Does subduction of mass transport deposits (MTDs) control seismic behavior of shallow–level megathrusts at convergent margins?

Abstract

We present a critical appraisal of the role of subducted, medium (10-1000 km 2) to giant (≥1000 km 2) and heterogeneous , mud-rich mass transport deposits (MTDs) in seismic behavior and mechanisms of shallow earthquakes along subduction plate interfaces (or subduction channels) at convergent margins. Our observations from exhumed ancient subduction complexes around the world show that incorporation of mud-rich MTDs with a "chaotic" internal fabric (i.e., sedimentary mélanges or olistostromes) into subduction zones strongly modifies the structural architecture of a subduction plate interface and the physical properties of subducted material. The size and distribution of subducted MTDs with respect to the thickness of a subduction plate interface are critical factors influencing seismic behavior at convergent margins. Heterogeneous fabric and compositions of subducted MTDs may diminish the effectiveness of seismic ruptures considerably through the redistribution of overpressured fluids and accumulated strain. This phenomenon possibly favors the slow end-member of the spectrum of fault slip behavior (e.g., Slow Slip Events, Very Low Frequency Earthquakes, Non-Volcanic Tremors, creeping) compared to regular earthquakes, particularly in the shallow parts (T b 250 °C) of a subduction plate interface.

Distinguishing the Mélange-Forming Processes in Subduction-Accretion Complexes: Constraints from the Anisotropy of Magnetic Susceptibility (AMS)

“…Highly sheared and disrupted tectonic mélanges and broken formations characterize the subduction plate boundary and/or associated megathrust shear zones, as well as out-of-sequence thrust faults in subduction complexes (e.g., [3][4][5][6][7][8][9][10]). Sedimentary mélanges or heterogeneous mass-transport deposits occur from slope instability in the upper part of frontal wedges of subduction-accretion complexes and in the trench inner-slope (e.g., [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]). Diapiric mélanges, including shale and mud diapirs, occur as the result of the upward rise of overpressured fluids [88] and [73]); (B) location of Figure 1A; (C) geological cross section across the Northern Apennines (modified from [89]).…”

“…The comparison of field and laboratory investigations validate the analysis of magnetic features as a diagnostic tool suitable to analytically distinguish the contribution of different mélange forming-processes and their mutual superposition, and to better understand the geodynamic evolution of subduction-accretion complexes.Geosciences 2019, 9, 381 2 of 23 migrating along the basal megathrust shear zone or channeled along megasplay faults (e.g., [29][30][31][32][33][34][35]). Each mélange type may subsequently be overprinted and structurally reworked by tectonic processes, such as shearing and tectonic mixing, during the evolution of the subduction complex, particularly when placed or recycled in the plate interface and/or involved within shear zones of (mega)thrust faults and splays, forming polygenetic mélanges (e.g., [26,34,[36][37][38][39][40][41][42][43][44]). As a consequence, the primary internal block-in-matrix fabric of mélanges is commonly obscured and strongly reworked, hampering the recognition of their primary process of formation.Although several helpful meso-to-mapscale structural criteria to distinguish chaotic rock unit types and related forming processes have been proposed and successfully applied on exhumed subduction-accretionary complexes on-land (e.g., [9,33,34,36,[38][39][40][41][45][46][47][48][49][50][51][52][53][54][55]…”

“…The topic is critical, since the subduction of heterogeneous material, such as chaotic rock units and mass transport deposits, has been reported as a significant factor affecting seismic style within a subduction plate interface and subduction shear zones (e.g., [8,26,[62][63][64]). In this paper, we present the results of the comparison between magnetic (AMS-anisotropy of magnetic susceptibility) and mesoscale structural fabrics of non-metamorphic tectonic, sedimentary, and polygenetic chaotic rock units, including broken formations and mélanges, of the exhumed Ligurian accretionary complex and the overlying wedge-top basin succession in Northern Apennines (northwest Italy; Figure 1), which represent an ancient analogue for the shallow portion of present-day subduction complexes (e.g., [18,26,33,34,40]). Our findings demonstrate that the magnetic fabric reveals peculiar configurations of the principal susceptibility axes orientations for the different types of chaotic rock units, which are fully comparable with the different mesoscale organization of fabrics distinguished by field observation (see [33]).…”

“…Geosciences 2019, 9, 381 2 of 23 migrating along the basal megathrust shear zone or channeled along megasplay faults (e.g., [29][30][31][32][33][34][35]). Each mélange type may subsequently be overprinted and structurally reworked by tectonic processes, such as shearing and tectonic mixing, during the evolution of the subduction complex, particularly when placed or recycled in the plate interface and/or involved within shear zones of (mega)thrust faults and splays, forming polygenetic mélanges (e.g., [26,34,[36][37][38][39][40][41][42][43][44]). As a consequence, the primary internal block-in-matrix fabric of mélanges is commonly obscured and strongly reworked, hampering the recognition of their primary process of formation.…”

“…Thus, the research and application of alternative analytical criteria to discriminate chaotic rock units and their forming processes are necessary and highly significant. The topic is critical, since the subduction of heterogeneous material, such as chaotic rock units and mass transport deposits, has been reported as a significant factor affecting seismic style within a subduction plate interface and subduction shear zones (e.g., [8,26,[62][63][64]).…”