4:00 PM - 4:15 PM
[STT37-03] Basement structure of the coastal zone of Sagami Bay, estimated by gravity and magnetic surveys
Keywords:ocean bottom gravity survey, aeromagnetic survey, gravity anomaly map, magnetic anomaly map, basement structure, Sagami Bay
The shallow basement structure of the coastal zone of Sagami Bay was estimated by ocean bottom gravity and aeromagnetic surveys.
Ocean bottom gravity surveys were conducted to better understand the shallow subsurface structure of the coastal zone of Sagami Bay in 2014 and 2015. These newly measured ocean bottom gravity data were compiled with the existing onshore and offshore gravity data to edit a gravity map. On a Bouguer anomaly map with an assumed density of 2.3 g/cm3, an ESE-WNW trending gravity high belt extends from Hayama in the Miura Peninsula to the mouth of Sagami River by way of Enoshima Island. This suggests a western continuation of the northern Hayama Unit (Takahashi, 2008), one of the Cenozoic accretionary complexes and the Kinugasa Fault which was traced as the southern boundary of the gravity high belt. A gravity high lies over the Kamegi Spur, western offshore of the Odawa Bay and implies the existence of a Cenozoic accretionary complex extending from the Miura Peninsula.
High-resolution aeromagnetic surveys have been also conducted to better understand the subsurface structures of the northern and eastern Sagami Bay areas in 2014 and 2016, respectively (Okuma et al., 2021b). Total magnetic intensity anomalies were reduced onto a smoothed observation surface. In the coastal zone of the eastern Sagami Bay, three magnetic boundaries were recognized to range from the Miura Peninsula to its western offshore areas on a compiled magnetic map, implying the extensions of the Kinugasa and Takeyama Faults to Enoshima Island. A magnetic high belt bounded by the northernmost and middle boundaries coincides almost with the gravity high belt and its southern edge (the middle boundary) corresponds to the Kinugasa Fault on the peninsula. Local magnetic anomalies are aligned in the NW direction from the northern coast of Odawa Bay on the peninsula to Enoshima Island. They form the southernmost boundary and correspond to the Takeyama Fault on the peninsula and the Zushi-oki Fault in the Sagami Bay. 3D magnetic imaging with effective source volume minimization (Nakatsuka and Okuma, 2014) was applied to the high-resolution aeromagnetic anomalies of the study area. Magnetic highs were imaged on the surface and/or at shallow depths along the magnetic high belt. Preliminary field magnetic susceptibility measurements were conducted to reveal the origin of the shallow magnetization highs and high magnetic susceptibilities of 20 - 30 x 10-3 (SI) were observed on outcrops of volcaniclastic layers in the northern Hayama Unit. Two pairs of magnetic anomalies with a reverse polarization reside over the Kamegi Spur and correspond to a gravity high.
Summarizing above, the northern Hayama Unit edged by the Kinugasa Fault to the south is traced and implied to extend from the Miura Peninsula to Enoshima Island by gravity and magnetic high belts. The Kamegi Spur in the Sagami Bay is characterized by the gravity high and conspicuous magnetic anomalies with a reverse polarization. As it corresponds to an uplift of acoustic basement (Sato, 2016), it is implied to be of volcanic origin. To conclude, ocean bottom gravity and high-resolution aeromagnetic surveys have renewed prior knowledge of gravity and magnetic anomalies in the study and were successful in clarifying the shallow basement structure of the coastal zone, related to active fault systems.
Ocean bottom gravity surveys were conducted to better understand the shallow subsurface structure of the coastal zone of Sagami Bay in 2014 and 2015. These newly measured ocean bottom gravity data were compiled with the existing onshore and offshore gravity data to edit a gravity map. On a Bouguer anomaly map with an assumed density of 2.3 g/cm3, an ESE-WNW trending gravity high belt extends from Hayama in the Miura Peninsula to the mouth of Sagami River by way of Enoshima Island. This suggests a western continuation of the northern Hayama Unit (Takahashi, 2008), one of the Cenozoic accretionary complexes and the Kinugasa Fault which was traced as the southern boundary of the gravity high belt. A gravity high lies over the Kamegi Spur, western offshore of the Odawa Bay and implies the existence of a Cenozoic accretionary complex extending from the Miura Peninsula.
High-resolution aeromagnetic surveys have been also conducted to better understand the subsurface structures of the northern and eastern Sagami Bay areas in 2014 and 2016, respectively (Okuma et al., 2021b). Total magnetic intensity anomalies were reduced onto a smoothed observation surface. In the coastal zone of the eastern Sagami Bay, three magnetic boundaries were recognized to range from the Miura Peninsula to its western offshore areas on a compiled magnetic map, implying the extensions of the Kinugasa and Takeyama Faults to Enoshima Island. A magnetic high belt bounded by the northernmost and middle boundaries coincides almost with the gravity high belt and its southern edge (the middle boundary) corresponds to the Kinugasa Fault on the peninsula. Local magnetic anomalies are aligned in the NW direction from the northern coast of Odawa Bay on the peninsula to Enoshima Island. They form the southernmost boundary and correspond to the Takeyama Fault on the peninsula and the Zushi-oki Fault in the Sagami Bay. 3D magnetic imaging with effective source volume minimization (Nakatsuka and Okuma, 2014) was applied to the high-resolution aeromagnetic anomalies of the study area. Magnetic highs were imaged on the surface and/or at shallow depths along the magnetic high belt. Preliminary field magnetic susceptibility measurements were conducted to reveal the origin of the shallow magnetization highs and high magnetic susceptibilities of 20 - 30 x 10-3 (SI) were observed on outcrops of volcaniclastic layers in the northern Hayama Unit. Two pairs of magnetic anomalies with a reverse polarization reside over the Kamegi Spur and correspond to a gravity high.
Summarizing above, the northern Hayama Unit edged by the Kinugasa Fault to the south is traced and implied to extend from the Miura Peninsula to Enoshima Island by gravity and magnetic high belts. The Kamegi Spur in the Sagami Bay is characterized by the gravity high and conspicuous magnetic anomalies with a reverse polarization. As it corresponds to an uplift of acoustic basement (Sato, 2016), it is implied to be of volcanic origin. To conclude, ocean bottom gravity and high-resolution aeromagnetic surveys have renewed prior knowledge of gravity and magnetic anomalies in the study and were successful in clarifying the shallow basement structure of the coastal zone, related to active fault systems.