ABSTRACT Continuous sediment, pollen, and charcoal records were developed from an 8.46-m-long sediment core taken from Hermit Lake in the northern Sangre de Cristo mountain range of Colorado. Presently, vegetation around the lake is upper subalpine forest, consisting of Picea engelmannii (Englemann spruce) with some Abies lasiocarpa (subalpine fir), and the lake lies >200 m below present tree line. We used several pollen ratios to reconstruct the relative position of the tree line and the occurrence of clay layers to infer landscape instability through time. Deglaciation of the Hermit Lake drainage began during the Bølling-Allerød interval. Between ca. 13.5 and 12.4 ka, high Artemisia (sagebrush) pollen abundance, low Picea / Pinus (spruce/pine; S/P) ratios, and sporadic occurrence of Picea macrofossils indicate alpine tundra-spruce conditions. Though the pollen record shows no transition to the Younger Dryas, the subsequent absence of Picea needle fragments suggests a lowering of tree line. By ca. 10.2 ka, a subalpine forest of Picea and Pinus grew there. Based on pollen ratios, tree line was higher than today from ca. 9.0 to ca. 3.8 ka, after which the tree line began to lower to its present elevation. Maximum expansion of the Picea-Abies subalpine forest, determined from both pollen and macrofossils, was coincident with the highest influx of charcoal particles and maximum deposition of postfire erosion (clay layers) into the lake. The period ca. 7.8–6.2 ka was the driest period, as shown by aquatic indicators, but pollen ratios suggest that ca. 6.2–3.8 ka was the warmest period of the Holocene, accompanied by high rates of burning, and consequently elevated erosion of clays into the lake. During the late Holocene, declining S/P ratios are interpreted as declining alpine tree line, while decreases in both Picea to Artemisia (S/Art) and Pinus to Artemisia (P/Art) ratios suggest climate cooling. Pollen evidence suggests expansion of the lower-elevation Colorado piñon ( Pinus edulis ), which has been documented as part of a widespread phenomenon noted by other studies.

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 10 4 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.

The Pecos greenstone belt, dated at ca. 1,720 Ma, consists of metamorphosed subaqueous basalts together with locally important felsic volcanic rocks, iron-formation, and metasedimentary rocks, some of volcanic provenance. Volcanic rocks define a compositionally bimodal suite of basalt (80 percent) and dacite-rhyolite (20 percent). Mafic volcanics are mostly fine-grained, massive to well-foliated amphibolite that locally display relict amygdules, pillows, and pillow breccia. Felsic volcanic rocks are mainly porphyritic flows and crystal-rich volcaniclastic units that contain variable amounts of quartz, K-feldspar, and plagioclase phenocrysts. The greenstone belt also includes a compositionally bimodal subvolcanic complex that intrudes, and is locally overlain by, portions of the volcano-sedimentary pile. The subvolcanic complex comprises concordant to discordant, hypabyssal intrusions of tonalite-trondhjemite (65 percent) and diabase-gabbro (30 percent). Ultramafic and mafic rocks with possible ophiolitic affinities constitute the remaining 5 percent of the subvolcanic complex. Greenstone-belt rocks have undergone regional metamorphism of upper greenschist to lower amphibolite grade and show the effects of at least three periods of deformation. The metamorphic sequence is cut by pre- to syn-orogenic granites and quartz porphyries dated at ca. 1,650 Ma and by syn- to post-orogenic granitic rocks dated at ca. 1,500 to 1,450 Ma. Mafic volcanic and subvolcanic rocks define four distinct chemical populations: (1) ultramafic, (2) high-Mg tholeiite, (3) tholeiite, and (4) calc-alkaline basalt (CAB). Geochemically, the high-Mg tholeiites are similar to Archean high-Mg tholeiites, whereas the tholeiites and calc-alkaline basalts are similar to their counterparts in modern arc systems. The ultramafic rocks and various basalt groups cannot be related to each other by closed-system fractional crystallization or batch melting of a single mantle source. Their incompatible-element ratios seem to demand at least three different mantle sources (one depleted). Felsic volcanics can be divided into three groups (one rhyolitic and two dacitic) on the basis of immobile-element abundances. The rhyolites and one dacite group can be produced by fractional crystallization of CAB. The other dacite group (Doctor Creek dacite) is not obviously or easily related, chemically or genetically, to the rest of the Pecos volcanic and subvolcanic rocks. It instead may belong to a younger (ca. 1,700 Ma) felsic-dominated volcanic succession that is widespread in northern New Mexico. Rocks of the Pecos greenstone belt may represent a remnant of a back-arc basin that opened far enough to form oceanic crust and tap a depleted mantle source.

We describe the structure of the eastern Española Basin and use stratigraphic and stratal attitude data to interpret its tectonic development. This area consists of a west-dipping half graben in the northern Rio Grande rift that includes several intrabasinal grabens, faults, and folds. The Embudo–Santa Clara–Pajarito fault system, a collection of northeast- and north-striking faults in the center of the Española Basin, defines the western boundary of the half graben and was active throughout rifting. Throw rates near the middle of the fault system (i.e., the Santa Clara and north Pajarito faults) and associated hanging-wall tilt rates progressively increased during the middle Miocene. East of Española, hanging-wall tilt rates decreased after 10–12 Ma, coinciding with increased throw rates on the Cañada del Almagre fault. This fault may have temporarily shunted slip from the north Pajarito fault during ca. 8–11 Ma, resulting in lower strain rates on the Santa Clara fault. East of the Embudo–Santa Clara–Pajarito fault system, deformation of the southern Barrancos monocline and the Cañada Ancha graben peaked during the early–middle Miocene and effectively ceased by the late Pliocene. The north-striking Gabeldon faulted monocline lies at the base of the Sangre de Cristo Mountains, where stratal dip relations indicate late Oligocene and Miocene tilting. Shifting of strain toward the Embudo–Santa Clara–Pajarito fault system culminated during the late Pliocene–Quaternary. Collectively, our data suggest that extensional tectonism in the eastern Española Basin increased in the early Miocene and probably peaked between 14–15 Ma and 9–10 Ma, preceding and partly accompanying major volcanism, and decreased in the Plio-Pleistocene.

Precambrian supracrustal rocks of the Cimarron Canyon area consist of quartzite, amphibolite, and a previously undescribed bimodal metavolcanic assemblage with associated metasedimentary rocks. These layered rocks, and the plutons that intrude them, differ on opposite sides of the Fowler Pass fault, a northwest-trending reverse fault of presumed Laramide age. The rocks east of the fault consist of a weakly metamorphosed sequence of felsic and mafic volcanic rocks and interlayered volcaniclastic sediments (now phyllite, chlorite schist and metasiltstone). The felsic rocks, which locally contain well-preserved eutaxitic fabric and bipyramidal quartz phenocrysts, are anomalously low in K 2 O and may represent highly altered tuffs. The metabasalts are tholeiitic and locally show amygdular texture. The metavolcanic rocks were intruded sequentially by small plutons that range in composition from gabbro to granodiorite and granite but do not appear to be comagmatic with granitic rocks west of the fault, the former being appreciably lower in potash and higher in lime and soda. West of the fault, the weakly to strongly foliated granitic rocks contain elongate roof pendants of quartzite (feldspathic and muscovitic near contacts) and smaller pendants of amphibolite of uncertain origin. Stratigraphic relations between the quartzites and the metavolcanic rocks, which lie on opposite sides of the Fowler Pass fault, are not determinable in this area. Lithologic similarity of these rocks to radiometrically dated supracrustal rocks in the Sangre de Cristo Mountains and Tusas Mountains to the west suggests a late early Proterozoic age for the deposition and metamorphism of the Precambrian framework of the Cimarron Mountains.