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Abstract

AThe morphology of a 1,200-m-high bedrock mountain front in the Rio Grande rift of northern New Mexico demonstrates the persistence through Pliocene-Quaternary time of temporal and spatial patterns of late Quaternary rupture along the range-bounding fault zone. Detailed mapping of the surface trace of the fault zone suggests a complex geometric segmentation pattern consisting of four primary segments, each containing two to four 5- to 10-km-long subsegments. The central to south-central part of a subsegment or segment is defined by a narrow zone of single- to double-strand fault scarps commonly at the base of a reentrant in the range front. The narrow zones typically change along strike into more complex zones of mixed piedmont fault scarps and multiple bedrock fault splays that bound structural benches. These diffuse terminations of subsegments or segments preferentially are at salients and/or abrupt deflections in the mountain front. Variations in fault-scarp morphology and displaced geomorphic surfaces suggest that several latest Pleistocene and Holocene ruptures are nonuniformly distributed along a 50-km-long section of fault scarps at or near the base of the mountain front. Morphologic age estimates from height-slope regressions and diffusion modeling of fault scarps suggest that: (1) one or several temporally clustered rupture(s) of mid- to early Holocene age may have extended 30 to 50 km across three primary segments of the range-bounding fault; and (2) a late to mid-Holocene rupture may have occurred on one 6- to 10-km-long subsegment. These two rupture lengths are associated with average vertical displacements of 1.2 and 0.8 m, respectively, which suggests potentially different scales of paleoearthquakes with estimated magnitudes of 6.7 to 7.1 (multiple segment) and 5.8 to 6.3 (single segment only), with probable recurrence intervals of 104 yr between events at a given site on the fault zone.

This time-space segmentation of the fault zone influences the morphology of large-scale tectonic landforms such as facets and spurs, which have developed in the adjacent bedrock escarpment over longer Quaternary time spans. Basal triangular facets above the central parts of subsegments or segments have greater relief and size, steeper mean slopes, fewer benches, less dissection, and thicker colluvial mantles, compared to facets at adjacent subsegment boundaries. Similar morphologic patterns characterize the overall profiles of the larger facet-spur systems of the range front. These patterns extend upward on the mountain front to at least the level of a prominent mid-escarpment bench that correlates with a 4.3-Ma basalt flow overlying an erosional surface at the northern end of the range block. The subsegment containing the morphologically youngest fault scarps also coincides with an unusually high, steep, and undissected set of basal facets, and the greatest amount of post-Pliocene vertical displacement, as estimated from the elevation of the mid-escarpment bench above correlative basalt flows in the adjacent basin. These collective relations suggest that cumulative amounts and rates (120 to 230 m/m.y.) of vertical displacements since mid-Pliocene time may increase by a factor of 1.5 to 2 near the south ends of some primary segments of the range-bounding fault zone. Post-Pliocene displacement rates are several times greater than those estimated solely from late Pleistocene and Holocene fault scarps (0.1 to 0.2 mm/yr versus 0.3 to 0.6 mm/yr, respectively), and are sufficient to generate most of the total relief of the Sangre de Cristo range block within a middle Miocene to Quaternary time interval. The width and internal complexity of the fault trace also increase northward along most primary segments; this structural asymmetry may reflect a small component of left-lateral slip related to unilateral northward propagation of seismogenic rupture from depth at non-conservative boundaries on the southern ends of some fault segments.

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