New Mexico Geological Society Annual Spring Meeting — Abstracts

Reconnaissance structural investigation of the Little Hatchet Mountains, SW Mexico, shows the presence of a low angle normal fault (Howells Ridge detachment). This fault was previously interpreted to be a thrust, but we suggest· that the latest movements on the fault acted to thin the crust. 1. It excises stratigraphic section (>1000 m of the U-Bar is missing); 2. It places a section of unmetamorphosed Lower Cretaceous through Lower Tertiary strata on top of Lower Cretaceous rocks that are pervasively intruded and contact metamorphosed by Laramide intrusions. And 3. It shows consistent brittle extensional fault geometries and extensional shear sense indicators. Because the fault is subparallel to bedding, generally subhorizontal and gently synform ally folded, we interpret it to have moved at a low angle orientation. Magnitude of displacement is unknown but it may be part of an upper crustal detachment system like those in Arizona that merge at depth with ductile extensional shear zones.

The low angle detachment fault cross cuts and is thus younger than the Paleocene Eocene Skunk Ranch Formation, and Laramide diorite-quartz monzonite intrusions (ca. 55-60 Ma). We.speculate that extension was roughly synchronous with intrusion of felsite dikes and sills (Ca. 30 Ma) for several reasons. 1. Latite dikes cross cut both lower and upper plate but are more abundant in the lower plate. 2. Latite dikes are subvertical and strike dominantly W-to NW, compatible with subhorizontal N-to NE-extension indicated by lineations on the detachment fault. And 3. This timing and extension direction are compatible with early stages of extension seen in adjacent areas of the southern Basin and Range.

NE extension in the Little Hatchets is interpreted to be part of a continuum where crustal thinning followed orogenic thickening in the hinterland of the southern Cordillera. The Granite Pass thrust, which places Paleozoic and Proterozoic rocks above Lower Cretaceous rocks, predated intrusion of the Granite Pass granite (ca 45 Ma) but apparently post-dated or was synchronous with intrusion of ca. 60 Ma diorite-quartz monzonite Laramide inttllsions. Contraction in the Little Hatchets continued through the early Tertiary based on deposition and subsequent thrusting of synorogenic early Tertiary deposits. Style of contraction (ramp-flat geometries versus basementcored Laramide uplifts) remains obscure until extensional fault geometries are better understood.

Kinematic studies of synkinematic calcite vein networks in· carbonates indicate that the incremental extension direction of the strain field rotated through more by 150 degrees during calcite veining.that may have spanned the last 60 Ma. Subvertical 020 veins (compatible with NNE thrusting) were emplaced first. These are cut by 050, then 090-120, then 150-180 subvertical veins suggesting that incremental extension rotated clockwise from 290 to 090. The 090-120 veins (record the dominant extensional strain (several to tens of percent) and are interpreted to be synchronous with WNW-striking latite dikes (ca. 30 Ma) and with detachment faulting. Subvertical E-striking dextral strike slip faults ( eg. Copper Dick fault) crosscut the detachment. These are, in turn cut, by N-striking range bounding faults which, along with the late N-striking calcite veins, record latest increments of Tertiary extension.