Topographic relief in arc-trench systems is thought to be formed by plate subduction; however, few quantitative investigations have so far been reported, with respect to the related mountain building ...process. This study applies apatite and zircon (U–Th)/He thermochronometry (AHe, ZHe, respectively) to Cretaceous granite rocks in the north part of the northeast (NE) Japan Arc to reveal its cooling/denudation history. Weighted mean AHe ages ranging from 88.6 to 0.9 Ma and ZHe ages from 83.9 to 7.4 Ma were determined for 10 rock samples. Using the AHe data, denudation rates were obtained for each sample. On the fore-arc side, denudation rates of < 0.05 mm/year were calculated, indicating a slow denudation process since the Paleogene. However, in the Ou Backbone Range and on the back-arc side, denudation rates at > 0.1–1.0 mm/year were computed, probably reflecting a recent uplift event since ~ 3–2 Ma. These data indicate a clear contrast in thermal and denudation histories between the tectonic units in this study area, similar to that previously reported from the southern part of NE Japan Arc. A comparison of the thermal/denudation histories between the N- and S- traverses, revealed the arc-parallel trend, the uplift model of the volcanic arc, and some minor variations of thermal/denudation histories in each tectonic unit. This study offers some further insights into the understanding of tectonic processes in an island-arc setting.
Natural monoclinic zirconia (baddeleyite) was irradiated with 340 MeV Au ions, and the irradiation-induced nanostructures (i.e., ion tracks and nanohillocks) were observed using transmission electron ...microscopy. The diameter of the nanohillocks was approximately 10 nm, which was similar to the maximum molten region size calculated using the analytical thermal spike model. Ion tracks were imaged as strained regions that maintained their crystalline structure. The cross-sections of most of the ion tracks were imaged as rectangular contrasts as large as 10 nm. These results strongly indicated that the molten region was recrystallized anisotropically, reflecting the lattice structure. Furthermore, low-density track cores were formed in the center of the ion tracks. The formation of low-density track cores can be attributed to the ejection of molten matter toward the surface. A comparison of the ion tracks in the synthetic zirconia nanoparticles and those in larger natural zirconia samples showed that the interface between the strained track contrast and the matrix was less clear in the former than in the latter. These findings suggest that the recrystallization process was affected by the size of the irradiated samples.
The South Fossa Magna region, central Japan, has been an active collision zone between the Honshu Arc and the Izu-Bonin Arc since the middle Miocene and provides an excellent setting for ...reconstructing the earliest stages of continent formation. Multi-system geo-thermochronometry was applied to different domains of the South Fossa Magna region, together with some previously published data, to reveal mountain formation processes, i.e., vertical crustal movements. Nine granitic samples yielded zircon U–Pb ages of 10.2–5.8 Ma (n = 2), apatite (U–Th)/He (AHe) ages of 42.8–2.6 Ma (n = 7), and apatite fission-track (AFT) ages of 44.1–3.0 Ma (n = 9). Thermal history inversion modeling based on the AHe and AFT data suggests rapid cooling events confined within the study region at ~6–2 Ma. The Kanto Mountains may have undergone a domal uplift in association with their collision with the Tanzawa Block at ~5 Ma. However, this uplift may have slowed down following the southward migration of the plate boundary and late Pliocene termination of the Tanzawa collision against Honshu Island. The Minobu Mountains and adjacent mountains may have been uplifted when motion of the Philippine Sea plate changed from northward to northwestward at ~3 Ma. Therefore, mountain formation in the South Fossa Magna region was mainly controlled by collisions of the Tanzawa and Izu Blocks and motional change of the Philippine Sea plate. Earlier collisions of the Kushigatayama Block at ~13 Ma and Misaka Block at ~10 Ma appear to have had little effect on mountain formation. Together with a clockwise rotation of the Kanto Mountains at 12–6 Ma, these observations suggest that horizontal deformation predominated during the earlier stage of arc-arc collision, and vertical movements due to buoyancy of the accreted crust resulting from crustal shortening and thickening developed at a later stage.
•Low-temperature thermochronology in an active arc-arc collision zone•Rapid cooling/exhumation events coeval with collision event and plate motion change•Mountain formation controlled by collision events and plate motions
We performed zircon U–Pb dating on the Pliocene Tanigawa-dake granites (Makihata and Tanigawa bodies) and the Cretaceous Minakami quartzdiorite, Northeast Japan Arc. Concordia ages were estimated to ...be 3.95 ± 0.11 Ma (± 2 sigma) for the Makihata body, 3.18 ± 0.13 Ma and 3.32 ± 0.15 Ma for the Tanigawa body, and 109.4 ± 2.2 Ma for the Minakami quartzdiorite. The Minakami quartzdiorite is possibly correlated to the bedrock in the Ashio belt because the age of the Minakami quartzdiorite is consistent with the zircon U–Pb ages of the earliest Tadamigawa granites (107–62 Ma) which are distributed to the northeast of the Tanigawa-dake region and belong to the Ashio belt. All the zircon U–Pb ages of the Tanigawa-dake granites are older than the previously reported cooling ages, i.e., K–Ar ages and zircon fission-track ages, being consistent with their difference in closure temperature. On the basis of these results, we concluded that the intrusive ages of the Tanigawa-dake granites are ~ 4–3 Ma, which are among the youngest exposed plutons on Earth. The U–Pb ages of the Makihata body and the Tanigawa body are different significantly in the 2 sigma error range. Thus, the Tanigawa body intruded later than the Makihata body by ~ 0.7 Myr.
Graphical Abstract
We applied apatite fission-track (AFT) thermochronometry to the Cretaceous-Paleogene granitic rocks across the southern part of the Northeast (NE) Japan Arc in order to elucidate the denudation ...history. As the results, AFT ages were estimated at 79.5–66.0 Ma on the fore-arc side, 29.8–5.5 Ma in the Ou Backbone range (OBR) and 19.1–4.6 Ma on the back-arc side. Thermal inverse modeling with HeFTy offered high-resolution thermal histories, which were converted into denudation rates under the assumption of appropriate geothermal gradients. On the fore-arc side, stable and slow denudation setting (denudation rates < 0.05 mm/yr) was deduced through the Paleogene. On the contrary, in the OBR and on back-arc side, rapid denudation rates from 1 to a few mm/yr were estimated at some localities reflecting the uplift event after 3–2 Ma. Besides, the uplifting style of OBR was constrained from the thermochronometric data; the doming uplift is more plausible than the tilted pop-up style, which was previously suggested based on geological observations. The Iide and Asahi mountains on the back-arc side probably have started to uplift at 3–1 Ma, coeval with the uplift of the OBR. This observation disagrees with the previous tectonic models in which the deformation on the back-arc side initiated at 5–3.5 Ma, prior to the deformation in the OBR. This disagreement possibly suggests a heterogenous tectonic history on the back-arc side. Keywords: Low-temperature thermochronology, Arc-trench system, Apatite fission-track thermochronometry, Northeast Japan Arc, Thermal inverse modeling
Thermochronometric studies (e.g., fission-track thermochronometry) on major orogenic belts, such as collision zones and/or cratons have been successfully conducted since 1970s. However, such studies ...in arc-trench systems were limited so far. According to the development of thermochronology in three decades, the applicability for these regions has been identified. Thus, recent thermochronometric studies in an island arc setting have been proceeding. This paper introduces the concept and the current status of thermochronometric studies for mountain building process on arc-trench settings, especially these case studies in island arcs.
Zircon U-Pb dating was carried out on granitic rocks from plutons in the Taiheizan Complex in Akita Prefecture, NE Japan. Two granodiorites from the Main Intrusive Rocks yielded weighted mean ...206Pb/238U dates of 103.4±1.0 Ma and 115.6±1.1 Ma (1SE). Three porphyrygranite from the Young Intrusive Rocks yielded weighted mean 206Pb/238U dates of 11.4±0.1 Ma, 4.7±Ma, and 4.8±0.1 Ma (1SE). The older and younger dates can be interpreted as the ages of early and later stages of granitic intrusion at the sampling locations. The Pliocene U-Pb dates suggest that the Nibetsu Body in the Young Intrusive Rocks is one of the youngest granitic plutons currently exposed on Earth.
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