Students Explore Sierra Los Alacranes, Sonora
These images document the landscape and rock outcrops in the vicinity of Sierra Los Alacranes, loosely translated as "mountain of the scorpions." I began taking students to this area in October, 1997. The geologic map shown below gives a sense of our accomplishments, but there is still much work to be done. Using U-Pb zircon geochronology, USGS colleague Wayne Premo and I have had good success distinguishing various pluton emplacement ages . Ongoing studies are unraveling the complex metamorphic history of this region.
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Panoramic view of the field area, looking southeast. Mexican Highway 2 (in the right foreground) passes between Sierra Las Tinajas Altas (left) and Sierra Los Alacarnes (highest range on the right).
Panoramic view (looking northeast) of the poorly accessible southwest side of Sierra Los Alacranes. Highway 2 is on the far side of the range. Granodiorite and granite plutons exposed in the northwest end of the range have yielded U-Pb zircon ages of 1722+/-12 Ma and 1696+/-11 Ma ; augen gneiss in the southwest end is 1725+/-19 Ma (see photographs below for typical field relationships).
Simplified geologic map of Sierra Los Alacranes and the southeastern part of Sierra Las Tinajas Altas. This is Figure 3 in GSA Special Paper #393, Chapter 4.
Early morning sunlight strikes steep cliff exposure of Paleocene(?) leucogratic granite of Sierra Las Tinajas Altas. Hills in the foreground are composed of Tertiary boulder-cobble conglomerate (these exposures extend to the roadcut shown below).
Miguel Espinoza measures bedding in tilted Tertiary conglomerate exposed in Highway 2 cut.
Miguel and Randal Burns look northwest toward Sierra Las Tinajas Altas from outcrop of 1725 Ma augen gneiss in Sierra Los Alacranes.
Typical exposure of mafic gneiss-amphibolite in northwest Siarra los Alacrenes, interlayered with thin bands of felsic gneiss and sills of 1696 Ma granite
Screen of mafic gneiss intruded by 1722 Ma granodiorite
Sample locality in southeastern Sierra Los Alacranes for granite augen gneiss geocron sample Alacranes #1. This rock yielded a U-Pb SHRIMP zircon age of 1725+/-19 Ma .
Augen gneiss in Sierra Las Tinajas Altas that strongly resembles dated sample Alacranes #1.
Two tilted tertiary basalt flows (Mesas de Malpais) occupy the pass between Sierra Las Tinajas Altas (left) and Sierra Los Alacranes (right). Red outcrops on the the left are altered andesite flows that overlie Tertiary conglomerate, which in turn rests upon Paleoproterozoic gneiss and Cretaceous quartz diorite.
Prominent fault breaks through conglomerate in the same roadcut.
Miguel and Randal stand in one of two major passes of Sierra Los Alacranes. View is to the south, with east-dipping banded gneiss sequence in the background.
Outcrop of fine-grained banded quartzo-feldspathic gneiss showing amphibolite layer broken by fault.
1722 Ma gneissic granodiorite with screens of banded gneiss host rock
Miguel points to example of southwest-vergent fold recorded by 1722 Ma granodiorite gneiss and younger granite dikes.
Augen gneiss displays intrusive relationship with mafic gneiss host.
Folded dike of augen gneiss within mafic gneiss-amphibolite host.
Another folded dike of 1725 Ma augen gneiss within mafic and felsic gneiss country rocks.