ridge slope of a mountain. The Hailin impact is a rare example of an asteroid impact solely into felsic magmatic rocks. Shock-metamorphic phenomena in the rocks and minerals provide key evidence for the impact origin of the crater. The discovery of this impact crater further deepens our understanding of the characteristics of impact structures in mountain areas.
ACKNOWLEDGMENTS
Junfeng Gan, from the Department of Geology, Hunan University of Science and Technology, China, and Haifeng Zhang, from the Second Institute of Oceanography, Ministry of Natural Resources, China, provided assistance in the field investigations. H. Mao acknowledges financial support from the Shanghai Key Laboratory Novel Extreme Condition Materials, China (Grant No. 22dz2260800) and the Shanghai Science and Technology Committee, China (Grant No. 22JC1410300).
AUTHOR DECLARATIONS
Author Contributions
Feng Yin: Conceptualization (equal); Investigation (equal). Ming Chen: Conceptualization (equal); Investigation (equal); Writing – original draft (equal). Wenge Yang: Supervision (equal); Writing – review & editing (equal). Ho-kwang Mao: Supervision (equal); Writing – review & editing (equal).