Seismic Investigation of the Yavapai-Mazatzal Transition Zone and the Jemez Lineament in Northeastern New Mexico Maria Beatrice Magnani and Alan Levander Department of Earth Science, Rice University, Houston, Texas Kate C. Miller and Tefera Eshete Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico A new seismic reflection profile of the Precambrian lithosphere under the Jemez Lineament (JL) (northeastern New Mexico, USA) shows impressive reflectivity throughout the crust. The upper crust is characterized by a 2 km thick undeformed Paleozoic and Mesozoic sedimen- tary sequence above the Precambrian basement. At a depth of 5–8 km, undulating reflec- tions image a Proterozoic nappe cropping out in the nearby Rincon Range. To the south the upper crust is seismically transparent except for south dipping reflections at 2–10 km depth. The middle-lower crust, from 10–45 km depth, shows oppositely dipping reflections that converge in the deep crust (35–37 km) roughly at the center of the profile. To the north the reflectivity dips southward at 25° to a depth of 33 km before fading in the lower crust. In the southern part of the profile a crustal-scale duplex structure extends horizontally for more than 60 km. We interpret the oppositely dipping reflections as the elements of a doubly ver- gent suture zone that resulted from the accretion of the Mazatzal island arc to the southern margin of the Yavapai proto-craton at ~1.65–1.68 Ga. Subhorizontal high amplitude reflec- tions at 10–15 km depth overprint all the reflections mentioned above. These reflections, the brightest in the profile, are interpreted as mafic sills. Although their age is unconstrained, we suggest that they could be either 1.1 Ga or Tertiary-aged intrusions related to the volcanic activ- ity along the JL. We further speculate that the Proterozoic lithospheric suture provided a pathway for the basaltic magma to penetrate the crust and reach the surface. 1. INTRODUCTION had a persistent influence on the tectonic and magmatic evo- lution of the Southern Rocky Mountain lithosphere for as Geologic and geophysical investigations in the southwest- long as 1.8–1.6 Ga since initial continental assembly. Here, the ern US suggest that Proterozoic accretionary boundaries have northeast-striking fabric established during cratonic assembly Book Title has provided, to varying degrees, a preferred path for the mod- Book Series ification of the lithosphere during the various Phanerozoic Copyright 2004 by the American Geophysical Union orogenic and magmatic events that modified the western mar- 10.1029/Series#LettersChapter# gin of the North American plate. 1 2 SEISMIC INVESTIGATION IN NE NEW MEXICO During cratonic assembly, tectonic elements of various Ma) and Tertiary extension, magmatism and uplift [see Karl- scales up to lithospheric-scale were incorporated in the Lau- strom and Humphreys, 1998 for a review]. rentia super-continent and the boundaries between these ele- The Proterozoic continental assembly produced an area of ments persisted as shallowly to steeply dipping structural and accretion ~1200 km wide that includes major crustal provinces chemical boundaries, some of which appear to have been defined on the basis of their composition and age: the Archean reactivated during subsequent magmatism or/and tectonic Wyoming Province (with protoliths and deformation between deformation. Geophysical, geological and geochemical data 2.5–3.5 Ga), the Mojave Province (pre-1.8 Ga crustal mate- show that these northeast-striking boundaries influenced the rial to a 1.75 Ga arc), the Yavapai Province (1.76–1.72 Ga sedimentation throughout the Paleozoic, for example, con- juvenile arc crust deformed during 1.70 Ga Yavapai orogeny) trolling the location of mineralization and mantle-derived and the Mazatzal province (1.7–1.6 Ga supracrustal rocks) magmatism in the Tertiary (see Colorado Mineral Belt, Jemez (see Karlstrom et al., [this volume] for a review). The exposed Lineament as examples) [Tweto and Sims, 1963]. boundaries separating these provinces have a general north- Despite the long-lived influence of some of these assembly east strike but exhibit extremely different characters. Unlike boundaries, the details of the nature, structure, and tectonic role the Cheyenne belt, which abruptly separates the Archean and of these features have not been determined clearly. The south- Proterozoic crust, boundaries between the Proterozoic ter- ern segment of the Continental Dynamics of ROcky Mountains ranes of the Mojave and Yavapai and Yavapai and Mazatzal are (CD-ROM) active source seismic experiment [Prodhel et al., diffuse and transitional zones where changes include isotopic this volume] targeted the boundary separating the Yavapai (Pb and Nd) differences of the crust, timing of deformation and Mazatzal terranes, two Paleoproterozoic island arc ter- (before 1.7 Ga for the Yavapai Province, between 1.66 and ranes with different geochemical signatures. As discussed 1.60 Ga for the Mazatzal Province), and character of crustal below, the location and nature of this boundary is still hotly xenoliths (P-T paths and rock types). The Yavapai-Mazatzal debated. We have interpreted from the reflection profiles that crustal boundary in the Southern Rocky Mountains is a 300 the Yavapai-Mazatzal boundary in central New Mexico rep- km wide region with tectonically intermixed rocks of the two resents a doubly vergent accretionary suture and that it coin- provinces. The northern edge of the transition zone is defined cides with the modern-day Jemez Lineament, a by the northern limit of the Mazatzal-age (1.7–1.65 Ga) defor- Tertiary-Quaternary volcanic trend stretching from Arizona mation in southern Colorado [Shaw and Karlstrom, 1999]. across New Mexico to southern Colorado. From the regional The southern margin of the transition zone coincides approx- point of view the geometry of the features recognized in the imately with the Jemez Lineament in northern New Mexico. seismic data provides an explanation for the elusive and dif- fuse character of this boundary. More generally the coinci- 3. THE JEMEZ LINEAMENT dence of the Proterozoic suture with the Tertiary volcanic centers of the Jemez Lineament suggests that collision zones The Jemez Lineament is defined on the basis of the NE- play a persistent (and active) role in the evolution of the lith- trending alignment of Tertiary-Quaternary volcanic centers osphere. In the case of the Proterozoic suture studied, the [Mayo, 1958] of variable-width extending more than 800 km weakness zone in the crust, and likely in the mantle, acted as from the White Mountains-Springerville, Arizona, volcanic a locus of tectonism and provided a preferred pathway for the field at the southern margin of the Colorado Plateau, through mantle to penetrate the crust. The seismic data document a the Jemez Mountains [Aldrich, 1986], to the Raton-Clayton multifaceted tectonic history with crustal scale structures that center and into the western Great Plains of New Mexico. The have survived since continental accretion, and Tertiary igneous volcanic rocks along the lineament range in age from 16.5 structures that are an expression of the processes that modify Ma to 1200 B.P. [Gardner and Goff, 1984] and are domi- the lithosphere today. nantly basaltic with a few silicic volcanoes. The alkali basalts show great petrologic variation along the lineament, attrib- 2. THE YAVAPAI-MAZATZAL TRANSITION-ZONE uted to distinct source zones in the mantle, different crustal contaminants and variable differentiation. Petrologic obser- The Southern Rocky Mountains region has experienced a vations on the alkali basalts at the northeastern end suggest a complex geologic history recording lithospheric assembly cratonic mantle source that is distinct from the asthenospheric during Paleo-Proterozoic time (1.8–1.6 Ga), intracratonic signature of the magmas in the southern end of the lineament magmatism during the Mesoproterozoic (1.44–1.35 Ga), incip- [Perry et al., 1987]. The volcanic activity along the Jemez ient rifting (1.1–0.5 Ga), development of the ancestral Rock- lineament exhibits no time progression: volcanism began at ies during the Paleozoic (350–290 Ma) and, more recently, 13.2 Ma in the middle of the lineament, at about 9.8 Ma at the Laramide tectonism during the Cretaceous-Paleogene (75–45 southwestern end, and at 8.2 Ma at the northeastern limit. MAGNANI ET AL. 3 Karlstrom and Daniel [1993] have proposed that the Jemez to account for differences in outcrops north and south of Las Lineament is a broad zone coinciding with the southern edge of Vegas, we chose different filter parameters for the two seg- the Yavapai-Mazatzal crustal boundary in northern New Mex- ments of the southern CD-ROM profile. However the pro- ico, as it also corresponds to a band of northeast-trending mag- cessing parameters of the two seismic segments are very netic highs located south of the volcanic centers [Zietz, 1982] that similar, permitting the combination of the final migrated sec- exhibit right lateral offsets across north-striking faults. tions into a composite display. Tomographic studies of the mantle underneath the Jemez lin- The seismic data generally exhibit good S/N ratio with eament in New Mexico show a pronounced low-velocity reflections visible at offsets up to 14 km and up to 10 s time anomaly of 1–2% within the regional 5–7% low-P-wave- on shot gathers. The most commonly identified noise on the velocity zone. The anomaly is 200 km long and 100 km wide data includes reverberations generated in the subsurface, sur- and lies in the depth range of 50–160 km [Spence and Gross, face (Rayleigh) waves, air waves, 60 Hz electrical power-line 1990] suggesting the presence of melt in upper mantle rocks. noise, and incoherent ambient or cultural noise.