Geophysical investigation of a Suture Zone: The Border Ranges Fault of southern Alaska

Abstract

The Border Ranges fault separates structurally complex accreted Cretaceous rocks from less deformed middle or late Paleozoic and younger rocks in the Cook‐Shelikof basin. Of the five types of geophysical data used to investigate this fault, gravity data give the clearest indication of its presence and crustal structure. For at least 400 km along the fault, gravity anomalies include a +20 to +30 mGal peak along the fault's upper plate and a −40 mGal trough along the lower plate. The paired anomaly can be modeled satisfactorily by a simple step, in a deep dense layer, that lies within 3 km of the projected offshore location of the fault. Relatively low‐density rocks lie along the fault's lower plate to a depth of about 10 km, and the upper part of the fault dips within 20° of vertical. Satellite altimetry data show that two circular geoid lows lie along the Border Ranges fault and coincide with lows in free air gravity data. Seismic refraction and seismic reflection data suggest that the large‐scale density anomalies that cause both types of lows must lie at depths greater than about 1 km within the margin. Three regional magnetic anomalies (Knik Arm, Seldovia, and Shelikof) terminate at the Border Ranges fault, suggesting that the fault truncates obliquely rocks that lie along its northwest side. Six seismic reflection lines cross the Border Ranges fault, but none of them shows reflections from it. The absence of such reflections probably results from the fault's steep dip and from the presence of strong water bottom multiples in the data. From the Late Jurassic until the early Late Cretaceous, the magmatic arc near the Cook‐Shelikof basin was inactive, and we infer that the predominant motion along the Border Ranges fault was strike slip. Resurgent Late Cretaceous magmatism was contemporaneous with uplift of rocks along the northwest side of the Border Ranges fault and with deformation of turbidite sequences in the fault's lower plate. We propose that during the Late Cretaceous, motion along the Border Ranges changed from strike slip to reverse. Cenozoic rocks near the fault show no evidence for post‐Cretaceous fault movement.

Interpretation of gravity and magnetic data and development of two-dimensional cross-sectional models for the Border Ranges fault system, south-central Alaska

“…Throughout much of the region, the BRFS is marked by a pronounced change in topography from the CIB to the Chugach Mountains. However, no seismicity or recent fault scarp appears to be associated with the BRFS (Pavlis, 1982;Fisher and von Huene, 1984;Pavlis and Roeske, 2007).…”

“…Most recently, high-angle normal movement toward the northwest appears to have occurred on the southwesternmost segment of the BRFS (Pavlis and Bruhn, 1983;Little and Naeser, 1989), correlated to late Neogene forearc basin development (Pavlis and Roeske, 2007). An absence of evidence for Neogene reactivation of the BRFS suggests that it did not act as a reverse fault after the Late Cretaceous Period (Fisher and von Huene, 1984). Haeussler and Saltus (2011) presented two observations based on contrasts of topography and magnetic anomalies across the BRFS to support the hypothesis of Pavlis and Bruhn (1983), that the BRFS was reactivated by normal faulting in the Neogene.…”

“…The CIB is bounded to the north by the Castle Mountain fault and to the west by the Bruin Bay fault (Figs. 1 and 2) (Fisher and von Huene, 1984;Shellenbaum et al, 2010). The three major subregions of the CIB are the Anchorage Lowland, Kenai Lowland, and Cook Inlet.…”

“…The buried location of the BRFS beneath the Cenozoic fi ll has been inferred from shallow and deep geophysical data (e.g., Shellenbaum et al, 2010). Previous studies in the Cook Inlet area suggested that the BRFS forms a 2-10-km-wide suture zone (Burns 1982;Fuis and Plafker, 1991) that dips steeply to vertically toward the north or northwest (Fisher and von Huene, 1984).…”

“…1; Shellenbaum et al, 2010). Surface exposure of the BRFS is limited, and only a few geophysical surveys (e.g., Burns, 1982;Fisher and von Huene, 1984) have been conducted across it. We know little about the subsurface structure of this feature or how it has infl uenced the past and present-day tectonics of the region.…”