Geological interpretation of volcanism and segmentation of the Mariana back‐arc spreading center between 12.7°N and 18.3°N

Abstract

The relationships between tectonic processes, magmatism, and hydrothermal venting along ∼600 km of the slow‐spreading Mariana back‐arc between 12.7°N and 18.3°N reveal a number of similarities and differences compared to slow‐spreading mid‐ocean ridges. Analysis of the volcanic geomorphology and structure highlights the complexity of the back‐arc spreading center. Here, ridge segmentation is controlled by large‐scale basement structures that appear to predate back‐arc rifting. These structures also control the orientation of the chains of cross‐arc volcanoes that characterize this region. Segment‐scale faulting is oriented perpendicular to the spreading direction, allowing precise spreading directions to be determined. Four morphologically distinct segment types are identified: dominantly magmatic segments (Type I); magmatic segments currently undergoing tectonic extension (Type II); dominantly tectonic segments (Type III); and tectonic segments currently undergoing magmatic extension (Type IV). Variations in axial morphology (including eruption styles, neovolcanic eruption volumes, and faulting) reflect magma supply, which is locally enhanced by cross‐arc volcanism associated with N‐S compression along the 16.5°N and 17.0°N segments. In contrast, cross‐arc seismicity is associated with N‐S extension and increased faulting along the 14.5°N segment, with structures that are interpreted to be oceanic core complexes—the first with high‐resolution bathymetry described in an active back‐arc basin. Hydrothermal venting associated with recent magmatism has been discovered along all segment types.

The Effect of Arc Proximity on Hydrothermal Activity Along Spreading Centers: New Evidence From the Mariana Back Arc (12.7°N–18.3°N)

“…The 11 segments of the southern Mariana BASC are arrayed en echelon, offset to the right except for a left offset of the northernmost one (Figure b). We follow Anderson et al () and identify each segment using the latitude value close to its center: 12.8°N, 13.3°N, 13.9°N, 14.5°N, 15.1°N, 15.5°N, 16.1°N, 16.5°N, 17.0°N, 17.8°N, and 18.2°N.…”

“…Anderson et al () describe four segment types. Magmatic segments ( M ; 12.8°N, 13.3°N) feature significant axial volcanic highs (∼500 m relief) with smooth volcanic terrain.…”

“…The 11 segments of the southern Mariana BASC are arrayed en echelon, offset to the right except for a left offset of the northernmost one ( Figure 1b). We follow Anderson et al (2017) and identify each segment using the latitude value close to its center: 12. 88N, 13.38N, 13.98N, 14.58N, 15.18N, 15.58N, 16.18N, 16.58N, 17 North Fiji, Auzende et al, 1994;East Scotia, Livermore, 2003;Manus, Martinez & Taylor, 2003;Lau Parson et al, 1990;Woodlark, Taylor et al, 1995;and Okinawa Trough, Letouzey & Kimura, 1986)…”

“…The Mariana BASC is an arcuate spreading ridge that echoes the shape of the adjacent volcanic and remnant arcs. Anderson et al () described the geologic setting of the southern Mariana BASC and refined the spreading axis location, based on a new bathymetric data set. Along its southern half, spreading rates decrease monotonically from 56 mm/yr at 11.6°N to 29 mm/yr at 18.3°N (Bird, ; Kato et al, ) (Figure ).…”

“…Segmentation is defined by offsets ranging from large (31–42 km) shear‐zone offsets to small deviations in axial linearity. Seven deep (mean = 4,800 ± 240 m) basins separate eight magmatic highs (mean = 3,660 ± 260 m) formed by volcanic construction (Anderson et al, ). The 11 segments of the southern Mariana BASC are arrayed en echelon, offset to the right except for a left offset of the northernmost one (Figure b).…”