Slab Break-off Mechanism for the Late Neogene Uplift of Honduras, Northern Central America

Robert D. Rogers

Rogers, R.D., 2000, Slab Break-off Mechanism for the Late Neogene Uplift of Honduras, Northern Central America, AGU Fall meeting, San Franciso, CA

Abstract
Honduras is characterized by broad highlands with elevations of between 1 to 2.8 km extending 400 km from behind the Central America volcanic arc to the Caribbean Sea.  Much of this elevated area is underlain by up to 2 km of ignimbrite and tuff deposits produced during a regional "flareup" that peaked at 15 Ma and ended by 10.5 Ma and was probably related to partial melting of continental crust beneath the area.  A record of regional uplift across Honduras is recorded in deeply entrenched meandering bedrock rivers, dissected erosion surfaces and incised bedrock channels that cut into the ignimbrites. A widespread network of incised bedrock channels has been mapped in the field, on aerial photos, and satellite images and compiled in GIS coverages.  Bedrock meanders are tectonically significant because they require uplift without significant tilting to form.  Bedrock channels incised into the ignimbrite date the initiation of the uplift after the peak of the ignimbrite flareup at 15 Ma.   Post 10.5 Ma extension related disruption of bedrock meanders in western Honduras brackets the age of uplift between 10.5 and 15 Ma.  I propose regional uplift initiated in Honduras and Nicaragua between 15 and 10.5 Ma involving the breakoff of the subducted Cocos slab beneath this area and the upwelling of hotter, less dense asthenosphere into the area of slab breakoff.  The presence and location of a 200-km-wide slab gap beneath northern Central America is constrained by tomographic work by van der Hilst (1990) and is projected onto maps of bedrock meanders and all dated late Neogene extrusive and intrusive units in Honduras.  A possible correlation of the slab gap to geomorphic and bedrock features of Nicaragua is also presented.

A.  Dissected Plateau in Honduras	B.  Model of Landform Evolution

Numbers indicate location of longitudinal profiles below.    Profiles below: Lower line - River  Upper line - interfluve elevation

North Coast rivers responding to present uplift.

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Rivers dissecting plateau to south.

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Rio Cuaca - Cuts Cordillera and actively capturing drainage from Sico and Mame.

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Rio Mame - Entrenched in dissected erosion surface formed on EarlyTertiary granite.	Rio Sico - Superimposed meanders on inverted topography along Caribbean Coast.  Rio Paulaya valley to south marks trace of Neogene Guayape fault system.	Rio Patuca - Superimposed meanders in dissected erosion surface of Jurassic metasedimentary strata.  Edge of Montana de Colon fold belt to right reflect strong differential weathering between limestone and breccia forming ridges and finer grained red beds in lowlands.

E.  Slab Breakoff as a Mechanism for Plateau Uplift and River Entrenchment

Gap in downgoing slab beneath Central America (image from Karason and van der Hilst, 2000).  Interpreted as a detached slab (van der Hilst, 1990).	Extent and depth to top of detached structure corresponds with regional distribution of geomorphic features developed in response to uplift.

Profiles 1. and 2. show the P-wave velocity constrast beneath Central America (dark is +3% and light is - 3%) perpendicular to the Middle America Trench. Contours on top of lower structure in upper mantle are based on largest gradient contrast.

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In contrast, the position of the subducted Cocos Plate is shown to depth of 200 km based on telesiesmic study of Burbach and Frolich (1986).

Ignimbrite deposited on low relief surface and accumulated to at least 2 kilometers thickness. Drainage basin reintegration occured after peak of flare-up and regional uplift entreched the drainage into plateau, pinning many rivers in their present locations. Extension in western Honduras begins after 10.5 Ma (Gordon and Muehlberger, 1994), faulting the plateau and disrupting previously entrenched drainage system.

Ignimbrite "flare-up" with peak activity between 13 and 15 Ma and 17 and 19 Ma.