| Abstrakt: |
For more than 65 years, geologists working in southern New Mexico have recognized that the sedimentary and volcanic rocks, unconformities, and structures exposed at San Diego Mountain and in the Tonuco uplift of northern Doña Ana County are keys to interpreting the Cenozoic evolution of the region. When combined with interpretations of outcrops from neighboring mountain ranges, a nearly complete record of major Cenozoic tectonic events can be pieced together, ranging from Laramide deformation to the evolution of the Rio Grande rift. Laramide contraction in south-central New Mexico produced the large Rio Grande uplift, which may rival some of the Wyoming Laramide uplifts in magnitude and style. Its northwest trend and northeast vergence are consistent with other Laramide uplifts that have been reconstructed across southern New Mexico. Its notable structural relief is confirmed by the thickness of 3.1 km of Paleozoic strata and an unknown amount of Proterozoic rocks that were eroded from its core. The sediments filled the complementary Love Ranch and Potrillo basins to a depth of 0.9-2.0 km, onlapped the uplift, and, in an imperfect way, recorded the erosional unroofing of the Rio Grande uplift. By middle Eocene time, Laramide uplifts were no longer active, were deeply eroded, and were at least partially buried in their own erosional debris. Intermediate-composition arc volcanism commenced at approximately 46 Ma, and by 37 Ma lava flows and lahar deposits had buried the Laramide structures to depths of as much as 0.6 km. The eruption of 36-34 Ma ignimbrite calderas and the deposition of ignimbrite outflow sheets, often interpreted to herald the onset of extension in southern New Mexico, may also be interpreted to represent the culmination of arc volcanism in a non-extensional stress field--a conclusion reached by other researchers in the southern Cordillera and southern Rocky Mountain regions. The ignimbrite outflow sheets, together with interbedded tuffaceous and fluvial sediment of the Bell Top Formation (36.0-28.6 Ma), accumulated on piedmont slopes adjacent to the Emory caldera and in a broad, shallow paleovalley across south-central New Mexico. Previous interpretations of the Bell Top Formation linked its origin to deposition in an extensional half graben (Mack et al., 1994a) or a volcano-tectonic depression (Seager, 1973), interpretations sometimes cited as evidence for late Eocene onset of extension in the Rio Grande rift. The geometry, facies distribution, low sedimentation rates, and lack of coeval faulting within the Bell Top Formation do not support an extensional setting for the formation. Instead, the outflow tuffs and sediment in the formation are interpreted to have filled a broad, shallow paleovalley and accumulated on a piedmont-slope landscape, both of non-extensional origin. Initiation of the San Andreas transform may have promoted earliest transtension across the western interior of North America, a hypothesis that is consistent with the onset of regional extension in the Rio Grande rift shortly after 30 Ma. Across south-central New Mexico, the outpouring of the Uvas Basaltic Andesite and Bear Springs Basalt (28 Ma), as well as a change to bimodal volcanism in the Mogollon-Datil volcanic field, mark significant extension. Together with other basaltic andesite units of southern New Mexico and northern Mexico, the basaltic lavas of these formations accumulated to form a broad plateau, fed largely by fissure eruptions. Local fissures, such as the ancestral Cedar Hills fault, probably had sufficient structural relief to produce small alluvial fans. By 27.4 Ma, accelerating extension is suggested by the initiation of a broad, deep basin in northern Doña Ana and southern Sierra counties. The basin was filled with the distal parts of an apron of ash-fall tuff and volcaniclastic sediment more than 0.5 km thick (Thurman Formation) derived from the Mt. Withington caldera located 60 km to the northwest. The onset of major faulting within the Rio Grande rift, at approximately 26 Ma, is recorded by the thick (1.5 km) "early rift" alluvial-fan and playa (closed basin) deposits exposed at San Diego Mountain and adjacent to the southern Caballo Mountains. The stratigraphy of these deposits, as well as the structures that transect them, indicate that oldest fault blocks within the rift, such as parts of the Caballo uplift, continued to rise throughout the Neogene up to the present. Other fault blocks were initiated at different times--middle Miocene, late Miocene, and perhaps Pliocene. The younger episodes of faulting not only initiated new uplifts but also fragmented all or parts of the older "early rift" closed basins, creating fault blocks such as the Tonuco, Rincon Hills, and Robledo uplifts, among many others. Although folding of middle Paleogene and Neogene rocks is not uncommon in the southern Rio Grande rift, all is extensional in origin, the product of draping or forced folding of strata across active normal faults. Extreme and perhaps very rapid local extension in the Tonuco uplift area is suggested by a rotated, uplifted, and abandoned low-angle normal fault on the Tonuco horst. At around 5 Ma, the broken and deeply eroded landscape was buried by "late rift" alluvial fans and fluvial deposits (Camp Rice Formation) associated with the ancestral Rio Grande as it entered southern New Mexico. At 0.8 Ma, this aggradational regime gave way to a degradational one, at least along the Rio Grande corridor. As a result, older structures, such as the Tonuco uplift, were exhumed, the Rio Grande and its tributaries were entrenched into modern valleys, and several generations of terraces along valley sideslopes formed in response to waxing and waning glacial cycles. The "late rift" Plio-Pleistocene deposits remain relatively undeformed, although locally they have been warped and broken along range-boundary faults by Pleistocene fault movements. Locally, Holocene movement has been documented. Broad, epeirogenic uplift of the Rio Grande rift and its flanks (perhaps 800 m or more) has accompanied the evolution of the rift since the late Paleogene. [ABSTRACT FROM AUTHOR] |
