Forearc basin stratigraphy resulting from syntectonic sedimentation during accretionary wedge growth: Insights from sandbox analogue experiments

Abstract Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure–low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks.

Download Full-text

To what degree the geometry and kinematics of accretionary wedges in analogue experiments is dependent on material properties

10.5194/se-2018-45 ◽

2018 ◽

Author(s):

Ziran Jiang ◽

Bin Deng ◽

Caiwei Fan ◽

Yu He ◽

Dong Lai ◽

...

Keyword(s):

Grain Size ◽

Material Properties ◽

Quartz Sand ◽

Accretionary Wedge ◽

Shape Factors ◽

Analogue Experiments ◽

Materials Used ◽

Grain Sorting ◽

Basal Detachment ◽

Geometry And Kinematics

Abstract. Cohesion and friction coefficients are fundamental parameters of granular materials used in analogue experiments. Thus, to test the physical characteristics and mechanical behaviour of the materials used in the experiments will help to better understand into what degree the results of experiments of geological processes depend on the material properties. Our test suggests significant differences between quartz sand and glass bead, in particular the shape factors (~ 1.55 of quartz sand to ~ 1.35 glass bead, angular to rounded) and grain sorting (moderately to well sorted). The glass beads show much better grain sorting and smaller shape factors than the quartz sand. Also they have smaller friction coefficient (~ 0.5 to ~ 0.6) and cohesion (20–30 Ma to 70–100 Ma), no matter of the grain size in our tested samples. The quartz sand shows much smaller friction coefficient (~ 0.6 to ~ 0.65), and smaller cohesion (~ 70 Pa to ~ 100 Pa) than that of smaller grain size sand. We have conducted four sets of analogue experiments with three repeats at the minimum. Our models show that material properties have important influence on the geometry and kinematics of the accretionary wedge. Although the difference in geometries are small, models with larger grain size develop wedges with higher wedge height, larger taper, shorter wedge length and less number of faults under the same amount of bulk shortening. In particular, models with basal detachment (even with 1 mm thickness), show significant difference in geometry and kinematics with that of quartz sand. We thus argue that the geometry and kinematics of the wedge appear to be significantly influenced by relative brittle and ductile strengths, and, to a lesser degree by the layering anisotropy. The basal detachment (even of tiny thickness) determines the first-order control on the location and development of accretionary wedge, in a contrast to the physical properties of brittle materials.

Download Full-text

STRUCTURAL AND EARTHQUAKE EVALUATIONS ALONG JAVA SUBDUCTION ZONE, INDONESIA

RISET Geologi dan Pertambangan ◽

10.14203/risetgeotam2020.v30.1072 ◽

2020 ◽

Vol 30 (1) ◽

pp. 65

Author(s):

Adi Patria ◽

Atin Nur Aulia

Keyword(s):

Subduction Zone ◽

Normal Fault ◽

Thrust Fault ◽

Earthquake Activity ◽

Accretionary Wedge ◽

Forearc Basin ◽

Plate Interface ◽

Seafloor Morphology ◽

Australian Plate ◽

Seismic Reflection Profiles

Java Subduction is a zone of trench perpendicular convergence of Australian Plate and Southeast Asia in the south of Java. It is characterized by an almost E-W trending trench with an eastward increase of convergence velocity. Three major earthquakes with tsunamis have been caused by deformation along this subduction zone. Although many studies have undertaken to understand the nature of the subduction system, a clear relationship between structures and earthquake activities remains poorly explained. In this study, we used bathymetry, residual bathymetry, and published seismic reflection profiles to evaluate structural and morphological elements, then link the observations to earthquake activity along Java Subduction Zone. Based on seafloor morphology, characteristics of the accretionary wedge and forearc basin varies along the trench in response to the variation of seafloor morphology. Features such as seamounts and ridges which were observed in the oceanic basin may be subducted beneath accretionary wedge and disrupt the morphology of accretionary wedge, forearc basin, and trench. Earthquake activities are generally dominated by normal fault solutions in the trench, which is attributed to plate bending faults while thrust fault solution is observed in the forearc basin area. Thrust fault activities in accretionary wedge are decreased to the east, where there is no thrust fault solution observed in the eastern end of the subduction zone. Few strike-slip focal mechanisms are observed and mainly located within the subducting oceanic plate. Structures and subducting oceanic features may control the earthquake activity where deformation occurred at the edge of these features. The two largest thrust fault earthquakes in 1994 and 2006 are interpreted as a result of deformation along with plate interface on soft or unconsolidated sediment above the incoming plate. The largest normal fault earthquake with a magnitude 8.3 is possibly caused by a crustal scale-fault that breaks the entire oceanic crust.ABSTRAK - Evaluasi struktur dan gempa bumi di sepanjang zona subduksi Jawa, Indonesia. Subduksi Jawa adalah zona konvergensi yang tegak lurus palung antara Lempeng Australia dan Asia Tenggara di selatan Jawa. Hal ini ditandai dengan palung berarah hampir barat–timur dengan peningkatan kecepatan konvergensi ke arah timur. Tiga gempa bumi besar dengan tsunami disebabkan oleh deformasi di sepanjang zona subduksi ini. Meskipun banyak penelitian telah dilakukan untuk memahami sifat sistem subduksi, hubungan antara struktur dan kegiatan gempa bumi masih kurang jelas. Dalam studi ini, kami menggunakan batimetri, batimetri residual, dan profil refleksi seismik untuk mengevaluasi elemen struktur dan morfologi, kemudian menghubungkan pengamatan dengan aktivitas gempa bumi di sepanjang zona subduksi Jawa. Berdasarkan morfologi dasar laut, karakteristik prisma akresi dan cekungan busur muka bervariasi di sepanjang palung sebagai respon terhadap variasi morfologi dasar laut. Fitur seperti seamount dan punggungan yang diamati di cekungan samudera menunjam di bawah prisma akresi dan mengganggu morfologi prisma akresi, cekungan busur muka, dan palung. Aktivitas gempa bumi umumnya didominasi oleh patahan normal di palung, yang dikaitkan dengan patahan tekukan lempeng sedangkan patahan naik diamati di daerah cekungan busur muka. Aktivitas sesar naik di dalam prisma akresi berkurang ke arah timur, di mana tidak ada patahan naik yang teramati di ujung timur zona subduksi. Beberapa mekanisme patahan mendatar diamati dan terutama terletak di dalam lempeng samudera yang menunjam. Struktur dan fitur di kerak samudra yang menunjam dapat mengontrol aktivitas gempa bumi di mana deformasi terjadi di tepian fitur ini. Dua gempa bumi besar dengan sifat patahan naik pada tahun 1994 dan 2006 ditafsirkan sebagai hasil dari deformasi di sepanjang antarmuka lempeng pada sedimen lunak atau tidak terkonsolidasi di atas lempeng yang masuk. Gempa bumi besar dengan sifat sesar normal magnitude 8,3 mungkin disebabkan oleh patahan skala-kerak yang menghancurkan seluruh kerak samudera.

Download Full-text