A comprehensive survey of faults, breccias, and fractures in and flanking the eastern Española Basin, Rio Grande rift, New Mexico

Abstract

A comprehensive survey of geologic structures formed in the Earth's brittle regime in the eastern Española Basin and flank of the Rio Grande rift, New Mexico, reveals a complex and protracted record of multiple tectonic events. Data and analyses from this representative rift flank-basin pair include measurements from 53 individual fault zones and 22 other brittle structures, such as breccia zones, joints, and veins, investigated at a total of just over 100 sites. Structures were examined and compared in poorly lithified Tertiary sedi ments, as well as in Paleozoic sedimentary and Proterozoic crystalline rocks. Data and analyses include geologic maps; field observations and measurements; orientation, kinematic, and paleostress analyses; statistical examination of fault trace lengths derived from aeromagnetic data; mineralogy and chemistry of host and fault rocks; and investigation of fault versus bolideimpact hypotheses for the origin of enigmatic breccias found in the Proterozoic basement rocks. Fault kinematic and paleostress analyses suggest a record of transitional, and perhaps partitioned, strains from the Laramide orogeny through Rio Grande rifting. Normal faults within Tertiary basin-fill sediments are consistent with more typical WNW-ESE Rio Grande rift extension, perhaps de coupled from bedrock structures due to strength contrasts favoring the formation of new faults in the relatively weak sediments. Analyses of the fault-length data indicate power-law length distributions similar to those reported from many geologic settings globally. Mineralogy and chemistry in Proterozoic fault-related rocks reveal geochemical changes tied to hydro thermal alteration and nearly isochemical transformation of feldspars to clay minerals. In sediments, faulted minerals are characterized by mechanical entrainment with minor secondary chemical changes. Enigmatic breccias in rift-flanking Protero zoic rocks are autoclastic and isochemical with respect to their protoliths and exist near shatter cones believed to be related to a previously reported pre-Pennsylvanian impact event. A weak iridium anomaly is associated with the breccias as well as adjacent protoliths, thus an impact shock wave cannot be ruled out for their origin. Major fault zones along the eastern rift-flank mountain front are discontinuous and unlikely to impede regional groundwater flow into Española Basin aquifers. The breccia bodies are not large enough to constitute aquifers, and no fault-or breccia-related geochemical anomalies were identified as potential contamination sources for ground or surface waters. The results of this work provide a broad picture of structural diversity and tectonic evolution along the eastern flank of the central Rio Grande rift and the adjacent Española Basin representative of the rift as a whole and many rifts worldwide.

Fault Kinematics of the Southern Rio Grande Rift: Exploring the Possibility of Fault Reactivation

“…Field observations and cross -cutting relationships provide good constraints that suggest the maximum horizontal extension was dominantly E -W during the late Tertiary. This indicates that normal faulting occurred after contractional and strike -slip faulting (Caine et al, 2017). Additionally, Caine (2017) suggests that the extensional slip reactivated major and minor structures that possibly formed during the Laramide Orogeny.…”

“…This indicates that normal faulting occurred after contractional and strike -slip faulting (Caine et al, 2017). Additionally, Caine (2017) suggests that the extensional slip reactivated major and minor structures that possibly formed during the Laramide Orogeny. The evidence includes closely located and similar orientation of reverse and strike -slip faults with major and minor normal faults and their related structures (Caine et al, 2017).…”

“…Smaller scale studies within the Tusas -Abiquiu segment have focused on the Proterozoic and Paleozoic rocks flanking the Española Basin (e.g. Caine et al, 2017). Other fault kinematic studies in the central rift assessed strain transfer within basins in the central Rio Grande rift (Minor et al, 2013).…”

“…The Española Basin is a result of multiple deformation events that happened in a progressive counterclockwise-rotating, far field reverse fault stress regime (Caine et al, 2017).…”

“…Additionally, Caine (2017) suggests that the extensional slip reactivated major and minor structures that possibly formed during the Laramide Orogeny. The evidence includes closely located and similar orientation of reverse and strike -slip faults with major and minor normal faults and their related structures (Caine et al, 2017). Finally, few of the more outstanding NE -striking, steeply dipping faults in the Española Proterozoic flank, such as the Santa Fe fault, contain slickenlines and shear sense data that indicate sinistral strike -slip movement.…”